Evaluation of long-term performance of sodium silicate

INOM TEKNIKOMRÅDETEXAMENSARBETE ENERGI OCH MILJÖOCH HUVUDOMRÅDETMILJÖTEKNIK,AVANCERAD NIVÅ, 30 HP

, STOCKHOLM SVERIGE 2019

Evaluation of long-term performance of sodium silicate grouted in embankment dams

JENNY FU

KTHSKOLAN FÖR ARKITEKTUR OCH SAMHÄLLSBYGGNAD

Abstract

Embankment dams is the most common type of dams in operation in

Sweden today. Due to the nature of embankment dams, seepage through

them will always occur. If the seepage velocity exceeds a critical velocity,

internal erosion is initiated, which could lead to damage in form of piping

and sinkholes. To treat this problem, remedial grouting has been

performed involving a combination of conventional grouts, i.e. cement and

cement-bentonite as well as sodium silicate, which is a chemical grout that

also known as water glass. Regarding the sodium silicate grout, there is

concern about the long-term permanence.

The aim of this thesis has been to study the potential performance of

sodium silicate grouted in embankment dams. The first part of this thesis

is a literature review of the general behavior of sodium silicate as a grout,

its degradation processes and the factors that could induce degradation.

The second part suggests monitoring methods to control and evaluate the

performance of the treated dam and the grout if degradation has occurred.

Findings from literature generally indicates a high risk of instability and

low permanence of sodium silicate when grouted in an embankment dam.

This type of grout will undergo degradation mainly in two forms: syneresis

induced shrinkage and leaching due to grout erosion or dissolution. As the

degradation has developed, an increase in permeability of the repaired

dam core is a potential consequence.

How the potential degradation of sodium silicate will affect the treated

dams is suggested to be observed by monitoring the permeability of the

grouted core. Applicable monitoring methods for this purpose are

measurements of pore pressure and temperature using piezometers. The

second direct method of monitoring a changed dam behavior is suggested

to be leakage analysis, in order to detect potentially increased leakage

because of the grout degradation. An indirect way to investigate the dam

performance is suggested to be monitoring of the grout state. Measurement

of ion concentration of sodium and silicon respectively in leakage water

using selective-ion electrodes will reveal any increase in ion concentration

due to the potential grout dissolution or leaching.

Keywords

Sodium silicate, water glass, embankment dams, remedial grouting,

dam monitoring

Sammanfattning

Fyllningsdammar är den vanligaste typen av dammar som används i

Sverige idag. Eftersom denna typ av dammar består av packad jord- och

bergmaterial kommer vattengenomströmning genom dammen alltid att

ske. När genomströmningshastigheten överstiger dess kritiska hastighet

kan inre erosion initieras. Piping och sjunkhål i dammen kan uppstå om

fortskridande inre erosion utvecklats. Reparationsåtgärder i form av

injektering har tillämpats för att behandla denna typ av problem kopplat

till inre erosion i svenska fyllningsdammar. En kombination av cement,

cement-bentonit och natriumsilikat som också kallas för vattenglas hade

tillämpats som injekteringsbruk vid reparation. Beständigheten av

natriumsilikat-bruk har emellertid varit ifrågasatt.

Detta examensarbete har utförts med syfte att analysera

beständigheten av natriumsilikat-bruk i fyllningsdammar. Första delen av

rapporten är en litteraturstudie som beskriver det generella beteendet av

natriumsilikat-baserat injekteringsmedel, dess nedbrytningsprocesser

samt att identifiera faktorer som kan ge upphov till nedbrytning av

natriumsilikat. Den andra delen är förslag till övervakningsmetoder för att

bevaka funktionen av den behandlade dammen vid en eventuell

nedbrytning av natriumsilikat-bruket.

Resultatet från litteraturstudien indikerar att det finns hög risk för

nedbrytning av natriumsilikat injekterat i fyllningsdammar. Urlakning och

krypning av injekteringsmassan är två olika nedbrytningsmekanismer. Ett

möjligt resultat av nedbrytningen av natriumsilikat är att den reparerade

tätkärnan får högre permeabilitet.

För att övervaka en eventuell förändrad funktion av dammen p.g.a.

nedbrytningen av injekteringsbruket har tre metoder föreslagits.

Övervakningen kan genomföras med en direkt metod för att bevaka

förändringar och inkluderar mätning av portryck och temperatur i

vattenståndsrör till följd av en förändrad permeabilitet av den reparerade

dammen och dess tätkärna. Övervakning av läckageflöde är en annan

direkt metod som föreslagits för att detektera förändring av injektering. I

den indirekta metoden mäts jonkoncentrationen av natrium- och

kiseljoner i läckagevatten från dammen för att erhålla indikationer på om

en pågående nedbrytning av natriumsilikatbruket äger rum.

Nyckelord

Natriumsilikat, vattenglas, fyllningsdammar, reparationsinjektering,

övervakning av dammar

Acknowledgements

This thesis project is funded by Energiforsk and made in collaboration with

Sweco Power Generation and Dams. I would like to thank Energiforsk that

made it possible to conduct this project.

I would like to thank my examiner Assoc. Prof. Fredrik Johansson for

his advice and guidance throughout this project. I would like to thank my

supervisors M.Sc. Ingvar Ekström, engineer and expert within

embankment dams and dam safety at Sweco, and my co-supervisor Dr

Marie Westberg Wilde, researcher within dams and dam safety at ÅF and

KTH for their invaluable help and encouragement of my learning. Also, I

would like to thank Peter Viklander, Adj. Prof. at LTU and Vattenfall

Vattenkraft AB for the valuable review comments on the performed work.

Finally, I would like to thank Mattias Jender, group manager for

Vattenkraft och Dammar, and everyone else at Sweco Power Generation

and Dams for being so welcoming and kind.

Contents

1. Introduction ............................................................................. 1

1.1. Aims and Objectives ............................................................................ 2

1.2. Limitations ............................................................................................. 2

1.3. Disposition ............................................................................................ 3

2. Embankment dams .................................................................. 5

2.1. Zoned embankment dams in general ................................................. 5 The impervious core.................................................................................. 6 Filter .......................................................................................................... 6 Supporting fill ............................................................................................ 7 Slope protection ........................................................................................ 8

2.2. Internal erosion ..................................................................................... 8

2.3. Remedial measures to repair the impervious core ......................... 10

2.4. General surveillance and monitoring related to internal erosion .. 10 Detection of a damage ............................................................................ 12

3. Methodology .......................................................................... 13

3.1. Evaluation of grout performance ...................................................... 14

3.2. Suggest monitoring and instrumentations ...................................... 14

4. Sodium Silicate grout ............................................................ 15

4.1. Grouting ............................................................................................... 15 Different aims of grouting ........................................................................ 15 Typical grouting materials ....................................................................... 16 Grout requirements ................................................................................. 18

4.2. The sodium silicate solution ............................................................. 19 Composition and properties of sodium silicate ........................................ 19 Setting time and neutralization of sodium silicate ................................... 20 Injection of sodium silicate ...................................................................... 21

4.3. Degradation of sodium silicate ......................................................... 22 Syneresis induced shrinkage .................................................................. 23

Grout leaching caused by gel dissolution and erosion by flowing water .. 29 Influence of grout degradation and the soil grain size ............................. 33

5. Monitoring to control the performance of the dam ............ 37

5.1. Analysis of ion content in leakage water .......................................... 38

5.2. Increased permeability of the core .................................................... 39 Pressure gradient and water level ........................................................... 39 Temperature measurement ..................................................................... 40

5.3. Leakage and turbidity monitoring ..................................................... 41

6. Discussion ............................................................................ 43

6.1. Degradation of sodium silicate in embankment dams .................... 43 Shrinkage due to syneresis ..................................................................... 45 Grout leaching ......................................................................................... 46

6.2. Suggested monitoring methods ........................................................ 47 Reoccurred damage or a new damage? ................................................. 48

7. Conclusions and suggestions for future research ............. 51

References ................................................................................ 53

INTRODUCTION | 1

1. Introduction

Embankment dams are the most common type of dams in operation in

Sweden today. This type of dam is constructed with compacted earth

material, therefore seepage through the dam will always occur even when

operating as designed. If the seepage velocity becomes high enough to

detach fines in the core and transport them, internal erosion may be

initiated.

ICOLD (2017) defined that internal erosion is the phenomena when the

fine soil particles in the core are carried away by seepage. Continuous

internal erosion can occur when filter criteria are not met, leading to

insufficient protection against mass movement and material loss. As

erosion progresses it may result in enlarged seepage paths and excessive

flow. As a result, damage in form of piping and sinkholes may occur.

Several dams in Sweden have experienced damage caused by progressive

internal erosion because the downstream filters were inadequate to

prevent movement of finer grains from the core (Nilsson et al., 1999).

