pile lateral capacities in limestone

[struct_eeyore]

Hi all,

Recently got a geotech report back that gives me an allowable lateral pile capacity of 1 ton. What is surprising to me, is that we have limestone bedrock 2-3' below ground surface, and we will be augering piles at least 10' into this bedrock. The SPT yields 50 blows per 4". I've limited geotech experience, I anticipated a much higher number.

 

[BridgeSmith]

It depends on how fractured the limestone is. You might ask the geotech if taking more core samples could help. With a small number of samples to work with, poor recovery (of rock they can do compression tests on) on a few cores will significantly reduce the reported strength, due to the large uncertainties involved.

That said, 50 blows for 4" (150 bpf) is really low for rock, indicating it may be heavily weathered, which would not be surprising with it that close to the surface. You may have something that more resembles crusher run base than intact rock.

 

[Okiryu]

I guess that your structure is very heavy to need piles even if you have bedrock at that such as shallow depths. Why shallow foundations don’t work in your design?

Also, if piles are really needed, are you socketing the piles with grout after predrilling? I am assuming that piles are precast concrete or steel piles and not drilled shafts. Lastly, What is the size of your pile foundation?