Conventional Field Testing Methods and Issues Amit Prashant Indian Institute of Technology Gandhinagar Short Course on Geotechnical Investigations for Structural Engineering 15 17 October, 2015 1 Conventional Field Testing 2 1
Conventional Field Testing In-situ shear strength tests Standard Penetration Test (SPT) Cone Penetration Test (CPT) Dynamic Cone Penetration Test (DCPT) Vane Shear Test (VST) Dilatometer Test (DMT) Pressure meter Test (PMT) Settlement test Plate Load Test Conventional Field Tests www.fhwa.dot.gov 4 2
Standard Penetration Test Kovacs, William D.; Salomone, Lawrence A. & Yokel, Felix Y. Energy Measurement in the Standard Penetration Test. 5 Standard Penetration Test http://www.tecopsa.com/ 6 3
Standard Penetration Test Components Drilling Equipment Inner diameter of hole 100 to 150 mm Casing may be used in case of soft/non-cohesive soils Split spoon sampler IS:9640-1980 Drive weight assembly Falling Weight = 63.5 Kg Fall height = 75 cm Others Lifting bail, Tongs, ropes, screw jack, etc. Procedure The bore hole is advanced to desired depth and bottom is cleaned. Split spoon sampler is attached to a drill rod and rested on bore hole bottom. Driving mass is dropped onto the drill rod repeatedly and the sampler is driven into soil for a distance of 450 mm. The number of blow for each 150 mm penetration 7 are recorded. Standard Penetration Test Procedure (Cont.) N-value First 150 mm penetration is considered as seating penetration The number of blows for the last two 150 mm penetration are added together and reported as N-value for the depth of bore hole. The split spoon sampler is recovered, and sample is collected from split barrel so as to preserve moisture content and sent to the laboratory for further analysis. SPT is repeated at every 750 mm or 1500 mm interval for larger depths. Under the following conditions the penetration is referred to as refusal and test is halted a)50 blows are required for any 150 mm penetration b)100 blows are required for last 300 mm penetration c) 10 successive blows produce no advancement 4
Precautions during SPT The ht. of free fall Must be 750 mm The fall of hammer must be free, frictionless and vertical Cutting shoe of the sampler must be free from wear & tear The bottom of the bore hole must be cleaned to collect undisturbed sample When SPT is done in a sandy soil below water table, the water level in the bore hole MUST be maintained higher than the ground water level. Otherwise: QUICK condition!! Very Low N value SPT Corrections Correction for Overburden Pressure : Correction for Dilatancy : [ ] 10 5
SPT Hammer Safety Hammer Donut Hammer geotechpedia.com cmeco.com 11 SPT Value 12 6
SPT Test Data No. of blows per 0.30m Data from different bore holes Interpretation from SPT: Cohesionless Soils N f D r (%) consistency 0-4 25-30 0-15 very loose 4-10 27-32 15-35 loose 10-30 30-35 35-65 medium 30-50 35-40 65-85 dense >50 38-43 85-100 very dense 7
Interpretation from SPT: Cohesive Soils not corrected for overburden c 6.25. N in kpa N c u (kpa) consistency visual identification 0-2 0-12 very soft Thumb can penetrate > 25 mm 2-4 12-25 soft Thumb can penetrate 25 mm 4-8 25-50 medium Thumb penetrates with moderate effort 8-15 50-100 stiff Thumb will indent 8 mm 15-30 100-200 very stiff Can indent with thumb nail; not thumb >30 >200 hard Cannot indent even with thumb nail Mayne and Kemper (1988) 0.689 N OCR 0.193 p ' MN/m 2 u Shear Strength from SPT-value Peck, Hansen, and Thornburn (1974) & IS:6403-1981 Recommendation 16 8
Total Settlement from SPT Data for Cohesionless soil Multiply the settlement by factor W' 17 Allowable Bearing Pressure from SPT value Terzaghi and Peck (1967): B 0.3 qn 1.37 N 3 R wrd1sa kn m 2B S a in mm and all other dimensions in meter. 2 2 Sa Permissible settlement in mm. (25 mm) Dw D f R w 0.5 1 R w 1 D f Df RD 1 depth correction factor 1 0.2 1.2 B 18 9
