Piles under horizontal load into bedrock

[trainguy]

Guys,

I am involved in a free standing structure, essentially an upside down L whose base must resist low vertical loads, horizontal load and moment. The team is heavily vested in mounting each of these onto a single caisson pile (drilled pier, filled with concrete), to an appropriate depth.

In a uniform cohesionless soil, the minimum calculated depth (controlled by deflection) is approx. 9m. (This is about equal to the structure's height above ground). However, the site has bedrock at around a 6m depth.

Is it common practice to neglect the soil in such a case, and design as a cantilever anchored into the bedrock, or are there more refined methods? (FEA with soil "springs" and a fixed base, etc.)

tg

 

[ishvaaag]

Certainly you may profit from the presence of rock and -6 m, and of the presence of the soil above it, except if very weak. In my view, ordinary FEM packages may give structural approximation enough, and only when nonlinear behaviour in the soil is paramount the more complicated geotechnical FEM programs should be of necessary resource.

 

[dcarr82775]

I wouldn't ignore the overburden soils. They are there and provide real resistance to horizontal/moment loads.

 

[SixDegrees]

I agree with dcarr, will be way too conservative to ignore the contribution of 6 m. of soil. Consider using a program like LPILE which will take into account the bedrock,overburden and caisson stiffness. The analysis will give you the deflection and bending moment in the caissons. You could also consider, going with caisson to bedrock and getting additional lateral resistance from increased diameter or a collar.