Category:701 Drilled Shafts

Substructure foundations may be designed to transmit loads to foundation strata by concrete columns cast in drilled holes.

This type of foundation is identified in Sec 701 of the Standard Specifications as Drilled Shafts. A drilled shaft is generally small in diameter compared to its depth. The design load may be assumed to be carried in end bearing only. There may be some design that includes side friction but this is not typical. Drilled shafts are to be constructed with either a temporary or permanent steel casing.

The shaft portion of a drilled shaft is usually founded on limestone and occasionally hard shale, with a smaller diameter socket into or limestone or shale. The generally assumed maximum (ultimate) bearing values for the founding material are 10 tons per square foot for firm hard shale and 50 tons per square foot for firm limestone. The inspector should carefully study all general specifications and special provisions pertaining to drilled shafts and become familiar with the designer's intent.

The integrity of the rock socket shall be verified by a foundation inspection hole. This is usually performed after the drilled shaft is drilled. Setting up over a drilled hole can be difficult. The contractor can perform the inspection hole in advance if they submit a procedure that assures the correct location is cored. If the integrity of the cores are questionable the Bridge Division Liaison should be contacted to see if the rock socket length should be extended.

Most problems with drilled shafts occur during the concrete pour. Great care should be made to assure the tremie is not breached and a proper head of concrete is maintained at all times.