MECHANICAL MODIFICATION

Dr. Supia Khatun

Department of Civil Engg.

Aliah University

Newtown Campus Kolkata

COMPACTION

OBJECTIVES

Deep compaction techniques are required when in–situ soil

extending to large depths does not meet the requirements of

performance criteria specified for the expected loading and

environmental conditions.

DYNAMIC COMPACTION

INTRODUCTION

•Dynamic compaction is a ground improvement technique that

densifies soils and fills by using a drop weight.

•The drop weight, typically hardened steel plates, are lifted by a

crane and repeatedly dropped on the ground surface.

•The drop locations are typically located on a grid pattern, the

spacing of which is determined by the subsurface conditions

and foundation loading and geometry.

•Treated granular soils and fills have increased density, friction

angle and stiffness.

Dynamic Compaction

Technique involves repeatedly dropping a large weight

from a crane

Weight may range from 6 to 172 tons

Drop height typically varies from 10 m to 40 m m to 40

m

Degree of densification achieved is a function of the energy

input (weight and drop height) as well as the saturation level,

fines content and permeability of the material

6 – 30 ton weight can densify the loose sands to a depth of 3

m to 12 m

Done systematically in a rectangular or triangular pattern in phases

Each phase can have no of passes; primary, secondary, tertiary, etc.

Spacing between impact points depend upon (5m to10 m)

Depth of compressible layer

Permeability of soil

Location of ground water level

Deeper layers are compacted at wider grid spacing, upper layers

are compacted with closer grid spacing

• Deep craters are formed by tamping

• Craters may be filled with sand after each pass

• Heave around craters is generally small

Energy transferred by propagation of Rayleigh (surface)

waves and volumic (shear and compression) waves

Rayleigh 67 %

Shear 26 %

Compression 7%

• Compressibility of saturated soil due to presence of micro

bubbles

• Gradual transition to liquefaction under repeated impacts

• Rapid dissipation of pore pressures due to high permeability

after soil fissuring

• Applicable to wide variety of soils

• Grouping of soils on the basis of grain sizes

• Mainly used to compact granular fills

• Particularly useful for compacting rockfills below water and for

bouldery soils where other methods can not be applied or are

difficult

• Waste dumps, sanitary landfills, and mine wastes

• In sanitary fills, settlements are caused either by compression of

voids or decaying of the trash material over time, DC is effective in

reducing the void ratio, and therefore reducing the immediate and

long term settlement.

• DC is also effective in reducing the decaying problem, since

collapse means less available oxygen for decaying process.

• For recent fills where organic decomposition is still underway, DC

increases the unit weight of the soil mass by collapsing voids and

decreasing the void ratio.

• For older fills where biological decomposition is complete, DC has

greatest effects by increasing unit weight and reducing long term

ground subsidence.

EVALUATION OF IMPROVEMENT

• The depth of improvement is proportional to the energy per

blow

• The improvement can be estimated through empirical

correlation, at design stage and is verified after compaction

through field tests such as Standard Penetration Tests (SPT),

Cone Penetration Test (CPT), etc.

Dmax = n√W H

Where,

Dmax = Max depth of improvement, m

n = Coefficient that caters for soil and equipment variability

W =Weight of tamper, tons

H = Height of fall of tamper, m

• The effectiveness of dynamic compaction can also be assessed

readily by the crater depth and requirement of backfill

GROUND VIBRATIONS

• Dynamic compaction generates surface waves with a dominant

frequency of 3 to 12 Hz

• These vibrations generate compression, shear and Rayleigh

waves

• The Raleigh waves contain about 67 percent of the total vibration

energy and become predominant over other wave types at

comparatively small distances from the source

• Raleigh waves have the largest practical interest for the design

engineers because building foundations are placed near the

ground surface

• The ground vibrations are quantified in terms of peak particle

velocity (PPV); the maximum velocity recorded in any of the

three coordinate axes

• The measurement of vibrations is necessary to determine any

risk to nearby structures

• The vibrations can be estimated through empirical correlations

or measured with the help of instruments such as portable

seismograph, accelerometers, velocity transducers, linear

variable displacement transducers (LVDT), etc.

