This document summarizes the findings of geotechnical investigations conducted for power plant projects located in the Thar Desert region of Rajasthan, India. The investigations characterized the stratified dune sand soils, which are poorly graded, highly permeable, and meta-stable with a collapsible structure. Deeper soils exhibited increasing density and cementation. Foundation design recommendations included embedment into stable sand below the water table to achieve adequate bearing capacity. Careful planning and testing was needed to address the complex, vulnerable behavior of these desert soils.

1. Dec. 17 , 2004 1
SANJAY GUPTA
RAVI SUNDARAM
Cengrs Geotechnica Pvt.Ltd., New Delhi

2. Dec. 17 , 2004 2
IGC 2004
GROUND
ENGINEERING
EMERGING
TECHNIQUES
( GREET )

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Emerging Techniques
Ground engineering
Geotechnical
investigations
Human body
Pathological
investigations
C

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Art of Geotechnical
Investigations
Investigation program:
Needs to be site specific
As per project requirements
Based upon geology of area

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The Charm of the Desert

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… and the hazards
The Work of the Wind : Aeolian Transportation

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Major Deserts on Earth

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The Thar Desert
Western & north-western Rajasthan &
Sindh, 640 km x 160 km area
Extends from the west of Aravalis to
Indus basin, south of Punjab
Extreme degree of aridity combined with
sand drifting action
Aeolian accumulations cover an
irregular rocky floor

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Dune Sands of the
Thar Desert

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Dune Morphology
Aeolian transportation
sorts particles
Major part (90 %) of particles in
active movement are fine sand
size
Deposition
Occurs when particles come to
rest – reduction in wind velocity
Fall in transportation potential
– “grainfall” deposition
Rolling down slope
– avalanching / grain flow

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Equilibrium develops between
sand supply and flow
Dune form responds to
changes in wind regime and
sediment supply
Dune Sand Development

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Dune Sand Development
Dune crest - more
sorted than flanks
in loosest form --
relative density < 20 %
Deeper levels more
compacted / stable
carbonate cementation

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Dune Sands of the Thar
Meta-Stable in Nature (Alam Singh)
Collapsible structure
Radical rearrangement of particles
and loss of volume on wetting, with
or without load application
Hydro-consolidation potential due
to cementation of sand with CaCO3

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Dune - Internal Structure

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SPT Values –
Stable/Unstable Dunes
Depth
(m)
Condition N – Value
Stable
Dune
Unstable
Dune
1 Dry 8-12 5-7
2 Dry 10-17 7-13
3 Dry 12-25 9-15
4 Dry 16-35 14-15
1 Submerged - 1-2
2 Submerged - 3-4
Ref : Alam Singh et al.

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Geotechical Investigation -
Suratgarh
Cengrs has been involved in the
geotechnical investigation for this power
plant since 1992
The focus has been to
Evaluate the soil behavior
Determine engineering properties
Decide foundation depths and ABP

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Suratgarh Power Plant
Constructed in three stages:
Stage I : 2 x 250 MW started 1992
Stage II : 2 x 250 MW started 1996
Stage III : 1 x 250 MW started 1998
Permissible settlement for most facilities
restricted to 25 mm
Units such as Power House, Boiler, Chimney,
Crusher House are heavily loaded

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Site Location

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Suratgarh Power Plant
Located in north-west Rajasthan
Dry, arid region of the Thar Desert
Covered by dune sands

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Geotechnical Investigations
Scope included: ( 1992 to 2004 )
Over 150 boreholes : 10 - 50 m depth
Over 30 plate load tests at various depths
Dynamic Cone Penetration Tests
Block Vibration Tests
Electrical Resistivity Tests
In-Situ Permeability Tests

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Borehole in Progress

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Site Stratigraphy
Stratum Depth,m Deposition
I GL to 8-10 m Fine sand & silty fine
sand (poorly graded)
II >8-10 m to 50 + Cemented fine sand
Weak to moderate
Cementation breaks on addition of water
Groundwater not met to 50 m depth

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Typical Sectional Profile

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Stratigraphy
Ground surface profile is roughly
parallel to profile of refusal strata
(N>100)
Aeolian deposition follows the trend of
the underlying cemented sand /refusal
Top 3-4 m: N < 10-12 : In unstable part
of dune

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Trend of SPT Values
Depth, m Range of
N-Values
Remarks
From To
0 3-4 < 10 Loose, unstable dune
3-4 6-8 15-30 Stable dune
6-8 9-11 40-100 Weak carbonate cementation
9-11 50+ > 100 Weak to medium carbonate
cementation

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Cementation , SPT
Increasing trend with depth

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Dune Sands
SPT values & relative density
function of overburden pressure
Excavation results in loosening
of strata / lower SPT values
reduction of overburden pressure

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Soil Gradation
Poorly Graded
Sand
• Near uniform
gradation
• Particle size in a
narrow band width
• Bulk of particles
are of 0.1 – 0.2
mm size