Remedial actions such as grouting have been performed to treat dam

cores damaged by internal erosion, using cement-based grout or combined

with chemical grouts, such as sodium silicate (Sjöström, 1999). According

to published reports, at least five embankment dams in Sweden including

Bastusel, Hällby, Suorva, Näs and Stenkullafors have been grouted with

sodium silicate, mainly during the 1980s (Ekström et al., 2016; Göthlin,

2004). Sodium silica is characterized by its relatively high penetrability

compared to other grouts available at the time. This property is suitable to

treat finer soils, e.g. the impervious core.

Although improved penetration was achieved using sodium silicate in

remedial grouting, there have been concerns regarding the internal

stability and permanence of its end-product. Hansson (1999) stated in a

report that sodium silicate would only be stable for one month. Its long-

2 | INTRODUCTION

term durability is thus questionable, indicating potential degradation of

the grout and thereby decreased remedial effect with time. In turn, it

implies a risk of reoccurred damage in the treated dam.

However, a systematic study of sodium silicate and its performance in

embankment dams has not been performed. This thesis project is therefore

carried out to compile information regarding the behavior of sodium

silicate based grout and its potential degradation process under typical

dam conditions; as well as to suggest monitoring and instrumentation to

control the state of sodium silicate in a dam and the performance of a dam

treated by sodium silicate grout.

1.1. Aims and Objectives

There are two aims with this thesis. The first is to investigate the possible

long-term performance of sodium silicate grout in embankment dams by

identifying potential degradation processes and identifying factors

inducing degradation. The second aim is to suggest monitoring

instrumentations to detect potentially changed dam functionality and

changed grout condition as a consequence of the degradation.

The following objectives have been established to achieve the aims by

answering the following questions:

• Under what conditions can sodium silicate become degraded and

how?

• What changes in a dam treated with sodium silicate are expected to

occur when the remedial effect is decreasing?

• How can potential negative changes of a dam related to decreased

remedial effect be detected by monitoring?

1.2. Limitations

This thesis project aims to describe the general behavior of sodium silicate

grout and the potential degradation of it based on available literature. In

addition, suggested monitoring and instrumentations to control the

performance of the dam with respect to the state of the remedial grouting

INTRODUCTION | 3

are based on already established and commercially available methods.

Hence, development of new instrumentation is not performed in this

thesis.

1.3. Disposition

The disposition of the thesis is as follows:

Chapter 2: Contains a description of typical embankment dams in

Sweden. The chapter also contains a description of the process of internal

erosion, the common type of remedial grouting to treat dam cores, and

general dam monitoring for controlling its performance related to internal

erosion.

Chapter 3: In this chapter, the methodology conducted to achieve the

stated aims is presented.

Chapter 4: A description of sodium silicate grout in general and its

potential degradation processes are presented in this chapter.

Chapter 5: This chapter presents suggestions of monitoring and

instrumentations to detect potential grout degradation.

Chapter 6: Contains a discussion of the susceptibility of sodium silica

grout to degradation under typical dam conditions. Also, the overall

representativeness as well as sensitivity of the suggested monitoring and

instrumentations are discussed.

Chapter 7: Conclusions.

EMBANKMENT DAMS | 5

2. Embankment dams

This chapter first describes the main parts of a zoned embankment dam.

After that, the internal erosion phenomena and its development in an

embankment dam is described. This is followed by a description of the

general principle of remedial grouting performed to treat dam cores that is

experiencing internal erosion. Lastly, existing monitoring related to

internal erosion are presented.

2.1. Zoned embankment dams in general

The following description is mainly based on Vattenfall (1988) and

RIDAS (2011) if no other sources are mentioned.

Embankment dams are constructed from compacted earth and/or rock

material. The history of embankment dams can be dated to as early as 504

B.C. in Ceylon, Sri Lanka for irrigation purposes (U.S. Bureau of

Reclamation, 2012). Sadd-El-Kafara in Egypt constructed around 2650

B.C. is an example of zoned embankment dam with long history (Mays,

2009). Until today, embankment dams are one of the most common types

of dams in operation. Since the construction of embankment dams often

uses locally available material close to the dam site, and the earth work can

be carried out by a local contractor without advanced equipment, the

construction can be both economically and constructively advantageous.

Normally, a zoned embankment dam contains four main zones as

illustrated in Figure 2.1. There is an impervious dam core impounding the

reservoir (1). The core is surrounded by one or more filters to prevent

migration of fine material from the core (2-4). The core and filter(s) are

surrounded by supporting fill to ensure the stability of the dam (5). Finally,

there is a slope protection to protect the supporting fill from erosion due to

6 |EMBANKMENT DAMS

wave action, rainfall and potential flood debris etc. (10). This zone can be

extended over the entire dam surface.

Figure 2.1: A typical zoned embankment dam founded on rock. Source:

Vattenfall (1988).

The impervious core

The impervious core in a zoned embankment dam is designed to prevent

excessive seepage. The core material must be selected with regard to both

permeability and workability. For a typical Swedish zoned dam, the

impervious core usually consists of broadly and well-graded moraine.

Moraine with a high content of silt and sand is a suitable material to meet

the required properties.

Before construction, the core material is subjected to modified Proctor

tests to investigate the water content in relation to the optimal degree of

compaction. During the construction, material segregation must be

prevented. Generally, a wide core is advantageous to ensure the resistance

to internal erosion. A thinner core is more sensitive to deficiencies during

construction. However, the design of the core must also consider the

material availability within a limited distance from the site. This affects the

design, since for example less competent core material can be acceptable,

if it is combined with a wider core and filters.

Filter

Seepage through an embankment dam will always occur due to the nature

of it. However, there is a risk that fines from the core is transported in the

downstream direction when seepage through the dam exceeds a certain

velocity. If the internal erosion process is not controlled or treated, the

EMBANKMENT DAMS | 7

development can damage the water retaining function and eventually

become a threat to the dam function. Filters and supporting fill at the

downstream side are meant to prevent such material transport from the

core.

Filters are placed between the core and the supporting fill, and both

zones consist of material with various properties. The core consists of well-

graded moraine, whereas the supporting fill commonly consists of rockfill.

Filter criteria must be met in transition between these two soil materials

bordering it. Therefore, there are often at least two filters, a fine filter

against the core and a coarse filter bordering the supporting fill.

Three main filter criteria are filtration ability, drainage ability and the

ability to prevent material segregation. The purpose of filtration ability is

to prevent the mitigation of fines in the core. To achieve this criterion, the

voids between the particles in the filter must be small enough to prevent

continuous transport of the core soil. At the same time, the filter should be

sufficiently pervious to drain seepage and prevent pore pressure to build

up. Thirdly, controlled placing of the filter is important in order to avoid

material segregation, which otherwise could influence the filter capability

and create layers of material with higher permeability. Fine filters are

usually designed from sand and coarse filters from gravel. Due to

environmental concerns, these materials are generally processed from

crushed rock.

Supporting fill

The purpose of the supporting fill is to sustain the stability of the dam and

to drain the seepage flow without erosion. To ensure this, coarse grained

material or rock are used.

The safety factor against sliding and for various loading conditions is

verified by stability calculations. For example, the loads to be considered

are the weight from the dam itself before its first impounding, pore-water

pressure due to seepage flow and due to extreme seepage flow respectively,

and eventual rapid draw-down of the impounded water.

The drainage capacity must be dealt with as it influences the dam

stability in several ways, e.g. a sudden increase in pore pressure may lead

8 |EMBANKMENT DAMS

to dam instability. Therefore, the risk of sudden excessive flow is also

considered when designing the drainage capacity of the supporting fill.

Slope protection

Slope protection are generally placed on the surface of a dam. The material

should consist mainly of rock material that are prone against surface

erosion. The protection requirement is highest on the upstream side, where

the erosion risk is high due to destructive wave action and ice load. The

purpose of the downstream protection concerns erosion caused by rainfall

and snow melting, and to increase the erosion resistance in case of

overtopping of the dam.

2.2. Internal erosion

Internal erosion is the phenomenon when seepage through an

embankment dam has become high enough to detach finer particles of the

core or its foundation to cause mass movement downstream (ICOLD,

2017). Well-developed internal erosion without treatment will result in

excessive flow. Turbid leakage containing eroded soil and sinkholes formed

on the surface of the upstream crest are often signs of material transport

due to internal erosion. Mechanisms leading to initiation of internal

erosion are concentrated leak, backward erosion, suffusion and contact

erosion. Once erosion has occurred, it will continue to detach and transport

fines if the hydraulic force is not reduced and material migration is not

limited by adequate downstream filters. Figure 2.2 illustrates the

continuous material loss due to internal erosion.

EMBANKMENT DAMS | 9

Figure 2.2: Material loss of the core and backward erosion piping due to internal

erosion. Source: Rönnqvist (2002).

As the internal erosion progresses, the erosive action will continue

towards the source of seepage at the upstream side. Furthermore, erosion

will continue throughout the dam core and a seepage tunnel within the

dam is formed and enlarged if the dam soil is able to hold such a seepage

tunnel. This type of damage is called piping, also illustrated in Figure 2.2.

As erosion and piping reaches the upstream filter, the filter material may

gradually move down to fill the pipe formed tunnel and lead to a sinkhole

at the upstream slope of the crest surface. Erosion caused by most of the

initiation modes can give rise to piping (ICOLD, 2017).

