SITE INVESTIGATION Validation and Interpretation of data Dr. G.Venkatappa Rao The Impact 1
The Need To determine the type of foundation To assess bearing capacity/settlement Location of Ground water table,issues related to construction Identification of potential problems to adjacent structures Identify environmental issues 3 Soil a Unique engineering material Does not possess a linear or unique stress-strain relationship Behaviour depends on pressure, time and environment Essentially different at every location Cannot be seen in entirety like other building materials Sensitive to disturbance 4 2
Sampling for Laboratory Testing volume/scale limitations with sampling. The total volume of the samples is approximately 1/250,000th of the stressed ground. Steps in Geotechnical Investigation Reconnaissance Preliminary Investigation Detailed Investigation Planning - Numbers,Depths,Locations,Types Drilling/Boring Field Testing/Sampling Laboratory Testing Report Writing Report Validation/Choice of design parameters 6 3
Sampling Methods a guide to sampling methods. Why Soil Classification Disturbed soil samples Classification Coarse grained or granular soils Fine grained or cohesive soils Boulders Gravel Sand Silt Clay 8 4
Soil as an Engineering Material Soils can rapidly change its properties depending on its microstructure and the way this microstructure interacts with pore water and external loads. Landslide in a quick clay. Strength loss of a sensitive clay, or quick clay, which liquifies during remolding. Sampling Disturbance distortion of bedding by a tube sampler. smooth penetration with correct sampler jerky penetration with correct sampler smooth penetration with incorrect sampler. 10 of 54 5
16-10-2017 Quality Evaluation Strength Results No.1 No.2 No.3 No.4 No.5 No.6 11 Quality Evaluation Stress strain curves 12 6
Quality evaluation -- e log p curves 13 Soil and Water Most geotechnical engineering problems are associated with water: water flow through voids and pores in the soil mass pore water pressures, effective stresses and consolidation seepage forces 7
Fig. 1 Alignment of boreholes 15 Standard Penetration Test (SPT) 14 Oct 2017 Site Investigation - Validation RAO G V 16 8
Procedures which may affect Measured N Values Inadequate cleaning of the borehole Not seating the sampler spoon on undisturbed material Driving of the sample spoon above the bottom of the casing Failure to maintain sufficient hydrostatic head in boring Attitude of operators Overdrive Sampler SPT is only partially made in original soil. Sludge may be trapped in the sampler and compressed as the sampler is driven, increasing the blow count. (This may also prevent sample recovery.) Incorrect "N" values obtained "N" values are increased in sands and reduced in cohesive soils The water table in the borehole must be at least equal to the piezometric level in the sand, otherwise the sand at the bottom of the borehole may be transformed into a loose state Blow counts for the same soil using the same rig can vary, depending on who is operating the rig, and perhaps the mood of operator and time of drilling Higher blow counts usually result from overdriven sampler Sampler plugged by gravel Higher blow counts result when gravel plugs sampler, resistance of loose sand could be highly overestimated. 17 Procedures which may affect Measured N Values Plugged casing Over-washing ahead of casing Drilling method Not using the standard inner drop Free fall of the drive weight is not attained Not using correct weight Weight does not strike the drive cap concentrically Not using a guide rod High "N" values may be recorded for loose sand when sampling below groundwater table. Hydrostatic pressure causes sand to rise and plug casing. Low blow count may result for dense sand since sand is loosened by overwashing. Drilling technique (e.g., cased holes vs. mud stabilized holes) may result in different "N" values for the same soil. Energy delivered per blow is not uniform. European countries have adopted an automatic trip hammer not currently in use in North America. Using more than 1/2 turns of rope around the drop and/or using wire cable will restrict the fall of the drive weight. Driller frequently supplies drive hammers with weights varying from the standard by as much as 4 Kg. Impact energy is reduced, increasing "N" values. Incorrect "N" value obtained. Not using a good tip on the If the tip is damaged and reduces the opening or increases sampling spoon 14 Oct 2017 the end area the "N" value can be increased. Site Investigation - Validation 18 9
