Undrained Shear Strength 1

[SCEIT]

What are some common undrained shear strengths (in psf) for over-consolidated clay till soils, with varying N60 values 20-70 BPF?

 

[dgillette]

Look in Terzaghi and Peck and NAVFAC DM-7 for correlations referenced to unconfined compression test.

For clay till, Hegedus E. and J. Peterson "Penetration Resistance and Shear Strength of Cohesive Soils" in Proc 1st Int. Symp. on Penetration Testing, Orlando, 1988.

In 1960, USBR published a correlation referenced to field vane; as I recall, it predicts higher strength than the others do. The report is pretty obscure. Most of their results, in psi, were between [2 + 0.95 N] and [6.5 + N], with N up to 16. No depth or energy adjustments were applied to N. I don't know what type of hammer they used at that time, probably donut. Much of the testing was done at a cemetery here in town (not filled up with bodies at that time) by an old friend of mine, who is now buried there. I suppose that's appropriate. :-(

 

[BigH]

Don't think that a correlation using a vane would be correct for overconsolidated till. Am assuming it is a clayey till rather than a sandy till. We used to use in the Toronto area tills a rule of thumb correlation of that each "N = 8 to 10", an allowable bearing pressure would be 1 tsf. So if you had N=30, qallowable would be 3 to 3.75 tsf (300 to 375 kPa). Using 6xSu/3 = allowable bearing, Su approx 1/2 of allowable bearing; so 150 to 175 or so.

 

[SCEIT]

How does unconfined compressive strength relate to undrained shear strength?

 

[dgillette]

Ooh! Time to back up to Strength of Materials class. Draw a Mohr circle with compressive strength qu on one side, and air (i.e., zero normal stress, zero shear stress) on the other. Then remember that the failure surface in frictional material is, in theory, inclined (45 deg + phi'/2) away from vertical, rather than 45 degrees. In theory, shear strength = [(qu/2)*cos(phi')], usually simplified as [qu/2] with no real loss of accuracy; in practice, there is sample disturbance, anisotropy, etc. that overshadow that. In the Hegedus and Peterson correlation, the data were pretty scattered of course, and I assume the same is true for the ones shown in DM-7, although I haven't actually seen the dots. Don't take the numbers too seriously.

Great Lakes area? I think H&P's data came from along south shore of Lk. Erie. (The paper shows H&P as being from Cleveland.)

 

[geohec]

You could make an estimate of undrained sher strength using Su=0.05*N-value (tsf).

I usually reduce the value even further by about 2/3.

Good luck!