• The frequency of the Raleigh waves decreases with increasing

distance from the point of impact

Tolerance Limits for Structures

British Standard 7385: Part 2-1993, lays down following safety

limits for various structures having different natural frequencies:

• Reinforced or framed structures industrial and heavy

commercial buildings at 4 Hz and above , 50 mm/s

• Un-reinforced or light framed structures residential or light

commercial type buildings at 4 Hz –15 Hz, 15-20 mm/s

• Un-reinforced or light framed residential or light commercial

type buildings at 15 Hz –40 Hz and above, 20-50 mm/s

Effect on Humans

MONITORING AND CONTROL

• Depth of improvement, d

• Impact energy, E

• Influence of cable drag

• Equipment limitations

• Influence of tamper size

• Grid spacing, S

• Time delay between passes

• Soil conditions

DESIGN AND ANALYSIS CONSIDERATIONS

Depth of Improvement

Depends on:

• Soil conditions

• Energy per drop

• Contact pressure of tamper

• Grid spacing

• Number of passes

• Time lag between passes

Impact Energy, E

• Weight of tamper times the height of drop

• Main parameter in determining the depth of

improvement

• Can be calculated from the equation

Dmax = n√W x H

(Free falling of weights)

Influence of Cable Drag

• Cable attached to the tamper causes friction and reduces

velocity of tamper

• Free fall of tamper is more efficient

Equipment limitations

• Crane capacity

• Height of drop

• Mass of tamper

• Tamper size

Grid Spacing

• Significant effect on depth of improvement

• First pass compacts deepest layer, should be equal to the

compressible layer

• Subsequent passes compact shallower layers, may require

lesser energy

• Ironing pass compacts top layer

Time Delay between Passes

• Allow pore pressures to dissipate

• Piezometers can be installed to monitor dissipation of pore

pressures following each pass

CASE STUDIES

Nice airport new runway - France

•An extension was made for the existing Nice airport by

constructing two new runways 3200 meters long, parallel to the

shore line on a reclaimed land.

•The soil conditions prevailing were loose fill, some stiff marls

and deposits of soft sandy silts.

•Hence there was a need for heavy dynamic compaction in and

around the runway.

•The project involved the placement of about 20,000,000 m³ of fill to

build a reclaimed platform of 200 ha. The borrow pit was situated at

13 km from the main site. The transport was made by means of a fleet

of 38 dumper trucks with trailer 145 tons total weight.

•The evolution of pore water pressure was continuously monitored at

various depth during DC. Works have been done in successive phases

with sufficient resting periods to avoid building excess pore pressure.

The volume versus DC energy governed the intensity of the treatment.

During Dynamic Compaction and after treatment numerous CPT, have

been performed to control fill characteristics.

Shuaiba IWPP III - Desalination Plant - Saudi Arabia

•Shuaiba Independent Water & Power Project (IWPP) was

planned to meet the growing demands of water and

electricity in Saudi Arabia‟s Shuaiba region, 110 km from

Jeddah.

•Site had two types of soil profiles. In the first profile there

was loose to dense silty sand and second profile was

composed of soft silt or very loose silty sand. This layer

was followed by the bedrock.

•The project consisted of 12 evaporators, 3 water tanks and a

number of related buildings. The tank‟s diameter and height were

respectively 106.6 m and 20 m. The design criteria stipulated a

bearing capacity and maximum settlement of respectively 200 kPa

and 75 mm for the tanks. For the other structures, the same were

required to be 150 kPa and 25 mm respectively.

•Due to the presence of loose sands and soft silts, it was decided to

optimize the foundation solution by implementing dynamic

compaction and dynamic replacement in the project. The choice of

this technique was dependant on the soil characteristics.

•Upon completion of soil improvement works, 75 pressure meter

tests (PMT) and one zone load test were used to demonstrate that the

acceptance criteria had been achieved. The results of the tests clearly

indicated that success of the ground improvement project, and the

ability of the foundations to safely support the design loads.

http://www.haywardbaker.com/solutions/ground-improvement

Vibration Methods

Blasting Methods

Vibro-compaction methods

• Compaction at selected locations using vibrations and vibratory

equipment results in compaction to large depths.

• The zone of compaction around a single float is a function of type

of float

• The success of in situ densification depends on grain size

distribution of the in situ soils, and that of backfill soil

Use of grain size analysis a soil to decide on compactability

• Soils in zones A and B can be compacted by the deep vibratory

compaction method vibro Compaction (also called

“vibroflotation”), while soils of zones C and D cannot be

compacted by vibration alone.

• Soils in zone C are often found on sites where liquefaction due

to earthquakes is of concern. These soils can be compacted

during the installation of Stone Columns.

• Soils in zone D are not compactable by vibration, but can be

substantially reinforced, stiffened and drained by installing

Stone Columns.

• Vibro floatation refers to compaction of soil using a vibrofloat in

horizontal motion from the vibrator inserted into the ground.

