Conventional Field Testing & Issues (SPT, CPT, DCPT, Geophysical methods)

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1 Conventional Field Testing & Issues (SPT, CPT, DCPT, Geophysical methods) Ajanta Sachan Assistant Professor Civil Engineering IIT Gandhinagar Conventional Field Testing 1

2 Field Test: In-situ shear strength Testing In-situ shear strength tests Standard Penetration Test (SPT) Cone Penetration Test (CPT) Dynamic Cone Penetration Test (DCPT) Vane Shear Test (VST) Common In Situ Testing Devices SPT VST DCPT CPT In bore holes 4 2

Standard Penetration Test IS: 2131-1981 5 Standard Penetration Test Components Drilling Equipment Inner diameter

Drive weight assembly Falling Weight = 63.

Procedure The bore hole is advanced to desired depth and bottom is cleaned.

3 Standard Penetration Test IS: 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: 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 are recorded. 3

4 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 100 blows are required for last 300 mm penetration 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 4

5 SPT Corrections Correction for Overburden Pressure : N ' C. N N' = Corrected value of observed N C N = Correction factor for overburden pressure Peck, Hanson and Thornburn (1974) N p' = Effective overburden pressure at a depth corresponding to N-value measurement SPT Corrections Correction for Overburden Pressure : (Alternative) Correction for Dilatancy : If the stratum consists of fine sand and silt below water table, for N' > 15, the dilatancy correction is applied as [ ] Alternative - 5

6 SPT Hammer Energy Correction Energy is dissipated in some fraction during the impact, and the output energy is usually in the range of 50% to 80% of energy input. For rope pully system with safety hammer E out E in 60% The N-value is standardized for 60 % energy output. For other hammers, the N-value may be corrected in ratio of their energy input N 60 E out Although IS is silent on this issue, the correction may be applied as per the requirement of the project. E 60 in %. N SPT Test Data No. of blows per 0.30m Data from different bore holes 6

7 SPT Test Data Interpretation from SPT IS

8 Interpretation from SPT: Cohesionless Soils N'' f' D r (%) consistency very loose loose medium dense > very dense Interpretation from SPT: Cohesive Soils not corrected for overburden c N in kpa N c u (kpa) consistency visual identification very soft Thumb can penetrate > 25 mm soft Thumb can penetrate 25 mm medium Thumb penetrates with moderate effort stiff Thumb will indent 8 mm very stiff Can indent with thumb nail; not thumb >30 >200 hard Cannot indent even with thumb nail Mayne and Kemper (1988) N OCR p ' MN/m 2 u 8

9 Total Settlement from SPT Data for Cohesionless soil Multiply the settlement by factor W' 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) 9

10 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 DCPT SPT Correlations for 50 mm dia. cone N cd = 1.5 N N cd = 1.75 N N cd = 2.0 N For depth < 3 m For depth 3 m to 6 m For depth > 6 m DCPT SPT Correlations for 62.5 mm dia cone Without bentonite slurry With circulating bentonite slurry N cbr = 1.5 N N cbr = 1.75 N N cbr = 2.0 N N cbr = N For depth < 4 m For depth 4 m to 9 m For depth > 9 m For all depths 10

11 DCPT Cone Penetration Test (CPT) IS: 4968 (Part III) 11

12 Depth Below Excavated Surface (m) 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 Friction Ratio f r Qc qc A q f q c q f c Qt Q A f x-sectional area off cone = 10 cm 2 c 10% Typical range 23 surface area of friction sleeve 0% Cohesive Granular 0 1 Interpreted Soil Profile Fine Sand w/ Shells (SP) CPT Cone Resistance, q c1 (MPa) SPT Blow Count, N 1(60) (Blows/300 mm) Relative Density, D r (%) Interbedded Fine Sand and Silty Sand (SP-SM) Fine Silty Sand (SM) Gray Silty Clay (CL) Sand (SP) Mean Mean-SD Mean+SD From CPT From SPT 12

13 Depth (m) CPT Profile for Piezocone Cone Tip 0 1 Interpreted Soil Profile EQ Drain Test Area 1 Sand Resistance, q c (MPa) Fricton Ratio, F r (%) Pore Pressure, u (kpa) Relative Density, D r Silty sand/sand Silt and Sandy Silt Sand to Silty Sand CPT Results & Soil Classification 13

14 Typical CPT Data 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 14

15 CPTU 29 Typical Measurements with CPTU 30 15

Measure torque required to quickly shear the vane pushed into soft clay.

