Geotechnical Testing Methods II

Transcription

1 Geotechnical Testing Methods II Ajanta Sachan Assistant Professor Civil Engineering IIT Gandhinagar FIELD TESTING 2 1

2 Field Test (In-situ Test) When it is difficult to obtain undisturbed samples. In case of Cohesionless soils, Sensitive clays, etc. Advantage: Testing on natural soil under undisturbed conditions Disadvantage: Testing conditions are not controlled Time dependent phenomenon are difficult to control due to large scale Measurements/instrumentation is tricky and rather a difficult task Field Test (In-situ Test) 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 2

3 Common In Situ Testing Devices SPT VST PMT DCPT CPT DMT In bore holes 5 Standard Penetration Test IS:

4 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 7are 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

5 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 : 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 5

6 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 - 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 60% E in 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 6

7 SPT Test Data No. of blows per 0.30m Data from different bore holes Interpretation from SPT: Cohesionless Soils N'' f' D r (%) consistency very loose loose medium dense > very dense 7

8 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 Cone Penetration Test (CPT) IS: 4968 (Part III) 8

9 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 q f c q f c Qt Q A f x-sectional area off cone = 10 cm 2 c 10% Typical range 17 surface area of friction sleeve 0% Cohesive Granular CPT Results & Soil Classification 9

10 Depth Below Excavated Surface (m) Typical CPT Data 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 10

11 Depth (m) CPT Profile for Piezocone 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) Fricton Ratio, F (%) r Pore Pressure, u (kpa) Relative Density, D r 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 11

12 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 12

13 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 25 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 D 3 Test in Progress Failure surface 13

14 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 mm dia. Flexible membrane Pressure meter Test (PMT) Determined: Elastic Young Mod, E Shear Mod, G Undrained shear strength, S u 14

15 Pressure meter Test (PMT) Measurements: 1. Fluid Pressure 2. Fluid volume change 29 Plate Load Test This test is used to estimate the Elastic Modulus and Bearing Capacity of soils which are not easily sampled. 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 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 soil from the test results. 15

16 ROCK TESTING Rock Testing Unconfined Compression Test Brazilian Test Point Load Test Direct Shear Test Slake Durability Test Schmidt Rebound Hardness Test Sound Velocity Test In-situ stress measurements in rocks 16

17 Specimen Preparation Equipments for Rock Testing Rock Core sizes: EX = mm AX = mm BX = mm NX = mm More: 35mm, 50mm, 75mm, 100 mm Core cutting & grinding machine: Cutting and grinding cylindrical rock specimens core size: EX to NX Polishing & Lapping machine Core drilling machine: Rock core preparation For regular and irregular Samples. core size: EX to 100mm Rock Samples Granite: High stiffness High strength Very brittle Limestone: Medium stiffness Medium strength Medium brittleness Shale: Low stiffness Low strength Ductile 17

18 Unconfined Compression Test This test is performed to obtain the unconfined compressive strength (UCS) of intact rock cores (slenderness ratio = 2). UCS is the maximum stress that that rock specimen can sustain. Rock specimen is kept in a loading frame, and if required heated to the desired test temperature. Axial load is continuously increased on the specimen until peak load and failure are obtained. Brazilian test: Tensile strength of Rock Brazilian test is performed to obtain the tensile strength of rock mass. Tensile strength of rock is imp to know for drilling, blasting of rocks, failure of roof and floor of tunnels, chambers & underground roadways; often weak rocks fail in tension exhibiting splitting mode of failure. In this test, a disc/cylinder is subjected to a line load, and fracture should initiate at the centre and progress towards periphery. If opposite, the test is discarded as considered that it did not fail in tension. 18

19 Point Load Test: compress. strength of irregular rock sp. When regular cores could not be obtained; only irregular pieces are available from the rock excavation, Point load test is performed to obtain the compressive strength of rock mass. The roughly chiseled spherical mass with dia. ranging between mm is tested between two hard conical tips in a rigid frame. Direct Shear Test: Normal stress versus Shear stress response of rock mass It measures peak and residual direct shear strength as a function of stress normal to the sheared plane. It can be used for testing for both: core & lump specimens. Shear box size: 300mm x 300mm x 100mm 19

20 Triaxial Shear Test: shear strength parameters (c, f) of rock mass Triaxial cells for testing rocks are designed to withstand a confining pressure 150 Kg/cm2. Mostly triaxial tests on rock specimens are performed under no volume change conditions. Stress-strain curve is obtained using deviator stress and axial strain. The modulus and failure deviator stress are estimated. Shear strength parameters (c & f ) are calculated adopting similar methods as in soils If the strain gauges are attached to measure the lateral strain, poisson s ratio (n) also can be obtained. Slake Durability Test: Resistance of rock mass to disintegration during wetting-drying Rock fragments of known weight placed in rotating drum apparatus, and rock pieces (approx 10 pieces, each 40-60gm weight) are circulated through wet and dry cycles. Re-weigh the rock fragments to determine the slake durability index (SDI). Mostly, this test allows the rock mass to get exposed up to two cycles of wetting and drying. 20

21 Schmidt Test: Hardness of rock Schmidt test is performed to determine the rebound hardness of rock. The plunger of the hammer is pressed against the specimen and the height of rebound of the plunger on a scale is taken as the measure of hardness. Sound Velocity Test: P & S-wave velocity of rocks It is non-destructive test and performed to determine the velocity of elastic wave propagation through rock in the laboratory. slenderness ratio used for the test is usually 3. Test can be conducted on dry, moist or saturated specimens. A transmitter and a receiver are attached at sides of rock specimen (a thin layer of grease is applied on the specimen s ends to have proper contact with transducers). The energy transmission between the transducers (transmitter and receiver) is used to determine the velocities of P and S wave. 21

22 In-situ stress measurements in rocks In-situ stress measurements in rocks: Testing methods on Field 22

23 In-situ stress measurements in rocks: Flat Jack Test In-situ stress measurements in rocks: Hydrofracturing Test 23

24 Thank You 24