3. Ground Improvement by Hydraulic

Ground Improvement by Consolidation

Methods of application:

Preloading with or without vertical drains

Electro-osmosis

Vacuum consolidation

Preloading with or without vertical drains

Preloading is usually accomplished by placing surcharge fills

To accelerate consolidation, vertical (sand or prefabricated wick) drains are often used with preloading

Principle and Mechanism

Coefficient of Surcharge: Ratio of weight used in preloading and wt.

of the final structure to be

constructed on the improved soil

Using a surcharge higher than

work load, soil always remains in an overconsolidated state

secondary compression for overconsolidated soil is much smaller than that of normally consolidated soil

Increasing the time of temporary overloading or size of the

overload, secondary settlement can be reduced /eliminated.

Preloading

• Simply place a surcharge fill on top of the soil that requires consolidation

• Once sufficient consolidation has taken place, the fill can be removed and construction takes place

• Surcharge fills are typically 10-25 feet thick and generally produces settlement of 1 to 3 feet.

• Most effective in clay soil

Advantages of preloading

• Requires only conventional earthmoving equipment

• Any grading contractor can perform the work

• Long track record of success

Disadvantages of preloading

• Surcharge fill must extend horizontally at least 10 m beyond the perimeter of the planned

construction, which may not be possible at confined sites

• Transport of large quantities of soil required

• Surcharge must remain in place for months or

years, thus delaying construction

8

Preloading

Preloading at West Kowloon Expressway, Hong Kong.

(5-10 m embankments for 2-5 months)

Vertical Drains

• Vertical drains are installed under a surcharge load to accelerate the drainage of impervious soils and thus speed up consolidation

• These drains provide a shorter path for the water to flow through to get away from the soil

• Time to drain clay layers can be reduced from

years to a couple of months

Vertical Drains

Sand drains

• sand drain is basically a hole drilled in a cohesive soil and filled with sand.

• Since the sand has larger particle size, its permeability is much higher, thus water will flow through it much more easily.

• The excess water is collected at the top and directed away from the jobsite.

• Simply drill the holes and fill them with sand, but if the soil is soft (which is frequently the case,) the holes will collapse.

Sand drains

 Typically 200-500 mm in dia.

 Formed by infilling sand in to a hole in the ground.

 Hole formed by driving ,jetting or augering.

 Typical spacing 1.5 - 6.0 m.

PVD (Prefabricated Vertical Drain)

• Geosynthetics used as a substitute to sand drain

• Installed by being pushed or vibrated into the ground

• Most are about 100

mm wide and 5 mm

thick

EQUIPMENT

PVD installation equipment can be developed to suit the soil condition, installation depth, specified scope of work and re- quired production rate. Technical data of typical medium-sized PVD installation equipment and accessories are shown below.

Installation Rig

Type of Base Machine : Excavator of suitable model CAT EL200B or larger model Size (CAT EL200B) : 3.18m x 4.45m

Weight (CAT EL200B) : 20 ton

Pushing Force : 5.5 – 20 ton

Mandrel Lifting and Pushing : Hydraulic gear drive

Mast Height : 8m

Typical Mandrel Dimensions

Weight of Guide and Mandrel : 1.5 to 4 ton Length of Mandrel : 12 to 20m Cross-sectional Area of Mandrel : 60 to 70 cm2

Maximum Installation Depth : 11 to 19m

INSTALLATION METHODS OF VERTICAL DRAINS

GROUP DESCRIPTION

PARTICULAR METHODS

REMARKS

DISPLACEMNT METHODS

Driving Vibration

Pull Down(static Force)

Washing

Combinations Of Above

A mandrel with or without a disposal shoe is used in each case

GROUP DESCRIPTION

PARTICULAR METHODS

REMARKS

DRILLING METHODS

Rotary drill, with or without a casing

Rotary anger, including continuous standard and hollow fight augers

Percussive(shell and auger) methods, with or

without casing Hand auger

A mandrel with or without a disposal shoe is used in each case.