There are three potential dam failure modes related to excessive flow

caused by internal erosion. The first failure mode is due to extremely

increased leakage to such an extent that the dam is no longer able to pass

it through safely. As a result, rock materials at the toe of the dam can

become unstable and continuous backward erosion could be initiated. In

the second failure mode, pore pressure increases due to excessive leakage

flow and it leads to decreased shear strength at the downstream slope.

Potentially, it can cause slope instability. The third failure mode is dam

breach due to pipe formation and sinkholes, which can lead to loss of

freeboard. Consequently, overtopping of the dam can develop. (RIDAS,

2011)

10 |EMBANKMENT DAMS

2.3. Remedial measures to repair the impervious core

Diaphragm wall, slurry wall, pile wall using sheet piles or secant piles and

grouting are examples of remedial measures to increase or restore the

impervious property of an embankment dam. Remedial grouting has

mostly been performed during the 1980s and 1990s in Sweden to treat

several dams experiencing internal erosion. The aim was both to seal the

excessive leak and repair the core.

Grouting of dam cores is performed by injecting the grout(s) into

boreholes drilled around the damaged area. Once the grout is injected and

has penetrated the voids and/or seepage channels, it will harden to seal

them. A combination of cement-based grout and chemical grout was

relatively common at the time, aimed to seal the main leak with cement

grout and seal the remaining part of the leak through the core with sodium

silicate. This was because penetration of cement or cement-bentonite grout

is insufficient for sealing the impervious core. Furthermore, the finish-

product of cement grout is much stiffer than the soil to be treated, thus

there is a potential risk of new seepage path between the grout and the soil.

Sodium silicate in the other hand is of higher penetrability and its finish-

product can become a soft gel, suitable for repairing the core.

As an example, Lagerlund (2007) reported that one dam has been

grouted after a sinkhole and sudden increase in leakage were observed.

Cement-based grout was injected to the outer rows of boreholes first in

order to seal the leak, as well as to prevent potential leak of sodium silicate.

Sodium silicate was injected into the middle rows of the boreholes to repair

the core after injection of cement. Totally, 42 m3 cement-bentonite and 164

m3 sodium silicate was injected respectively.

2.4. General surveillance and monitoring related to

internal erosion

Dam surveillance is a vital part within dam safety work. The purpose of the

surveillance is to control dam performance and evaluate dam safety by

detecting and identifying signs that indicate changes in a dam, before a

EMBANKMENT DAMS | 11

damage occurs. Visual inspection and monitoring by instrumentations are

two main parts of dam surveillance.

Common visual detections related to internal erosion are sinkholes in

the dam crest or in the upstream slope and wet spots found in the

downstream slope etc. Increased leakage and turbid leakage are two

parameters of abnormality of the dam, since turbidity of leakage that

contains eroded material is a result of material transport. Sinkhole is a

clear indication that damage has occurred due to extensive internal erosion

and piping. Such visual inspection provides qualitative information of the

dam performance (DSIG, 2012).

Quantitative information of a damage, e.g. quantity and velocity of

seepage in a dam can be achieved by monitoring with instruments.

Common instrumentations include measurements of pore-water pressure

by piezometer, surface settlement by levelling, leakage monitoring by weirs

and settlement inside a dam by settlement gauges (ICOLD, 1988).

Piezometers are often installed in the supporting fill, but sometimes they

can even be installed in the core, as illustrated in Figure 2.3.

Figure 2.3: The potential location to install piezometers for dam monitoring.

The brown part illustrates the dam core.

Continuous measurements are in some cases required to achieve

relatively accurate interpretation of monitoring data. For instance,

quantity of leakage downstream of the dam must be measured regularly to

minimize influence of e.g. seasonal changes such as rainfall and snow melt.

12 |EMBANKMENT DAMS

Detection of a damage

Visual inspection and monitoring are combined to assess performance of a

dam and the quantity of a potential damage, e.g. seepage velocity through

temperature measurements respective measurements of the amount of

leakage. Once a damage has occurred and become detectable by visual

inspection, for instance through a sinkhole, it means that the damage has

already become extensive. Therefore, monitoring needs to be performed

over time to detect early signs of a change or a potential damage.

Furthermore, measurement data needs to be interpreted accurately. Also,

location and cause of a change or an abnormality needs to be addressed.

Once internal erosion has occurred, water level measurement and

temperature measurement carried out with piezometers installed

alongside the filters and/or the supporting fill downstream can help to

determine the location of this damage and the amount of leakage.

Sometimes, several piezometers are required to be installed to achieve

accurate result. Also, turbidity test is a way to determine whether a sudden

increased leakage is actually caused by internal erosion and material loss

of the core.

METHODOLOGY | 13

3. Methodology

This thesis project is divided into two parts. The first part is an evaluation

of the performance of sodium silicate grout by a literature review. The

second part aims to suggest suitable monitoring methods to follow up the

performance of a dam grouted with sodium silicate. The evaluation of the

performance of sodium silicate grout aims to identify potential grout

degradation process and factors causing respective types of degradation.

Suggestions of dam monitoring are based on the potential influence on a

dam when grout degrades. The overall workflow is illustrated in Figure 3.1.

Here, the treated dam does not refer to any real dam, but a theoretical

embankment dam grouted with sodium silicate.

Figure 3.1: Workflow to conduct the evaluation and monitoring of grouted

dam.

14 |METHODOLOGY

3.1. Evaluation of grout performance

The first step of the evaluation is a description of the background of sodium

silicate grout including the common composition, the hardening process

and chemical reactions of sodium silicate. The second step is to identify

grout degradation processes and explain why it occurs. Definition of grout

degradation in this context is changed grout behavior that would affect

remediation of the treated dam negatively. Identification of factors that

induce potential degradation is the third step. These factors are those that

could be existing under conditions typical for an embankment dam.

3.2. Suggest monitoring and instrumentations

The second part of this project is to suggest how performance of dams

repaired with sodium silicate can be monitored and evaluated. If sodium

silicate grout will undergo degradation, it would potentially cause changes

and even reoccurred damage in the treated dam when grout degradation

reaches a certain degree.

Changes of a dam behavior due to decreased grouting efficiency can

develop to a damage in several stages, from a gradually decreased remedial

effect to a reoccurred damage in form of e.g. excessive flow. The

development from initiation to continuation and finally progression

leading to a damage is identified. Parameters indicating changed dam

behavior at each of the damage development stage are also identified.

Monitoring and instrumentations to evaluate the dam function are

suggested based on these identified parameters controlling dam behavior

at respective damage development stage. The suggestion includes both

direct methods to monitor the dam behavior itself and indirect methods to

monitor potential grout degradation which could lead to changes in the

grouted dam.

SODIUM SILICATE GROUT | 15

4. Sodium Silicate grout

Soil solidification and soil stabilization are two common applications of

grouting with cement-based grouts and/or chemical grouts. Injection with

sodium silicate has been common due to its high potential of good

penetrability in fine soil and the possibility of grout setting control.

Nevertheless, its long-term performance has been uncertain which gives

rise to concerns, especially to grouting work aimed for permanent support

and remediation.

This chapter will describe grouting in general, properties of sodium

silicate grout, observed uncertain performance and factors that could cause

degradation of sodium silicate grout.

4.1. Grouting

The definition of grouting is the injection of a fluid material into a soil or

rock formations that will harden to change their original physical

characteristics (Karol, 2003). In practice, the change of the physical

characteristics of a certain geological formation by grouting often refers to

improvements of this formation, e.g. decrease of its permeability to gain

improved waterproofing ability, solidify loose soil particles and enhance

the soil strength.

Different aims of grouting

The following description is based on Karol (2003) if no other sources are

mentioned.

Preventing or reducing water flow through a geological formation or

soil can be achieved by decreasing the permeability. Tunnels under the

groundwater table, as well as embankment and concrete dams are

examples of structures that may experience different types of water inflow

through both the structures themselves, their foundation and the

16 | SODIUM SILICATE GROUT

impervious dam cores. Water intrusion and leakage of an unacceptable

extent can lead to problems affecting both stability and functionality of a

structure negatively. Grouting is a measure to treat such a problem, by

sealing the formation and the fissures where leakage is taking place. For

sealing purpose, the grout should be selected with consideration of void

size in relation to the penetrability of the grout. Furthermore, grouting is a

relatively beneficial method if the fissures where leakage occurs is known

(Bell, 2007).

For seepage control purpose, the grout is expected to be in constant

contact with groundwater and/or reservoir water. Therefore, this grout

must not be susceptible nor sensitive to hydraulic pressure. It should also

be chemically stable against influence from the water and groundwater.

Typical grouting materials

The following description is based on Palmström and Stille (2010) if no

other sources are mentioned.

