Geosynthetics for Accelerated

Pre-consolidation of soils

Prof K. Rajagopal
Department of Civil Engineering
IIT Madras, Chennai
e-mail: gopalkr@iitm.ac.in
 Synopsis
• Construction on soft clay foundation soils
needs to deal with low bearing capacity, large
settlements, large lateral flows, etc.

• Accelerated pre-consolidation is used to

increase the strength and reduce the potential
of settlements after the construction
 Accelerated
Pre-consolidation Techniques

Sand drains
Pre-fabricated vertical drains
Vacuum assisted pre-consolidation
 Schematic of
Accelerated Consolidation
surcharge fill

Sand blanket

T = c t v
2 sand
v
d columns of
High
2
permeability
t=T d v

c v

Accelerated drainage achieved by reducing drainage path

 Properties of sand in drains
• Sand in the drains (200 to 600 mm diameter)
should be at least 1000 times more permeable
than the native foundation soil (California DoT)

Sieve size (mm) % passing

12 mm 90-100
2.38 (No. 8 US sieve) 25-100
0.59 (No. 30) 5-50
0.297 (No. 50) 0-20
0.149 (No. 100) 0-3
 Properties of sand in blanket
• Blanket soil should be more permeable
than the sand drain soil to drain away the
water coming from the ground
• Thickness about 300 to 500 mm
Sieve size % passing
(mm)
9.5 m (3/8”) 80-100
2.38 mm 5-50
0.59 mm 0-20
0.297 mm 0-5
Governing Equation and Solution
 2u 
∂ ∂ ∂
2 2
u  u ∂u
ch  ∂ 2 + ∂ 2  + cv ∂ 2 = ∂t
 x y  z
Solution given by Carillo (1942)

Uav = 1 – (1-Uv)(1-Ur)

Uav = average degree of consolidation due to

combined radial and vertical drainage
Uv = degree of consolidation due to vertical drainage
Ur = degree of consolidation due to radial drainage
 Degrees of consolidation
• Degree of consolidation in vertical direction
(Uv) is related to time factor Tv (Terzaghi
1943)
• Tv = (π/4) U2; U ≤ 53%
• Tv = 1.781−0.933 [log10(100-U%)], U > 53%
 Barron (1952) solution for radial
consolidation – equal strain theory
 − 8T r 
U r = 1 − exp  F (n) 
3n − 1
2 2

F (n) = n ln(n ) −
n −1
2 2
4n
Tr = time factor for radial consolidation = crt/de2

n = re/rw; re = radius of the unit cell area,

rw = radius of the sand drain
de = diameter of the unit cell area
Radial consolidation is faster due to:
• Higher permeability in horizontal direction
• Shorter drainage path

Likely Problems
• Sand in the drains gets clogged easily
• Continuous drainage path is disturbed due
to internal shear movements
• Takes long time to install the sand drains
• Smear effects which reduce drainage
Soil embankment placed as surcharge to drive the
consolidation process
PVDs for pre-consolidation

Corrugated plastic core for

drainage

Geotextile filter
 Typical Properties of PVDs
• Typically 100 mm wide and 5 mm thick
• Tensile stength 5 to 15 kN
• Typical discharge rate 2 to 5 liters/minute
(ASTM D4716)
• Consists of a drainage core and a filter
cover all around
• PVDs come in rolls of about 100 m length
• Pushed rapidly into the ground to desired
depth
PVD roll – connection to base anchor plate to push it into
ground
View of PVD installation rig
Close-up view of PVD roll and the installation rig
Another close-up view
Crawler mounted Rig for
installing the PVDs
Connection arrangements for wick drain
installation
Koerner (2000)
Manufacture of PVDs
General view after installation of PVD’s at a site
 Advantages of PVDs
 Continuous flow path even after shear
movement
 No clogging due to geotextile filter
 Reinforcement action due to the tensile
strength of the PVD
 PVD arrangement – Plan view
Square pattern Triangular pattern
s
d
d

s
D D

s
Diameter of Influence Diameter of influence
area = 1.128 s area = 1.05 s
Triangular pattern is more efficient
 Hansbo’s equation (Hansbo, 1979)
Time for consolidation, Consolidation due to vertical
drainage is neglected

D 2  ln ( D d ) − 3 − ( d D )
 2

t  ln 1
8ch  1 − ( d D )2 4  1−U

Neglecting the small quantity (d/D)2

t D  ln D − 0.75  ln 1
2

8Ch  d  1−U
Where, ch – Coefficient of consolidation (Horizontal)
d – Equivalent diameter of the PVD
D – Diameter of the influence area
U – The avereage degree of consolidation
 Design Example
Given PVD size = 100mm × 5 mm
Data: Consolidation to be achieved = 80%
Time available = 1 year
Coefficient of Consolidation ch = 10 m2/year