16 DOWNHOLE SEISMIC PIEZOCONE PENETRATION TEST (SCPTU) 31 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 = mm. d soft clay 32 vane 16

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.

17 Vane Shear Test Interpretation: Undrained shear strength - 2. T cu D 1 3. H 2. D. H. For H = 2.D c u T D 3 Test in Progress Failure surface Plate Load Test This test is used to estimate the Modulus of subgrade reaction and Bearing Capacity of soils. Bearing Capacity Estimation: The load is applied such that the rate of penetration remains constant. A load-settlement curve is produced. Equations have been developed to obtain undrained shear strength from ultimate bearing capacity. Modulus of Subgrade Reaction Estimation: The load is applied to the plate in increments of one fifth of the design load. Time-settlement and loadsettlement curves are then produced to estimate the 17

18 Plate Load Test IS: Bearing Plate: Rough mild steel bearing plate in circular or square shape Dimension: 30 cm, 45 cm, 60 cm, or 75 cm. Thickness > 25 mm Smaller size for stiff or dense soil. Larger size for soft or loose soil Bottom of the plate is grooved for increased roughness. Concrete blocks may be used to replace bearing plates. Plate Load Test IS: Test Pit: Usually to the depth of foundation level. Width equal to five times the test plate Carefully leveled and cleaned bottom. Protected against disturbance or change in natural formation Section Plan 18

IITGN Plate load test Plate Load Test: Bearing Capacity In case of

capacity may be estimated from the peak load in load-settlement

In case of partially cohesive soils and loose to medium dense soils

19 IITGN Plate load test Plate Load Test: Bearing Capacity In case of dense cohesionless soil and highly cohesive soils ultimate bearing capacity may be estimated from the peak load in load-settlement curve. In case of partially cohesive soils and loose to medium dense soils the ultimate bearing capacity load may be estimated by assuming the load settlement curve so as to be a bilinear relationship

20 Plate Load Test: Bearing Capacity A more precise determination of bearing capacity load is possible if the load-settlement curve is plotted in log-log scale and the relationship is assume to be bilinear. The intersection point is taken as the yield point or the bearing capacity load. For cohesioless soil q q uf up B B f p For cohesive soil q uf q up Geophysical Methods Seismic Reflection Method Seismic Refraction Method Cross-Hole Test Down Hole Test & Up-Hole Test Spectral Analysis of Surface Wave (SASW) Seismic Cone Penetration Test (SCPT) 20

P Wave (Compression/Primary Wave) longitudinal, primary or

point in the direction of wave propagation by compressional

S Wave (Shear/Secondary Wave) transverse, secondary or shear

21 P Wave (Compression/Primary Wave) longitudinal, primary or compressional wave Material particles oscillate about a fixed point in the direction of wave propagation by compressional and dilatational strain. S Wave (Shear/Secondary Wave) transverse, secondary or shear wave Particle motion is at right angles to the direction of wave propagation and occurs by pure strain. 21

22 Rayleigh Waves (used in MASW) Love Waves 22

23 Wave Velocities P-wave velocity V p Shear Wave velocity V s V p > V s Soil Properties from Wave Velocity Shear Modulus G 2. V s Density of soil Constrained Modulus, M 2. V p V 3V 4V Young s Modulus, E 2 2 V V V Poisson s Ratio, 2 V s p s 2V p 2 2 p s V 2 2 p s s 23

24 Typical Wave Velocities in Geomaterials 47 Seismic Measurement-Systems 1. Geophone 2. Cable 3. Hammer (Source) 4. Processing and Control Unit 24

Seismic Reflection Method Depths greater than ~50 feet Seismic reflection is particularly suited to marine applications (e.g. lakes, rivers, oceans, etc.

