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49119 Problematic Soils and Ground Improvement Techniques

OUTLINE

Preloading

Soft Ground Improvement Using Preloading

(Including Surcharge, Vertical Drain, and Vacuum)
Behzad Fatahi
PhD, MEng, BEng (Hons), CPEng, MIEAust, NPER University of Technology Sydney (UTS), and Coffey Geotechnics Pty Ltd, Sydney Office

Preloading and Vertical Drains Preloading with Vacuum Selected Examples

Ground Improvement Using Preloading

Process of loading the ground with surcharge equal or more than future structure load and then removing the surcharge after end of the required degree of consolidation

Ground Improvement Using Preloading - continued

e
1.0

Real Soil Consolidation Behaviour

e
1.0

0.9
Slope Cr

0.9

0.8
Compression line Slope Cc Re-compression or swelling line Slope Cr
1 10

Compression line Slope Cc Re-compression or swelling line Slope Cr Pressure (kPa)

1 10 100 1000

0.8

0.7

0.7

0.6

o
100

Pressure (kPa)
1000

0.6

Ground Improvement Using Preloading - continued Ideal Soil Consolidation Behaviour

Ground Improvement Using Preloading - continued Ideal Soil Consolidation Behaviour

e
1.0

e
1.0

Slope Cr
0.9 0.9

0.8

Compression line Slope Cc Re-compression or swelling line Slope Cr

1 10

Soil is normally consolidated if the current stress in the soil is the maximum ever experienced by the soil. Soil is over-consolidated if it has been subjected to a larger stress than the current stress.
Normally consolidated (on the line)

0.8

0.7

0.7

Over-consolidated

0.6

o
100

Pressure (kPa)
1000

0.6

Pressure (kPa)
1 10 100 1000

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Consolidation
Example 1 :
84 kPa
2m 4m
Clay Sand

Part a)
84kPa
2m 4m
Clay Sand

Consolidation
Settlement 584 days after loading? t=18 kN/m3 t=16 kN/m3 cv=4m2/year Initial stress: = 218+ 216 = 68 kPa u = 4 9.8 = 39.2 kPa i = 68 -39.2 = 28.8 kPa Normally consolidated: f = 28.8 + 84 = 112.8 kPa Cc=0.2, Cr=0.05, eo=1.1 e=0.2log(112.8/28.8)=0.119 Stf=(0.119/2.1) 4 = 0.227 m
Tv = cv t d p2 = 0.4

t=18 kN/m3 t=16 kN/m3 cv=4 m2/year

Normally consolidated: Cc=0.2, Cr=0.05, eo=1.1

a) Calculate the settlement of the clay layer 584 days after application of 84kPa surface loading just before it removal? b) Calculate the long term settlement of the clay layer after removal of 84 kPa load and application of 50kPa surface loading?

Calculation of Settlement
00 20

Part a)
84kPa
2m 4m
Clay Sand

Consolidation
Settlement 584 days after loading? t=18 kN/m3 t=16 kN/m3 cv=4m2/year
Normally consolidated: Cc=0.2, Cr=0.05, eo=1.1

Initial stress: = 218+ 216 = 68 kPa u = 4 9.8 = 39.2 kPa i = 68 -39.2 = 28.8 kPa f = 28.8 + 84 = 112.8 kPa e=0.2log(112.8/28.8)=0.119 Stf=(0.119/2.1) 4 = 0.227 m
Tv = cv t d p2 = 0.4

40

U (%)

60

0.7
80

Time for 100% settlement? Assume Tv=3.0

0.0 0.2 0.4 0.6 0.8 1.0

100

T DP t= v cv

3 4 = = 12 years 4

Dimensionless time, Tv

U = 0.70 Stt = U Stf = 0.700.227=0.159m

Part b)
2m 4m
Clay Sand

Consolidation
Long term settlement after reloading? 50 kPa t=18 kN/m3 t=16 kN/m3 cv=4m2/year
Initially over consolidated: Cc=0.2, Cr=0.05, eo=1.1

Ground Improvement Using Preloading - continued

Preloading (method 1)
Preload soft soil equal to the eventual structural load Wait until primary consolidation is complete how long? Remove the preload, allow soil to swell Construct structure OC consolidation settlement is expected

Initial stress: = 218+ 216 = 68 kPa u = 4 9.8 = 39.2 kPa i = 68 -39.2 = 28.8 kPa f = 28.8 + 50 = 78.8 kPa pc = 28.8+ 0.784=87.6 kPa

Heavy Preloading (method 2)

Preloading much greater than the eventual structural load Wait until settlement is equal to the expected total settlement that would have occurred if the structure load had been applied Remove the total surcharge and Construct structure

If the soil was not initially loaded e=0.05log(78.8/28.8)=0.022

Soil is normally consolidated

1 4 2 e=0.2log(78.8/28.8)=0.087 = = 4 years 4

Stf=(0.022/2.1) 4 = 0.042 m

Stf=(0.087/2.1) 4 = 0.167 m

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Ground Improvement Using Preloading - continued The average effective stress just before unloading in both methods are equal, therefore, the settlement values are the same. Methods 2 is more time effective but more surcharge fill material is required (time-cost analysis).

