Geotechnical exploration

Common erros in collecting Geotechnical information

Sub-surface exploration

To determine the suitability or otherwise of soil or rock formation, available at the

location selected for the proposed structure by means of conducting field tests and

to collect undisturbed & disturbed samples for conducting laboratory tests.

To determine

• The engineering properties of soil /rock strata• Location & thickness of each formation• Location of ground water level• Geological condition – like Joint, fault, fissures – their dimensions, frequency

and orientation

To calculate • Bearing capacity of formation• Depth of foundation• Probable settlement• Depth & extend of scour• Construction difficulties• Remedial measures

Method of conducting Geotechnical exploration

• Hand boring• Auger - Clay strata• Shell - Sand strata

• Mechanical - type• Continuous flight auger - Clay strata• Wash boring – mainly for bore well • Calyx drilling - Rock strata• Percussion drilling - Gravelly strata• Rotary drilling - Rock strata

Depth of boring• Not less than 1.5B from bottom of foundation• Sufficient depth into firm soil or rock – problematic material• 3m in hard rock• Not less than 15m for Pile• Distance - twice the depth below bed of the last main foundation – to assess

the effect of approach embankment

Number of bores

• One bore at each abutment• One bore in each pier

Sampling methods

• Undisturbed soil samples – UDS - Clay • Disturbed soil sample - Sand, Gravel• Rock cores -

Laboratory Tests for soil samples

On UDS soil samplesName of test Application

• Sieve Analysis Soil classification, scour depth• Atterberg limits Soil classification, Cohesion • Specific Gravity • Natural Moisture Contend Comparison with Liquid limit• Natural wet density • UCC Cohesion & Angle of internal friction• Consolidation - Settlement calculation & Ground improvement• Triaxial Shear test - Drained shear strength (embankment)• Direct Shear test - Angle of internal friction• Lab Vane shear test - Shear strength & Cohesion

On Disturbed samples• Sieve analysis• Atterberg Limits

Laboratory Tests forn rock samples

Crushing strength – SBC calculation Point Load Index - Crushing strength Rebound Hammer – Crushing strength, classification Density

Field Tests Soil

Standard Penetration Test Vane shear test Static Cone Penetration Test Dynamic Cone Penetration Test Pressure meter Load test

Rock

Crushing strength Load test Pressure Meter

Important Geotechnical properties

• Cohesion less soil – Sand, Gravel

• Angle of internal friction • Density

• Cohesive soil – Clay

• Cohesion, c• Coefficient of consolidation• Density• Pre-consolidation pressure• Voids ratio

Standard Penetration Test

Measuring the resistance offered by the soil formation - to penetrate a split spoon sampler

to a known distance (30cm) giving blows by means lifting & dropping a hammer of known

weight (63.5Kg) free falling through a known height (75cm)

SPT spoon specification Split spoon length - >50cm Cutting edge - >5cm Adopter – Provision for the release of air & water Cutting edge – Not blended Material Hardened steel

Point of refusal

• SPT is halted if• 50 blows are required for any 15cm penetration• 10 successive blows produce no advance

Corrections to be applied for arriving design N value

• Overburden correction

• Dilatancy correction

N” = 15 + (N’ – 15) 2

Common errors in conducting SPT test

1. Weight of hammer weight differ from the standard weight 63.5 Kg

2. Use of damaged cutting edge – higher “N” value

3. Split spoon length less than standard length – higher “N” value

4. Non functioning of pressure releasing value – high “N” value

5. Height of free fall less than standard height (75 cm) – high “N” value

6. Height of free fall greater than standard height – low “N” value

7. Poor cleaning of bore before conducting SPT test – low or high N value

Sampling interval

1. 1.5 m interval irrespective of layer thickness

2. If the layer changes within 1.5m then it shall also be collected

3. Continuous coring of rock strata

Geotechnical exploration in rock strata

• Types of boring– Wash boring– Calyx drilling– Rotary drilling

• Types of core barrel– Single tube core barrel– Double tube– Triple tube

• Types of drilling bit– Diamond impregnated– Tungsten carbide

Boring work in rock strata is very difficult than soil strata this because collection of

rock core samples from the bores is entirely depends on the selection of appropriate

drilling method , type of core barrel and use of correct drilling bit. Selection of

wrong drilling method, core barrel and drill pit make the boring work a failure.

Laboratory test could be conducted only on the samples which have length two

times its diameter. Samples having less than required length are of no use.

Information regarding grade of weathering, record of observation on the presence

of cracks in the core samples including its orientation with the drilling direction,

spacing of cracks and its dimension shall be carefully done by the rig operator.

Rock Quality Designation index & Percentage of Core recovery

Core recovery (%) = Length of Recovered Core X 100 Length of Run

Percentage of core recovery shall always be greater than 70 If it is less than 70 then it shows correct drilling procedure is not

adopted.

RQD (%) = Sum of recovered core in pieces > 100mm X 100 Length of Run

Soil Type Size mm

Clay <0.002 Silt 0.075 – 0.002 Fine sand 0.425 – 0.075 Medium sand 2.36 – 0.425 Coarse sand 4.75 – 2.36 Gravel 76 – 4.75

Important Codes

SPT - IS 2131DCPT – IS 4968 Part I & IIUCC - IS 2720 Part 10SCPT – IS 4434Consolidation – IS 2720 Part 15Sieve AnalysisAtterberg Limits - IS 2720Natural Moisture ContentSubsoil profile – IRC:75Bore log - IRC :75RQD – IS 11315Vane shear test – IS 4434Boring – IRC 78 - 2000