Continued…

GROUP DESCRIPTION

PARTICULAR METHODS

REMARKS

WASHING METHODS

Rotary wash jet Washed open

ended case Weighted wash jet

head on flexible hose

Methods in which sand is washed in via the jet pipe are not suitable for

prefabricated drains

Continued…

Vertical Drain

Installation of drains on a barge

Typical installation of PVD

• Typically spaced 3 m on centers

Prefabricated Vertical Drains Available in US

•Alidrain

•Aliwick

•Ameridrain

•Colbond Drain

•Mebradrain

The drain which is

placed inside the

mandrel with tip

anchor

The details of PVD and tip anchor.

Horizontal Stripdrain

Instrumentation of Vertical Drains

Settlement Platform Permanent Fill

Soft Clay Vertical Drain

Firm Soil

Piezometers

Surcharge

Drainage Blanket

Not to Scale

How are PVD designed?

• The vertical wick drains are usually placed in a triangular configuration of 3 to 12 feet (1 to 4

meters) - depending on the desired consolidation time. As a result of this method of accelerating the consolidation process, uneven post-

construction settlements can be virtually

eliminated.

LAYOUT CONFIGURATION AND DRAIN INFLUENCE ZONE

PVDs are installed in either square or triangular patterns. A square pattern is more simple for setting out in the field. Triangular pattern however provides more uniform consolidation between drains.

Relationship of drain influence zone (D) to drain spacing (S) can be expressed by;

For square pattern D = 1.13 S For triangular pattern D = 1.05 S

TYPICAL APPLICATIONS

PVDs with surcharge as pre-loading method has been successfully appplied in various projects.

PVDs are typically used as ground improve- ment system in -

• Construction of road, railway, embankment, airport and ports

• Industrial projects

• Land reclamation projects

Case history – EADS Airbus Plant, Hamburg General overview of Airbus site

• Settlement ≥ 2,0 – 5,5 m

• Dyke construction to +6.5 in 8.5 month and to + 9.00 in 16 month

• Columns GCC Settlement 0,7 – 1.84 m

Temporary sheet pile wall – in 5 month – dyke construction in 3 years

Dyke construction to +6.5 in 8.5 month and to + 9.00 in 16 month

Columns GCC Settlement 0,7– 1.84 m

Settlement≥ 2,0 – 5,5 m

Basic design and alternate concept of Moebius–Menard

Subsoil characteristics

How to move on the mud !

Case history – EADS Airbus Plant, Hamburg

Case history – EADS Airbus Plant, Hamburg

Case history – EADS Airbus Plant, Hamburg

Case Study For Ground Improvement Using PVD With Preloading For Coal & Iron

Ore Stackyard

Project Details

Development of New Port at Gangavaram at 15 km south of Visakhapatnam Port , AP Development of Port Facilities included development of backup facility for coal and iron ore storage and stacking and handling

The Proposed Heights were – Coal Stacks : 12.00 m

Iron Stacks : 10.00 m

Sub-soil stratification

• Ascertain Design

Parameters

• 8 Nos Boreholes

Geotechnical Investigation

• Dredged Sand: 0.20~0.30 m thick

• Marine Clay with Shells:

1.00~3.00 m thick

• Soft Marine Clay:

7.00~15.00 m thick

• Below 12- 18m N values increased to a tune of 30 Stratification

Improvement Required

Natural Moisture content

12 - 81 %

Specific Gravity 2.52 – 2.65 Bulk Density 1.24 –

1.52 g/cc

Gravel 00 %

Sand 2 – 31 %

Silt + Clay 7 – 63 %

Liquid Limit 21 – 102 %

Plastic Limit 15 – 47 %

Initial Void Ratio, e₀ 0.627 – 2.249 Compression Index, Cc 0.38 –

0.92 Coefficient of

Consolidation, C_v

0.72 –

1.95 m²/y r

Cohesion, Ccu 0.19 –

1.05 kg/

c m² Angle of Friction, Φcu 18 – 29 Deg Shear Strength from

VST 0.095 –

0.991 kg/

c m²

Ground Improvement Scheme

Depth of

PVD 10.00 m to 18.00 m below OGL

Spacing of (TriangularPVD

)

