Pilot Holes for Drilled Pier Foundations in Karst Conditions

Abstract

Development on land underlain by limestone formations that exhibit karst conditions can present unique engineering and construction challenges. These challenges often include concerns regarding questionable subgrade for foundation support and economical methods for foundation construction. Over the past decade, many structures have been constructed in the Central Texas area over limestone formations exhibiting karst features such as voids and solution cavities, as well as soil seams and layers. The most common geologic formation to exhibit such properties in Central Texas is the Edwards Group limestone. Significant foundation design challenges may be encountered for structures constructed over such formations, particularly larger structures with significant column loads. Many such structures often require a pilot hole program for evaluation of the bearing stratum below each foundation unit. Through multiple projects in the Central Texas area, the authors have compiled experience with three major categories of pilot hole programs, including 1) the use of geotechnical drilling rigs, 2) air percussion drilling equipment, and 3) drilling pilot holes during construction with pier drilling equipment. This paper discusses the benefits and drawbacks of utilizing each of these three different pilot hole methods, and presents several case histories. The authors conclude that the use of pilot holes is an effective means of evaluating subsurface conditions at individual foundation locations in situations where other means of site evaluation cannot adequately characterize subsurface variations. The authors caution that construction documents and cost estimates should consider the probability of foundation modifications due to karst features which will be identified in the pilot hole program.