In general, grouting materials are classified in two main groups,

according to their physical properties: cement-based grouts and chemical

grouts. The cement grout mix is assumed to behave as a Bingham fluid of

suspension type. It means that the fluid contains particles, and

penetrability of this fluid is partly affected by the particle size. Due to this

fluid property, the penetration of cement-based grout is limited. This

means that cement is suitable for soil fractions ranging from coarse sand

to gravel. Bell (2007) stated that cement cannot enter fissures smaller than

approximately 0.1 mm. The finished-product of cement grout is another

limiting factor, since it is hard and behaves as a rigid body. Consequently,

there is a risk of new seepage developing along the surface of the grouted

area, if the grout is more rigid than the treated medium. Because of this

concern, bentonite is sometimes added to the cement grout mixture in

order to achieve a more elastic behavior. These two described phenomena

may indicate that cement is less suitable for repair of finer soils, e.g. an

impervious core of an embankment dam, which usually have a high content

of silt.


Chemical grouts are often assumed to behave as Newtonian fluids and

are characterized by solution type. Sodium silicate which often goes under

the name “water glass” is a chemical grout. In Sweden, sodium silicate has

been applied for remedial treatment on cores of several embankment dams

due to internal erosion of the core caused by inadequate filter design. This

remediation with sodium silicate was performed mainly during the 1980s

and 1990s (Ekström et al., 2016).

Since the sodium silicate grout is a solution, it has better penetrability

than the grouts of suspension type. The penetration of sodium silicate is

mainly controlled by the viscosity, which can be controlled by adjusting the

grout composition. The penetrability of four common grouts in relation to

grain size is illustrated in Figure 4.1. The silicate-based grout can penetrate

fine sand, but not into silt. It has however a notable better penetration that

can successfully be used in fractions down to coarse sand.

Figure 4.1: Soil particle size limitations on grout penetration. Source: Bell

(2007).

Usually, the sodium silicate solution and a reagent are mixed and

injected into the soil, because hardening or gelation of sodium silicate

requires reaction with the reagent to decrease the grout’s pH-value. A

decrease in pH results in that silicate ions polymerize, and the solution

turns into a gel. A chemical grout such as sodium silicate is associated with

time-dependent flow properties. The time required for sodium silicate to

harden can also be controlled through the choice of reagent and adjusting

the grout composition.


In addition, grouting by a combination of cement and sodium silicate

are not unusual. For example, remedial grouting performed to treat several

dams in Sweden was based on a combination of these two grouts. The

principle behind this type of grouting is that the cement-bentonite grout

should seal the larger voids and at the same time prevent sodium silicate

solution to leak away.

Grout requirements

The following description is based on Karol (2003) and Palmström and

Stille (2010) if no other sources are stated.

Selection of a grout should consider both its workability and its grouting

efficiency. Workability concerns the application aspects, i.e. the

penetration of the grout as well as the setting time which the grout needs

to harden (Göthlin, 2004). Grouting efficiency refers to the strength and

durability of the grout, meaning that a grout should obtain enough internal

strength and is durable enough to withstand influence from its

surroundings (Krizek and Madden, 1985). Criteria for grouting can thus be

categorized into either mechanical or chemical properties.

Mechanical properties

1. The penetrability of the grout should be high enough to penetrate

the treated zone thoroughly. This criterion is directly related to the

grouting workability. Viscosity is the determining factor affecting

penetrability in case a grout in solution is injected. For sodium

silicate grout, viscosity is controlled by the silicate concentration

in the grout as well as the weight ratio between sodium and silicate

in the grout.

2. Strength provided by the grout should be sufficiently high. When

a grout has completely filled the soil voids and pores, it forms a

continuous latticework which binds the grains together. By doing

so, the grout will increase the shear strength and resistance against

deformation of the soil grains. If the grout provides only low shear

strength to the geological formation, there is risk of internal

erosion at high water pressure and a high hydraulic gradient.


3. Permanence of a chemical grout should be sufficiently high against

mechanical influence. For example, the grout can be influenced

mechanically due to freezing-thawing cycles, and/or wetting-

drying cycles that leads to mechanical deterioration. The grout is

most sensitive when water which once contained in the grout no

longer is bound to it chemically.

Chemical properties

1. The grout must be chemically stable to withstand chemical

influence. An appropriate grout should be durable against the

influence of the environment which a grout is injected into, e.g.

groundwater. If not, there will be risks that the grout becomes

unstable and leaches due to reaction with groundwater or water.

2. The grout setting time should be under control. Controllable

gelation process of sodium silicate has the impact on the final

outcome and completion of grouting. Without control, the grouted

formation could undergo uneven grouting and the result might

differ from the desired outcome.

4.2. The sodium silicate solution

This section describes in general the composition of sodium silicate-based

grout, the chemical reactions behind the hardening and gelation of it, as

well as injection procedures of sodium silicate.

Composition and properties of sodium silicate

As its name indicates, sodium silicate is a mixture of sodium and silicate in

a water solution. Commercially, this product is known as water glass and it

is available in form of powder or solution. The chemical formula is 𝑛𝑆𝑖𝑂2 ·

𝑁𝑎2𝑂, where n is the ratio that can refer to both molar and weight ratio

between silicate and sodium, since these two values are almost identical.

For grouting purpose, the ratio often varies between 3 to 4, but never

exceeds 4 (Karol, 2003; Tallard and Caron, 1977; Hamouda and Amiri,

2014). Depending on the applied reagent, the finished-product becomes

either a soft gel or a hard gel. In the case of grouting the impervious core

of embankment dams, a soft finished-product is more advantageous.


Sodium silicate is alkaline, because silicate is only weakly acidic, while

sodium is strongly alkaline. When the grout reacts with a reagent which is

often an acid or a metal salt, the solution will harden. Adding of such a

reagent will decrease the pH by neutralizing the sodium ion in the sodium

silicate solution, thereby releasing free silicate ions. At lower pH, these free

silicate ions are polymerized into a longer chain of polysilicic ions, which

turns into a gel by hardening (Yonekura and Miwa, 1993).

Sodium silicate is a flexible material and it is possible to adjust the

composition in order to optimize its grouting ability such as viscosity and

strength. The silicate concentration controls both density and viscosity of

the solution. The higher the content, the higher its viscosity becomes. At

higher concentrations, molecules are denser, resulting in higher viscosity

(Weldes and Lange, 1969).

Compared to cement, chemical grouts have limited strength. The

strength of the end-product of sodium silicate is positively related to the

concentration of both silicate and reagent in the solution. The more sodium

that is neutralized by the reagent, the higher the strength becomes.

Consequently, a higher silicate content can lead to higher strength of the

end-product. However, there is a risk that the strength will be low by

lowering the silicate content in order to achieve low viscosity and thereby

a high penetrability (Zheng, 2000).

Setting time and neutralization of sodium silicate

Setting time is the time required for sodium silicate to harden and stabilize.

This property is influenced by the grout composition in several ways. Also,

permanence of the finished grout product can be influenced by the

reactivity, i.e. the degree of sodium neutralization when sodium silicate has

undergone gelation.

The most accepted theory of grout gelation in the references is that

sodium silicate hardens through pH decrease. First, free silicic ions are

discharged from the grout solution by neutralization of sodium ions. When

acids or salts based reagent is applied, the pH of the grout in solution

decreases. At low or neutral pH, silicate ions will polymerize to form silica.


(Tallard and Caron, 1977; Lagerblad et al., 1995; Yonekura and Miwa,

1993)

Since the gelification of silicate sodium is achieved through neutralizing

the sodium ions, the degree of neutralization is an important parameter

affecting performance of the grout after injection. It is desirable to have as

low sodium content as possible, thereby a high silicate/sodium molar ratio.

However, a ratio above 4 will lead to an unstable end-product. With respect

to this, the molecular upper limit of sodium silicate should not exceed 4

(Tallard and Caron, 1977). The degree of neutralization decides the

strength and durability. The higher the neutralization, the stronger the gel

and the treated soil becomes. A relatively high degree of neutralization can

be achieved with a high amount of reagent.

The reagent, which is also called reactant, activator or coagulant in the

literatures has an impact on the gel setting time as well as on the degree of

sodium neutralization. Generally, the higher reagent content, the faster is

the setting time. For a given reagent, higher silicate concentration will also

lead to faster setting time. Further, Yonekura and Miwa (1993) categorized

the reagents into inorganic reactant, inorganic gas reactant and organic

reactant with regard to both the setting time and the degree of sodium ion

neutralization.

1. Inorganic reagents: the gelation process is fastest with inorganic

reagent, such as calcium chloride. However, after the gelation, the

remaining of sodium ions is high within the gel structure, meaning

a risk of instability.

2. Organic gas reagents: the gelation process is fast. Some sodium

ions remain in the gel network after gelation.

3. Organic reagents: the setting time is longest with organic reagents.

However, little sodium ions are remained afterwards.

Injection of sodium silicate

Injection of sodium silicate is performed with either the one-step or the

two-step method. One-step injection means that the grout and reagent are

mixed first and then injected together. This method is more frequently

performed today than the two-step method, and the mixture is usually

prepared just before injection on the site. The grout will harden and form


a gel with relative slow rate because the gelation initiation is delayed (US

Army Corps of Engineers, 1995). The slower rate of gel formation would

allow more control of grout penetration, leading to more even and

thorough penetration.