Equivalent diameter of circular drain having same circumference ,

2 ( 100 + 5 )
d=
π
=
66.84 mm 0.0668 m
 Design of PVD …
By Hansbo′s equation

Time, 1 year D 2  ln
8 × 10  ( D
0.0668 )
− 0.75  ln
 (1
1 − 0.8 )
47.706
(
D 2  ln D
0.0668 )
− 0.75 


D RHS
2 10.59
4 53.46
3.5 39.3
3.8 47.5

Diameter of the influence area, D = 3.8 m

 Design of PVD …

Time for Consolidation Vs Spacing of drains

1000
Time for consolidation (days)

750 U=90%
U=80%
U=70%
U=60%
500
U=50%

250

0
0 1 2 3 4 5 6
Wick drain spacing, D (m)
 Design of PVD …
Spacing of PVDs
1. Square pattern = 3.8/1.128 = 3.4 m
2. Triangular pattern = 3.8/1.05 = 3.6 m
Triangular pattern is preferred as spacing is greater and overlapping
of influence areas is less. Coverage is better with triangular
pattern.
 Pre-consolidation
by
Vacuum Application
Atmospheric pressure is used in place of
external surcharge by creating vacuum in
the foundation soil
Two types of field installation methods

Membrane System: Entire volume of soil to be treated

is covered with a geomembrane and bentonite filled
trench all around, M/s Menard Soil Treatment company,
France

Membraneless System: Vacuum pipe is directly

connected to PVD without any membrane, BeauDrain
system, M/s Cofra, The Netherlands
 FIELD CONSTRUCTION METHOD
• PVDs or perforated pipes are installed vertically at 1 to 3 m
c/c spacing
• Horizontal drain pipes are installed connecting all the vertical
drains
• Vacuum is applied using 14 HP pump for each 600 to 900
sq.m. area to be treated
• Soft clay soils as deep as 20 m can be treated.
• 90% consolidation can be achieved within 2 to 3 months.
• Secondary compressions can also be achieved
Vacuum consolidation system with membrane cover
Naidu, Rajagopal and Robinson (2008)
 Vacuum pump Working platform Vacuum pump

Hose

Cap

PVD

Vacuum consolidation system without membrane cover

Naidu, Rajagopal and Robinson (2008)
Horizontal drain pipes
Bentonite filled peripheral trench
 Increase in effective Stress due to
vacuum application
horizontal vacuum
pipelines

1.0 m
sand layer
cut-off 3.0 m
walls

Let dry and saturated unit weights of both soils

be 18 and 20 kN/m3.
Initial water table is at a depth of 3m below the ground.
After vacuum application, it will raise to drain pipe level,
i.e. 1m below ground level
Stresses in sand layer before the application of vacuum
Total stress, σv = Pa + γdry* z = 100 + 18*z kPa
Pore pressure u = Pa = 100 kPa
Effective stress, σ′v = σv-u = 18*z kPa

Stresses in sand layer after the application of vacuum

After the application of vacuum, water level will raise from 3.0 m to 1.0 m below the
ground level.
Total stress, σv = Pa + γdry* 1 + γsub*(z-1) = 100 + 18*1 + (z-1)*(20) kPa = 98+20*z kPa
Pore pressure, u = (z-1)*γw = (z-1)*10
Effective stress, σ′v = σv-u = 98 + 20*z – 10z+10 = 108 + 10*z

Increase in effective stress in sand layer

∴ increase in effective stress within sand layer due to vacuum =
∆σ′ = 108+10*z – 18*z = 108 – 8*z

i.e. change in effective stress is 100 kPa at 1 m depth

and 84 kPa at 3 m depth.
Stresses in clay layer (below water table) before
the vacuum application
Total stress, σv = Pa + 3*18+20*z, (z is measured from top of clay
layer)
Pore pressure, u = Pa + 10*z
Effective stress, σ′v = σv−u =54+10*z
Stresses in clay layer after vacuum application
Total stress, σv = Pa + 18+ 2*20+20*z = Pa + 58 + 20*z
Pore pressure, u = 20+10*z
Effective stress, σ′v = σv−u = Pa+38+10*z=138+10*z
∴ increase in effective stress in clay layer,
∆σ′ = 138+10*z−54−10*z = 84 kPa

This increase in effective stress is constant with

depth!!!!!
 Advantages of vacuum consolidation
• Increase in effective stress is isotropic and is constant with
depth
• No inherent increase in shear stress. Lateral ground
movements are compressive rather than expansive.
• Vacuum consolidation creates more uniform surface
settlements.
• No surcharge fill is necessary to drive the system.
• Because of the increase in effective stress in the drainage
layer above the ground level, this layer acts like a semi-rigid
mat and hence construction equipment can be moved on the
site without waiting for consolidation to take place.
• Can get rid of secondary compressions which is not possible
with conventional pre-consolidation.
 Some case studies of different vacuum consolidation applications

