Geotechnical Investigations

By - Rajiv Kumar, ADG/IRICEN

 Geotech Investigation for Bridges
 For Bridges
• “Code of Practice for the Design of Sub-structure and
Foundations of Bridges”
• Clause - 6
 For Buildings
• IS:1892 – 1979 (Code of Practice for Sub-surface Investigation
for Foundations)
 For Earthwork
• “Comprehensive Guidelines and Specifications for Railway
Formation” (No. RDSO/2020/GE:IRS-0004) – September, 2020
• Chapter – 1.0
 Geotech Investigation for Bridges

Sub-structure and Foundation Code

6.2 Sub-soil Investigation
6.2.1 Scope

 To determine the Nature, Extent and Engineering Properties

 Of Soil/Rock strata

 And Depth of Ground Water table

 For development of a reliable and satisfactory design of bridge

foundation.
 6.2.1.1 Sub-structure and Foundation Code

Reference Standards

 IS:1892 (1979) – Sub-surface Investigation for Foundations.

 IS:6935 (1973) – Method of determination of Water Level in a

Bore Hole.

 IS:2131 (1981) – Standard Penetration Test.

 IS:2720 – Total 41 Parts - Method of Tests for Soils.

 IS:1498 (1970) – Classification and Identification of Soils for

General Engineering purpose.

 IRC:78 (Part-VII) – Standard Specification and Code of Practice

for Road Bridges – Foundation and Sub-structure.
6.2.1.2 Stages of Sub-surface Investigation

6.2.1.3 Reconnaissance Survey

• Bad locations like unstable hill side, talus formation,

swampy areas, peaty ground etc. are avoided.

• Data from available sources e.g. geological maps,

topological maps etc. is studied.
Colluvial Soils/Talus –
These are formed due to
gravity forces. In Mountains
and hills, on steep
slopes soil creeps down
under gravity force due to
variation of moisture
content (hence loosening
of soil). Such soils are
deposited in the lower part
of the mountains
6.2.1.2 Stages of Sub-surface Investigation

6.2.1.4 Preliminary Survey

• To determine depth, thickness, extent and composition

of each soil stratum.

• To determine location of rock and ground water table.

• To obtain approximate information about strength &

compressibility characteristics of various strata.

• Objective is to select the type, location and principal

dimensions of all major structures.
6.2.1.2 Stages of Sub-surface Investigation

6.2.1.5 Final Location Survey

• Undisturbed samples are collected to conduct various

tests (viz. Shear tests, consolidation tests etc.).

• Exploration shall cover the entire length of the bridge and

also extend at either end for about twice the depth below
bed of the end foundations, to assess the effect of
approach embankment on the end foundations.
6.2.1.6 Information obtained by Sub-surface investigation

• Site Plan – Showing location of foundations and abutments

etc.

• Cross section along proposed bridge showing various levels

(rail level, top of superstructure, HFL, LWL, foundation level
etc.).

• Sub-surface profile with engineering properties of the

founding strata.

• Environmental Factors – Geological history, Seismicity,

Hydrological information etc.
6.2.1.6 Information obtained by Sub-surface investigation

• Modulus of Elasticity and Modulus of Sub-grade Reaction.

• Review of performance of similar structure, if any, in the

locality.

• Information necessary to assess the possible effects of the

new structure on the existing structures in the
neighborhood.
 IRS – Bridge Substructure & Foundation Code
Cl. 6.2.2: Open Foundation
 Investigation by trial pit can be carried out.

Cl. 6.2.3: Deep Foundations

 Exploratory bore holes by deep boring equipment.