Two-step process, also referred as the Joosten-method is performed by

first injecting the sodium silicate, and at the second step injecting the

reagent into the same zone to form a gel and stabilize the grout. A salt

reactant, often calcium chloride in solution form is used. This process

enhances the soil strength most but is the most expensive approach (US

Army Corps of Engineers, 1995; Tallard and Caron, 1977). The chemical

reaction with calcium chloride is shown in Eq. 1.

𝑁𝑎2𝑂 ∙ 𝑛𝑆𝑖𝑂2 + 𝐶𝑎𝐶𝑙2 + 𝐻2𝑂 → 𝐶𝑎(𝑂𝐻)2 + 𝑛𝑆𝑖𝑂2 + 2𝑁𝑎𝐶𝑙 [Eq. 1]

The two-step process using calcium chloride results in an instantaneous

harden reaction. This is advantageous when aiming to seal a sudden

leakage and stop the flow. But too rapid gelation can lead to less thorough

penetration of the grout, thus it can result in uneven gelation which limit

the remedial effect.

4.3. Degradation of sodium silicate

Permanence of sodium silicate is a concern since the grout gel can undergo

different types of degradation. Potentially, it can lead to worsened

remediation. Göthlin (2004) reviewed monitoring data of measured

sodium concentration in a dam where both the core and its foundation had

been grouted with sodium silicate. The leakage from the foundation

showed much higher sodium content compared to sodium concentration

measured at the downstream weir. Göthlin concluded that this was an

indication that the grout had leached from the dam foundation. Göthlin’s

analysis based on field data has thus indicated an uncertain behavior and

questionable long-term permanence of sodium silicate in an embankment

dam.


Other reviewed literatures that described results from laboratory tests

have shown that the soil grouted with sodium silicate generally experiences

increased permeability with time. Avcı (2017) measured permeability of

soil specimens grouted with sodium silicate under 120 days in the

laboratory. All specimens were observed to experience an increase in

permeability. Krizek and Madden (1985) also measured permeability of

several sand specimens treated with sodium silicate for 600 days in the lab.

Continuous increased permeability was also observed in this experiment.

Both studies concluded that the increased permeability was related to

grout degradation.

The gradually increased permeability after grouting with sodium

silicate is mainly caused by syneresis or leaching of the grout, or a

combination of both. Syneresis can induce gel shrinkage, and sodium

silicate leaches when it has dissolved or partly dissolved. Table 1 has

summarized the identified factors that could give rise to these two types of

grout degradation according to the references (Avci, 2017; Einstein and

Schnitter, 1970; Karol, 2003; Krizek and Madden, 1985; Lagerblad et al.,

1995; Littlejohn et al., 1997; Yonekura and Kaga, 1992).

A decrease in strength is another consequence of grout degradation,

mainly due to grout leaching. Strength of several soil specimens grouted

with sodium silicate were decreasing continuously under 1000 days,

observed by Yonekura and Kaga (1992) in the laboratory. This was because

sodium silicate leached, therefore it was no longer able to support the soil.

This decrease was significant, because in this experiment, the strength of

the soil measured in the end of this experiment was only half of its initial

value.

Syneresis induced shrinkage

Syneresis is the phenomenon related to silicate gel when water once

contained in the grout gel leaks, leading to decreased gel volume and

shrinkage of the gel. Syneresis of silicate gel will occur because the electric

charge of the silicate molecules decreases, resulting in repulsion between

different parts in the molecular structure. This means either shrinkage, or

that the pressure of the grout decrease (Sjöblom, 1995). Furthermore,


Table 1: Factors that can induce syneresis or leaching of the sodium silicate.

Degradation Factors causing grout degradation

Syneresis induced gel

shrinkage

A certain range of silicate content can cause

higher syneresis.

The type of applied reagent to harden the

grout. Certain reagent can lead to much higher

syneresis than others.

Cement or concrete that contains calcium ion

may induce high syneresis.

Leaching of grout when

flowing water erodes the

grout, or when the gel

dissolves.

High pH can cause grout dissolution.

Insufficient curing of the grout can lead to

lower grout strength, thus it is more

susceptible to erosion caused by flowing water.

Insufficient neutralization of sodium can cause

dissolution.

Being in contact with water or groundwater can

cause dissolution.

syneresis can also lead to higher susceptibility of the grouted soil to erosion

caused by flowing water (Krizek and Madden, 1985).

Avcı (2017) and Littlejohn et al. (1997) found that syneresis of the pure

grout gel can be as high as 80% and 60% respectively. Both studies suggest

that syneresis is related to the silicate content in the grout. Theoretically,

sodium silicate can also experience shrinkage when it is in contact with

water and in contact with concrete.

Influence of syneresis or gel shrinkage on the grouted soil is also

dependent on grain size of the treated soil. Generally, syneresis has larger

influence on coarser soil but less effect on fine soils. It means that

sometimes syneresis would not develop sufficiently to induce higher soil

permeability. This is because the particles of fine soil are placed closely,


therefore they can provide more support to prevent shrinkage or loss of the

grouting material (Einstein and Schnitter, 1970; Karol, 2003).

Syneresis of a silicate gel will always occur, but the degree of it can be

controlled by several factors. The factors that may lead to a high syneresis

rate are identified to be the silicate concentration of the grout, the type of

reagent used to harden the grout solution and the influence of calcium ion

from e.g. cement grout or concrete.

Grout composition and silicate content

Avcı (2017) studied the permeability of soils grouted with sodium silicate

under 720 days. These studies showed that the syneresis would increase

with increased silicate concentration. But when syneresis reached a peak,

the rate decreased. Figure 4.2 shows the relationship between syneresis

and silicate concentration in Avcı’s study, where the highest syneresis rate

was at 80%.

Figure 4.2: The relationship between syneresis and the silicate content in

the grout. Source: Avcı (2017).

Littlejohn et al. (1997) provided the same information, that there was a

relationship between the syneresis following the same trend as Avcı’s


study, as shown in Figure 4.3. The highest syneresis observed in this study

was at 60%. The degree of syneresis is defined as the ratio between the

volume water leaking from the grout gel and the initial grout volume in

both studies.

b)

Figure 4.3: Degree of syneresis due to silicate concentration. Source:

Littlejohn et al. (1997).

A study by Yonekura and Kaga (1992) led to the same conclusion. A15

and A20 shown in Figure 4.4 are sodium silicate grouts, but A15 contained

less silicate than A20. A20 showed to experience higher syneresis than A15

for all 1000 days during the observation. After 1000 days, syneresis was

measured to be around 4% and 6% respectively as shown in Figure 4.3.

This value of syneresis seems to be much lower compared to Avcı (2017)

and Littlejohn et al. (1997), but the degree of syneresis was defined

differently here. The definition of syneresis was the ratio between weight

of leaked water and the weight of the initial grout gel. Yonekura and Kaga’s

study also showed that shrinkage developed with increasing rate under the

first 100 days of the observation. After the first 100 days, syneresis rate

ceased and became relatively constant.


Figure 4.4: Syneresis of four silicate based grouts. A15: sodium silicate of

lower silicate content. A20: sodium silicate of higher silicate content. CH: silica

sol. CSN: colloidal silica. Source: Yonekura and Kaga (1992).

These three independent studies have suggested that syneresis has a

wide range. In different labs, syneresis was observed to range from 4% up

to 80%. Thereby, shrinkage may not always be significant. Since syneresis

rate is also shown to be related to the silicate content, it is theoretically

possible to regulate syneresis by adjusting the grout composition.

Type of reagent

The second factor leading to high syneresis is the reagent used to harden

sodium silicate. Krizek and Madden (1985) measured the syneresis of five

sodium silicate grouts under 21 days, where different reagents were used

for respective grout sample. The sample using Terraset as the reagent was

observed to experience much higher syneresis rate than the other four

samples. Terraset had also led to continuously increased syneresis rate,

whereas syneresis of the other four samples already stabilized under these

21 days. See Figure 4.5, where Terraset is expressed as a line marked with

the black triangles. At the 21st day of this experiment, syneresis of the


Terraset sample was observed to be above 30%, whereas the other four

samples experienced syneresis around or below 10%. One conclusion is

that certain types of reagent can lead to much higher syneresis.

This finding suggests that when sodium silicate will be used but

syneresis might be a concern, the influence of the reagent to be applied on

syneresis should be examined. In Sweden, the commercial reagent

Dynagrout was commonly used in relation to remedial grouting of dams.

How this reagent would affect the syneresis and the grout can be important

to examine in future studies.

Figure 4.5: Syneresis in relation to different reagents. Source: Krizek and

Madden (1985).


Influence of calcium ion

Lagerblad et al. (1995) observed in the lab that when sodium silicate gel

and concrete were placed in water together, the grout gel became enriched

with calcium ions and sodium ions had leached. Shrinkage of the gel had

also occurred. One reason was because sodium ions in the grout gel was

replaced by calcium ions from the concrete. It resulted in a calcium silicate

gel. However, calcium silicate gel is more polymerized than sodium silicate

gel. This could have resulted that the calcium gel is not able to hold as much

water as the sodium gel, thereby the grout gel can shrink once the sodium

gel becomes a calcium gel due to the concrete (Lagerblad et al., 1995).