1.00 m c/c below stacker reclaimers

1.50 m c/c in other area

Consolidati For

on Period

1.00 m spacing:

65 days

For 1.50 m spacing:

174 days

Thickness of Sand

Mat ^{300 mm}

Horizontal Drainage

System

Geotextile pipes filled by boulders / gravels;

PVD laid horizontally

Machinery

Used Hydraulic Stitchers

Post treatment Assessment & Analysis

Ana

Readings every 4 days when loading started.

Later at every 7 days Settlement Recorders

Plate Type: 13

Nos Magnetic: 7 Nos

Piezometers

Casagrande : 5

Nos Vibrating Wire : 14 Nos

Post Treatment Assessment

SECTION - C-C'

Hard / Stiff Strata

PreloadAsPerDesign

GL

Soft Clay 6m

9m 9m 3mMS 6m CP PS

VWP

AREA - B

AREA - A Section 3

IN 8

Section 2 Section 1

CP2(9m) VWP4(6m)

CP3(6m) CP1(9m)

CP4(9m) MS6

VWP5(6m)

VWP6(9m)

Vwp8(9m) VWP7(6m)

VWP9(6m)

VWP10(6m) VWP11(9m)

VWP3(6m) VWP2(6m) VWP1(9m)

MS3

MS4

MS5

MS1

MS2 PS3

PS4

PS5

PS6

PS7 PS10

PS9

PS8 PS1

PS2 IN 3

IN 6 IN 9

)

PS13 C'

C IN 4 IN 2

Section 4

IN 7 IN 1

IN 5

Section 5

IN 10 CP 5 (6m )

VWP13(6m) VWP12(9m

MS7 VWP14 (9m) PS12 PS11

Analysis of Data

Excess Pore Pressure Settlement

%U UmaxUt UtUi x100

x100

%U  S100

S_t

Asaoka Method

Settlement at equal time intervalΔ t

Points (Si, Si-1) are plotted

Interception of this line with line having slope = 1

Settlement S₁₀₀

Hyperbolic Method Graph of Time

/ settlement Vs Settlement Graph in the

form of Hyperbole Inverse of

slope of Hyperbola Settlement

S₁₀₀

800 700 600 500 400 300 200 100 0

0 100 200 300 400 500 600 700 800 Settlements (Si-1)

Settlements(Si)

Asaoka Method - PS2

1.0 0 0.9 0 0.8 0 0.7 0 0.6 0 0.5 0 0.4 0 0.3 0 0.2 0 0.1 0 0.0 0

0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0 4 5 0 5 0 0

T i m e ( D a y s ) Time /Settlements

H y p e r b o l i c M e t h o d - P S 2

Analysis of Data – Pore Pressure

1.40 1.30 1.20 1.10 1.00 0.90 0.80 0.70 0.60 0.50 0.40

0 25 50 75 100 125 150 175 200 225 250 275 300

CP 1 CP 2 CP 3 CP 4

1.40 1.30 1.20 1.10 1.00 0.90 0.80 0.70 0.60 0.50 0.40

0 25 50 75 100125150175200225250275300 VP 2 VP 11 VP 10 VP 9 VP 8 VP 5

Piezome

t er Ui Umax Ut % U

CP 1 0.543 0.884 0.665 64.22

CP 3 0.579 0.981 0.688 72.89

CP 4 0.843 1.035 0.889 76.04

VP 2 0.597 1.062 0.753 66.45

CP 2 0.807 1.189 1.039 39.27

VP 5 0.534 0.918 0.793 32.55

VP 8 0.621 0.961 0.835 37.06

VP 10 0.567 1.004 0.833 39.13

VP 11 0.920 1.226 1.070 50.98

VP 9 0.610 1.222 0.894 53.59

Analysis of Data

0

100

200

300

400

500

600

0 25 50 75 100 125 150 175 200 225 250 275 300

Settlement(mm)