Utilization of a top pile driving vibrator in a vertical mode is less

efficient.

• Utilization of the concept of frequency of vibrofloat matching

that of natural frequency of in-situ soil is also done in vibro-

compaction (Eg: Miller Resonate compaction technique).

• Vibro-replacement uses the same equipment as in vibro-

compaction and uses water/air as the jetting medium, and graded

stone aggregate as backfill.

Vibro Compaction

The objective in Vibro-compaction is to achieve densification

of coarse grained soils with less than 10-15% silt. The effect

of the process is based on the fact that particles of non-

cohesive soil can be rearranged by vibration.

Applicable soils

• Coarse grained soils with silt/clay content less than 10-

15%

Effects

• Increased shear strength, Increased stiffness, Reduced

liquefaction potential

Common applications

• Buildings, Chemical plants, Storage tanks & silos, Pipelines,

Wharf structures, embankments, Roads

• Both land / offshore applications

• Maximum depth 60 m

Vibro Replacement

Vibro Replacement is a technique of constructing stone columns

through fill material and weak soils to improve their load bearing

and settlement characteristics. Unlike clean granular soils, fine

grained soils (such as clays and silts) do not densify effectively

under vibrations. Hence, it is necessary to form stone columns to

reinforce and improve fill materials, weak cohesive and mixed

soils.

Principle

Reinforcement and Drainage

Applicable soils

Mixed deposits of clay, silt and sand, Soft and ultra soft silts (slimes) Soft

and ultra soft clays, Garbage fills

Effects

Increased shear strength, Increased stiffness, Reduced liquefaction potential

Common applications

Airport taxiways and runways, Chemical plants, Storage tanks & silos,

Pipelines, Bridge abutments and approaches, Offshore bridge abutments,

Road and railway embankments, Both land / offshore applications

Maximum depth 20-40 m

STONE COLUMNS

Stone columns, which are sometimes referred to as granular columns, sand columns,

or granular piles, are columns of compacted sand or gravel that, are inserted into a

soft foundation soil using a variety of installation techniques .

Why stone columns?

To increase bearing capacity

Reduced settlements

Accelerated consolidation settlements

Simplicity of its construction method

Environmentally friendly

Economical

Stone Column Installation Methods

Vibro-Replacement (Wet, Top Feed Method )

Vibro-Displacement (Dry, Top and Bottom Feed Method )

Stone columns are installed using either top- or bottom-feed systems, either

with or without jetted water.

The top-feed method is used when a stable hole can be formed by the vibratory

probe. With the dry method (top or bottom-feed), the probe is inserted into the

ground and penetrates to the target depth under its own weight and compressed

air jetting

Most widely used methods for installation of stone columns

are:

Dry – top - feed method

process schematic Dry – Bottom - feed method

process schematic

In the displacement or dry method, native soil is displaced laterally by a vibratory

probe using compressed air. This installation method is appropriate where ground

water level is low and in situ soil is firm.

Displacement or Dry method

In the replacement or wet method, native soil is replaced by stone columns in a

regular pattern where the holes are constructed using a vibratory probe

accompanied by a water jet.

Replacement or Wet method

Wet - top - feed method process schematic

Application of Stone Column

Stone column acts as vertical drains and thus speeding up the process of

consolidation, replaces the soft soil by a stronger material and initial

compaction of soil during the process of installation thereby increasing the unit

weight. Stone columns also mitigate the potential for liquefaction and damage

by preventing build up high pore pressure by providing drainage path.

Advantages of Stone Column

Weak soil, which has very low shear strength and high compressibility to support

structures require strengthening to be capable of carrying loads from structures.

Stone columns are ideally suited for structures, because:

To reduction of total and differential settlements.

To reduction of liquefaction potential of cohesionless soil.

To increase the bearing capacity of a site to make it possible to use shallow

foundation on the soil.

To increase the stiffness.

To improve the drainage conditions and environment control.

To control the deformation and accelerate consolidation.

Limitation of stone column

Limited to soft soils with undrained cohesion equal to 15 kPa.

Penetration of surrounding soft soil into the stone column.

Excessive bulging of stone column.

Squeezing of stone into surrounding soil .

Load

Stone column

75

Stone column

σrL σrL 4d

Load

76

77

Pattern:-

Preloading and vertical drains

When highly compressible, normally consolidated clayey soil

layers lie at limited/large depths, large consolidation settlements

are expected as the result of the loads from large buildings,

highway embankments, or earth dams etc. Pre-compression and

provision of vertical drains in soft soil may be used to minimize

post construction settlement.

