Drilled Shaft Side Friction in Gravelly Soils

To evaluate side friction, 28 axial tension (uplift) load tests were performed on drilled shafts in soil profiles ranging from uniform medium sand through well-graded sandy gravel. Typical load–displacement curves for skin friction in gravelly soils were developed. Measured load capacities were compared with capacities computed using equations proposed by Reese and O’Neill, Meyerhof, and Kulhawy. Reasonable agreement between measured and computed capacities was generally found for sandy profiles. However, measured capacities were typically two to four times higher than predicted at sites where the gravel fraction was over 50%. Additional load test data for gravelly soils were collected and combined with the data from Utah load tests. Based on this data set, modifications to the design equations were then developed to predict ultimate side friction capacity better while still maintaining a margin of safety.