Drilled Piles used as Rock Supports for Sheet Piles - An investigation into the structural behavior and gap effects of drilled piles, with a comparative analysis of rock dowel design approaches

Abstract

Drilled piles are used as rock supports for deep excavations where passive resis tance is insufficient due to proximity to bedrock. Conventional methods primarily include the use of rock dowels; however, there is growing industry interest in adapting drilled piles as a primary alternative rather than just a backup. Avail able design methods in standards and guidelines do not adequately address the forces that occur in the bedrock, especially when gaps exist between the sheet pile wall and the bedrock due to uneven levels or obstacles that hinder the sheet pile. This thesis aims to investigates how varying horizontal and vertical gaps between the sheet pile and bedrock affect the structural behavior of drilled piles acting as supports. Numerical simulations in PLAXIS 2D were performed using data from the Sahlgrenska Life project, and interviews were conducted to provide further insight into the installation processes. The results show that the horizontal distance, b2, between the sheet pile and drilled pile contributes to minimal changes in forces within the two structures. For the vertical gap, b1, between the toe of the sheet pile and the bedrock, the drilled pile exhibits an increase in shear force as the gap increases, while the bending moment remains relatively unaffected. However, this applies only to cases where redistributed net earth pressure remains low. For cases with higher redistributed earth pressure, the bending moment increases with a gradient of approximately 3·b1, whereas the shear force remains relatively unchanged. This indicates that the conventional design method for rock dowels does not show the same correlation between design shear force Qed and design bending moment Med. Variations in toe modeling were also examined, concluding that modeling the drilled pile as close to realistic conditions as possible resulted in unrealistic shear forces within the drilled pile. This is assumed to be caused by numerical point loading effects due to the significant stiffness contrast between the soil layer and the bedrock.