 First boring to extend to a depth sufficient to disclose deep problem
layers.
 Soft strata shall be penetrated completely even when covered with a
surface layer of higher bearing capacity
 Samples to be collected at every 1.5m depth or change of strata.
 Depth of boring: Up to 1.5 to 2.0 times the width of footing, below the
foundation level.
 Geotech Investigation for Buildings
IS:1892 – “Code of Practice for Sub-surface Investigation for
Foundations”
1.1 Scope

To determine -
 Sequence and extent of each soil and rock stratum in the region likely
to be affected by the proposed work,
 Nature of each stratum and engineering properties of soil and rock
which may affect design and mode of construction of proposed
structures and their foundations, and
 Location of ground water and possible corrosive effects of soil and
water on foundation materials.
 Geotech Investigation for Earthwork
“Comprehensive Guidelines and Specifications for Railway Formation”
Specification no. RDSO/2020/GE: IRS-2004 (September -2020)
Chapter 1.0 Soil Exploration and Survey

1.1 Objectives
 To determine type of soil and their suitability for earthwork in formation.
To identify suitable borrow area for subgrade and blanket material. To
determine method of compaction of subgrade.
 To avoid known troublesome spots (unstable hill sides, swampy areas, soft
soil, peat etc.)
 To determine ground water table position and its seasonal variation and
general hydrology of area such as flood plains, river streams, etc.
 To determine behaviour of existing track or roads and causes of geo-
technical problems in them, if any.
 “RDSO Guidelines and Specifications for Railway Formation”
1.0 Soil Exploration and Survey
1.1 Objectives
 To determine soil type for their suitability for earthwork in formation
 To avoid known troublesome spots (unstable hill sides, swampy areas, soft
rock areas, peat lands etc.)
 To determine method of compaction of subgrade.
 To identify suitable alignment from view point of stability, safety, economy
in construction and maintenance.
 To identify suitable borrow area for subgrade and blanket material.
 To determine depth of various strata of soil and bed rock level.
 To determine ground water table position and its seasonal variation and
general hydrology of area such as flood plains, river streams, etc.
 To determine behaviour of existing track or roads and causes of geo-
technical problems in them, if any.
Methods for Geo-technical Investigation
• Direct Methods
o Open excavation (Test Pits, Trenches etc.)

• Indirect Methods
o Geophysical methods
o Penetration tests

• Semi-direct Methods
o Boring and Drilling
Trial Pit (Open Excavation)
 Cheapest way of site exploration & do not require any
specialized equipment.
 Obtain undisturbed & disturbed samples.
 Allows visual inspection of any change of strata &
facilitate in-situ testing.
 Suitable for exploration of shallow depth only generally
upto 3-4 m depth. Beyond 6 m depth it becomes
uneconomical.
 If the side wall of trial pits is expected to fall, provide
adequate bracing or create stepped trial pits.
 Provide adequate protection when they are left open at
the end of each working day.
 Indirect Method

• Geophysical methods
(A) Seismic Refraction Method
(B) Electrical Resistivity Test
(C) Ground Penetrating Radar (GPR)
 (A) Seismic Refraction Method
 Velocity of propagation of refracted seismic waves through various layers of sub-strata is
measured and using this, the thicknesses and types of these layers is determined.

 Test is conducted by impacting the surface by hammer blow or by a small explosive charge;
depending on local ground conditions and required depth of peneteration.

 The impact on ground creates seismic waves, which propagate downward through the
ground until they are reflected or refracted off subsurface layers.

 Refracted waves are detected by arrays of 24 or 48 geophones spaced at regular intervals of

1 - 10 m, depending on penetration depth. Slide 76

 Geophones output data are compiled and processed by the seismograph.

Generation of seismic
Impulse by hammer
Geo-Phones
Installation of Geo-phones
Seismogram
 (B) Electrical Resistivity Test

• This method is based on measuring the Electrical Resistivity of different layers in

sub-strata.

• Two current electrodes are inserted about 20cms into ground through which a
known electrical current is introduced, thereby producing an electric field within
the ground.

• Measurements are made on the basis of potential (voltage) drop across the two
intermediate potential electrodes.

• The survey data is processed to produce details of thickness and resistivity of

subsurface layers in graphical forms.
Electrical Resistivity
Measuring
Equipment
Electrical Resistivity
Measuring
Setup
(C) Ground Penetrating Radar (GPR)

Video
 Penetration Tests
 A standard object is driven into the ground with standard impact energy,
imparted by manual or mechanised methods.
 The resistance to penetration is measured.

