Rapid Characterization of Unsaturated Soils using Simple Equipment. Xiong Zhang, Ph.D., P.E. August 12, 2016

Transcription

1 Rapid Characterization of Unsaturated Soils using Simple Equipment Xiong Zhang, Ph.D., P.E. August 12, 2016

2 Outline Introduction Limitations of suction-controlled triaxial test Novel methods for soil characterization and modelling Modified state surface approach (MSSA) Optical method for volume measurement High suction tensiometer for suction measurement Undrained tests and data analysis Conclusions 2

3 GCTS USTX-2000 for Unsaturated Soils Independent control: s1, s3, ua, and uw Volume measurements: soil skeleton, water and/or air $100,000-$150,000 3

4 Divide-and-Conquer Approach When a process is influenced by several parameters, to examine the influence of each individual parameter, one has to conduct tests which involve keeping all the influencing parameters constant except for the target parameter. Varying p Varying s Varying q (1-3) months/test (20-30) tests = 2-5 years/ soil 4

5 Historical Development of Unsaturated Soil Mechanics Bishop (1959), Bishop and Donald (1961) Jennings and Burland (1962) Matyas and Radhakrishna (1968) Fredlund and Morgenstern (1976, 1977) Alonso et al. (1990) Fredlund and Rahardjo (1993) Vaunat et al. (2000), Khalili et al. (2004), Nuth and Laloui (2008), Lu and Likos (2004), Wheeler et al Sheng et al. (2008), Alonso et al. (2010), Zhang (2015) 5

6 Barcelona Basic Model Developed by Alonso, Gens, and Josa in 1990 The first elasto-plastic model for unsaturated soils It is later called the Barcelona Basic Model (BBM) The very most cited paper (about 1,700 times) in unsaturated soil community Sheng et al. (2004): All existing models are variants of the BBM 6

7 Suction-Controlled Triaxial Tests Advantages Designed upon Divide-and-Conquer approach Stress path can be controlled: p, q, ua, and uw Results easy to analyze Disadvantages Require sophisticated equipment ($60-120K) Time-consuming (3-5 years/soil) Results theoretically incorrect* *Zhang, X. (2015). Limitations of the suction controlled tests in the characterization of constitutive behavior of unsaturated soils. International Journal for Numerical and Analytical Methods in Geomechanics (DOI: /nag.2401). 7

8 Failure of Divide-and-Conquer Approach What is your record? God s number = 20 World s record= 5.25s 8

9 Suction-Controlled Isotropic Compression Test s yield curve LC 1 Idealized stress path G a b c H d e f I e s G a b c m yield point H d 4 d n e Idealized results * p 0 m n r s Actual stress path 7 p Actual results 5 e r f 6 f s I 7 I p Stress Path Test Results 9

10 Step vs. Continuous Loadings H (m) m Impermeable umax C V = m 2 /s Drainable Step p (kpa) 50 Step q c = 50 kpa umax 0 Continuous 100 t (hour) Continuous 10

11 Drained and Undrained Loadings e s * p 0 A A D Constant w Loading Constant w Unloading LC 1 LC 2 * ( p 0 ) 1 B B J C p e E k s A k F D B C H s J J I l(s 3 ) D C O G l(s 1 ) ln p p 11

12 Multiple Undrained Tests with Different w v A D G E H s B V Test 1 X C Test 2 l(s 3 ) F Test 3 l(s 2 ) l(s 1 ) I * p 0 * p 1 * p 2 ln p 12

13 An Optical Method to Measure Volume 13

14 Principle of Photogrammetry-Based Method Frame Rod Water Specimen Acrylic Cell 14

15 Shape Changes in Soil Specimen 15

16 Development of Shear Band 16

17 High Suction Tensiometer EPXO Pressure Transducer Epoxy Ceramic Disc 0.2 mm Water Reservoir Epoxy Diaphram Housing Housing Ceramic Disc Stainless Steel Ring EPXO Pressure Transducer Cables 17

18 Suction (kpa) Reaction Time and Maximum Suction Reaction time <2 seconds Maximum Suction =1100 kpa Tensiometer Tensiometer Time (s) 18

19 Undrained Test Setup <$1,400 <$1,100 19

20 Undrained Test Results v A D G E H s B Test 1 V X Measured Soil Responses Measured Stress Paths * p 0 * p 1 C * p 2 Test 2 F Test 3 l(s 1 ) I l(s 2 ) l(s 3 ) ln p 20

21 Calibration of Model Parameters n ( ) ( ˆ ) F X w v v i 1 i i i 2 n si p at pi wi vi N ( 0) ks ln l( si ) ln c i 1 pat p n si p at pi wi vi N ( 0) ks ln l ( 0) ( 1 r) exp( si ) r ln c i 1 p at p

22 Yield Curve for Compacted Fairbanks Silt 22

23 Experimental vs. Predicted Test Results 23

24 Experimental vs. Predicted Test Results 24

25 Conclusions Suction-controlled triaxial tests cannot produce correct results needed for the constitutive modeling MSSA is proposed to study unsaturated soil behavior Undrained triaxial tests used for soil characterization to get correct results Optical method developed for volume measurements High suction tensiometer for direct suction measurements Shape of the LC yield curves in the BBM verified 25

26 Conclusions The proposed method is: More cost-effective ($2,500 vs. $60-120K) More efficient (4-5 hours vs. 1-3 months/test) Theoretically correct Easy to understand and use 26

27 THANK YOU27