Best Practices for Pre-Drilling Drilled Shaft Foundations in Karst Geology

Bowling Green, Kentucky, is situated on one of the most well-developed karst landscapes in the world. Karst presents significant challenges to geotechnical engineers when deep foundations are required. When using end-bearing drilled shafts in this region, the bearing layer is typically carbonate limestone. Due to the soluble nature of this material, voids and seams within the rock are common. It is critically important that the bearing surface for the drilled shaft not be immediately underlain by voids or thick seams, which could result in punching failure through a thin layer of rock. To determine if there are voids beneath the bearing surface, a two-to-three-inch diameter test hole is drilled in the bottom of the shaft to a depth of 5 to 10 ft, depending on the design load. This test hole is then probed with a “scratch rod” to check for voids or seems. The design geotechnical engineer will provide acceptance criteria for the number and thickness of seems allowed and at what depth beneath the bearing surface. This process consumes time, delays production (especially when voids are found), and sometimes needs to be repeated several times on a single shaft. Another option for testing the bedrock is to pre-drill each drilled shaft location with an air track drilling rig. An experienced driller working with a field engineer can find even small voids and seams in the bedrock layer and continue to drill until they achieve the appropriate thickness of competent rock to establish the bearing elevation. The exact depth of each anomaly is provided on the digital depth gauge on the drill rig and logged. This can be done in advance of the foundation drillers and can provide them with a pre-determined target depth for each shaft. This paper will present the advantages, disadvantages, and best practices to effectively utilize pre-drilling of drilled shafts to reduce cost by increasing production, improve up-front budgetary estimates to minimize change orders, and improve jobsite safety.