Rock-socketing into competent foundation layers is a common practice throughout Texas. Throughout the state competent foundation layer will vary from very hard, intact, non-weathered bedrock; to very soft, friable with poor jointing conditions, and/or extremely fractured “bedrock-like” conditions; to Intermediate Geomaterials (IGMs, as defined in AASHTO Article 10.8.2.2.3) displaying characteristics of both rock and soil. Foundation designer is responsible for determining if (within socket) only side or end resistance can be considered in their determination of total resistance; or in cases of softer competent foundation, they can incorporate part or all of both. When encountering fractured strong rock, or softer cohesive IGMs such as shale and/or severely weathered limestone; note that alternative methods are specified by AASHTO and GEC-10 (2010 and 2018). In stratified or visibly jointed rock bearing layers, it’s difficult to determine how much of actual load will be transferred to base of the drilled shaft and in lieu of load testing at locations, practical design should assume that the axial load will be resisted entirely by side resistance.

When relying on a rock layer for capacity, minimum rock socket to use on any project is 1 diameter length into such rock. Should rock be located near the ground surface, shafts should be drilled at minimum 10 feet or 3 diameters in length (whichever greater).