No abstract

Rock-socket pile design predominantly depends on the shaft resistance to support the load at the serviceability state. However, due to limited understanding of the pile–rock interactions, the pile capacity is normally calculated using empirical correlations. In this study, the load-bearing mechanisms of rock-socketed piles were investigated through a miniaturised pile–load test setup in a soft synthetic rock. X-ray CT imaging and numerical discrete element modelling were used to investigate the micro-mechanics influencing the load-bearing mechanisms at the pile–rock interface. The numerical pile model was established based on suitable constitutive models capable of simulating the soft rock behaviour. The analysis of X-ray CT images at various displacements revealed three different interface mechanisms, namely sliding, local shearing and progressive shearing. The numerical model validated this observed micro-mechanics in the rock asperities through the evolution of damage and micro-cracks. Insights from the experimental and numerical results indicated that the height of the rock asperities significantly dictates the failure mode. Results also illustrated that the shaft load–displacement response primarily depends on the forces acting on the leading edges of the pile asperity. In particular, it was observed that the bottom leading edge carried a predominant portion of the shaft loads due to its connectivity with the rock at the base of the pile. Though negligible, the forces on the trailing edges provided valuable information on the contribution of residual shaft resistance by the debris at the interface. Moreover, the numerical studies revealed the different failure modes at the pile–rock interface. The discussions presented in this study provide novel insights into the load-bearing mechanisms of piles socketed in soft rocks, which will help to improve design guidelines in the future.

The design of rock-socketed piles at the serviceability state primarily depends on the shaft resistance (skin friction) at the interface, which may be influenced by the presence of weak smear (e.g., bentonite filter cake, soil, and remolded weak rock). To date, two-dimensional direct shear interface testing has been utilized to interpret the effect of smear on the shaft response of bored piles. However, without the nondestructive testing and three-dimensional (3D) visualization of smeared interfaces, the effect of smear at the pile-rock interface cannot be accurately assessed. This study presents a novel methodology to cast and test small-scale rock-socketed piles with 3D smeared pile-rock interfaces using in situ X-ray computed tomography (CT) imaging. A new smear casting apparatus has been designed to incorporate desired smear fabrics distribution around the model piles comprising idealized saw-tooth asperities. Different materials were trialed to represent the smeared interfaces and the most suitable one for this study was chosen to be a mixture of petroleum jelly and kaolin through the evaluation of the load-displacement behavior and the X-ray CT images. Various smear configurations were cast on the leading faces of the pile asperities (smear-dominant, balanced rock-smear, and rock-dominant) and their placement and volume were compared with the design values to provide confidence in the proposed experimental methodology. Moreover, the micromechanics evolving at the fully smeared leading-faced interface were assessed in detail using the vertical load-displacement behavior and the corresponding X-ray CT images acquired during multi-stage in situ loading. Based on the observations, the interface mechanics for the smeared shafts were classified into smear compression, subsequent smear compression with shearing, and rock shearing. The proposed experimental methodology opens new avenues for studying the smear fabric effect on the load-bearing mechanisms of smeared piles socketed in soft rock.