UC Irvine

The determination of internal shaft reactions is critical to designing and assessing the structural performance of deep foundations. At zones of strong geomaterial stiffness contrast, Winkler-spring-type analyses predict pronounced changes in the shear and moment profiles for laterally-loaded foundation elements. In particular, the sudden deamplification of internal moments when transitioning from a soft to stiff layer is accompanied by amplification of shaft shear. This “shear spike” results in dense transverse reinforcement designs and poses severe constructability challenges due to reinforcement congestion, increasing the risk of defective concrete on the outside of the cage. This study presents an experimental research program of three large-scale, instrumented drilled shafts with simulated rock. Each shaft had a different transverse reinforcement design intended to bound the amplitude of the predicted amplified shear demand. State-of-the-art instrumentation and monitoring program was implemented to capture the behavior of the test shafts during loading. As part of the instrumentation program, a new sensor was developed that aimed at measuring concrete internal strains in three dimensions. The instrumentation is novel in that it represents the first attempt to determine experimentally the 3D strain field through embedded sensors with immediate application to a broad array of shaft foundation engineering problems. The three large-scale pile specimens were tested to structural failure and subsequently retested after additional soil was placed to raise the fill height above the rock socket. The originally predicted shear failure did not occur; rather, a flexure-triggered failure through the formation of a plastic hinge above the rock-socket was observed. Test results suggested that the shafts experienced a flexure-dominated failure irrespective of the transverse reinforcement detailing. Retesting with 64% more soil compared to the original soil thickness provided an additional 80% capacity compared to Pult of the damaged specimen. Results obtained from the experimental study were used to calibrate a three-dimensional finite element model in PLAXIS 3D to better understand the load-transfer mechanism within the rock socket. Good agreement was observed between the numerically generated results and experimental data. A selected set of parametric studies was performed to investigate the effects of the interface input parameters (i.e., cohesion, and friction angle) and the influence of shaft/rock relative stiffness on the shaft shear response.