Disaster Mitigation Geotechnology. Laboratory soil characterisation

Transcription

1 Disaster Mitigation Geotechnology 10 Laboratory soil characterisation

2 What is laboratory soil characterisation? Determining soil properties / soil characteristics by means of laboratory tests Field investigation Soil sampling Laboratory characterisation Soil properties / characteristics expressed as soil parameters Modelling Analysis Design Soil characterisation Feedback Construction

3 Soil Properties and Soil Parameters e Yield stress Peak strength Strain softening Swelling index Compression index ln p Strain hardening Critical State? Residual strength e.g. Soil parameter c u (undrained shear strength) distribution Watebe et al. (009)

4 Laboratory tests for soil characterisation - Physical tests - Index tests (PL, LL, PI) - Physical properties (density, unit weight, etc.) - XRD, SEM, MIP, etc. - Mechanical tests Today s topic X-Ray Diffraction Scanning Electron Microscope Mercury Intrusion Porosimetry - Other - Hydraulic tests (permeability, water retention, etc.) - Thermal tests (thermal conductivity, heat capacity, etc.) - Chemical tests (ph, leaching, etc)

5 Mechanical tests Static / pseudo-dynamic tests - Direct shear, simple shear, ring shear ( 直接せん断 = 一面せん断 単純せん断 リングせん断試験 ) - Unconfined compression ( 一軸圧縮試験 ) - Triaxial compression/extension ( 三軸圧縮 / 伸張試験 ) - Hollow cylinder shear ( 中空ねじり試験 ) - Oedometer (1-D compression) ( 圧密試験 ) - Plane strain shear ( 平面ひずみ試験 ) (for research) - True triaxial ( 真の三軸 ) (for research) Dynamic tests - Resonant column ( 共振試験 ) - Bender element ( ベンダーエレメント )

6 Direct shear (box shear) vh v h :? (Potts & Zdravkovic, 001) - Only controls vh and v Constant pressure (for sand) vs Constant volume (for clay) - c, f determined. - Strains cannot be measured. Only displacements. - Significant non-uniformity within a specimen. Sand (Ohshima et al., 1999) Clay (Takada, 1993)

7 Incidentally v h :? vh Not knowing h, Mohr s stress circle cannot be drawn. The c, f values cannot be rigorously determined, unless the vh-plane coincides with the plane of maximum /. 3 =? ( h, vh) =? f from triaxial test f from direct shear test c u from triaxial test ( v, vh) c u from direct shear test 1 =?

8 Ring shear Similar to direct shear: - Only controls vh and v - c, f determined. - Strains cannot be measured. Only displacements. - Significant non-uniformity within a specimen. (Bishop et al., 1971) But, - No displacement limit Residual strength - hv automatically generated.

9 Unconfined compression Applicable only to clay or cemented soils - Undrained shear strength, S u - Young s modulus, E (often E 50 ) qu S u z 1 0.5q u E 0 x y E 50 f (Constant) 3 (Constant)

10 Triaxial compression / extension Applicable to any soils - -DOF(degree of freedom)s each for stress and strain ( v, h, v, h ) all controlled & measured. - Can change confining stress - Requires more skill and time than unconfined compression Load cell Bender element system (also in other side of soil specimen) LVDTs Tie rod Perspex wall Ram Bearing Suction cap Mid-height PWP transducer Radial belt Soil specimen Porous stone Drainage (Global) displacement transducer z 1 (Increase) z 3 (Constant) Ram pressure chamber filled with oil To oil/air interface or CRS-pump x y x y 3 3 (Constant) (Constant) Triaxial compression 1 1 (Increase) (Increase) Triaxial extension

11 Anisotropy on shearing direction Triaxial Extension sue Direct Shear sus su=(suc+sus+sue)/4 su=(suc+sue)/ or su=sus Strength anisotropy sue/suc 0.7 Triaxial Compression suc

12 Deviator stress q (kpa) Deviator stress q (kpa) m (a) Osaka Bay clay (b) 193m 14m 14m m 83m 56m 56m m 56m 83m 83m 14m 14m m m m: ' v0 =344kPa 83m: ' v0 =51kPa 14m: ' v0 =935kPa 193m: ' v0 =130kPa Axial stress (%) Effective mean stress p' (kpa)

13 Hollow cylinder (HC) apparatus Applicable to most soils - 4-DOFs each for stress and strain ( z, r, q, zq, z, r, q, zq ) all controlled & measured. - Can rotate principal stress axes Anisotropy - Cyclic zq loading relevant to earthquake motions - Very difficult to perform! Axial force z Torque qz zq q Outer cell pressure Inner cell pressure (b) 1 r z 3 (a) r q (c) = +b( - )

