Axial load transfer in large scale physical model drilled shaft foundations

Abstract

Drilled shaft foundations are a widely implemented type of deep foundation used to support heavy loads from superstructures such as buildings and bridges. Uncertainties exist in the axial load transfer behavior of drilled shafts subject to loading and unloading cycles. A better understanding of the changes in soil reload stiffness can lead to a better understanding of the load deformation response. A study was undertaken using a 16-inch diameter by 28-inch tall physical model drilled shaft foundation. A total of nine static load tests were performed in a five-foot diameter by five-foot deep calibration chamber where soil conditions could be controlled. Soils tested were a loose clean sand, dense clean sand and compacted silty sand. A bladder system was used to apply vertical effective stress to the soil to simulate depths of embedment up to 70 feet below the ground surface. Based on test results, a relationship between the reload stiffness and displacement was developed. The relationship indicates that the reload soil stiffness is initially large before decreasing to a constant value as displacement increases depending on soil configuration. Results from three different soil configurations were normalized based on the constant behavior observed at large displacements. A relationship between the normalized reload stiffness versus displacement was created. This relationship can be used to predict the reload stiffness for model drilled shaft foundations in coarse grained soils.