Ground Improvement Using Preloading - continued Example 2 :

A very wide road should be constructed on top of 10m deep soft clay overlaying a sandstone layer. The total pressure induced by road embankment and traffic is 60kPa. Calculate the consolidation settlement of the road for the following two options. Option 1) placing 4m of surcharge with unit weight of 20 kN/m3 for 36months Option 2) Placing 7.5m of surcharge with unit weight of 20 kN/m3 for 10months
(for simplicity conduct the consolidation settlement calculations for one layer).

60kPa

Method 1
Cc=0.3, Cr=0.05, eo=1.0 cv=4 m2/year

t=15 kN/m3

Method 2

Preload and Surcharge - Continued Low bearing capacity Excessive and differential settlements
H
f

Preload and Surcharge - Continued

Staged-construction is required for high surcharge embankments to prevent failure

Lower Bound

Fill Height

H2 H1

Preloading Technique

H0

d un Bo er pp U

Disadvantages of Excessive Preloading

t1

t2

t3
Time

tc

Some difficulties associated with surcharging only

Preloading method sometimes may not work alone due to a thick soft clay layer Preloading method sometimes may not work alone because of very low permeability of the clay layer which makes the consolidation process very long and not practical Sometime the required surcharge will be very high which has cost consequences. Sometimes rate of undrained shear strength gain is very small so the rapid placement of the surcharge embankment will cause foundation failure (very important).

Preloading and Vertical Drain System

Decrease the length of the drainage path. Horizontal permeability of soil is normally greater than vertical permeability

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Preloading and Vertical Drains

Combination of vertical and Radial Consolidation

The average degree of consolidation due to vertical and radial consolidation can be estimated using Carillo (1942) equation:

Vertical Drains
Shorten the length of the drainage path

(1-Uave)=(1-Uv)(1-Ur)
where, Uave is the average degree of consolidation Uv is the vertical degree of consolidation Ur is the radial degree of consolidation

Accelerate the rate of pore water pressure dissipation

Accelerate the rate of consolidation / settlement

Different Types of Vertical Drains Sand Drains

They were widely used between 1930 -1980 with diameter changing between 20 - 60 cm and with 1.5m to 6m spacing.

Sand Drains
Closed Mandrel Method: soil is displaced by pushing a closed
end tube and filling it with sand

Open Mandrel Method: soil is removed after an open end tube is pushed into the ground and then filled with sand Sand Drain

Prefabricated Vertical Drains (PVD)

PVDs consist of a plastic core and a filter all around. The filter material can be paper, fibrous material or porous plastic.

Prefabricated Vertical Drains (PVDs)

Composed of plastic core with a longitudinal channel wick functioning as drain, and a sleeve of fibrous material as a filter protecting the core Installation Rig

PVD Installation
Drain Delivery Arrangement

Cross section of mandrel and drain

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Wick Drain

Potential benefits of vertical drains

Surcharge Soft ground consolidates under load Without drains With Vertical Drains Surcharge

Settlement

Without vertical drains

vertical drains with surcharge Time

Main Factors Influencing PVD performance

Disturbance of soft soil because of installation methods of PVDs. Permeability of soil next to the PVD decreases, and also pore water pressure is generated in the soil during installation (Smear Zone). PVD wall can be damaged and disturbed and the drain permeability reduces (drain is assumed as free drainage boundary).
Smear Zone Equivalent PVD

Influence zone of drains (de)

Square pattern Triangular pattern

(Equilibrium Equation)

Equivalent Drain Diameter

Eq. (2)

Smear Zone Parameters

(after Xiao, 2001)

Eq. (3)

Eq. (1)

Drain size

w (mm)
95 98 98 94 93

t (mm) Eq. (1)

5 4 5 4 4 63.6 64.9 65.5 62.3 61.7

Drain diameter (mm, dw) Eq. (2) Eq. (3) 50.0 51.0 51.0 51.8 51.5 52.5 49.0 49.8 48.5 49.3

Kh & kv = horizontal and vertical permeability of soil rs = radius of smear zone rm=equivalent radios of mandrel ks= permeability of smear zone

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Simulation of Surcharge and PVDs

For accurate design of ground improvement using surcharge and PVD, to consider the smear zone effects, numerical analysis using computer codes is required. As the 3D simulation of embankment plus PVDs on the soft ground is very time consuming, simplified 2D equivalent plan-strain analysis is very common in practice.
2 0.6m 2 1 1

Converting 3D Axisymmetric Vertical Drain Parameters to Plan-Strain Parameters Plan-

