Effective strength parameters of clay material 3

[tipsabc]

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.

 

[jdonville]

tipsabc,

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.

http://www.ttlassoc.com

 

[BigH]

Good advice from jdonville - I usually use Su/(0.22 to 0.25) = pc - as a first step. You next need to understand why you need this information. Is it for bearing capacity so you can plug into the equations? If so, it is usually not the bearing capacity that governs (except for very soft to soft clays) and you would use the Su values anyway for design (remember that NC clays are "weaker" in the short term than in the long term). Even for stage loading, etc., where you are taught about how the increase can be effected - not usually done in practice due to the very difficult situation of getting a good estimate of the pore water pressure increases due to the new loading (same with supporting foundations in the short term - which is why Su analyses are used).

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.

 

[GeoPaveTraffic]

tipsabc, as implied by the previous posts, you CANNOT estimate the drained (effective) strength of a soil based on the undrained strength. The two parameters are not related.

BighH makes a good point about why you want the parameters as does jdonville about talking to the original geotech.

 

[BigH]

As a small aside - what is the going rate these days of an undrained triaxial with pore pressure measurements (computer run - vs "man" run). Just interested to know.

 

[jdonville]

BigH,

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.

http://www.ttlassoc.com

 

[BigH]

Jeff - now when you see this cost (and I am sure it is fair - we used to be about $350 back in 1992) - you can see why, when my whole fees, including drilling is only $2000 (and that is a good job) - I can't do it! These are good tests for appropriate jobs - i.e., not "run of the mill" so to speak.