 Standard Penetration Test (SPT)

 (IS:2131-2002)
 Static Cone Penetration Test (SCPT)
 IS:4968 – 2007 (Part-3)
 Dynamic Cone Penetration Test (DCPT)
 IS:4968 – 2007 (Part-1 & 2)
Standard Penetration Test
 Standard Penetration Test

• A cylindrical penetration tool is driven into the ground by 450 mm by

dropping a 63.5 kg Hammer from 750 mm height.
• Blows required for every 150mm penetration are recorded. Readings
for 1st 150 mm ignored as seating drive.
• Number of blows for last 300mm reported as N value
• N Value can be co-related with C-Ø values.
• Performed every 1.5m depth, as well as at change of strata.
• Suitable for granular soils. Not so accurate for cohesive soils.
• A Bore hole is to be drilled for test at required depth.
• Samples can also be collected by Split Spoon Sampler.
Standard Penetration Test
• The boring log shows refusal and the test is
halted, if:
 50 blows are required for any 150mm penetration
 100 blows are required for 300mm penetration
 10 successive blows produce no advance
 SPT Sampling
 As per IS:2131-2002.
 Employs a Split spoon sampler (As per IS:9640-1980).
 Correction Factor - 1

Due to Overburden

The N value for cohesion-less soil

shall be corrected (N’) for
overburden as per Figure given
in IS:2131 – 2002.

N’ = N x Correction Factor
 Correction Factor - 2

Due to Dilatancy
(Bulk expansion under stress)

 In the case of saturated fine sand or silt below water-table, apparently high
values may be noted for N (Terzaghi).

 The corrected SPT value after applying correction factor 1 need to be further
corrected as under say Correction Factor-2 ;

N’’ = 15 + ½ x (N’ – 15)

Where, N’ – Corrected SPT value; and

N’’ – Corrected SPT value after application of Correction Factor-2
Use SPT values to find Ø value
Ref.: IS:6403-1981 (Fig. 1)
 N Value Correlation for Granular Soils

RD
Sl.
Condition N (Relative Ø
No.
Density)

1 Very Loose 0–4 0 – 15% Less than 28o

2 Loose 4 – 10 15 – 35% 28° – 30°

3 Medium 10 – 30 35 – 65% 30° – 36°

4 Dense 30 – 50 65 – 85% 36° – 42°

5 Very Dense > 50 > 85% > 42°

 N Value Correlation for Clay

Sl. No. Condition N qu (kN/m2)

1 Very Soft 0–2 < 25

2 Soft 2–4 25 – 50
3 Medium 4–8 50 – 100
4 Stiff 8 – 15 100 – 200
5 Very Stiff 15 – 30 200 – 400
6 Hard > 30 > 400

Correlation for clays is less reliable and therefore to find out

the in-situ strength of clays, Field Vane Shear is used
 Dynamic Cone Penetration Test

• 50 mm dia Cone is driven into the soil by 65Kg hammer to fall freely through
750mm and Number of blows for every 100mm penetration recorded (dynamic cone
resistance).
• Process is repeated till the cone is driven to the required depth.
• Refusal: Driving may be stopped when the number of blows exceeds 35 for 100mm
penetration.
• Ncd (Cone Penetration Value) is number of blows for 300 mm penetration.
• The test gives continuous record of Ncd with depth.
• No samples are obtained in this case.
• Used for soft clays & silts & fine to medium sand deposits
 Dynamic Cone Penetration Test with Bentonite Slurry

 DCPT with Bentonite Slurry is conducted

to eliminate frictional resistance on the drill
rods. This becomes necessary, generally, for
depth more than 6m.
 In this, 62.5mm Dia. 600 Cone is driven with
the arrangement for drilling mud to flow
through the cone.
 Number of blows required for 300mm
penetration is denoted by Ncbr.
 It is relatively quicker and economical
test.
Empirical Correlation given by Central Building Research
Institute (CBRI)Roorkee
for medium to fine sand

o Ncd = 1.5 Nfor Depth < 3m

o Ncd = 1.75 N for Depth 3 to 6m
o Ncd = 2.0 Nfor Depth > 6m

o Ncbr = 1.5 N for Depth < 4m

o Ncbr = 1.75 N for Depth 4 to 9m
Also called Static Cone
Peneteration (SCPT) Test
or only Cone
Peneteration Test (CPT).