Surcharge

Tensile Fabric Tencate WX600kN/m watertable

Berm Berm
20m

Berm

PVDs Soft Clay

(Indraratna and Redana,1997)

Example Surcharge Embankment

3D section of PVD and soil

Simplified Design Method For Preload with Vertical Drains

For simplicity it is assumed that each vertical drain is independent and is located in centre of a soil cylinder. The governing Equation for the radial consolidation is:

Simplified Design Method For Vertical Drains

By solving the horizontal consolidation equation the radial degree of consolidation can be calculated as:
8Th

2u 1 u u = ch 2 + r r r t

Uh = 1 e F

where,

Th =

c h .t de
2

u=u0 at t=0 at all place u=u0 In the draIn at any tIme

The governing Equation for the vertical consolidation is:

2u u = cv z 2 t

According to Hansbo(1979); F = F(n) + Fs + Fr where, F(n) : Due to spacing of drains, n=re/rd Fs : Due to smear effect Fr : due to well resistance There are graphs and equation to calculate F, and Uh

u=u0 at t=0 at all place u=u0 In the drainage boundaries at any tIme

Simplified Design Method For Vertical Drains

Simplified Design Method For Vertical Drains

Example 2 A road embankment is constructed on top of a 9.2m thick layer of clay, sandwiched between silty sand at top, and dense sand at the bottom. The required degree of consolidation before the embankment consolidation is 90% within 9 months. For this purpose, sand drains of 450mm diameter, need to be installed in a square arrangement. From the laboratory tests, assume that ch=0.288 m2/month and cv=0.187 m2/month. Estimate the spacing of the drains.

VarIatIon Of Uh and Uv with tIme factor for varIous n values

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Vacuum Consolidation

Vacuum Consolidation - Continued

Initial conditions

Geotextile on top of clay

Sand mat installation

Vertical Drain installation

Vacuum Consolidation - Continued

Vacuum Consolidation - Continued

Conditions of Application of the Method : Compressible Saturated Clay, Silt, Peat Low permeability Watertable close to surface No or few sand contents or pockets ( air and water leakage ) Light to medium loads Benefits : Vacuum Load equivalent to 4m of surcharge (Saving in Earthmoving) Isotropic load No risk of slope failure: high embankments built up in reduced period on soft soils Time Saving

Geomembrane installation

Sealing Trenches

Membrane Welding

Pumping Stations

Vacuum Consolidation - Continued

Vacuum Consolidation - Continued

Classical Surcharge Vacuum Method

4m

Settlement

without vertical drains

Classical Surcharge

Vacuum Method

vertical drains with surcharge

vertical drains with surcharge and vacuum preloading

Failure Surface

No Failure

Time

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Vacuum Consolidation - Continued

Vacuum Consolidation - Continued

If H > H limit , FAILURE

If H > Hlimite ,>FAILURE If H H limit , FAILURE

Vacuum Consolidation - Continued

Vacuum Pressure

Design of transition zone between different ground improvement areas Transition Zone Design for Bridge Abutments
6.2m for FRB1-2 4.8m for FRB3

Vacuum Method No limitation : High Surcharge built up in limited period

Surcharge Fill (zone 2)

1m

(zone 1)
2.5m (Design Height)

H+

Surcharge with PVDs

DSM Zone Bridge Piles

No Failure
X? 50m

Vacuum Preloading Ballina Bypass

Australias first use of vacuum preloading More cost effective than piles, stone columns or deep soil mixing 25m deep clay deposit, at the Southern Abutment of Emigrant Creek crossing Low bearing strength Unsuitable for surcharge preloading Poor groundwater quality Treatment required

Vacuum Preloading Ballina Bypass

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Vacuum Preloading Ballina Bypass

Settlement 5mm/day before vacuum preloading 22mm/day during vacuum preloading 1.2mm/day after vacuum preloading 5m (20%) total settlement at deepest clay deposit System stability and full vacuum pressure maintained throughout operation Vacuum preloading suitable for sites with deep deposits of soft clay

Electro-Osmosis
Removal of excess pore pressure from soil through electric currents Cathodes and anodes inserted into soil Pore water attracted to cathodes Electro-osmotic conductivity much greater than hydraulic Preload (surcharge) conductivity
Anodes system in soil

Electro-osmosis cell

Inefficient and costly so rarely used

Clay layer

Sand layer Water flows from anodes to cathodes

Cathodic PVD System

Conclusions
Ground improvement techniques allowing previously unworkable sites to be utilised Traditional preloading, preloading with vertical drains and vacuum preloading all viable consolidation techniques. Electro-osmosis still needs further development to be cost effective and viable. Traditional preloading slowest, accurate, less monitoring Preloading with vertical drains faster, more monitoring, research still being conducted Vacuum preloading fastest, constant monitoring, still needs a lot of research Thorough geological and geotechnical investigations needed before making decision.