Groung Investigation and Ground Improvement Techniques for Developing Mining Pond Lands Economically

8/4/2019 Groung Investigation and Ground Improvement Techniques for Developing Mining Pond Lands Economically

1/12

GROUND INVESTIGATION AND GROUND IMPROVEMENT TECHNIQUES FOR

DEVELOPING MINING POND LANDS ECONOMICALLY

Nathan Narendranathan1 and Bandula Samarasinghe2

1.Managing Director, Infra Tech Pty Ltd, 2 Jones Street, OConnor, WA 6163, Australia. ([email protected])

2.

Group General Manager (Corporate Sustainability and R&D), Infra Tech Pty Ltd, 2 Jones Street, OConnor, WA6163, Australia. ([email protected])

ABSTRACT

Brown field sites such as mining tailings ponds, old quarries and old landfills are common in most countries. The lack

of cost effective and technically sound approaches have meant that the developers had only one choice of ground

improvements, which is piling for all structures to be erected on such sites with deep loose or soft deposits. This paper

provides an overview of how mining pond land can be assessed rapidly and strengthened economically to enable

building of low rise, medium rise or even high rise structures that will not experience large settlement and differential

settlements. Case histories from Malaysia and Australia will be presented on site investigations using shear wave

techniques and Controlled Dynamic Compaction and other ground improvement techniques for effective ground

strengthening.

1. INTRODUCTION

Mining for earths resources such as metals, precious stones, construction materials and salt has been in progress for

thousands of years. Processes relating to mining involve excavation, separations, chemical or physical processing and

finally the storage of waste products. These waste products could be solids or soft slimes. The soft wet slimes or pastes

are termed as tailings and the storage locations of those are called tailings ponds or tailings dams. Tailings ponds

normally contain very soft slimes and sediments and remain in this condition for decades after the mine closure. This

not only poses a hazard but also creates impediments in developing these locations after mining has ceased. Several

countries have requirements for mining pond or tailing pond closures before a mining company can be released from

their obligations. However, in the past such regulations did not exist and hence in most countries there is a legacy of old

disused mining ponds. This paper provides an overview of the various geotechnical issues associated with developing

such sites and some of the ongoing research being undertaken in the authors organisation.

2. SITE INVESTIGATIONS AND POINTS TO NOTE

The first step in the process of tailings pond rehabilitation is the characterisation of the spatial distribution of the

material depths, variability in depths, depth to hard layer, and assessing the physical and chemical properties of the

sediments or slimes. Properties of interest to geotechnical engineers are the strength, solids content or moisture content,

permeability and compressibility, to name a few.

Also, resolving the practical issues such as access into the slime pond is vital and need to be addressed early in the

developments. Floating access platforms made of bamboo poles and geosynthetics are possible options. Floating

Marsh Buggies and air cushion vehicles such as hovercraft can be used for very soft areas. Figure 1 shows an air

cushion supported work platform operating in a slimy environment as associated with tailings ponds.


2/12

Figure 1: Air cushion supported work platform

The authors recommend the use of non invasive geophysical investigations as an economical means of obtaining a

preliminary assessment of the sub soil strata. Echo sounding, seismic wave measurements, electromagnetics are some

techniques that have been used with success as they can provide inidcation of sediments and hard layer profiles. Figures2, 3 and 4 provides information relating to different geophysical techniques that may be used in investigations.

Ground Penetrating

radar

Figure 2: Typical Ground Penetrating Radar (GPR) profile


3/12

Figure 3: Typical seismic shear wave velocity profile

Figure 4: Clay mine pond filled with slurry investigated by Electro Magnetics

The geophysical investigative techniques will provide a rapid and cost effective means of looking at depth variations,

layering, and changes in strengths. Seismic shear wave technique has been very promising in the above approach.

Geophysical techniques can be complemented by relatively inexpensive Mackintosh Probing. These methods can

precede traditional drilling and sampling or probing by Dutch Cone Penetrometers.

