Drilled Pier or Caisson End Bearing in Claystone

[bigfootstu]

I have run into an interesting situation recently. From some of my research, regional definitions of "claystone" vary - I have seen it referred to as claystone soil and claystone rock. Call me a purest, conservative or whatever you like, but I would say that claystone would be a soft (or weak) rock with a minimum SPT of 50/12". I understand that technically, claystone can be weathered as a residual soil and have seen soils with an SPT of 20 still referred to as claystone, but can't imagine the end bearing of such a material would be close to 20 to 25 ksf.

My problem is this, I need some crude rules of thumb for end bearing i.e. 50/12" (or less) is appr. 20 to 25 ksf, etc. Also some general rules of thumb for visual classifications for claystone, siltstone, mudstone, etc., would be helpful.

 

[epongra2]

Can't help you with the end bearing values, but I will offer some advice on you rock classification.

Claystone is an indurated rock composed primarily of clay-sized particles. It lacks the fissility of shale, and therefore may appear massive.

The term "mudstone" is similar to claystone, and the two are often used synonymously, in practice anyways. Mudstone is also massive (i.e. lacks the fissility of shale) and is composed of either clay or silt sized particles. So, mudstone is used when you don't know whether clay or silt size particles dominate.

Siltstone is simply a massive rock composed of silt-sized particles that, once again, lacks the fissility or fine laminations of shale.

The engineer vs. geologist classification of "soil vs. bedrock" is classic. Highly weathered bedrock at or near the surface is just that, highly weathered rock. The distinction between weathered rock and residual soil is a qualitative judgement call made by a "qualified" person.

 

[BigH]

Nothing to add to your request on definitions of Claystone, etc., but might want to check out the link below on drilled shafts in weak rock. One of the authors was Michael O'Neil.

http://www.dot.state.co.us/Research/2005-3.pdf