Cause Issues during SPT test Inadequate cleaning of hole Failure to maintain adequate head of water in borehole Effects SPT in disturbed soil. Soil gets trapped in sampler and may be compressed as sampler is driven, reducing recovery Bottom of borehole may become quick Kulhawy and Mayne, 1990 Influence on SPT N Value Increases Decreases Careless measure of drop Hammer energy varies Increases Hammer weight inaccurate Hammer energy varies Increases or decreases Hammer strikes drill rod collar eccentrically Hammer energy reduced Increases Ungreased sheaves/shaft, new stiff rope on weight, more than two turns on cathead, incomplete release of rope each drop Sampler driven above bottom of casing Careless blow count Use of non-standard sampler Hammer energy reduced Sampler driven in disturbed, artificially densified soil Inaccurate results Correlations with standard sampler invalid Increases Increases greatly Increases or decreases Increases or decreases Coarse gravel or cobbles in soil Sampler becomes clogged or impeded Increases Use of bent drill rods Inhibited transfer of energy of sampler Increases 19 Cone Penetration Test (CPT) IS: 4968 (Part III) 10
CPT Cones Drive Cone Dutch Cone Electrical Piezocone 21 CPT Procedure Push the sounding rod with cone into the ground for some specified depth. Then push the cone with friction sleeve for another specified depth (> 35 mm). Repeat the process with/without friction sleeve. Pushing rate = 1 cm/s Mantle tube is push simultaneously such that it is always above the cone and friction sleeve. Tip Load, Q c = Load from pressure gauge reading + Wt. of cone + Wt. of connecting sounding rods Tip resistance With friction sleeve add its self weight as well Q t = Q c + Q f Frictional resistance q Friction Ratio f r Qc qc A q f q c f c Qt Q A f x-sectional area off cone = 10 cm 2 c 10% Typical range 22 surface area of friction sleeve 0% Cohesive Granular 11
Failure Modes around Advancing Cone 23 CPTU 24 12
Typical Measurements with CPTU 25 CPT Interpretations 26 13
Depth (m) 20-10-2015 CPT Interpretations 27 CPT Profile for Piezocone 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Interpreted Soil Profile EQ Drain Test Area 1 Sand Silty sand/sand Silt and Sandy Silt Sand to Silty Sand Cone Tip Resistance, q c (MPa) 0 2 4 6 8 1012 Fricton Ratio, F r (%) 0 1 2 3 4 5 6 Pore Pressure, u (kpa) -100 0 100 200 Relative Density, D r 0 0.2 0.4 0.6 0.8 1 14
Depth Below Excavated Surface (m) 0 0.2 0.4 0.6 0.8 1 1 2 3 4 5 6 7 8 9 0 10 1 2 3 4 5 6 7 8 9 0 10 1 2 3 4 5 6 7 8 9 0 10 20-10-2015 CPT Versus SPT CPT: Advantages over SPT provides much better resolution, reliability versatility; pore water pressure, dynamic soil properties CPT: Disadvantages Does not give a sample Will not work with soil with gravel Need to mobilize a special rig 0 1 IITGN Short Course on Geotechnical CPT Cone Investigations Resistance, for qstructural c1 SPT Engineering Blow Count, N 1(60) Interpreted (MPa) (Blows/300 mm) Soil Profile 0 2 4 6 8 10 12 14 0 10 20 30 Fine Sand w/ Shells (SP) Relative Density, D r (%) 0 20 40 60 80 100 2 3 4 Interbedded Fine Sand and Silty Sand (SP-SM) 5 6 7 Fine Silty Sand (SM) 8 9 10 Gray Silty Clay (CL) Sand (SP) Mean Mean-SD Mean+SD From CPT From SPT 15
CPT-SPT Correlation 31 Downhole Seismic Piezocone Penetration Test (SCPTU) 32 16
CPT Correlations Soil Profiling and Soil Type Equivalent SPT N60 Profiles Soil Unit Weight Undrained Shear Strength (s u ) Soil Sensitivity Stress History - Overconsolidation Ratio (OCR) In-Situ Stress Ratio (Ko) Friction Angle Relative Density (D r ) Stiffness and Modulus Modulus from S and P Wave Velocity Hydraulic Conductivity (k) Consolidation Characteristics Refer the given handout 33 Continuous Sampling on the Side of CPT http://geopractica.co.za/portfolio/namoya-mine-drc/ http://www.alfakat.gr/en/projects/16_geo_boula.htm 34 17
Dynamic Cone Penetration Test (DCPT) Components: 1) Cone (dia = 50 mm) ~usually made of steel IS: 4968 (Part I, II) SPT DCPT 2) Driving rods/drill rods ~marked at every 100 mm Hollow (split spoon) Solid (no samples) DCPT Procedure Cone drill rod driving head assembly is installed vertically on the ground and hammer is dropped from standard height repeatedly The blow counts are recorded for every 100 mm penetration. A sum of three consecutive values i.e. 300 mm is noted as the dynamic cone resistance, N cd at that depth. The cone is driven up to refusal or the project specified depth. In the end, the drill rod is withdrawn. The cone is left in the ground if unthreaded or recovered if threaded. No sample recovered Fast testing less project cost / cover large area in due time Use of bentonite slurry is optional, which is used to reduce friction on the driving rods. Modified cone is used in this case: diameter = 62.5 mm 18
