A New Experimental Technique to Investigate the Load-Bearing Mechanisms of Smeared Rock-Socketed Piles Using In Situ X-ray CT Imaging

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.