No of Days

Settlement ^{PS1 PS2 PS4 PS6 PS9 PS10}

Settleme

nt Marker Observed Settleme nt

Asaoka Method Hyperbolic Method

S₁₀₀ % U S₁₀₀ % U

PS 1 532 460 115.65 833 63.87

PS 2 385 380 101.32 556 69.24

PS 4 261 335 77.91 500 52.20

PS 10 390 450 86.67 732 53.28

PS 6 239 260 91.92 735 32.52

PS 9 289 340 85.00 667 43.33

Conclusions

• Plate Settlement Recorders are more reliable than the Magnetic Settlement Recorders for marine clays.

 With the application of the load the pore pressure increased and dropped down slowly with time. The pore pressure variation indicated about 55 - 60 % dissipation i.e. degree of consolidation.

 Hyperbolic Method is more comparable with the Pore Pressure Dissipation Results. Further the results obtained with theoretical slope of hyperbola as 1.00 are more closer to the predicted by pore water pressure analysis.

 The consolidation settlements worked out theoretically from laboratory test results were much higher than that predicted by Asaoka and Hyperbolic Method

Electro-osmosis

The process of dewatering assisted by the application of a direct electric current is known as electro-

osmosis thus resulting in consolidation

soft clays whose moisture content cannot be reduced by conventional dewatering methods.

In Electro-osmosis method, Electrodes are installed in the soil and a DC current supplied which results water movement from anode to the cathode

A wellpoint system or ejector well system used as

cathode which collects and removes the water from the

ground.

Mechanism

Electro-osmosis transports water of the clay pore space to the cathodically charged electrode

When these cations move toward the cathode, they also bring water molecules along with them

These water molecules clump around the cations as a consequence of their dipolar nature

Macroscopic effect of this process is reduction of water content at anode and an increase in water content at the cathode

Free water appears at the interface between the clay and the cathode surface

Molybdenum Electrodes

Graphite Electrode

Shapes of Electrodes Metallic Electrodes

Electrodes Before Installation

Connecting Wires

DC Current Source

Flow of Water under Electro-osmosis

Advantages and Limitations

Advantages

 Can be used for dewatering of silty and clayey soils which are difficult to drain by gravity.

 Method is fast and instantaneous.

 Environment-friendly method

 Equipments required are few in number and easy to carry to the site.

 Method useful for all types of soils.

 Efficiency of this method is very high.

 Less man-power required to implement this method.

Limitations

• Practical application limited since very costly.

• Before actual application on site Laboratory tests and site tests are imperative.

• Huge amount of electricity.

needed

• Highly skilled labour needed

• Electrodes replacement needed from time to time.

• Method becomes ineffective If the moisture content of the soil is extremely low

Vacuum consolidation

Vacuum consolidation

^,

Both liquid and gas (water and air) are extracted from the ground by suction

This Suction is induced by the creation of vacuum on the ground surface and assisted by a system of vertical and horizontal drains

Vacuum is applied to the pore phase in a sealed membrane system

The vacuum causes water to drain out from the soil and creates negative pore water pressure in the soil

This leads to an increase in effective stress to the magnitude of the induced negative pore water pressure, without the

increase of total stress

•For rapid pre-consolidation, vertical drains (Prefabricated Vertical Drains) along with the vacuum preloading are used

•Vertical drains helps to distribute the vacuum pressures to the deeper layers and drain out water from the sub soil

•Vacuum preloading with PVD substantially reduces the lateral displacement and potential shear failure

•Maximum achievable

vacuum pressure in the field

is only about 80kPa Schematic view vacuum consolidation technique

VACUUM (J.M. COGNON PATENT)

Vacuum Consolidation (high fines contents soils)