Since the engineering properties will be governed by the geochemistry of the tailings, it is very important to carry out

chemical and geochemical analysis of the tailings to assess the components in the tailings. This can be done by X ray

diffraction, scanning electron microscopy along with more conventional mass spectrometry techniques. The authors

recommend the use of assessing the specific surface area and mineral types by Methylene Blue Test (MBT), which is a

rapid and inexpensive filed test. Correlations can be developed between engineering properties and MBT values basedon the exchangeable ions determination.

An adequate investigation is a pre requisite for an in-depth understanding of the tailings properties and hence will

influence the selection and design of economical ground improvement processes.

While using investigative techniques such as Dutch Cone Penetrometer for probing to measure the cone resistance and

sleeve friction is very useful it should also be borne in mind that the empirical equations correlating the cone resistance

and sleeve friction to various engineering properties such as bearing capacity, elastic modulus etc. were established for

soils and not mining pond slimes. These tests could be used to get a relative assessment of the ground conditions which


4/12

should be followed by some form of sampling and testing, especially if local experience or knowledge base is not

available.

3. RHEOLOGY AND ENGINEERING PROPERTIES OF MINING SLIMES

Though mine tailings and slimes are derivatives of soils and rocks, it should be borne in mind that the mining and mineprocessing operations and any chemicals used in the processing might have rendered the tailings with different

properties to soft clays or silts. The geochemistry and electro chemistry of the slimes could be changed thereby

affecting the application of traditional engineering analytical equations or even ground improvement techniques. In

general many of the tailings will tend to have a large negative charge on them. Furthermore the amount of

exchangeable ions (cat ions) will determine how stable the tailings will be after strengthening and will in fact have a

major influence on the strengthening process. Chemical or other additives used in the processing of the mineral may

also be present in the slimes and this need to be factored into. The grading of the fines and the types of minerals present

in the slimes will have an effect on the engineering properties.

Depending on the age of the tailings the solid content could vary from 15% to 50%. The sedimentation process is often

very slow due to the fact that the fine particles possess large negative charge which prevents rapid agglomeration and

setting. Sampling of these slimes and carrying out tests such as one dimensional consolidation test is quite common in

the geotechnical industry. The consolidation test data are used to assess compressibility, prediction of settlements etc

using conventional one dimensional small strain consolidation theory.

It should be noted that conventional consolidation theory will not give representative predictions of settlements,

consolidation times etc. since this approach does not account for the change in permeability of the tailings as

consolidations proceed. In addition the coupling of the soil particle settlement and fluid movement is also not

accounted for. It is necessary to use large strain consolidation theory which accounts for the change in permeability of

the setting sediments or tailings. In the 1960s and 1970s, treatment of the mathematics associated with large strain

consolidation theory was difficult and hence simplified finite difference modelling was used. However with the

development of powerful computers it is now possible to use 2D and 3D consolidation analysis using large strain

consolidation theory.

In a technical and contractual sense application of inappropriate predictive tools can not only lead to uncertainties in

settlement prediction but also contractual disputes between clients and ground improvement specialists.

4. TRADITIONAL APPROACHES FOR BUILDING ON BROWN FIELD SITES

4.1 LEAVE AS IS WITH SURCHARGE FOR SEDIMENTATION

In the past the view has been to leave the site to sediment for decades with some amount of surcharge fill placed in

stages. This will seldom work since the low permeability of most tailings will require several decades before

appreciable improvements can be seen to the strength and compressibility. Relying on evaporation also is not reliable

since a dry crust forms during intense evaporation and this crust reduces further long term evaporation of the underling

slimes. Figure 5 shows a coal mine tailing pond after 20 years of closure.