Replacement of sodium ions by calcium ions occurs due to ion

exchange. Calcium silicate is thermodynamically more stable than sodium

silicate since the electrostatic energy between calcium and oxygen atoms

in a silicate silanol group is higher than that of sodium and oxygen.

Therefore, the sodium silicate group is more susceptible to dissociation

than the calcium silicate group (Wang and Gillott, 1993; Lagerblad et al.,

1995).

Grout leaching caused by gel dissolution and erosion by

flowing water

Lagerblad et al. (1995) observed that the volume of the grout decreases not

only because of syneresis, but also dissolution of the grout gel, especially

when the gel is in constant contact with water. Furthermore, the grout gel

can be eroded when it is subjected to a high hydraulic gradient. The degree

of erosion is controlled by the gel strength, and the stronger the gel is, the

less it is affected by the seepage. Stronger grout gel can be achieved with

longer curing time.

Influence of the pH

The hardening of sodium silicate is achieved by decreasing its pH by

applying a reagent. This means a potential risk that instability and even

dissolution can occur when the grout gel experiences high pH. The

increased pH can lead to depolymerization of the grout, potentially

resulting in an unstable gel behavior.


Lagerblad et al. (1995) found that two sodium silicate grout samples lost

continuously weight and became partially dissolved under 49 days during

the experiment, when the grout sample was placed in water together with

concrete. pH of the water increased gradually from 10 to 11.5 during the

experiment. Lagerblad et al. (1995) explained that the dissolution was a

result of increased pH due to the concrete. This experiment also showed

that sodium ions had leached from the grout gel. The gel had in the other

hand gained more calcium. It indicated that calcium from the concrete

would form a calcium silicate gel. Potentially, syneresis and shrinkage

might have occurred.

The grout gel will gain higher strength at lower pH for a certain silicate

content. Hamouda and Amiri (2014) found in an experiment that sodium

silicate gel gained a maximum strength around 3000 Pa when it was

experiencing a pH at 10.10. The maximum strength was only 700 Pa for

another grout sample when the pH increased to 10.70.

Consequently, the long-term permanence of the silicate grout can be

questionable when the grouted area has been treated with both the cement

and the sodium silicate grout. Since cement is an alkaline product which

also contains calcium, there is a risk of both dissolution and shrinkage of

the grout gel. At higher pH, the sodium silicate can become more

susceptible to e.g. a hydraulic gradient and seepage due to the potentially

decreased grout strength.

Gel strength and the curing time

Strength of the grout and the treated soil against a hydraulic gradient is

related to the curing time, which is the time the sodium silicate being

grouted in the soil. In the short-term, the longer a soil is grouted, the higher

strength is achieved to withstand a certain hydraulic pressure. Krizek and

Madden (1985) performed an experiment where five sodium silicate

grouted soil specimens were all experiencing a hydraulic gradient of 100.

Curing time of each specimen was ranged from 5 minutes to 6 hours. The

specimen allowed to cure for 6 hours could resist this hydraulic gradient,

whereas the specimens cured only for 5 and 10 minutes respectively were

eroded totally by this hydraulic gradient. The remaining two grout


specimens that had been cured for 30 minutes and 1 hour respectively,

both experienced partial erosion at this hydraulic gradient.

However, the sodium silicate grout in an embankment dam can be

expected to undergo relatively short curing time because of the constant

seepage. Thus, in the beginning of the grouting, the strength can be

relatively low and erosion of the grout may occur due to seepage through

the treated area.

There is another risk regarding the grout efficiency related to the

flowing water. The grout solution used to treat several dams in Sweden is

fluent and requires relatively long setting time to harden (Najder, 2012).

This suggests a risk that grout could have leaked before it could harden to

seal the damaged area.

Insufficient degree of sodium neutralization

There is often some sodium remaining in the grout gel structure, despite

the neutralization with the reagent. The residue of sodium within the gel

structure can lead to dissolution of the grout gel by depolymerization.

Consequently, it will result in a weaker gel, or even a breakdown of it.

(Yonekura and Miwa, 1993)

Yonekura and Kaga (1992) observed the strength of four sand

specimens grouted with both sodium silicate and two other chemical

grouts under 1000 days. Here, A15 and A20 were the two sodium silicate

grouts, but A20 contained higher amount of silicate and the reagent. The

two other studied grouts were silica sol (CH) and colloidal silica (CSN). The

measurement under 1000 days showed that A20 and A15 experiencing

continuously decreased strength. In the end of this observation, the

strength had decreased to 1 MPa, whereas initial strength for both A15 and

A20 was around 2 MPa. On the other hand, soil specimens grouted with

silica sol respective colloidal gained higher strength with time.

One difference between sodium silicate and the two other chemical

grouts in Yonekura and Kaga’s study was that only the former one

contained sodium. This was considered to be the main reason leading to

the decrease in strength for soils grouted with sodium silicate. The

remaining sodium due to insufficient sodium neutralization gave rise to the


depolymerization of the grout gel, in turn resulting in leaching of the grout

gel.

The degree of grout leaching was recorded by measuring the silicate

content in the water, which the grouts were being exposed to. Figure 4.6

shows leaching of all four studied soil specimens. In the figure, it can be

seen that sodium silicate had a leaching above 40%, whereas silica sol and

colloidal silica had 2% and 1% respectively at the 1000th day of this

experiment.

Figure 4.6: Silicate leaching from four types of grouts under 1000 days. A15:

sodium silicate of lower concentration. A20: sodium silicate of higher

concentration. CH: silica sol. CSN: colloidal silica. Source: Yonekura and Kaga

(1992).

To conclude, sodium silicate will be leached with time from the grouted

soil due to residue of sodium after the gelation. Since neutralization of all

the sodium is hard to reach, there will always be some sodium remaining

in the grout gel. However, according to Yonekura and Miwa (1993), the

organic reagents would neutralize 80%-90% of sodium in the grout (see

section 4.2.2), which is the highest neutralization rate compared to the


other types of reagents. This suggests that the risk of sodium residue and

leaching of the grout gel might be less by applying an organic reagent.

Instability due to flowing water and groundwater

A seepage through a soil grouted with sodium silicate can cause erosion if

the hydraulic gradient is sufficiently high. Lagerblad et al. (1995) found

that when a specimen of pure grout gel was exposed to flowing water, the

chemical test showed that the salts in the grout had leached. After 7 days

of exposure to the flowing water, the weight of this grout sample decreased

from 73.4 g to 17.0 g, indicating that a large part of the grout sample had

been eroded.

Since glyoxal was the reagent used in this experiment, the instability of

glyoxal is another potential factor leading to grout dissolution because

glyoxal is soluble in contact with groundwater. Glycolic acid is formed from

glyoxal to harden the grout when the glyoxal reacts with sodium silicate.

However, glycolic acid is soluble in water due to e.g. diffusion, meaning

that the silicate gel containing glycolic acid also becomes soluble (Sjöblom,

1995)

To conclude, dissolution of the sodium silicate gel due to water is

complex. The grout gel might be eroded by flowing water, and the reagent

can be unstable when exposed to water. However, this reviewed study by

Lagerblad et al. (1995) and Sjöblom (1995) mainly provided information

for a case when using glyoxal as the reagent, but in Sweden Dynagrout that

consisted of sodium aluminate was the reagent used to treat several dams.

One reason was that this type of reagent would result in a soft gel which

was considered to be suitable to repair the impervious core. However, how

other types of reagents e.g. Dynagrout may be affected by groundwater

remains to be examined.

Influence of grout degradation and the soil grain size

Pure grout gel can undergo degradation in form of syneresis and leaching

due to dissolution or erosion of the grout gel. However, fine soils would be

less affected from syneresis or grout leaching compared to the coarse soils

(Einstein and Schnitter 1970; Littlejohn et al. 1997; Karol, 2003).


Einstein and Schnitter (1970) observed three sands specimens grouted

with sodium silicate under 5 months, whereas these specimens were

ranged from fine to coarse soils. The result showed that the specimen of

the finest soil experienced least grout leaching compared to the other two

specimens. Also, the permeability of this soil sample increased from 10-7

cm/s to 5·10-6 cm/s, which was not considered to be a significant increase.

It was concluded that influence of leaching to some extent is a function of

the grain size and the size of the pores, since finer and tighter placed soil

particles could give more support to the grout to prevent it from leaching.

(Einstein and Schnitter, 1970)

Figure 4.7 shows syneresis presented by Littlejohn et al. (1997) of both

a pure grout gel as well as three treated soils ranging from gravel to sand.

The pure grout gel experience highest degree of syneresis at 70%, whereas

the sand sample experienced lowest degree of syneresis at 3%. The

difference in syneresis between the pure grout gel and a grouted soil

sample is thus large.

Figure 4.7: Syneresis in relation to soil grain size. Source: Littlejohn et al.

