Effect of Pressurized Drilling Methods on the Lateral Load Capacity of Drilled Shafts - A Practical Insight and an Analytical Approach-Article

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The paper examines how pressurized drilling methods affect the soil around drilled shafts and, consequently, their lateral load capacity. It combines observations from two Manhattan construction projects, lateral-load testing and finite-element analysis.<br /><br />In both case studies, 50-cm-diameter steel casings were advanced through granular soils to bedrock. High-pressure water mixed with polymer disturbed and weakened soil near the casing, creating gaps that extended about 2.5 m—roughly five shaft diameters. High-pressure air caused less disturbance immediately around the casing, but could weaken soil farther away through piping. In one project, the observed disturbance prompted shafts near an existing building to be relocated to protect its shallow foundations.<br /><br />Tests on larger shafts installed using augers showed a mismatch between measured behavior and models that assumed continuous soil support. Analyses that removed soil support around the shaft better matched the test results, suggesting installation-related gaps or weakened soil had reduced lateral resistance. Further modeling found that even a relatively narrow weakened zone could substantially increase shaft displacement, especially when its soil stiffness was very low.<br /><br />The author proposes accounting for this effect through a “free-length” in lateral design: assume the soil near the ground surface provides no lateral support over a specified depth, and begin crediting soil resistance below that depth. For pressure-drilled shafts up to 50 cm in granular soils, the paper recommends a free length of five shaft diameters. This is a practical design suggestion, not a universal rule; the appropriate value depends on soil conditions, shaft size and installation method. The paper emphasizes that engineers should design for the as-built foundation, recognizing that construction can significantly alter the soil–shaft interaction.

Keywords

pressurized drilling

drilled shafts

lateral load capacity

soil disturbance

granular soils

finite-element analysis

lateral load testing

free length design

shaft installation

soil-structure interaction