5/12

Figure 5: Coal mine tailing pond after 20 years

4.2 PILING

There is a view that piling of any structures built on tailings ponds will prevent settlement. This is partially correct and

may not always be the most functional solution. See Figure 6, showing differential settlement. Most tailings ponds are

filled up to create a platform that is above the flood levels or to match surrounding developments and roads. The newly

placed fill will trigger settlements in the underlying soft slimes. This settlement (unless accelerated) will occur over 10

to 25 years depending on the permeability of the slimes. Piles supporting any structures will then be subjected to down

drag forces (called negative friction) and hence will be overloaded. If the piles are designed to withstand these negative

friction forces then the cost of piling tends to be expensive. Furthermore settlement of the fill platform will cause

breakage and differential settlement between piled structures and other un piled pipelines, roads, aprons etc that are

supported directly on the fill.

Figure 6: Photo of differential settlement behind piled bridge

Thus it is apparent that ground improvement of mine tailings ponds will be required if sustainable and economical infra

structure developments are to be undertaken in such areas. The significance of this for a country like Malaysia can be

appreciated when it is realised that many of the mining areas have been located very close to large cities and towns as

shown in the mine location plan in Figure 7.


6/12

Figure 7: Mining areas in Peninsula Malaysia (From Mineral Distribution Map of peninsula Malaysia 6th

edition 1973)

5. GROUND IMPROVEMENT SOLUTIONS

The approaches and technologies available for ground strengthening are constantly evolving. These techniques must

consider the following factors.

Type of soils Type and magnitude of loads Time available for ground improvement Constraints placed by surrounding developments Availability of skilled geotechnical personnel Cost

Some of these technologies applied in Malaysia and Australia are discussed in this paper. Figure 8 shows a slime pond

with fresh slime being pumped in for storage.


7/12

Figure 8: A slime pond with fresh slime being pumped in for storage

5.1 HIGH IMPACT ENERGY DYNAMIC COMPACTION (HIEDYC)

There are many techniques that use high impact dynamic energy for the compaction of ground. The methods of

delivering dynamic compaction energy to the ground include dropping heavy weights on the ground from a height

traditionally called Dynamic Compaction (DC), imparting dynamic energy on a compaction plate placed on the ground

by varied means and imparting energy on the ground by rolling a non circular drum on the ground to be compacted,

referred to as HIEDYC in this paper. The method of compaction adopted depends on many factors, such as the type of

soil, depth to water table, strength requirement and the depth of improvement required.

A heavy drum with a square cross section has been pulled using a prime mover so that the roller continues to thump the

ground imparting the energy as it advances. Compactors with various cross sections have been adapted such as

triangular and pentagonal. Over the years, these shapes have been refined to maximise the ground compaction depth and

minimise vibration when being towed at appropriate speeds.

The depth of improvement and strengths achieved in different compaction techniques depend on the type and condition

of the soil, size and shape of the compactor and the travel speed. Based on these factors, the cost and time required for

compaction vary as well.

HIEDYC compaction has been applied to sands, silts, silty sands, sandy clays and clays at moisture contents not

exceeding OMC +4%. In general, it was assumed that HIEDYC process is applicable to sands only. However, HIEDYC

has been applied successfully for the compaction of clay fills according to authors past project experience.

Figure 9 shows the application of HIEDYC in clay fills at Senai Desaru Expressway in Malaysia, a 77km long highway

in the Malaysian road network (Infra Tech Pty Ltd 2008).


8/12

Figure 9: Application of HIEDYC on clay fills at Senai Desaru Expressway in Malaysia and Cowal Gold Mine, New

South Wales

A summary of typical soil properties from some projects where the successful compaction achieved using HIEDYC

deep compaction is presented in Table 1.

Table 1: Summary of typical soil properties from some ITPL projects where the compaction was successfully achieved

using HIEDYC deep compaction.