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Soil Gradation
Range of Particle Size : 0.3 - 0.075 mm
Average Particle size : 0.15 mm
Silt content : < 5-8%
max 15-20%
D60 : 0.14-0.18 mm,
average 0.15 mm
D10 : 0.05-0.09 mm,
average 0.07 mm

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Permeability
Dune sand is highly permeable
Field permeability test – constant head
method – conducted at various depths
in boreholes
Coefficient of permeability, kk ranges
from 1.5 ~ 5.9 x 10-4 m/sec
Variation of kk with depth is insignificant

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Electrical Resistivity
Test by Wenner’s
Four Electrode
Configuration
Dry aeolian sands have high
electrical resistivity
Resistivity increases with depth

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Electrical Resistivity
Electrode
Spacing,
m
Apparent
Resistivity
ohm-m
Soil Conditions Field
N-Values
1 100-200 Loose sand 6-10
3 400-450 Medium dense sand 10-15
5 500-600 Medium dense to
dense sand
25-35
10 1100-2000 Dense to very dense
cemented sand
50-75
15 >2000 Very dense moderately
cemented sand
>100

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Plate Load Test
PLT under Dry & SaturatedPLT under Dry & Saturated
ConditionCondition
• Water added to saturate soils
at loading intensity of 2 kg/cm2
• Increase in settlement upon
saturation nearly 2.8 times as
against 2.0 for normal soils
(Rw = 0.5 )
• Computed Rw factor = 0.36
• Rw of 0.3 is reported in
literature

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Block Vibration Test :
Set Up

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Boreholes Profile: TG
Top 4 m: SPT values 7 –11: in
Unstable part of Dune
Deeper soils to 8-9 m depth :
SPT values 18-45 : Stable
Dune
Below 8-9 m depth: Weakly
Cemented Sand

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Forced Vibration Test
Vertical Vibration Test Horizontal Vibration Test
Test at 3 m depth
•Loose Unstable
Dune
•Two peaks in
vertical tests
•Freq 1 – 30 c/s
•Freq 2 – 42 c/s
•Sand settling
under influence
of vibrations

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Forced Vibration Test
Test at 8 m depth
•Stable Dune
•Results used
effectively for
TG design
•Freq ~ 47 c/s
•Comparable to
2nd peak
Vertical Vibration Test Horizontal Vibration Test

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ANALYSIS
Dune Sands have low bearing capacities
Traditional and standard methods
over-predict allowable bearing pressures
Higher settlements – other soils
Behavior not governed by normal soil-water
relationship
Under dynamic loading, behavior is more
complex

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Analysis
Suitable Reduction Factors should be
applied to account for:
Collapsible nature of soils
Additional settlement on partial saturation
rains, leakage from water reservoirs, etc.
Reduction in over burden pressure
loosening effect of soil

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Location on Dune
On the dune crest, particles are in
active movement
so conditions are loose and unstable
Along the dune flanks, extent of loose
soils is relatively less compared to crest

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Extent of Cutting/Filling
If FFL is below EGL, cutting will be
done. Therefore,
unstable part of dune will be removed,
SPT values at founding level will be
relatively high
high bearing capacity will be achieved

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Extent of Cutting/Filling
If FFL is above EGL, filling will be
done. Therefore,
Fill will be placed over unstable dune
SPT values will be relatively low to
greater depth below FFL
To achieve desired bearing capacity,
greater embedment depth below FFL
will be required

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Design Net Bearing
Pressures
Stage Structure Foundation
Depth below
FFL, m
Net Allowable
Bearing
Pressure,
kPa
I TG Foundations 7.0 230
I Chimney 12.0 550
I Coal Handling Plant 5.0 175
I Switchyard 3.0 200

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Design Net Bearing
Pressures
Stage Structure Foundation
Depth below
FFL, m
Net Allowable
Bearing
Pressure,
kPa
II Crusher House 9.0 500
II Coal Conveyor 7.0 300
II Water Treatment
Plant
1.0 175
III Boiler 7.0 250
III Ash Silos 7.5 200

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Concluding Remarks
Dune Sands are:
vulnerable to disturbances
meta-stable, have collapsible structure
Careful planning, detailed testing and
thorough understanding required to
ensure foundation stability

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Concluding Remarks
Foundation embedment depth should
be decided based on nature of dune
and its stability in addition to SPT
values
Both statically and dynamically loaded
foundations should be seated on the
stable part of dune

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Concluding Remarks
Response under earthquake is a matter
of concern
Further research is required to
understand the complex behavior of
these arid soils of the Thar Desert

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Source of Data
Presentation is
based on:
Detailed soil
investigation projects
for RRVUNL
Stage 1 (1992)
Stage 2 (1996)
Stage 3 (1998)
Over 16 detailed
investigations for
various turnkey
plants (1993-2004)

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Acknowledgements
Thanks to:
Rajasthan Rajya
Vidyut Utpadan
Nigam ( RRVUN )
Development
Consultants (P) Ltd
Various turn key
agencies

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Thank You !!Thank You !!