(1997), derived from Caron (1975).

Avci (2017) and Littlejohn et al. (1997) have observed and suggested

that high syneresis of a pure grout gel could occur, as described in the


earlier section. But when analyzing the effect of syneresis or leaching on a

grouted soil, the soil grain size should also be considered to examine the

potential influence or decreased efficiency on this treated soil.

MONITORING TO CONTROL PERFORMANCE OF THE DAM | 37

5. Monitoring to control the performance of the

dam

The main purpose of remedial grouting in a damaged dam is usually to

repair the core, ensure and restore its intended permeability in order to

stop excessive leakage flow. If sufficient grouting efficiency is not achieved

or the grouting efficiency decreases when sodium silicate grouted in the

dam has degraded, it could lead to a decreased remedial effect, meaning a

risk of reoccurred damage.

Damage caused by decreased grouting efficiency occurs and develops

mainly in three stages as shown in Figure 5.1. The first stage is instability

of the sodium silicate grout occurring in form of dissolution, leaching and

syneresis of the sodium silicate grout. This will gradually result in larger

voids and cavities in the grouted area. At the second stage, the larger voids

and cavities will lead to higher permeability in the treated core as the grout

continues to degrade, which allows more seepage passing through the

remediated area. At the final stage, a damage in form of excessive flow and

sudden increased in leakage will occur, caused by the loss of the remedial

effect.

Monitoring can be used to identify the phenomena described above,

and to conclude whether an unwanted change is taking place in a dam and

determine the stage of the change. This chapter describes the methods

suitable to monitor grout efficiency and its long-term performance.

38 |MONITORING TO CONTROL PERFORMANCE OF THE DAM

Figure 5.1: Different stages of damage development related to degradation of

sodium silicate and possible indicators to detect degradation.

5.1. Analysis of ion content in leakage water

The first stage of a decreasing grout efficiency is leaching of sodium silicate

caused by erosion or dissolution. Excessive leaching of grout will lead to an

increased permeability, and therefore more water is able to seep through

or adjacent to the treated part of the core. Potential leaching and

dissolution of sodium silicate can be detected by chemical analysis, e.g.

measuring the ion content in the leakage water, since leaching of sodium

silicate will lead to a higher concentration of, for instance, sodium ions that

are being released from the grout as it dissolves. Where leakage monitoring

is intended to specifically identify potential changes in a grouted area, the

measurement of the ion content should be a long-term commitment to

obtain reference values.

The ion content can be measured in leakage water collected in the

downstream drainage system and weir. The concentration of specific ions,

in this case sodium respective silicon ions can be quantified by using ion-


selective electrodes. Such an electrode instrument has a ligand. High

affinity for the analyte ion(s), i.e. sodium and silicon can be selected

respectively for the ligand to bind to identify the ion content in the water.

The bound analyte ion can be quantified by calculating the electric

potential of respective ion (Harris, 2007).

The measured results should be registered and change in concentration

should be analyzed. Such measurement should be performed continuously

to avoid unappropriated interpretation of measurements. Measurement of

ion content is an indirect way of monitoring the dam behavior, by

monitoring the gradual change of the grout competence itself.

The grouting of sodium silicate could lead to increased sodium or

silicate content in leakage in the beginning of injection. Therefore, the time

required for the grout to stabilize should be studied in the lab before the

grouting, and the measurement of ion content of the leakage should be

performed when the grout has stabilized.

Also, the measurement can also be performed at all the gauges of the

dam, in order to compare the measured results from different parts of the

dam. Once an increase or decrease in the concentration is observed, it can

provide information where the leakage containing potentially leaked grout

may have seeped through the treated area.

5.2. Increased permeability of the core

A gradual increase in seepage and seepage velocity may occur as the core

becomes more permeable. This change in seepage can be detected by

monitoring the pressure gradient and temperature of the core using

piezometers installed in the downstream filters and/or the supporting fill.

Pressure gradient and water level

Pore pressure measured by piezometers can provide information of the

condition of the dam. Possible consequences of the first stage of grout

deterioration is a gradually increased seepage and increase in seepage

velocity. Compared to a normally functioning dam that is not experiencing

40 |MONITORING TO CONTROL PERFORMANCE OF THE DAM

internal damage, the pressure distribution of a damaged dam will be seen

as being offset towards the downstream side.

Pore pressure can be measure by piezometers installed at selected

locations. Comparison with earlier measured pressure can provide

information regarding potential changes in the pressure distribution,

thereby indicating an ongoing change in the dam. It is however unusual to

place monitoring equipment in the core of a dam, but it can be valuable

when remediating a damaged area of the dam and for long-term evaluation

of grout performance.

Temperature measurement

The temperature in a dam is controlled by a combination of air

temperature at the surface of the dam and the water temperature in the

reservoir. Below the surface of the dam, the influence of the air

temperature is relatively small, and the dam temperature shows a relatively

small variation over the year, which decreases with increased depth.

However, when a large increase in seepage occur, the reservoir water

temperature will influence the dam temperature at the level where the

leakage occurs to a larger extent. Compared to the earlier temperature

measurement at the same time of a year, in the same piezometer and same

level, an increase or decrease in temperature indicates a damage in form of

larger seepage (Johansson, 1997).

Temperature measurements can also be performed in piezometers

installed upstream and downstream of the core respectively. Since it

requires some time for seepage to travel from the upstream to the

downstream side of the core, it will result in a temperature difference of

the seepage water at respective sides. If the temperature difference is

notably small in one measuring section, it may indicate faster seepage

occurring at this section, thereby a higher permeability, potentially due to

damage (Lagerlund, 2007).


5.3. Leakage and turbidity monitoring

When the grout efficiency has reached a lower limit, the treated dam will

again experience a negatively changed function due to internal erosion.

This can directly be detected in the same way as to monitor the overall dam

behavior.

Fine material transported to the downstream side of the dam is a result

of internal erosion, which can also be induced because the grout has lost

its efficiency to repair and seal the core. At this stage, the damage can be

observed by turbid flow downstream of the dam in a weir, commonly in

combination with increased leakage. If erosion continues and progresses,

it may not only lead to mass movement, but also increased leakage.

DISCUSSION | 43

6. Discussion

This chapter is divided into two parts. The first part of this chapter

discusses how potential degradation of the sodium silicate in a dam may

affect the dam’s functionality. The second part of this chapter discusses the

suggested instrumentations to monitor the performance of the sodium

silicate grout and a treated dam.

6.1. Degradation of sodium silicate in embankment dams

The reviewed literature has shown that sodium silicate grout can undergo

degradation, mainly in form of syneresis and leaching. In several studies,

the soil samples grouted with sodium silicate have experienced increased

permeability due to syneresis and leaching of the grout. When grouting an

embankment dam with sodium silicate, an increase in permeability can be

expected when syneresis and leaching has occurred at a high enough level.

A potential consequence is that the remediation will lose its effect and the

repaired damage may reoccur.

However, the described behavior of sodium silicate and its degradation

are mainly based on the performed literature review, and most of the

conclusions are based on laboratory tests conducted by other researchers.

Such laboratory environment differs from the field conditions in many

ways. It is worth to discuss how the actual field conditions can influence

the potential degradation. Some factors that need to be addressed in the

field to identify the potential of grout degradation under typical dam

conditions are presented in Table 2.

44 |DISCUSSION

Table 2: Some factors to be identified in the field in relation to the potential of

sodium silicate grout degradation.

Degradation Factors causing grout

degradation

Factors to be

identified in the

field

Syneresis

induced gel

shrinkage

Grout composition incl.

amount of silicate in the

grout.

Find the silicate

content of the applied

sodium silicate grout.

Type of reagent to harden the

grout.

Study the behavior of

the applied reagent in

relation to syneresis.

Being in contact with cement

or concrete that contain

calcium ions.

Study the influence of

grouted cement or

cement-bentonite on

the syneresis under

field conditions.

Leaching,

erosion and

dissolution of

the grout gel

Influence of high pH. Address the common

pH-value of the

reservoir water.

Address the influence

of cement-grout on

the pH by measuring

pH of the leakage

water. If dissolution

of the grout gel has

occurred, suggest a

range of pH causing

the dissolution.

Insufficient grout strength

subjected to a hydraulic

pressure due to e.g.

insufficient curing time.

Identify the common

hydraulic gradient

and seepage velocity

in the field. Perform

DISCUSSION | 45

leaching test under

similar conditions.

Insufficient neutralization of

sodium.

This might be hard to

determine. But one

way is to check what

reagent was applied.

For example, organic

reagent will result in

least sodium residue

after the reaction.

Being in contact with water. Perform continuous

monitoring of ion

concentration in

leakage to observe

any increase in for

example sodium

concentration.

Shrinkage due to syneresis

Syneresis is a natural phenomenon related to silicate gel. The phenomenon

is that the gel expels water that results in a decrease of the gel volume,

leading to shrinkage. When sufficiently high shrinkage has developed, it

can result in new seepage channels between the grout and the soil, thereby

resulting in an insufficient sealing effect. In some extreme cases, the pure

grout gel can experience syneresis up to 80% according to the references.

