Laterally loaded piles - Driven piles in weathered weak rock

[AnaKa]

Dear geos,

I am currently busy with lateral response of steel tubular piles driven in very weak/weak siltstone/sandstone. In principal the horizontal response of the piles is based on the p-y curves which are developed based on the well know API method. API describes design methods for clay or sand (so or c or φ would play a role).

My initial thought was to model the rock material as a sand, since the upperlimit of cu=1000kPa in case of a clay is not representing my rock material when UCS values get more that 2MPa. I get the strength characteristics of the rock with the Mohr-Coulomb fit, I get a set of c and φ.

But it looks like on upper layers, where the effective stresses are low, the spring stiffness is very low... which puts my approach into serious doubts.

Is there any of you experienced with p-y in very weak/weak siltstone/sandstone? How would you treat a subject like that? Literature gives limited info on drilled shafts under static loading.

 

[VoyageofDiscovery]

It may not be the right approach to rely on the soil in this case.

 

[FixedEarth]

Foundation Analysis by R.F. Scott & Foundation Design by Cernica covers this topic very well. Basically, you have a driven pile with a shear and moment loading, assuming it is in a free head condition. The above two books have the beam on elastic foundation equations by Hetenyi. This analysis simply consists of 3 algebraic equations and solves for shear, moment and deflection with depth. Each equation consists of two terms - influence of shear load and influence of applied moment. You will need to develop your own range for lateral subgrade modulus values & alos obtain the pile's E and I values. Then simply use a spreadsheet and solve for your three equations with depth. You may decide to use about 50 to 60 depth intervals or nodes.

I am puzzled, however, about your last statement on drilled shafts - I thought you have a driven pile? Good luck.

 

[VoyageofDiscovery]

Missed the stone part.

 

[AnaKa]

Dear "FixedEarth", I dont have the books you mention but it looks similar to the winkler model. I will look it up, cause I am not familiar with that but are the springs there elastic?
I am considering a supported beam on non linear springs. The values of these non linear springs are actually my p-y curves. And American Petroleum Institute is suggesting formulas to build-up these springs for steel piles especially in case your soil is sand, or clay. Several software's give the freedom to insert p-y manually, but how should i create my values in case of rock? According to which guidelines?
My question is if any of you knows any literature source/guidelines for that? Regarding your question for the pile.. it is a steel tubular pile driven in weak weathered siltstone, exposed on cyclic loading. I only find references in literature for p-y in sock for drilled shafts.

Dear "VoyageofDiscovery"... I appreciate the thought... although as a geotechnical I am obliged to persist the try of understanding soil better and better!

I know it is a tricky subject... I know that driving piles in weak rock is also a recent hobby (due to the mechanical engineering progress and the bigger hammers) so not much literature is available... I know that most of the engineers treat the horizontal loading in rocks as a sand... but a sand with a small φ at the first 2-3 meters will give very small spring stiffness and I dont think that it is realistic... First meters are VERY critical though for a free cantilever static system like a monopile!!!!