Lateral Pressure in Rocks - Bridge Drilled Shaft Design

[Enjoy learning_not working]

Hello All,

I have a question where I would like all your expert opinions or direction to references that would be helpful.

Background: I am currently working (EIT) for a railroad bridge project and need to analyze the abutment backwalls and drilled shafts. The problem I am having is in regards to a question that I cant find a good answer to.

Q. Do I apply lateral pressures to the abutment Drilled Shafts if it is adjacent to deep layer of bedrock?
Till present through research I found that Rocks do indeed exert high lateral pressures if they are highly fractured, have faults tilting towards the wall, are highly weathered. In this case, I use a similar approach to how we apply soil lateral earth pressures, Ka. However, if the rock is of good quality without significant fractures, the rock can stand in itself and would not exert lateral pressures.

So, with that I will present the current scenario I have. The bridge is for a railroad extension. Draft geotechnical report states that there is bedrock (limestone) approximately 10-12 feet from existing ground with clayey soil in between. The first rock layer is highly weathered but has good RQD (approx. 65-70%) and there is slight to moderately weathered rock profile after that. However, in their draft report, they present the L-Pile design parameters for the upper layers and only present rock parameters such as qu, rock mass parameter, strain factor and RQD for the two rock profiles (200 tsf and 800 tsf). The drilled shaft is 5 feet in diameter and will be rock socketed a min of 1D. The DS will be tipped at the moderately weathered surface.

I have attached few snips to lay out a sketch and a portion of the boring log.

To now analyze the drilled shaft, do I apply lateral pressures due to railroad surcharge load (Cooper E80) all the way down to the cap and the drilled shaft or assume since the Geotech engineer did not include the Phi and c parameters in their draft report, there is no lateral pressures exerted from the rock.

Thanks and appreciate y'all answers.

 

[EireChch]

Assessing extent of surcharges and pressures at depth were developed for use in soils really. I think applying a boussinesq type distribution would be overly conservative.

In reality, i think there would be negligible lateral stress on the pile, but the only way to prove would be to do a 2D plaxis model. Coudl do 3d but probably dont need to.

and just because someone else did something (i.e. not provide parameters in rock) does not discharge you of a need to do something.