Evaluation of uplift behavior for drilled shaft socketed into rock via centrifuge model tests

Abstract

Rock-socketed drilled shafts are widely used in various geological settings all around the world to transfer the heavy load from the superstructure. Their load-carrying behavior under compressive loading is well-understood. However, a working pile is not always subjected to a compressive load. Some structures, such as transmission towers, jetting structure, submerged platforms, are constructed on pile foundation that have to resist uplift load. Despite the importance of these structures, the behavior of drilled shaft under uplift load is insufficiently understood and design procedures provide little guidance in situation involving uplift loading. Therefore, in this study, research was conducted to investigate the uplift behavior of rock-socketed drilled shaft.

A series of uplift load tests were conducted using KAIST geotechnical centrifuge facility. The artificial model rocks were constructed by mixing dry silica sand, ultra rapid hardening cement (or dry mortar), retarding agent and water. The properties of artificial rocks were verified through various testing such as uniaxial compression test and Free-Free Resonant Column (FFRC) test. The model drilled shafts were made by aluminum tubing with a roughened surface having an equivalent structural stiffness as that of a typical concrete piles. Roughness of socket wall is an important factor in development of shear resistance. The surface of model pile was roughened using a process called knurling, which produce crosshatch texture, the tooth depth of which is controlled by the choice of knurling tool in the machine. The groove depth was designed based on average value of socket wall roughness for Korean domestic drilled shafts. All the model piles were instrumented with closely spaced strain gages. A hydraulic loading system was created to apply large uplift load up to failure condition. The testing program includes model drilled shafts socketed in different rock conditions where their strength of rock, socket depth, embedment ratio are varies.

This thesis presents the experimental results from centrifuge modeling of instrumented uplift load test for drilled shaft socketed into various rock conditions. Evaluation of ultimate uplift capacity from load-displacement behavior observed at the shaft head using appropriate criteria to determine the ultimate capacity, followed by assessment of the effect of intact rock strength, socket depth, and embedment ratio of layered rock on the ultimate capacity. Also axial load distribution in the pile and the skin friction distribution were interpreted using the strain gage measurements. In addition, load-transfer (t-z) curves which describe the rock-pile interactions and load-displacement behavior were investigated.

Based on the information obtained in this study, improved design recommendations for computation of side resistance and load-transfer for rock-socketed drilled shaft under uplift load can be suggested.