Procedures which may affect Measured N Values Use of drill rods heavier than standard Not recording blow counts and penetration accurately Incorrect drilling procedures Using drill holes that are too large Inadequate supervision Improper logging of soils With heavier rods more energy is absorbed by the rods causing an increase in the blow count. Incorrect "N" values obtained. The SPT was originally developed from wash boring techniques. Drilling procedures which seriously disturb the soil will affect the "N" value, e.g. drilling with cable tool equipment. Holes greater than 10 cm in diameter are not recommended. Use of larger diameters may result in decrease in the blow count. Frequently a sampler will be impeded by gravel or cobbles causing a sudden increase in blow count; this is not recognized by an inexperienced observer. (Accurate recording of drilling, sampling and depth is always required.) Not describing the sample correctly Using too large a pump Too high a pump capacity will loosen the soil at the base of the hole causing a decrease in blow count. 19 (After US Navy 1985) Use of existing Correlations Grain size plasticity Plasticity -- consolidation behaviour Grain size permeability PI vs Strength N value vs Relative Density N value vs angle of friction 20 10
14 Oct 2017 Site Investigation - Validation RAO G V 21 Consistency of Saturated Cohesive Soils Consistency N 70 q u, kpa Remarks Very Soft 0 2 < 25 Extruded between fingers when squeezed Soft 3 5 25 50 Very easily deformed by NC Young Clay squeezing by light finger pressure Medium 6 9 50 100 Moulded by strong finger pressure Stiff 10 16 100 200 Hard to deform by hand squeezing Very Stiff 17 30 200 400 Very hard to deform by Increasin Aged / hand squeezing, g OCR Cemented indented by thumb nail Hard > 30 > 400 Nearly impossible to deform by hand 22 11
Empirical Values of Ø, D r and Unit Weight of Granular Soils Description Relative Density D r Very Loose Loose Medium Dense Very Dense 0 0.15 0.35 0.65 0.85 SPT, N 70 Fine 1 2 3 6 7 15 16 30? Ø Medium 2 3 4 7 8 20 21 40 > 40 Coarse 3 6 5 9 10 25 26 45 > 45 Fine 26 28 28 30 30 34 33 38 Medium 27 28 30 32 32 36 36 42 < 50 Coarse 28-30 30-34 33 40 40 50 g14 wet Oct, kn/m 2017 3 11 Site 16 Investigation 14 18 - Validation 17 20 17 22 20 23 23 SPT Vs UCS -- Clays 24 12
16-10-2017 Strength correlations Granular Soils g 25 Consolidation Test Relationships g g 26 13
The Triaxial Test UU Unconsolidated-Undrained (i.e. undrained strength, c u and u ). CU Consolidated-Undrained. CD Consolidated-Drained (i.e. drained strength, c' and '). The Triaxial Test Analysis of the test results are based on the recorded magnitudes of the minimum principal stress, given by the confining pressure, and the maximum principal stress, taken as the sum of the confining stress and the axial load at failure. The effective stresses can then be computed at any point during the load history if the pore water pressure is measured. 14
16-10-2017 Triaxial Shear Parameters 29 Permeability Relations -- SAND 30 15
16-10-2017 Correlations of Consolidation Characteristics SILTS & CLAYS 31 Testing Permeability 16
Typical Issues In Delhi Filled up soil vs. Natural soil In Delhi Structure partly on rock & soil At IIT Delhi Library Building Solar passive houses Block VI SPT Vs Plate load test Depth of exploration vis-à-vis foundation depth 34 Additional Information Information for ground improvement Successful technologies Soil chemistry Soil water chemistry Liquefaction potential Backfill materials 35 17
Caveat Geotechnical investigation does not end with the Report Continues..as Construction begins.. At least throughout construction 36 THANK YOU 37 18