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Sub-Soil Investigation

by

Prof. S. R. Gandhi

INFORMATIONS

REQUIRED Variations in Ground levels and Site Grading

Drainage Pattern

Shear strength parameters for baring capacity

Compressibility and Consolidation parameters

Modulus of Sub-grade reaction

Dynamic Soil Parameters

Earth Pressure Coefficients

Permeability Constant

Chemical Analysis of Soil/Ground water

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Planning Subsoil Investigation

• Number of Boreholes

• One for each important structure

• Information for every 50m of loaded area

• Borehole for medium size structure only if variations

• Few boreholes for future expansion

• Depth of Boreholes

• 1.5 to 2 times times the width of foundation (beyond

foundation depth)

• Few up to hard strata( N > 100 ) or rock

• 5 to 10m beyond foundation level on rock

Methods of Investigation

• Geophysical Investigation

• Open Pits

• Bore Holes

• Field In-situ tests

• Special Tests(Block vibration, Cross Hole

Shear Wave Velo., etc.)

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Geophysical Investigation

• Gravity Survey

• Magnetic Survey

• Electrical Resistivity Survey

• Seismic Survey

• Temperature Logging

• Radioactivity Logging

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Selection of Agency

• Not by lowest quotation

• Short listing of Agencies based on

equipments’ condition and laboratory

facilities

• Qualified/Trained manpower

• Constant supervision at site

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Precautions to be taken

• Check straightness of the drill rods

• Effective circulation of bentonite slurry

• Maintain slurry above water table

• Clean bottom before in-situ test/sampling

• Proper packing of samples

In-situ Tests

• Standard Penetration Test (sandy soils)

• Vane Shear Test (clayey soil)

• Cone Penetration Test

• Pressuremeter Test

• Plate Load Test

• Block Vibration Test

• Cross Hole Test

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Pic.1.1. Non Standard

Equipment

No safety measures Prof. V.S. Raju

Pic. 1.2 International Standard

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Pic. 1.5 Bent SPT Sampler

In one of the sites in 2013

Prof. V.S. Raju

Project: 10 Towers, 15 storeys each

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Pic. 1.3 SPT Hammer

Dropped Manually (2013)

Pic. 1.4. SPT with Auto Trip Hammer

+ Standard Rig

Prof. V.S. Raju

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Strength Parameters For Cohesionless Soils

SPT (N) Relative Density (Degrees)

0-4 Very loose <28°

4-10 Loose 20-30°

10-30 Medium 30-36°

30-50 Dense 36-41°

>50 Very Dense >41°

STRENGTH PARAMETERS FOR COHIVE

SOILS

Standard

Penetration

value (N)

Relative

Consistency

Consistency

Index (Ic)*

Unconfined

Compression

(qu) **

kPa

0 – 2 Very Soft 0-0.5 25

2 – 4 Soft 0.5-0.75 25-50

4 – 8 Med. Stiff 0.75-0.9 50-100

8 – 16 Stiff 0.9-1.0 100-200

16 – 32 Very Stiff 1.0-Ics 200-400

> 32 Hard > Ics () >400

* Adopted from German Standard DIN 1054

** Soil Mechanics by Lambe & Whitman

() Ics = Consistency Index at Shrinkage Limit

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Un-disturbed Sampling

• Mostly in cohesive layers

• Soft to stiff consistency

• To assess strength and deformability

parameters

• Requires quality equipment and utmost care

Source of Disturbance

• During drilling operations

• Improper cleaning of the borehole bottom

• During driving of the sampler tube

• Stress release due to removal of overburden

• During ejecting the sample from sampling tube

• During storage of sample

• During preparation of sample for testing

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LABORATORY TESTS

• Grain Size Analysis

• Liquid and Plastic Limits

• Unconfined Compression Test

• Triaxial Compression Test

• Consolidation Test

• Chemical Analysis on Soil/Water samples

• Shrinkage Limit and Freeswell Test

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Prof. V.S. Raju

Pic. 1.8 Electric Cone plus pore

pressure sensor

Pic. 1.7 Conventional Static

Cone Penetrometer-

Mechanical Cone 41

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INTERPRETATION OF CONE DATA

(Cohesive)

Consistency qc (kN/m sq.) SPT (N) Indrained

cohesion

(kPa)

Very soft <200 <2 <12.5

Soft 200-400 2-4 12.5-25

Medium stiff 400-800 4-8 25-50

Stiff 800-1500 8-15 50-100

Very stiff 1500-3000 15-30 100-200

Hard >3000 >30 >200

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Interpretation of Cone Data

(Cohesionless soil)

qc (kPa) SPT (N) Relative

Density

Classification °

<200 <4 <0.2 Very loose <30

200-400 4-10 0.2-0.4 Loose 30-35

400-1200 10-30 0.4-0.6 Medium dense 35-40

1200-2000 30-50 0.6-0.8 Dense 40-45

>2000 >50 0.8-1.0 Very dense >45

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Dynamic Soil Parameters Soil Type Baring

Capacity

(kPa)

Coefficient of

Uniform compression

cz (kN/m3)

Weak soil 100 20,000

200 40,000

Medium soil 300 50,000

400 60,000

Hard soil 500 70,000

Rocks >500 >70,000