Hi,
I'm scheduling triaxial tests on silty sand (SM) and while I understand that consolidated drained (CD) test is required to obtain the soil hardening parameters, the test may take a long time due to the fine content in the soil sample. Therefore, I have a few questions:
1. What if I use consolidated undrained (CU) test instead? I know it is not ideal but otherwise how do people come across this long testing time issue? Especially for clay?
2. Is there any guidance on fine content % vs CD test time?
3. Due to the difficulty to obtain quality undisturbed sample for SM, I'm planning to remold the sample to the insitu dry density and natural moisture content, then perform a multistage triaxial test. Again I understand that a multistage triaxial test is not ideal, but I think it is still possible to use it to derive soil hardening parameters. Any thoughts on this approach would be much appreciated.
Thanks!
Dear Teng Wai Vu,
Doing a a CU test is fine for determining Hardening Soil parameters, it only requires a bit more work.The CU test will give you an undrained E50 whereas the HS model requires a drained E50. So the procedure would be as follows:
1) Make a first estimate of E50,drained = 0.7*E50,undrained2) Simulate with this E50, drained a CU test in SoilTest3) Determined E50,undrained from the test simulation4) If this E50,undrained is not equal to the value from your lab test, adjust E50,drained and go to 2) After 3-4 adjustments you should have an E50,drained for the HS model that will produce the E50,undrained as found in your lab test.
I'm not aware of a general guidance of fines content vs test time as that probably depends on grain size distribution and grain shape.
The disadvantage of doing a test on a completely remoulded sample is that you will loose valuable information on for instance overconsolidation, structure (cementation) and anisotropy. There is not much you can do about that, but you should be aware. If determining HS parameters is your goal, doing 2 single stage CU tests at different cell pressures is enough and probably easier to do.
With kind regards,
Dennis Waterman
Thanks very much Dennis,
If we are performing a CU test with pore water pressure measurement, do we still need to follow your procedure? Since the fact that the sample is sheared with excess pore water pressure makes it an undrained behavior, are we only able to obtain undrained E50? Or, does pore water pressure measurement make it possible to derive the drained stiffness directly?
As you mention it's a CU test .... the U stands for Undrained. Whether you measure the excess pore pressures or not does not suddenly make the test drained. It's an undrained test because there is a change of excess pore pressures - hence the result is an undrained E50.