This approach has resulted in a number of techniques involving

•Pre-compression or Pre-loading

•Sand drains

•Pre-fabricated Vertical Drains

•Vacuum consolidation

•High Vacuum Densification Method (HVDM)

Embankment on Clay Foundation Effect of Surcharge Treatment

Se

ttl

e

m

en

t

Time for Equivalent Settlement With Surcharge – Remove Surcharge at This Time

Time Time for Total Settlement Without Surcharge

The principle of pre-compression is explained in shown below

The proposed structural load per unit area is Δσ‟(p) and the

thickness of the clay layer undergoing consolidation is Hc.

The maximum primary consolidation settlement caused by the

structural load is then

The settlement-time relationship under the structural load is shown in

figure 1(b). However, if a surcharge of Δσ‟(p) + Δσ‟(f) is placed on

the ground, the primary consolidation settlement will be

Sequence of steps in “Precompression”:

• The total settlement of Sc(p) will occur at time t2, which is

much shorter than t1.

• Hence, if a temporary total surcharge of Δσ‟(p) + Δσ‟(f) is

applied on the ground surface for time t2, the settlement will

be equal to Sc(p) .

• At that time, if the surcharge is removed and a structure with

a permanent load per unit area Δσ‟(p) is built and no

appreciable settlement will occur.

Note: The total surcharge Δσ‟(p) + Δσ‟(f) can be applied by

means of temporary fills.

Derivation of equations for obtaining Δσ’(f) and t2:

From the figure 1(b), under a surcharge of Δσ‟(p) + Δσ‟(f ) the

degree of consolidation at time t2 after the application of load is

Figure gives magnitudes of U for varies combinations

of Δσ‟(p) / σ‟o and Δσ‟(f) / Δσ‟(p )

Example:

During construction of a highway bridge, the average permanent

load on the clay layer is expected to increase by about 115

kN/m3. The average effective overburden pressure at the middle

of the clay layer is 210 kN/m3. Here, Hc = 12m,Cc = 0.81, eo =

2.7 and Cv = 1.08m2/month. The clay is normally consolidated.

Determine.

a.The total primary consolidation settlement of the bridge

without precompression.

b.The surcharge, Δσ‟(f), needed to eliminate the entire primary

consolidation settlement in nine months by precompression.

Solution

Part a

The total primary consolidation settlement may be calculated from

=0.81×10

1+2.7 log

210+115

115

=0.415 m=415mm

Part b We have

Cv = 1.08 m2/month. H = 6m (two way drainage) t2 = 9 months.

According to Figure , for Tv = 0.27, the value of U is

40%.

According to Figure , for U=40% and Δσ̕(p)/σ̕o = 0.548,

Δσ̕(f)/σ(̕p) =2.5; Δσ(̕f) = (2.5)(115) =287.5kN/m2

we have,

Δσ‟(p) = 115 kN/m2

and Δσ‟o = 210kN/m2

so

Assuming a bulk density of 20 kN/m3 for fill material and a height

of 5m gives a pre-load of 100 kN/m2.

Vacuum Assisted Consolidation

Vacuum Assisted Consolidation is a new technology with the aim

to replace the standard preloading technique. Instead of increasing

the effective stress in the soil mass by increasing the total stress

with a conventional surcharge, a negative pressure preloads the

soil by reducing the pore pressure while maintaining a constant

total stress. This technique mainly consists of placing an airtight

membrane over the soft cohesive soil to consolidate and create a

vacuum underneath it by pumping.

Vacuum consolidation

Prefabricated Vertical Drains

Prefabricated vertical drain can be defined as any prefabricated

material or product consisting of a synthetic filter jacket surrounding

a plastic core. Because of their shape,. they are also known as band

or wick drains. They are manufactured in rolls of 200-300 m and are

inserted into ground to required depths using special drain stitcher

rigs. Generally, installation takes place up to full depth of

compressible soils. PVDs have replaced conventional sand drains for

soil consolidation due to their easy & speedy installation and unlike

sand drains, they act as a integral unit during the process of

consolidation.

INTRODUCTION

• Prefabricated Vertical Drains (PVDs) or „Wick Drains' are composed of

a plastic core encased by a geotextile for the purpose of expediting

consolidation of slow draining soils.

• They are typically coupled with surcharging to expedite

preconstruction soil consolidation. Surcharging means to pre-load soft

soils by applying a temporary load to the ground that exerts stress of

usually equivalent or greater magnitude than the anticipated design

stresses.

• The surcharge will increase pore water pressures initially, but with time

the water will drain away and the soil voids will compress. These

prefabricated wick drains are used to shorten pore water travel

distance, reducing the preloading time.