Used for soft clays and

silts &
Fine to medium sand
deposits.
Gives value of cone
peneteration resistance
and skin friction
resistance separately.
 Applicability of Penetration Tests
• SPT needs Bore hole. For soft and loose soils, soil properties may get
disturbed by Boring. Also not suitable for Clayey soils.
• Sample can be collected in SPT through Split Spoon Sampler

• Static CPT provides Cone Friction resistance as well as End Tip Resistance
to get total strength. Skin Friction values can be evaluated which is useful
for determining length of friction piles.
• SCPT suitable for Clayey soils but not suitable for Gravel, Boulders,
Cohesionless soil and Dense Sand.

• DCPT is quick and easy and very suitable for Preliminary Survey over large
area to identify variability of subsoil profile and presence of weak pockets.
• DCPT is suitable for Gravel, Boulders Cohesionless soil and Dense Sand
also. DCPT is not suitable for cohesive and very loose cohesionless soils.
Semi-direct Methods : Boring and Drilling

• Used for large depth exploration.

• Disturbed as well as undisturbed samples can be obtained depending upon
method of boring.

• Selection of boring method will depend upon following:

 The materials to be encountered and the relative efficiency of the
various boring methods in such materials.
 The available facility and desired accuracy
 Possible disturbance to the sample material.
 (A) Displacement Boring
• It is combined method of sampling & boring operation.
• Closed bottom sampler (slit cup or piston type) is forced in to
the ground up to the desired depth.
• Then the sampler is detached from soil below it, by rotating
the piston, & finally the piston is released or withdrawn.
• The sampler is then again forced further down & sample is
taken.
Features:
 Simple and economical
 Suitable if excessive caving does not occur. Not suitable for
loose sand.
 Major changes of soil character can be detected by means of
penetration resistance.
 Used for 25mm to 75mm holes.
(B) Auger Boring

• Fast and economical. Uses simple, light,

inexpensive instruments for large to small
holes.

• The diameter of augers used varies from 75

mm to 300 mm.

• Suitable for soft to stiff cohesive soils.

• Not suitable for very hard or cemented

soils, very soft soils and cohesionless soils.
• Augers can be Hand operated or Power
operated.

• Generally, hand auger boring is used in field

where it is not necessary to bore a hole deeper
than 6m.

• In case of situations when deeper holes are to

be bored into the ground, power augers are
used.

• For large augers, trucks mounted auger type

drilling machine are used to construct large
shafts of several mm diameter.
 Continuous Flight Augers (CFA)

Video

Solid Stem CFA Hollow Stem CFA

(C) Wash Boring
• inexpensive and portable handling and
drilling equipment.
• First an open hole is formed in the ground
for soil sampling/ drilling
• The hole is advanced by chopping and
twisting action of the chopping bit. Sample
is taken out by water jet under pressure
through the rods operated inside the hole.
• By noting change of colour of soil coming
out, the change of soil character can be
identified by experienced person.
• Gives completely disturbed samples and
not suitable for gravels & boulders.
(D) Rotary Drilling

• This is almost same as wash boring, with two

main differences:
o Drill bit is used in lieu of Chopping bit.

o The bentonite slurry is used to help in

drilling and to stabilizes the bore hole.
Hence, no casing is required.

• Suitable for bore holes of diameter 10cm or

more, in most of the rocks. It is uneconomical
for holes less than 10cm diameter.
 (E) Percussion Drilling

• In case of hard soils or soft rock,

auger boring or wash boring cannot
be employed.

• In percussion drilling, bore hole is

advanced by alternatively lifting and
dropping a heavy drilling bit. Water
is added to the hole during boring and
the slurry of pulverised material is
bailed out at intervals.

The slurry is removed using bailers.

