Effective strength parameters of clay material
Effective strength parameters of clay material
(OP)
Hi, everyone,
How to evaluate the effective strength parameters (phi & c) of clay material from the given undrained shear strength (cu)? Sure, the SPT N values are also available. How to estimate k0 at rest? And any reference books or papers? Thank you.
How to evaluate the effective strength parameters (phi & c) of clay material from the given undrained shear strength (cu)? Sure, the SPT N values are also available. How to estimate k0 at rest? And any reference books or papers? Thank you.





RE: Effective strength parameters of clay material
First off, get your geotech to provide you the values you are looking for.
For total stress (undrained condition) analysis, the undrained friction angle is generally taken to be zero. The unit cohesion is the cu value (hence the "u", for undrained).
For effective stress (drained condition) analysis, the effective cohesion, c', is generaly taken as zero. This approach is purposefully conservative, but keeps the engineer from relying on cohesive forces for long-term strength.
Ideally you would have a relatively undisturbed sample, such as from a Shelby tube, to run a one-dimesional consolidation test and a CU' triaxial test in order to experimentally determine pc and phi'.
If the geotech cannot or will not provide the values, the following ideas may help you out. I strongly recommend that you get an experienced geotech to look over your work and sign off on it before proceeding to construction.
The US Army Corps of Engineers Engineering Manual EM 1110-2-2504 Figure 3-4 shows ranges of effective friction angle vs. the plasticity index for normally consolidated clays. You will need to perform Atterberg limits tests on the soil sample. Based on the information presented in the figure, the soil should have a minimum effective friction angle of 20 degrees, regardless of the PI.
The preconsolidation pressure of a clay can be estimated using the relation
pc = cu / (0.11 + 0.0037 * PI)
presented in NAVFAC 7.1, page 141.
If the current overburden stress, measured to the midpoint depth of your sample, is less than the pc value then your sample may be overconsolidated. As always, use experienced judgement with this approach.
Good Luck!
Jeff
Jeffrey T. Donville, PE
TTL Associates, Inc.
www.ttlassoc.com
RE: Effective strength parameters of clay material
Settlements usually govern and you will not need the effective stress parameters for these analyses.
I find that it is "stimulating" to get the effective stress parameters and the stress paths, etc. but usually it is not necessary for most projects. And, who has the money to spend for undrained triaxials with porepressure measurements? - okay for big jobs but the run of the mill ones - doubt it.
RE: Effective strength parameters of clay material
BighH makes a good point about why you want the parameters as does jdonville about talking to the original geotech.
RE: Effective strength parameters of clay material
RE: Effective strength parameters of clay material
My company currently charges an undiscounted rate of $785 per CU' triaxial test. We currently do not electronically instrument our cells. (We will naturally "sharpen our pencil" and discount the price somewhat if the test is included in a more comprehensive investigation program.)
Jeff
Jeffrey T. Donville, PE
TTL Associates, Inc.
www.ttlassoc.com
RE: Effective strength parameters of clay material