k_rm values for l-pile analysis of weak rock

[rwjackson]

In the absence of axial strain measurements, is there a reasonable relationship between the k_rm parameter required for L-pile analysis of weak rock and other known parameters such as unconfined strength and RQD?

 

[TDAA]

The L-Pile software recommends:

Parameter k rm for Weak Rock
The parameter k rm typically ranges between 0.0005 and 0.00005.

What sort of weak rock are you working with?

 

[rwjackson]

Thanks very much for your response. I'm familiar with the range you mentioned. I'm trying to pin it down a little further. The rock is predominantly limestone and dolomite with unconfined strength in the range 200 to about 7000 psi. RQD is typically in the range 0 to 50. We have one site underlain by a metagabbro with strength of about 200 psi and RQD of 0.

 

[jdonville]

rwjackson,

The LPILE Technical Manual suggests that you should consult Reese's paper "Analysis of Piles in Weak Rock" presented in the November 1997 issue of the ASCE Journal of Geotechnical and Geoenvironmental Engineering. This paper is available from ASCE ($) here:

http://ascelibrary.aip.org/getabs/s...00123000011001010000001&idtype=cvips&gifs=Yes

Jeff

 

[rwjackson]

jdonville,

Thanks. I have Reese's paper. However, he presents two case studies, one of which involves a limestone with krm=0.0005, and the other an apparently similar limestone with krm=0.00005. I assume that's where the range in LPILE originates. Any feel for how you would decide where in that range your material lies?

 

[jdonville]

rwjackson,

Darn. I haven't knowingly read the paper, but hoped that it would have shed some light on this topic. Perhaps you can query Ensoft. They have been helpful with me before with support for LPILE.

However (proceed at your own risk here, as I'm basically looking at this for the first time), based on the text presented in the Tech. Manual, the p-y curve for weak rock is based on a wedge failure (page 3-84). I will posit that this failure wedge is similar to that developed for clay (page 3-15).

From Eq. 3.89, Pur is a function of the diameter of the element, the compressive strength of the rock, a strength reduction factor based on RQD, and the depth of the layer below the rock surface.

K_ir and k_ir are both defined by the depth below the rock surface and the element diameter (Eqs. 3.91, 3.92, 3.93).

It seems likely that k_rm (within the range previously stated) is simply a fudge factor that controls the curvature of the p-y curve between the initial "elastic" portion and the limiting value of Pur.

You could construct the p-y curve based on your data and the equations in the manual in a spreadsheet and plot the resulting curve on the screen. Vary the k_rm value and see what effect it has on the curve.

Hope this was helpful,

Jeff

 

[adtgeo]

My experience with the k_rm (and the e50 in the clay models, for that matter) is that they do not affect the results very much. Typically, I find that changing the k_rm value affects the result far less than other uncertainties within the analysis. Hence, I often use the default values selected by the program, unless there is an overriding reason not to.

Try running your analysis with both the minimum and maximum "typical" k_rm values. The difference in the results are probably minimal.