• It can be used in most of the soils

and rocks and can drill any material.
Chisel Bailer
Boring and Drilling
(A) Displacement Boring
(B) Auger Boring
• Post Hole Auger
• Helical Auger-CFA
 Solid Stem CFA
 Hollow Stem CFA

(C) Wash Boring

(D) Rotary Drilling
(E) Percussion Drilling
 6. Common Laboratory Tests

Type of Test Ref. IS CODE

6.1 Index Properties Tests
1. Soil classification IS: 1498
2. Sample preparation IS: 2720 (Part I)
3. Determination of Water content IS: 2720 (Part II)
4. Determination of Specific gravity IS: 2720 (Part III)
5. Grain size analysis IS: 2720 (Part IV)
6. Liquid limit & Plastic limits IS: 2720 (Part V)
7. Shrinkage factors IS: 2720 (Part VI)
8. Determination of organic matter IS: 2720 (Part XXII)
9. Dry density (Sand replacement method) IS: 2720 (Part XXVIII)
10. Dry density (Core cutter method) IS: 2720 (Part XXIX)
 Type of Test Reference of IS CODE
6.2 Mechanical Properties Tests
11. Shear strength test:
- Direct Shear IS: 2720 (Part XIII)
- Unconfined compression IS: 2720 (Part X)
- Tri-axial compression IS: 2720 (Part XI-XII)
12. Consolidation properties IS: 2720 (Part XV)
13. Permeability IS: 2720 (Part XVII)
14. Compaction tests
- Light compaction IS: 2720 (Part VII)
- Heavy compaction IS: 2720 (Part VII)
- Relative Density IS: 2720 (Part XIV)
15. Lab CBR values IS: 2720 (Part XVI)
 Type of Test Reference of IS CODE
6.3 Chemical Tests
14. Total soluble solids IS: 2720 (Part XXI)
15. Cation exchange capacity IS: 2720 (Part XXIV)
16. Corrosivity tests:
- pH value IS: 2720 (Part XXVI)
- Sulphate content IS: 2720 (Part XXVII)
 Extent of Geotechnical Investigations

 No rigid rules can be set for spacing of borings and

depth of borings.
 should be such as to reveal major changes in thickness or properties
of the strata over the base area of the structure and its immediate
surroundings.
 Major factors to be kept in mind are:
 Type, Size and Criticality of structure;
 Magnitude and distribution of load imposed by the structure;
and
 Nature of subsurface conditions.
Sub-surface Investigation for Foundations
(IS:1892 – 1979)

Cl. 2.3.1: Number of Trial pits/ Bore holes

 For a compact building covering an area of 0.4 Hectares :

One Bore Hole in each corner and one at the Centre.

 For smaller and less important buildings: One Bore

hole/Trial pit in the Centre.

 For very large areas covering Industrial and Residential

colonies: To be decided based on the geological nature of
the terrain.
 Sub-surface Investigation for Foundations
(IS:1892 – 1979)
Cl. 2.3.2: Depth of exploration
 In weak soils: exploration should be continued up to a depth at which
loads can be carried by the stratum in question, without undesirable
settlement and Shear failure.
 Normally up to 1.5 times the width of footing, below the bottom of
foundation.

Cl 2.4.1: Pile and Well Foundations (Depth of exploration)

 Up to a depth 1.5 times the width of structure, from the toe of pile or
bottom of well.
 If rock is encountered, bore at least 3m in the rock to ensure that it is
bedrock and not boulder.

• In any case, the exploration should be continued to the depth upto which the
seasonal variation affect the soil.
 Normally up to 1.5 times
the width of footing, below
the bottom of foundation.
L≥B
 If a number of loaded areas
are in close proximity: B A B
Whole of the area may be
considered as loaded and GL
exploration should be
continued up to 1.5 times
the lower dimension of the D = 1½ B When A ≥ 4 B
loaded area.
D = 1½ L When A < 2B
B
B A B A B

A
W

B L

GL GL

D = 1½ W A > 2B and ≤ 4B
Thumb Rule for Depth of Boring

D = Depth of foundation
Z = Depth of exploration/investigation
 = Increase in vertical stress due to load
imposed by the structure, at depth Z
0’ = Effective overburden pressure

