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How Do You Design A Rock Socket?

How Do You Design A Rock Socket?

How Do You Design A Rock Socket?

A client (a heavy/highway contractor) in Eastern Pennsylvania (U.S.A.) wants to use a (steel) soldier pile and (heavy timber) lagging system to retain (about 10'-0") fill for a temporary roadway embankment.  The soldiers will be set into shale, so they can't be driven.  I've searched rather extensively for design examples, methodology, criteria, etc. in order to design rock sockets, but I've been unsuccessful.  Any input appreciated.  Thanks very much.  WideMike.

RE: How Do You Design A Rock Socket?

The lateral pressure will cause shear and bending at the interface, and friction will add some compression. Also, part of the tip will be likely damaged by the driving procedure. So what said, with the minimum strengths of the shale will mark a limit to both the soldier pile sizing and its embedment, for which I would select a safety factor bigger for the steel than usual, and about 3 for the limit strength of the shale. Shale is directionally anisotropic, hence a good determination of the strength would be very important if you plan to stay around minimum embedment lengths. Since the top of the embedment will be likely somewhat distroyed, at least 3 depths deep shouldn't be an unreasonable minimum embedment, except if unworkable and strong enough. The situation is --from a structural design standpoint -- not much unlike that of cantilever embedments on masonry.

RE: How Do You Design A Rock Socket?

Hello WideMike:

Is the plan to drill into the shale and set the pile into the shale and then concrete to the top of the embedment depth. Check Tomlinson "Pile Design and Construction Practice".

How strong is the shale. Generally, one can drive and embed pile as least one to 2m in depth as the top of shale is normally weathered.

You can also consder driving and use of inclined screw anchors/piles to provide for lateral resistance.


RE: How Do You Design A Rock Socket?


Forget about the above responses; they will not help you very much. To design soldier beams socketed into rock, you can check the earth pressure loading diagrams in AASHTO's Standard Specifications for Highway Bridges. There are diagrams for cantilevered beams in cohesionless and cohesive soils, for mixed soils, and for soils over rock, for both permanent and temporary applications, for soldier beams (discrete elements) and continuous steel sheeting. Make sure you chose the correct diagram. Due to the nature of shale bedrock in Eastern PA and NJ, I will often treat the shale as a stiff clay. Frequently the shale bedrock is very mixed with clay. Some shale is very fractured and drills easily; some drills hard and is more competent.

How high is your cantilevered excavation?  Is there sloping behind the sheeting wall? Are there surcharges to be added? Where is excavation subgrade with respect to the top of the shale bedrock?  If top of rock is below subgrade, then you will also have to consider the pssive resistance of the dirt above the shale. If subgrade is below top of rock, you will have to consider the stability of the exposed rock bench.

Make sure you specify the entire drill hole to be filled with lean concrete or flowable fill after the soldier beam is set.

RE: How Do You Design A Rock Socket?

I would think if the rock was intact with excellent quality (80%+RQD)and the H Pile socket was filled solid with concrete, the encased portion design loads would like perhaps a typical "fixed end" connection. Basically, Uniform Varying Triangular Intensities. The sloping high points would of course would be on the opposite sides from each other. Sum up the forces and moments for equilibrium. I never looked at AASHTO, but I would interested myself as to their encased loading diagram for a rock socket and its pile embedment criteria. GOOD LUCK.

RE: How Do You Design A Rock Socket?

Pells et al (1998), Foundations on sandstones and shale in Sydney region, Australian Geomechanics, Dec., 1998
gives details for "rock socketed" piles.

For lateral capacity analysis, you may try using the
"modulus of subgrade reaction approach", setting
some spring constants for the rock socketed portion.

RE: How Do You Design A Rock Socket?

WideMike:  Greetings from Bangkok.  I suggest you might want to read Dr. Jorj Osterberg's article in July 2000 Geo-Strata Magazine but out by the Geo-Institute.  He also had an erratum in the October 2000 issue.  Interesting read.

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