Underground Singapore 2011 5
SHORTCOMINGS OF THE HARDENING SOIL
The HS model is a great improvement over the MC model. However, it has its own set of short comings. Some of them are listed below. a. b. c. d. e. f.
Over-estimation of undrained shear strength, cu of soft clay Under-estimation of pore pressure of soft clay Over-estimation of cu/p ratio for normally consolidated clay Under-estimation of cu/p ratio for over-consolidated clay Over-estimation of undrained shear strength cu under simple shear and triaxial extension condition For stress paths below yield surface, the HS soil becomes elastic and has all the shortcomings associated with an elastic soil.
Figure 22 shows the stress paths of a CU test. The MC model greatly over-predictes the cu. The HS model fares a little better. The over-prediction by the HS model is due to the location of the elliptical cap adopted by the model as shown in Figure 23. The Modified Cam Clay also uses an elliptical yield surface but predicts a much lower cu than the HS model. This problem could be overcome by using a lower friction angle or by setting c = cu and =0. This method works well for excavation analysis in most cases. However, it would not be appropriate for problems involving consolidation analysis and changes in strength with time. Figure 24(a) shows that the HS model over-predicts the cu/p ratio for normally consolidated clay and under-predicts the ratio for over-consolidated clay. This issue may not be crucial for excavation analysis. However, for problems with complex stress paths involving unloading-reloading and changes in effective stress, it may be prudent to scrutinise the results carefully. The HS model also over-predicts cu tested under simple shear or triaxial extension conditions as compared to the cu from real soils as shown in Figure 24(b).
q (kPa)
It is interesting to note that from tests on real soil and simulations on HS soil, the moduli obtained from the simple shear, triaxial extension and triaxial compression tests are different. Test results also show that the strength and modulus obtained under plane strain condition are also different from those obtained from triaxial test. Therefore, it should be reminded that the parameters obtained from the conventional UU, CU, CD and consolidation tests can only be used as a crude approximation.
qD
Mohr-Coulomb Soil Hardening Soil
Real Soil
u at qD = UAB Effective Stress
u at qD = UAC
u at qD = UAD
D
C
B
A
Total Stress
p or p’ (kPa)
Figure 22. Stress paths of a CU test.
Figure 23. Effect of yield surface on cu.
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