Project type of soilPlastic

limit

Liquid

limitPI

%

finesOMC MDD

Compacted

moisture

content

Remarks

Senai Desaru

Expressway,

Malaysia

Light

yellowish

red motled

white sandy

silty clay

35.97 51.10 15.13 49.8 15.0 1.81 17% to 21% HIEDYC Penta

mostly compacted

in wetter than

OMC

AMC

Henderson,

WA,

Australia

GW

CL

ML

NP NP NP 7 10.5 2.03 4% to 6% HIEDYC Penta

mostly compacted

very dry22 35 13 95 NT NT

25 32 7 9 NT NT

Cowal Gold

Mine, NSW,

Australia

Orange

brown silty

clay, tracegravel

14 35 21 Natural

moisture

HIEDYC Penta

mostly compacted

in wet state

Mermaid

Marine

Supply Base,

Dampier,

WA,

Australia

Sandy clay

Gravelly

sand

Silty clay

18 31 13 62 NT NT Natural

moisture

HIEDYC Tria

mostly compacted

in dry stateNP NP NP 5 10.1 1.922

18 52 34 99 NT NT

NP Non plastic

NT Not tested

Source: Project geotechnical reports of Infra Tech Group

The data in Table 1 demonstrates the applicability of HIEDYC compaction for sands, clays and silts provided the

appropriate HIEDYC module is used. It was also noted that the soils were compacted at moisture contents ranging

between 5% wetter to very dry natural moisture state. In addition to soils, HIEDYC has also been used to compact old

landfills filled with building rubble (Infra Tech Pty Ltd 2009-2).as well as ponds with slimes (Infra Tech Pty Ltd 2004

and Infra Tech Pty Ltd 2009-3).

To decide on the most optimum high impact energy ground improvement solutions, it is necessary to have not only anunderstanding of the operational aspect of HIEDYC rollers, but also a sound understanding of geotechnical engineering.

Due to the requirement of matching the equipment to the ground to be compacted to achieve performance criteria, there

are several configurations of HIEDYC modules that have been deployed in the projects that are used as case studies in


9/12

this paper. Authors organisation operates these equipment under the trade mark HIEDYC and has three basic

modules, namely Tria, Qadra and Penta. Table 2 provides basic information of these modules with an overview of their

capabilities and applications. Photographs of these three modules; Penta, Qadra and Tria are shown in Figure 10.

Table 2: HIEDYC modules, capabilities and applications

ModuleNumber of

sides

Mass

(tonnes)

Mining engineering applications Civil engineering applications

Tria 3 17 Pit floors/rock crushing, haul roads, soil

dumps, rock dumps, tailings

consolidation and strengthening.

Subgrade compaction, earthwork

compaction, coarse sand and silt

compaction and clay soils or slime

ponds in conjunction with Prefabricated

Vertical Drains

Qadra 4 14 Haul roads, soil dumps, tailings


Subgrade compaction for road

pavements, earthwork compaction and

sand and silt compaction not greater

than 1.5m

Penta 5 16 Pit floors/rock crushing, haul roads, soil

dumps, rock dumps, tailings


Subgrade compaction, earthwork

compaction, coarse sand and silt

compaction and clay soils or slime

ponds in conjunction with Prefabricated

Vertical Drains

Figure 10: Photographs of HIEDYC modules, (L to R) Tria, Qadra and Penta

Application of high impact energy compaction when used in combination with other techniques can be effective andalso the application of this technique can be made much broader, facilitating the compaction of varied types of soils.

One such application is the use of prefabricated vertical drains in conjunction with HIEDYC for deep compaction of

clay soils as shown in Figure 11.

Figure 11: Installation of PVD in old tine mine pond in Kepong and HIEDYC deep compaction over PVD


10/12

5.2 CONTROLLED DYNAMIC COMPACTION (CDYC)

Another form of deep dynamic compaction suitable for mining pond improvements is Controlled Dynamic Compaction,

(CDYC).

CDYC is the acronym for Controlled Dynamic Compaction technique adapted by Infra Tech Pty Ltd. CDYC has been

found to achieve deeper compaction than that achieved by HIEDYC and was developed by the authors organisation tocater for deep ground improvements.

