Top-Down Construction of a Bridge in Clay Shale

Published case histories of expansive soil/rock materials have historically reported the effects of vertical expansion, and the lessons learned from vertical expansion of soil/rock materials are numerous. In this case the abutment walls of the Las Colinas Bridge were constructed using the increasingly popular, top-down construction method which, while facilitating the construction process, restricts lateral earth movement. As a result of restraining the lateral earth movement, high horizontal stresses in the clay shale were transferred to the bridge resulting in significant structural problems in the bridge deck and guard rails. This is a case study of a bridge built over and contemporaneous to the development of the President George Bush Turnpike (PGBT) northwest of Dallas, Texas. The geologic formation that impacted this bridge is the Eagle Ford Group (Eagle Ford) a highly plastic, residual clay soil grading to clay shale at relatively shallow depths. This paper covers the studies of the in-situ lateral pressures using the Marchetti flat plate dilatometer test (DMT) in conjunction with monitoring soil mass movement using inclinometers. The paper compares its findings with information in the literature from studies and research that have experienced comparable lateral earth pressures.