However, the degree of syneresis can theoretically be controlled by

adjusting the grout composition and by avoiding inappropriate reagents

that may induce high syneresis. Also, how syneresis will affect a treated

dam is related to the treated soil and its grain size.

The degree of syneresis is mainly controlled by three factors: content of

silicate in the grout, the type of reagent used to harden the gel and the

presence of calcium ions. This suggests that syneresis can potentially be

46 |DISCUSSION

limited by adjusting the grout composition to avoid a high degree of

syneresis. Furthermore, the influence of the applied reagent on grout

syneresis can be studied in the laboratory together with some other types

of reagents, to check whether this applied reagent can lead to high

syneresis. Cement grout can be a concern when it is applied with sodium

silicate to grout the damaged dam core, because its calcium ions can react

with the silicate grout, thereby leading to syneresis.

For coarser-grained soil, shrinkage induced by syneresis is more likely

to create seepage channels, thereby decreasing the initial grouting

efficiency. Generally, fine-grained soil is less susceptible to grout

degradation than coarser soils. The impervious cores in embankment dams

consist of moraine and are of fine grain size, mostly of silt. Therefore, the

impact of syneresis on the impervious core can be considered to be

relatively limited with regards to the soil grain size. Some reviewed

literatures have concluded that syneresis has caused decreased grouting

efficiency, but other references conclude that the gel shrinkage on fine soil

specimens is so small that it has little negative influence on the grouted

soil. The actual effect of syneresis needs to be addressed in the field to

conclude whether syneresis can be a threat to the dam remediation.

Grout leaching

Leaching caused by dissolution of sodium silicate has been observed in

independent studies. Leaching occurs when the grout gel is dissolved or

when it is eroded by flowing water of a high hydraulic gradient, especially

when the grout gel has not gained sufficient strength.

Insufficient degree of neutralization and high pH are identified to be

two factors leading to grout dissolution. Theoretically, neutralization can

be optimized through an appropriate composition, e.g. using the organic

type of reagent. Also, a higher degree of neutralization can be achieved by

lowering the amount of sodium in the grout. However, total sodium

neutralization is hard to achieve, and in practice it is hard to control how

much sodium that has been neutralized. Thus, the risk of dissolution due

to it should always be considered.

DISCUSSION | 47

Since the cement grout is alkaline, it is a source of high pH when cement

and sodium silicate are grouted to treat the same damaged area. The

cement may therefore cause the grout to dissolve by increasing the pH. If

the seepage water is of high pH, it is another source leading to grout

dissolution. Therefore, the common pH of the reservoir water should be

addressed when considering the risk of grout dissolution.

Sjöblom (1995) suggested that sodium silicate itself is sensitive to water

or groundwater, it tends to dissolve when it is exposed to groundwater.

However, when grouting an embankment dam, seepage is expected, which

means a risk that the grout exposed to the seepage can dissolve.

Erosion due to flowing water were observed at hydraulic gradients of 50

and 100 respectively in the references. This indicates that the sodium

silicate grout can be unstable when subjected to high hydraulic gradients.

Since seepage through a dam will apply a certain hydraulic gradient, it

means that seepage may erode the grout, especially when the grout

strength is low.

6.2. Suggested monitoring methods

As discussed in section 6.1, the long-term performance of sodium silicate

grouted in embankment dams can be subjected to many factors. Therefore,

the risk of grout degradation can be considered to be high. There is a need

to monitor the grouted area of the core to control the grout efficiency.

The grout long-term performance is suggested to be observed by both

direct and indirect monitoring methods. Indirect methods are monitoring

of sodium silicate stability and the leakage through the dam. The

monitoring of the grout is suggested to be performed by measuring the

sodium ion content in the leakage, since leaching of the grout will lead to

an increase in sodium ions. Leakage monitoring is suggested to be

performed by visual inspection to observe any increase in leakage, as well

as turbidity monitoring in case of material loss from the core. Direct

methods are monitoring of the pore water pressure in the core and the

seepage velocity, aiming to detect any sign of increased permeability of the

core that can be related to the worsened remedial effect.

48 |DISCUSSION

Monitoring by measuring the ion concentration in leakage is based on

the hypothesis that the grout degradation will lead to increased sodium or

silicate content in the leakage. Since in the beginning of grouting the

leakage can show to have a relatively high sodium or silicate content, this

measurement should be conducted when the grout has stabilized.

However, the time required for the grout to stabilize and thereafter for

leakage to represent the actual ion content can be uncertain, leading to

uncertainties when interpreting the measurement results. Thus, it raises

questions regarding when a measurement can be performed, and how the

measured values should be analyzed.

Furthermore, the ion content may not always increase due to the grout

degradation, sometimes it can also a decrease in the concentration as

Göthlin’s study from 2004 has revealed. When aiming to monitor the

sodium silicate grout, a decrease in ion content can be hard to interpret,

and not providing very relevant information regarding the state of the

grout. To conclude, this type of monitoring requires more knowledge and

understanding of the grout degradation.

Since there are not commercially available monitoring methods to

monitor the behavior of a treated dam to its grouts, the measurement of

pore pressure and the temperature of the dam are also suggested. These

types of monitoring can be measured through the piezometers. The

piezometers are usually installed in the supporting fill of the dam.

However, sometimes the piezometers can be required to be installed in the

core, to obtain more accurate information of the dam core. However, the

installation can lead to further damage of the core and should not be

performed if the condition is not critical.

Reoccurred damage or a new damage?

It is of importance to identify the reason and location of the damage, in

order to determine whether the damage is caused by a decreasing grout

performance, or if the cause of the changes is outside of the previously

treated area. Internal erosion can be initiated even though remedial

grouting has been performed, since the cause of initiation itself is not

addressed or treated.

DISCUSSION | 49

Identification and localization of a damage requires detailed

investigation. The investigation can be carried out by reviewing monitoring

data of the ion concentration, pore pressure distribution, and dam

temperature as well as leakage water analysis, i.e. a combination of the

above suggested monitoring. Furthermore, deformation measurements of

the treated part of the dam can also be useful to perform to monitor the

dam behavior.

Measurements of ion content in the leakage water is suggested to be

carried out according to the stated development stages of a damage related

to decreased grouting efficiency, as described in Chapter 5. However,

leakage water at the downstream side of the dam is a combination of both

the actual seepage water through the embankment, groundwater and from

other sources, e.g. rainfall, snow melt and surface runoff. Influence of the

latter two sources of water should be included and studied in case any

abnormality of ion content is observed under continuous monitoring.

Therefore, the leakage water through the grouted zone should be collected

and isolated from the surface runoff such as the rainfall and snow melt to

limit the influence of other water that is not the actual seepage.

Pore pressure measurement is typically carried out by measuring water

level by piezometers. Such instrumentation can provide information at

locations with existing boreholes. But the pore pressure of the entire dam

cannot be monitored or described with only piezometers. Therefore, it may

not reflect the actual seepage distribution or pore pressure in the dam

body. The reliability of monitoring results achieved by only piezometer

measurement must often be verified by other measurements, for example

an increased leakage, or an increased seasonal temperature variation in the

treated zone of the dam body.

CONCLUSIONS AND SUGGESTIONS FOR FUTURE RESEARCH| 51

7. Conclusions and suggestions for future

research

The presented literature review of sodium silicate grout has shown that the

grout most likely is susceptible to degradation under typical embankment

dam conditions. Degradation of sodium silicate based grout can occur in

form of shrinkage and leaching, which potentially can lead to increased

permeability of the treated dam core. However, how the degradation will

affect the dam is also dependent on other factors, such as the grain size of

the treated soil. This thesis study has gathered some information of the

potential behavior of sodium silicate grout from the literature on research

performed in-situ and in laboratory, and discussed how this type of grout

could degrade in relation to a treated dam. But how this type of grout

behaves in the field under typical dam conditions remains to be examined.

There is a need of controlling the dams treated with sodium silicate by

monitoring, with consideration of the relatively high risk to a decreased

grout efficiency.

Monitoring methods to control the grout’s long-term efficiency and

performance of the treated dam are suggested to be monitoring the ion

content in the leakage, measuring the permeability of the repaired core by

measuring pore water pressure distribution and the dam temperature, as

well as by performing regular visual inspection. However, there are

uncertainties when performing the measurement of the ion concentration

in the leakage, since it requires more understanding of the grout’s behavior

when it degrades. Potentially, it will lead to difficulties for interpreting the

measurement results.

Although there are some uncertainties regarding the accuracy of these

methods, there is still no commercially available methods allowing direct

monitoring of grout injected in the dam core. Monitoring of pore water

pressure distribution, temperature of the dam and leakage analysis are well

52 |CONCLUSIONS AND SUGGESTIONS FOR FUTURE RESEARCH

proven methods that will provide relevant information to monitor the

overall dam performance and grouting efficiency. To conclude, more

studies are required regarding the monitoring methods, aiming to capture

a relevant picture of the silicate based grout and the treated dam.

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