The CDYC technique consists of dropping a weight by hydraulics on a steel impact plate varying in diameter between

1.0m and 1.5m. The hammer weight and the drop height can be changed. In general 10 to 30 blows a minute can be

imparted on the steel plate by the drop hammer. The greater efficiency of energy transfer (compared to traditional free

fall Dynamic compaction) is due to the fact that the impact plate is constantly in contact with the ground. Figure 12

shows the assembly and the CDYC operation in progress, while the Figure 13 shows the stages of CDYC compaction

from a slimy pond to a developable land.

Figure 12: CDYC equipment being assembled and CDYC ground improvement work in progress

Figure 13: CDYC work stages in a swampy site transforming a slimy pond in to a developable land

The ground strength after CDYC can be assessed by Electric Friction Cone Penetrometer (EFCPT) .or seismic wave

velocity profiling. Figure 14 show the ground being pegged for testing by EFCPT and the EFCPT equipment

conducting testing.


11/12

Figure 14: Pegging of EFCPT locations and Tests in Progress after CDYC deep compaction and EFCPT in progress

5.3 ELECTRO OSMOSIS

Electro osmotic (EO) consolidation means the consolidation of soft clays by the application of electric current. It wasstudied and applied for the first time by Leo Casagrande (1948). It is inherent that fine grained clay particles with large

interfacial surface will consolidate and generate significant settlement when loaded. Electro osmosis was originally

developed as a means of dewatering fine grained soils for the consolidation and strengthening of soft saturated clayey

soils. Electro osmotic dewatering essentially involves applying an electric potential across the sediment layer. It is the

process where in positively charged ions move from anode to cathode. I.e. Water moves from anode to cathode where it

can be collected and pumped out of soil. Electro osmotic flow depends on nature of soil, water content, and pH value

and on ionic type concentration in the pore water. Due to the applied electric potential, the electrolysis of water occurs

at the electrodes. Electro osmotic transfer of water through clay is a result of diffuse double layer cations in the clay

pores being attracted to a negatively charged electrode or cathode.

When electrodes are placed across saturated clay mass and direct current is applied, water in the clay porespace is transported towards cathode by electro osmosis.

In addition frictional drag is created by the motion of ions as they move through the clay pores helping totransport additional water.

The flow generated by the electric gradient is called electro osmotic flow.The ability to consolidate soft sediments without the need for high surcharge fill makes Electro osmosis desirable

especially when working on very soft ground where surcharge weight cannot be supported. Figure 15 shows electro

osmosis in progress for soft clay deposits.


12/12

Figure 15: Electro osmosis in progress for soft clay deposits

6. SUMMARY

To determine economical and sustainable solutions for the development of mining pond lands, it is necessary to

understand the soil types through adequate investigations, understand that tailings rheology and consolidation

mechanisms. Each site will require site specific solutions. HIEDYC, CDYC and EO techniques offer good options that

can achieve the desired outcome of transforming the mining ponds into developable lands economically and

sustainably.

REFERENCES

Infra Tech Pty Ltd (2004), Closure report for Westport Container Terminal CT4, Malaysia (unpublished).

Infra Tech Pty Ltd (2008), Closure report for Senai Desaru Expressway Project, Malaysia for Randhill Engineers.

(unpublished).

Infra Tech Pty Ltd, (2009 -1), Closure Report for Digesters 6 & 7, Beenyup, WA for Thiess, SKM Consulting and

W2WA Alliance. (unpublished).

Infra Tech Pty Ltd, (2009 -2), Closure Report for Lake Coogee Stage 1 Development for VDM Consulting and DM

Civil. (unpublished).

Infra Tech Pty Ltd (2009-3), Closure Report for Mermaid Marine, Dampier, WA for Ertech Contractors. (unpublished).

Documents

Groung Investigation and Ground Improvement Techniques for Developing Mining Pond Lands Economically