Vane Shear Test (VST) bore hole measuring (torque) head For clays, and mainly for soft clays. Measure torque required to quickly shear the vane pushed into soft clay. undrained vane h2d torque undrained shear strength c u Typical d = 20-100 mm. d soft clay 37 vane Vane Shear Test Interpretation: Undrained shear strength - 2. T cu D 1 3. H 2. D. H. For H = 2.D c u T 0.273 D 3 Test in Progress Failure surface 19
Dilatometer Test (DMT) Insert DMT using SPT drilling equipment to the desired depth and pressure the cell Measure pressure when the membrane is flushed with plate and when it enters ground by 1.1 mm. Decrease the pressure & measure the pressure when membrane is again flushed with plate. Determined: Elastic Modulus Soil Type and state 39 60 mm dia. Flexible membrane Pressure meter Test (PMT) Determined: Elastic Young Mod, E Shear Mod, G Undrained shear strength, S u 20
Pressure meter Test (PMT) Measurements: 1. Fluid Pressure 2. Fluid volume change 41 42 21
Plate Load Test This test is used to estimate the Elastic Modulus and Bearing Capacity of soils which are not easily sampled. Modulus Estimation: The load is applied to the plate in increments of one fifth of the design load. Time-settlement and load-settlement curves are then produced to estimate modulus of subgrade reaction (K) at 1.25 mm settlement. Geophysical Methods 44 22
P or Primary Waves longitudinal, primary or compressional wave Material particles oscillate about a fixed point in the direction of wave propagation by compressional and dilatational strain. 45 S or Secondary Waves transverse, secondary or shear wave Particle motion is at right angles to the direction of wave propagation and occurs by pure strain. 46 23
Rayleigh Waves (used in MASW) 47 Love Waves 48 24
Wave Velocities P-wave velocity V p Shear Wave velocity V s V p > V s 49 Soil Properties from Wave Velocity Shear Modulus G 2. V s Density of soil Constrained Modulus, V 3V 4V E 2 2 Vp Vs Young s Modulus, 2 2 Vp 2Vs 2 2 2 Poisson s Ratio, Vp Vs 50 M 2. V p 2 2 2 s p s 25
Typical Wave Velocities in Geomaterials 51 Seismic Measurement-Systems 1. Geophone 2. Cable 3. Hammer (Source) 4. Processing and Control Unit 52 26
Seismic Methods Seismic Reflection Method Seismic Refraction Method Cross-Hole Test Down Hole Test & Up-Hole Test Spectral Analysis of Surface Wave (SASW) Multichannel Analysis of Surface Waves (MASW) method 53 Waves from point source 54 27
Snell s Law Critical Angle of Refraction 1 V 1 A sin V2 55 Seismic Refraction Method Depths less than ~ 30 m Cost Effective as compared to Reflection method (<3to5 times) Used for computation of layer thickness of soil http://www.geologicresources.com/seismic_refraction_method.html 56 28
Measurement at a Geophone 57 Shot Record uniform deposit 58 29
Time (s) 20-10-2015 Shot Record real deposit Source 59 Cross-Hole Test Sensors are placed at one elevation in one or more boring. Source is triggered in another boring at the same elevation. S wave travels horizontally from source to receiving hole, and the arrivals of S waves are noted Shear wave velocity (Vs) is calculated by dividing the distance between the bore holes and the travel time. 60 30
Cross-Hole Record http://www.structuremag.org/article.aspx?articleid=994 61 Down Hole Test Sensors are placed at various depths in the boring. Source is located above the receivers, at the ground surface. Only one bore hole is required. A source rich in S wave should be used (P wave travels faster than S wave) Up-Hole method: source of energy is deep in boring and the receiver is at the ground surface 62 http://www.geophysics.co.uk/mets3.html 31
Seismic Cone Penetration Test (SCPT) A Down-Hole Test Seismic cone is pushed into the ground Shear wave is generated at the top and the time required for the shear wave to reach the seismometer in the cone is measured Computer in the SCPT rig collects and processes all the data & shear wave velocity is measured 63 http://geoprobe.com/how-seismic-cone-penetration-equipment-works Down-Hole Test Record SPT Velocity (m/s) Time (s) 64 http://www.belirti.com/english/downhole.htm 32
Thank You 65 33