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16-10-2017
1
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