Authors Location of field Results

study
Chu et al. Airport runway in The undrained shear strength of the soil
2000 Coast of Tianjin, increased to two to three times after the
China. application of vacuum load for 4 months.
Tang and Airport runway in Two field pilot tests were conducted and
shang 2000 China achieved same settlements in half of the time by
vacuum treatment
Hirochika Road embankment in To examine the effect of the vacuum
Hayashi et Japan consolidation method in peat ground, which is
al. 2003 composed of highly organic soil

Yan and Oil storage tank, Achieved degree of consolidation 90% within
Chu 2003 storage yard and three months.
roads in China
Song and Sewage disposal Reported lateral deformation due to vacuum
Kim 2004 plant in Korea preloading and concluded that vacuum pressure
is applied to the ground isotropically
Gao. C 2004 Huanghua port, Conducted pilot tests and suggested Vacuum
China preloading is suited to uniformly distribute soft
soils.
 Encased stone column and vacuum consolidation
Stone column: Most common soil reinforcement technique in soft clay soils.
– Load carrying capacity depends on lateral confinement
– Possibility of contamination which affects the strength and drainage path

Encased Stone Column: Additional confinement and filtration effects

Vacuum consolidation:
 Vacuum applied in a sealed membrane system.
 Increase in effective stresses without increase in total stresses
 Vacuum pressure is isotropic and constant with depth
Research objectives
 Study the improvement in the strength and stiffness of soft clays under
vacuum consolidation
 Study the possibility of using vacuum pressure along with encased
stone columns for accelerated consolidation of soft clays.
 Analyse the performance of stone columns after the application of
vacuum pressure.
 Practical recommendations for field applications.

Experimental study

 Soil Used- Soft clay with undrained shear strength 5-10 kPa (Prepared
from clay slurry having 1.5 times liquid limit water content and
consolidated at a pressure of 10 kPa)
 Type of Stone column – Displacement type, stones of 2-10 mm size,
encased with a woven geotextile.
 Type of geotextile used- Woven type geotextile having tensile strength
of 50 kN/m at approximately 12% strain
 Lateral Deformations of Soil under Vacuum Consolidation
Sl. Test Details of test
No. designation
1 Test 1 One-dimensional consolidation test under a pressure increment
of 65 kPa
2 Test 2 Consolidation under vacuum pressure of 65 kPa under 1-D
conditions
3 Test 3 Consolidation under vacuum pressure of 65 kPa allowing
lateral deformations
4 Test 4 Isotropic consolidation under a confining pressure of 65 kPa

Fig: 1 Cross sectional view of

Double wall cylinder
 Critical State Parameters
Parameters Value
slope of compression line, (λ) 0.203
slope of unloading line, (κ) 0.018
Specific volume of isotropic consolidation at p’=1 , 2.9
(N)
slope of critical state line, (M) 0.93
Specific vol. of soil at critical state line at p’=1, (Γ) 2.77

Data from vacuum application on triaxial test samples

Vacuum Undrained Shear Cu/Uv Predicted from

Pressure , Uv Strength , Cu (kPa) CSSM (kPa)
(kPa)
40 9.5 0.24 9.2
65 14.1 0.22 12.8
80 19.7 0.25 19.6
Initial vane shear strength ≅ 2.5 to 3 kPa
 Γ − N + λ ln p′ 
c u = 0.5 M exp 
 λ 
Experimental Program on Stone Columns

1. Unit cell tests (202 mm diameter and 480 mm height columns)

To determine the strength and stiffness of stone column after vacuum and
surcharge loading
a) Pressure levels varied 40 kPa, 65 kPa and 80 kPa
b) Diameter of the encased stone column varied (50 mm , 60 mm and 75
mm)
2. Large tank testing (1.2 m × 1.2 m) to simulate multiple stone columns and various
points of vacuum application. Pressure,kPa
0 100 200 300 400 500 600
0

4
Settlement ,mm

10

12

14
clay GEC without vacuum GEC with vacuum
 Water contents after surcharge and vacuum loading
Water content (%)
Position
65 kPa Surcharge loading 65 kPa vacuum loading

Top 41.71 39.31

Middle 45.12 39.41
Bottom 45.14 38.45

Initial water content ≅ 48% Pressure,kPa

0 500 1000 1500
0
Load capacity & stiffness with
5
vacuum treatment is much higher
 Higher water content at deeper 10

Settlement,mm
depths under surcharge loading
15
Water content with vacuum
treatment is constant with depth 20

Effect of vacuum treatment is 25

constant with depth while surcharge is Surcharge Loading-65kPa
more effective near the surface 30
Vacuum loading-65kPa
35

Surcharge vs. vacuum consolidation

 Summary
• This lecture has discussed some
principles of accelerated consolidation
• Geosynthetics are extensively used in
these applications