Shear strength

[killswitchengage]

Hello
When it comes to shear strength of saturated clays, i know for sure that CD tests are rarely performed especially with a Direct shear test .
To my knowledge no DST can compute PWP during shear , and even with a normalized shear rate no one can prove if PWP is always kept nil or negligible .
Can anyone confirm what i just stated ?

 

[GeoPaveTraffic]

No, you are wrong; at least in the US.

Direct shear tests are often ran to determine drianed shear strengths in clays. Based on the rate of consolidation, there are equations that allow you to determine how slowly the sample needs to be sheared to prevent the formation of significant pore water pressure.

As for proving that pore water pressure is negligible, I agree that it would be very difficult to prove it.

Personally I prefer to have consolidated drained triaxial tests ran instead of direct shear tests, but as stated there are still a lot of direct shear tests being performed.

Mike Lambert

 

[fattdad]

yeah, the OP doesn't make sense! CD tests are drained, so there is no pore water pressure to build up during shear. The strain rate for direct shear testing is related to some known value of T50, to minimize the development of excess pore water pressure.

I just can't quite figure the intent of knowing pore water pressure in drained direct shear testing? That said, I do understand the intent of CU-bar testing.

f-d

ípapß gordo ainÆt no madre flaca!

 

[killswitchengage]

thanks geopav and fattdad
Monitoring PWP in a drained test helps us insure that no buildup of excess PWP is generated . If this rule is not respected than we will be computing shear strength in total stresses rather than effective stresses ,because simply water is holding (if you will) the pressure exerted on the soil .