Then, Z should be such that:

  10% of 0’;

Generally  5% of 0’
(for compressible strata)
 7. Preparation of Bore Log (Cl. 6.5.1 of IS:1892)
Site plan and section along with following information should be
given:
a) Agency;
b) Location with map and plan reference;
c) Pit number;
d) Reduced level (RL) of ground surface or other reference point;
e) Dates, started and completed;
f) Supervision;
g)  Scales of plans and sections;
h) Dimensions, types of sheeting and other material of stabilization, advancing the
exploration, such as by hand tools, blasting, boring etc.;
i) General description of strata met with;
j) The level at which the subsoil water table is met with;
k) Inflow of water, methods of controlling the water, required capacity of pumps;
l) Position and altitude of contacts, faults, strong joints, slicken-sides etc.;
m)  Dip and strike of bedding, and of cleavage; and
n)  Any other information and remarks.
Appendix-D IS:1892
Bore Log Sample - 3, Bridge Work
Ref- IS-1892 Clause- 5.1
 Cohesionless Soil
(a) Lab Tests
Classification Test IS:1498
(i) Grain Size Analysis IS:2720 (Part-IV)
Liquid Limit and Plastic Limit IS:2720 (Part-III)
(ii) Specific Gravity IS:2720 (Part-V)
Shear Strength Parameters (as applicable)
Direct Shear Test, or IS:2720 (Part-XIII)
(iii) Unconfined Compression Test, or IS:2720 (Part-X)
Tri-axial Shear Test IS:2720 (Part-XII)
(iv) Permeability Test (if dewatering required) IS:2720 (Part-XVII)
(b) Field Tests
(i) Plate Load Test IS:1888
Standard Penetration Test, or IS:2131
(ii) Dynamic Cone Penetration Test, or IS:4968 (Part-I or Part-II)
Static Cone Penetration Test IS:4968 (Part-III)
 Cohesive Soil
(a) Lab Tests
Classification Test IS:1498
(i) Grain Size Analysis IS:2720 (Part-IV)
Liquid Limit and Plastic Limit IS:2720 (Part-III)
(ii) Specific Gravity IS:2720 (Part-V)
Shear Strength Parameters
Direct Shear Test IS:2720 (Part-XIII)
(iii) Unconfined Compression Test IS:2720 (Part-X)
Tri-axial Sher Test IS:2720 (Part-XII)
(iv) Permeability Test (if dewatering required) IS:2720 (Part-XVII)
(v) Consolidation Test IS:2720 (Part-V)
(b) Field Tests
(i) Plate Load Test IS:1888
Standard Penetration Test, or IS:2131
(ii) Dynamic Cone Penetration Test, or IS:4968 (Part-I or Part-II)
Static Cone Penetration Test IS:4968 (Part-III)
(iii) Vane Shear Test (For soft or cohesive soil) IS:4434
Generally for cohesive soils more reliance on laboratory tests as undisturbed
samples can be easily taken whereas field tests duration being less so
consolidation does not take place , except for soft clay deposits where its not
possible/very difficult to take undisturbed samples vane shear test gives reliable
results. For cohesinless soils its difficult to take undisturbed samples so field tests
data are preferred.

For type of tests to be

conducted in different
locations in field have
been mentioned in
Appendix A of IS Code-
1892-1979. A long list ,a
snapshot of small
portion shown here.
Colluvial Soils/Talus –
These are formed due to
gravity forces. In Mountains
and hills, on steep
slopes soil creeps down
under gravity force due to
variation of moisture
content (hence loosening
of soil). Such soils are
deposited in the lower part
of the mountains
TALUS Formation

• Talus slopes are formed by soil transported by gravitational forces

consisting of rock fragments; generally top 1-2m of loose
soil,weathered small stones, kankars, fragmented rocks etc loose type
material .
It’s also defined as a type of slope in which debris piles up to a
characteristic angle of repose. When new debris is added to the slope,
thereby locally increasing the angle, the slope adjusts by movement of
the debris to reestablish the angle.