Drilled shafts in limestone 3

[SLMPicker]

What is a good resource for design and analysis of drilled shafts in limestone? Or any type of foundation in limestone, for that matter. A little background - I've been designing foundations (among outher things) for 2 years, and have just moved from Louisiana to Texas, and limestone is a material with which I've not dealt. Any help would be appreciated. Thanks...

 

[dik]

I've used caissons for end bearing on limestone as well as socketed caissons; the socketed caissons use the skin friction between the concrete and the rock. Depending on the weathered surface a portion of the socket may be discounted. It's been so long since I've used socketed caissons, that I don't recollect the friction value used.

The installation I'm familiar with is a matter of drilling the main shaft to the limestone bedrock and then drilling a smaller shaft, approx 6" smaller diameter to provide a bit of a shoulder. The smaller shaft is drilled to the depth required to provide the necessary surface area.

Sometimes the end bearing is used in conjunction with the friction, but best to check with a local geotekkie to get an opinion due to the differences in moduli for shear and compression.

Also sometime a small 2" drill is used to determine if there is soundness in the limestone to a suitable depth. Again local practice could dictate this.

 

[KAM_imported]

The best document by far is FHWA Publication No. IF-99-025, Drilled Shafts: Constuction Procedures and Design Methods by Mike O'Neill and Lymon Reese (two volumes). It can be obtained thru the ADSC by calling 214-343-2091, or check their website at

http://www.adsc-iafd.com.

It addresses design of shafts in sands, clays, weak rocks, hard rocks, and intermediate geomaterials.

 

[Qshake]

I agree with KAM on the publication. However, I have these caveats on the matter. Here in the Midwest (a little above Texas) Drilled Shafts are not really very common although we're seeing more of them these days. I think this is mainly due to the amount of construction going on near existing facilities which require a non-intrusive method of constructing a foundation. On the other hand, another main reasons is cost. Drilled shafts are expensive and rock sockets are even more expensive. Yet another is qualified contractors. A big vicious circle - more shafts, more contractors ultimately lead to lower cost.

I have worked in Texas a little but I am aware that just about everything (in the bridge business) down there is drilled shafts. This probably keeps the cost down.

 

[veer]

I think in limestones you have to be careful abaout cavities

 

[Rjakiel]

Small diameter grouted piles (mini-piles, micro-piles) can be used in limestone and can be reinforced to achieve relatively high capacities. FHWA also has a manual describing the design procedures for these piles, including bond capacities for the sockets in various types of rock.