CONCEPT

-Soil is too soft for surcharge

-Time does not allow for step loading -Surcharge soil not available

-Available area does not allow for berns

• PARAMETERS

1 – Depth

2 – Drainage path

3 – Condition of impervious soil 4 – Watertable near surface

• 5 - Absence of pervious continuous layer 6 – Cohesion

7 - Consolidation

parameters (oedometer, CPT)

• e_O, C_C, C_V, C_R, C_α, t,

• CPT dissipation test

8 – Theoretical depression value 9 – Field coefficient vacuum

10 – Reach consolidation to

effective pressure in every layer 11 – Target approach

Vacuum Consolidation

Advantages of Vacuum preloading technique over the Surcharge preloading technique

•Ground improvement with vacuum preloading does not require any fill material and there is no need of heavy machinery

•Construction period is generally shorter

•The increase in effective stress under vacuum preloading is isotropic. Therefore, the corresponding lateral displacement is in the inward direction and there is no risk of shear failure

•Application of Vacuum Preloading improves Bearing capacity of soil by 100% in the case of soft clays and eliminates 70% of the total estimated settlement of design load

•The overall cost of vacuum preloading is only about 2/3^rd of that with surcharge preloading

VACUUM CONSOLIDATION IN PEAT

CASE STUDTY IN DUBLIN

0.85m sq.

spacing

1.2m sq.

spacing

TCD/NRA VACUUM CONSOLIDATION FIELD TRIAL

TCD/NRA VACUUM CONSOLIDATION

FIELD TRIAL - INSTRUMENTATION

Pumping system: 30

^th

Nov 2009 – 23

^rd

Jun 2010

• 1.5kW Centrifugal pump

• 38mm diameter jet pump

Pumping system: 29

^th

Jul 2010 – 29

^th

Oct 2010

• 2.2kW Liquid ring pump

• 1.5kW Centrifugal pump

• 38mm diameter jet pump

• Prior to starting the TCD/NRA vacuum

consolidation field trial, four months of baseline monitoring were conducted.

• The vacuum consolidation trial was run from 30

^th

November 2009 and was terminated on 29

^th

October 2010

MONITORING BEFORE PUMPING

Flooding in August 2009

• The TCD/NRA vacuum preloading field

trial commenced on the 30th November 2009.

• The results for the eleven months of pumping presented here.

• Rain, water table, vacuum, settlement and pore water pressure are presented.

MONITORING DURING PUMPING

Visual 1 st June 2010

CRACKS AT EDGE

Ground Improvement by Consolidation

Key Issues associated with consolidation:

 Stability during surcharge placement,

 Clogging of vertical drains, and

 Maintenance of the vacuum.

System Stability

To safeguard against stability problems, the surcharge loads are often placed in stages

Each stage of loading is added only after the soil has acquired sufficient strength under the influence of the previous stage to support the new load

Build up and dissipation of the excess pore water pressure and the accompanying soil deformations are monitored to pinpoint the time for stage placement

In case of electro-osmosis or vacuum consolidation no stability problem is anticipated

Clogging of Drains

•Clogging of the vertical drain is a key issue affecting the feasibility of the system of ground improvement

•Major advantage of the plastic wick drains over sand drains is their flexibility and ability to sustain large deformations of the consolidating cohesive soil, which may otherwise shear and clog the sand drains, rendering them ineffective

•The hydraulic conductivity of the wick drains is influenced by the potential crimping of the material when large deformations take place or clogging of the drainage channels due to an

ineffective filter jacket

•A non-wooven geotextile fabric is usually used to provide filtering and ensure the hydraulic conductivity of the

prefabricated drains

Maintenance of the Vacuum

• Maintaining the vacuum by providing an all-around seal is

critical for the successful application of vacuum consolidation

• Resistance of the membrane to tear during and after placement is an important factor

• Membrane is covered by a layer of soil or by water ponding to prevent its tear by vehicles, animals or birds attacks, or

vandalism

• Vacuum is generated by circulation of air through a series of specially designed drains, installed to the depth of the layer to be consolidated (new system developed recently in France eliminated the need for the membrane)