• The intent is to accelerate primary settlement. Pore water will flow

laterally to the nearest drain, as opposed to vertical flow to an

underlying or overlying drainage layer. The drain flow is a result from

the pressures generated in the pore water.

Prefabricated PVC wick drain

Historical background

The concept of drainage through vertical drains was initially

developed in 1920s. The property of sand being more permeable

than clayey/silty soil was utilized by creating sand columns in

lesser permeable soils as these sand columns functioned as drains.

The first pre-fabricated vertical drain (wick) was developed by

Walter Kjellman in 1940s. It consisted of few channels imprinted

into a stiff card board core. Drains using a synthetic drainage core

with longitudinal channels or grooves enveloped in a paper or non-

woven filter were introduced in early 1970s after further

development in wick drains.

Necessity of Vertical PVD Drains

• Soil stabilization using vertical wick drains is applied in areas

with compressible and water saturated soils such as clay and

silty clays. These soils are characterized by a very weak soil

skeleton and a large pore space, usually filled with water (pore

water).

• When a load such as a railway embankment is placed on soft

compressible soils, increase in load results in an increase of

pore water pressure and in impermeable soils, this water

dissipates very slowly, gradually flowing from the stressed

zone.

At the same time, increased pore pressure may cause instability

and slip plane failures may occur. Also, significant soil settlements

may occur which can create serious problem. Risk of instability

affects the safe rate of fill placement during construction. To

increase the rate of consolidation process, flow path needs to be

shortened so that pore water is released quickly.

Components of Vertical PVC Drains

There are two components of pre-fabricated PVC drain namely, core

and filter jacket. By combining the features of both core and filter

jacket, pre-fabricated PVC drain system provides effective, fast and

reliable performance for soil improvement.

Core

It is also called drain body which is a unique, corrugated, flexible and

made of polypropylene specifically designed to provide high

discharge capacity, high tensile and compressive strength

Filter jacket

It is strong and durable non-woven, thermically bonded

polypropylene fabric wrapped around the core. The fabric is of

random texture having high tensile strength, high permeability and

effective filtering properties. It acts as a filter to allow passage of

ground water into the drain core while eliminating movement of soil

particles and preventing piping. It also serves as an outer skin to

maintain the cross sectional shape and hydraulic capacity of the core

channels.

Advantages of Vertical PVC Drain System

• There are following advantages of using PVC drain system

• Minimum disturbance to the soil layers during installation.

• High water discharge capacity.

• Customized cores and filters to suit the various soil conditions.

• High compressive strength core prevents the collapse of the flow

path

• Proven performance under different soil conditions.

• Fast and easy installation.

• Deep installation exceeding 40 m in depth

• The unique and flexible core will not pinch off or flatten during the

consolidation process

• The installation rate of PVDs is typically 5,000 linear meters per day,

which results in a significantly lower project cost

• There is no risk of shear failure of PVDs during settlement, while

sand drains are vulnerable to shear failure during settlement.

• PVDs have discharge capacities, typically 30×10-6 to 90×10-6 m3/s,

while a 0.35 m diameter sand drain has a discharge capacity of 20 ×

10-6 m3/s.

• For typical projects, the cost of PVD is around 1/5th to 1/10th that of

300 mm to 450 mm sand drain.

• The size of wick drain is typically 100 mm in width and 3 to 9 mm in

thickness.

Patterns and spacing of PVC drains

Vertical wick drains are usually placed in a square/triangular

configuration, as shown below, the pattern size (Ds) being converted

into the equivalent drainage spacing (D) determined from the diameter

of the ground cylinders around a drain. The relationship between D

and Ds is as follows:

D = 1.13 x Ds for a square pattern

D = 1.05 x Ds for a triangular pattern

• The drain spacing may be varied to optimise the design. Closer

drain spacing decreases consolidation time but increases

installation costs

• Generally 1-3 metres centre to centre spacing is adopted.

INSTALLATION METHOD

• Place the drainage wick through or around the anchoring

device and tuck the loose end of the wick up into the mandrel

about 6 to 8 inches (150 to 200 mm).

• Move the machine/mandrel to the specified vertical drainage

wick location and insert the mandrel with anchoring device in

place using static force (and /or vibratory force if necessary)

into the ground to the desired depth.

• Extract the mandrel, leaving the anchoring device and the

completed or installed vertical drainage wick in place.

• Check the vertical drainage wick installation machine mast to

make sure it is plumb. Use hydraulic controls to correct if not

within specification