LPile, Weak Rock and compressive strength

[structdtl]

I am designing a drilled shaft to bear in a soil layer the geotech recommends modeling in Lpile as "weak rock".

The allowable end bearing pressure given for this layer is 14ksf (= 97 psi) and for input in Lpile the compressive strength= 50 psi, RQD 20% and k rm value = 0.0007.

I run LPile and there are a few warnings:

1. "The input value for k_rm used by the weak rock criteria is greater than 0.0005. This value is outside the recommended range of 0.00005 to 0.005. You should check your input data for accuracy."

Not sure the reason for this warning. Even though 0.0007 is higher than 0.0005, it is still within the recommended range.

2. "An unreasonable input value for compressive strength has been specified for a soil using the weak rock criteria. The input value is less than 100 psi. You should check your input data for correctness"

Is there a specific compressive strength range for this soil layer? some literature available? should there be a correlation between the allowable end bearing pressure given for axial load design with the compressive strength value for input in LPile?

 

[EireChch]

50psi is too low to be rock. BS5930 defines rock as a UCS above 0.6MPa, which is around 90psi so close to 100psi. Not sure what ASTM says.

Why has your geotech recommended modelling it as weak rock! The soil models available should be appropriate

 

[MTNClimber]

I would notify your geotechnical engineer that their parameters are setting off alarms in LPile. It could be that they don't use the software so they're just throwing conservative numbers at you and guessing which p-y curve model to use.

What type of rock are you dealing with?