Pile suitable for bouldery, gravelly soil

[bones206]

I’m one of the structural engineers for a project in California, where piles are needed to anchor a concrete “protective structure” subject to large, somewhat dynamic lateral loads on the order of hundreds of kips due to mudslide. I have a soil boring that shows bouldery, gravelly silt conditions, but I’ve been told by the folk who bid the job that there will be no budget for a geotechnical consultant. I have 2 weeks to come up with a design, and I have limited experience with pile supported structures.

Since this is an extremely affluent, densely populated area, I suspect that the noise and vibration of driven piles could open a political can of worms, and I’m sure the bouldery soil would preclude them anyways.

Can anyone recommend a type of pile with high lateral resistance, constructible in these soil conditions, and without high levels of noise and vibration that the local residents will tolerate?

 

[BridgeSmith]

I'll echo the sentiments of others here, that designing slide mitigation structures is a specialized area of practice that should not be attempted by someone who does not have the expertise. I've been designing bridges for 16 years, including numerous foundations, but I would not go near something like what you describe.

If you feel you must come up with something, I'll second Okiryu's suggestion to consider prebored and grouted piles. That, or just regular drilled shafts would solve the noise issue. If you have info on the depth of the slide and the soil properties, you can estimate the soil pressures. I would recommend being very conservative with that estimate. Just as an example, we had one of those consultants who specializes in slope stability and slide mitigation recently use a value of 5 times the passive resistance coefficient, Kp to estimate loads for a drilled shaft group (coupled using a CIP concrete cap) protecting bridge piers in a slide area. If you have enough information to estimate the loads, a program such as Lpile or Allpile can analyze the lateral geotechnical resistance and bending moment.

 

[bones206]

BUGGAR - I'd rather not disclose who I work for. I'd probably be violating some kind of corporate policy and receive a nastygram from someone in the PR department. I work for an EPC firm.

In general, I agree with everyone's advice about not designing a specialized structure without having the requisite specific experience. I was brought on to this job mainly to perform dynamic impact analysis, which I do have a fair amount of experience with. A pile/anchor type had not been selected for the conceptual design, which I needed to include in the model to try to capture the overall force-displacement behavior. Since I was told there would be no geotechnical assistance, I posed my question here in hopes of at least narrowing down the most feasible type of pile.

I had initially looked at driven H-piles, generated p-y curves in L-PILE based on sand (conservative for displacement) and had reasonable looking results in the dynamic analysis. My hope was to take that proof-of-concept analysis and refine it with geotech-provided parameters at a later date, or if that never happened, put huge disclaimers on the results and let my management take it for what it's worth.

 

[Okiryu]

"Since I was told there would be no geotechnical assistance..."

Seismic conditions....mudslide...I think that your company has to solve this issue (not having a geotech on board) before start thinking on the type of piles that may be required for this project...

Just curious to know: what is the reason why you do not have a geotech on board for this geotechnical issue?

 

[bones206]

That was my first question too. I think it was due to the emergent nature of the job. From design kickoff to construction was only a few weeks. I have to believe a geotech will become involved one way or another before this is done playing out.

 

[BUGGAR]

I mentioned pockets of corrosive soils in coastal Southern California. Add this to your caveats, or budget in a cathodic protection system. Caltrans has standard drawings (free) for these for their tie back anchors and similar.