$Seismic Refraction Method Depths less than ~100 feet Cost Effective as compared to Reflection method (<3to5 times) Used for computation of$

25 Seismic Reflection Method Depths greater than ~50 feet Seismic reflection is particularly suited to marine applications (e.g. lakes, rivers, oceans, etc.) The inability of water to transmit shear waves makes collection of high quality reflection data possible even at very shallow depths that would be impractical to impossible on land. Seismic Refraction Method Depths less than ~100 feet Cost Effective as compared to Reflection method (<3to5 times) Used for computation of layer thickness of soil 25

$Differences in Seismic Reflection and Seismic Refraction Method Seismic Reflection uses field$ $Seismic Refraction involves measuring the travel time of the component of seismic energy which$ $travels down to the top of rock (or other distinct density contrast), is refracted along the top$

26 Differences in Seismic Reflection and Seismic Refraction Method Seismic Reflection uses field equipment similar to seismic refraction, but field and data processing procedures are employed to maximize the energy reflected along near vertical ray paths by subsurface density contrasts. Seismic Refraction involves measuring the travel time of the component of seismic energy which travels down to the top of rock (or other distinct density contrast), is refracted along the top of rock, and returns to the surface. 26

27 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. Cross-Hole Test 27

A source rich in S wave should be used (P wave travels faster than S wave) Up-Hole method: source of

28 Down Hole Test Down Hole method: 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 Down Hole Test 28

Seismic Cone Penetration Test (SCPT) Seismic cone is pushed into the

required for the shear wave to reach the seismometer in the seismic

the data & shear wave velocity is measured http://geoprobe.

29 Seismic Cone Penetration Test (SCPT) Seismic cone is pushed into the ground During the penetration, shear wave is generated and the time required for the shear wave to reach the seismometer in the seismic cone is measured Computer in the SCPT rig collects and processes all the data & shear wave velocity is measured Seismic Cone Penetration Test (SCPT) 29

30 Seismic Cone Penetration Test (SCPT) Seismic Cone Penetration Test (SCPT) 30

31 Seismic Cone Penetration Test (SCPT) Seismic Cone Penetration Test (SCPT) 31

32 32

SASW Test (Spectral Analysis of Surface Waves) SASW does not require Boring like other tests Sensors are spread along a line on the surface & the source is also located on the surface Sensors receive

33 SASW Test (Spectral Analysis of Surface Waves) SASW does not require Boring like other tests Sensors are spread along a line on the surface & the source is also located on the surface Sensors receive Rayleigh waves, which are the surface waves Dispersion curve (phase velocity Vs frequency) is created. Then individual dispersion curves from all receivers are combined into a single composite dispersion curve, called field dispersion curve. Forward-modeling procedure is then used to match the field dispersion curve with a onedimensional layered system of varying soil layer stiffnesses and thicknesses. The shear wave velocity profile that generates a dispersion curve that most closely matches the field dispersion curve is then presented as the shear wave velocity profile for the site. MASW Test (Multichannel Analysis of Surface Waves) 33

34 MASW Test (Multichannel Analysis of Surface Waves) MASW does not require Boring like SASW. 24 or more channels (Sensors) are placed over a few to a few meters of distance (eg: m) Sensors receive Rayleigh waves, which are the surface waves. Dispersion curve (phase velocity Vs frequency) from each sensor is created. MASW deals with various frequencies range (eg: 3-30 Hz) Active MASW method generates surface waves actively through an impact source like sledge hammer, where as Passive MASW method utilizes surface waves generated by traffic, thunder, tidal motions etc. Investigation depth is usually shallower than 30 m with the active method. MASW utilizes dispersion properties of surface waves for the purpose of shear wave velocity (Vs) profiling in 1D (depth) or 2D (depth and surface location) of soil strata. Thank You 34

Geophysical Site Investigation (Seismic methods) Amit Prashant Indian Institute of Technology Gandhinagar

Geophysical Site Investigation (Seismic methods) Amit Prashant Indian Institute of Technology Gandhinagar Short Course on Geotechnical Aspects of Earthquake Engineering 04 08 March, 2013 Seismic Waves