14 Oedometer (1-D compression test) Applicable to most soils - Confine the specimen in a rigid ring no horizontal strain - 1-D compressibility (consolidation characteristics) is investigated. v (Increase) h K v 0 (Increase) h K v 0 (Increase) (Potts and Zdravkovic, 001)

15 Plane strain shear Only direct shear is commonly adopted in practice. - No normal strain allowed at least in one direction (direct shear is thus a particular type of plane strain shear) - More relevant to -D problems (such as in linear structures than axi-symmetric triaxial tests - A plane strain condition tends to give larger strength parameter values than in triaxial tests Plane-strain extension (PSE) Direct simple shear (DSS) Plane-strain compression (PSC)

16 True triaxial test Limited to research purposes only - 3-DOFs each for stress and strain ( x, y, z, x, y, z ) all controlled & measured. - Influence of intermediate principal stress can be investigated. - Requires very complex design to prevent platen interferences. - Famous study by Lade (1975) No one managed to make true triaxial apparatus that really works Lade came closest to it, but not quite. Anyway, how ingenious he was to use balsa! (by Bruce Menzies) z x y

17 Comparison of stresses & strains There is no all-mighty apparatus combine their use, at most. Stresses Strains x y z xy yz Direct shear Ring shear Unconfined C Triaxial Hollow C Oedometer Plane strain True triaxial : Controlled or measured : Controlled or measured but not independently : Not controlled but measured : Neither controlled or measured 0: Meant to be zero (in theory) zx x y z xy yz zx

18 Elastic wave velocity measurement G tan G sec G 0 Unloading & reloading x u(x) t x u x u G t u 3 x u V t u s G V s x u G t u (Visco-plastic elasticity) (Elasticity) Wave propagation in an elastic medium Wave equation Very small strain amplitude

19 Amplitude of signals in arbitrary units Bender elements Piezo-ceramic elements to measure shear wave velocity Soil Specimen Bender elements Transmitter & Receiver BE hv hh v (or z) h (or r) Input Output TE4: After consolidation f = 9 khz, vh-direction 0 Function Generator Oscilloscope -50 Beginning of signal First arrival t = msec Time [msec]

20 Resonant column Apply excitation with varying frequency, and search for resonance frequency Shear modulus, G or Young s modulus, E (Hight et al., 1997) - Amplitude (strain level) can be changed - Not as portable and versatile as BE. F Active K a F Active C a Passive Active K a F C a (a) Fixed-free (b) Fixed-base-spring top (c) Free-free Various types of resonant column

21 Example of resonant column device incorporated in hollow cylinder apparatus Displacement Transducer Bellofram cylinder Bellofram cylinder Ram Clamp Stepper motor for torsion Sprocket and torque transmission chain Rotary tension cylinder Rotary table Tie rod Hardin oscillator Oscillator Cam Proximity transducers Soil specimen Acrylic chamber wall Specimen Load cell Load cell Foundation Oscilloscope & function generator for RC Outer cell and pore water pressure transducers To foundation

22 Examples of laboratory characterisation scheme Example 1: Ground vibration analysis Soil Base rock Either of cyclic triaxial cyclic HC resonant column G & D (damping ratio) at different strains From cyclic HC (Iwasaki et al., 1978)

23 Example : Long-term (consolidation) settlement in soft clay Along the centre line: Case 1 Case Case 3 Compression with large lateral strain allowed Compression with small lateral strain allowed K 0 -compression (Ohta, 009) Case 1: May involve significant shear deformation and shear failure Long term: Oedometer (k, p c, C c, C s ) + Triaxial tests (c, f) Short term: Unconfined compression or direct shear (S u ) Case 3: No worry for shear failure: Long term settlement only Oedometer (k, p c, C c, C s )

24 Stress 応力比 ratio, / v0 vh ' / v Example 3: Liquefaction resistance R 0 Liquefaction of sand (Ishihara, 1985; reproduced after Iai et al., 1991) 0.1 Dr=50% Dr=70% FLIP K = 0.5 K = 1.0 K = 繰り返し回数 N Number of cycles Undrained cyclic triaxial or Undrained cyclic HC Liquefaction resistance, R 0

25 Laboratory characterisation Summary - No single testing apparatus is all-mighty, unfortunately. - Need to choose and combine an appropriate testing methods according to a problem in interest - Even if a single parameter (such as G and f) is determined from different types of tests, its value may differ, due to anisotropy and intermediate principal stress effects and non-uniformity within a specimen.