Category:701 Drilled Shafts

Substructure foundations may be designed to transmit loads to foundation strata by concrete columns cast in drilled holes. See EPG 751.37 Drilled Shafts for design guidance and additional information.

This type of foundation is identified in Sec 701 of the Standard Specifications as Drilled Shafts. A drilled shaft is generally considered a deep foundation. Drilled shafts are to be constructed with or without a casing. When casing is used it can be either a temporary or permanent steel casing. Requirements for plan reporting of steel casing are given in EPG 751.37.1.3 Casing.

The shaft portion of a drilled shaft is usually founded on rock (limestone, dolomite or other suitable material with q_u ≥ 100 ksf) or weak rock (shale or other suitable material with 5 ksf ≤ q_u ≤ 100 ksf) with a smaller diameter rock socket drilled into same. 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 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 should be contacted to see if the rock socket length should be extended.

Most problems with drilled shafts occur during the concrete pour. The concrete placement requirements in Sec 701 should be reviewed carefully.