Effect of Smear Fabric on Rock-Socketed Pile Capacity: 3D Micromechanical Perspective

Abstract

The performance of rock-socketed piles is primarily governed by the micromechanical interactions at the pile–rock interface (PRI), where the presence of a construction-induced weak smear material interface can significantly influence the load-transfer mechanisms. Although PRI behavior has been investigated for decades, a fundamental understanding of smear fabric effects remains limited. Utilizing recent advances in small-scale pile load testing, integrated with three-dimensional X-ray computed tomography imaging, this study investigates the smear effects in piles constructed in soft rocks by examining their location, varying thicknesses, and potential distributions along the interface. The experimental results indicated that increasing the smear thickness on the leading asperity face delayed the peak shaft mobilization by shifting the mechanism from asperity-dominated to smear-dominated. On the trailing face, it accelerated the peak shaft mobilization through loss of confinement and subsequent degradation of leading asperity. When smear occupied both faces, the response was controlled within a thickened smear envelope, restricting effective rock engagement to only the advancing leading asperity. Across all cases, the leading faces were the dominant contributors to shaft resistances. At serviceability conditions, selective cleaning of the shaft limited to only the leading or trailing faces, or with no cleaning, resulted in reductions of shaft resistances approximately 4%, 33%, and 55%, respectively, compared with a clean shaft. Interestingly, smear areal coverage controlled the ultimate shaft resistance even when present as discrete patches, with distinct interface mechanisms emerging based on their distribution along the asperities. Continuous smear patches along the asperities suppressed early shaft mobilization and enabled sustained peak load transfer, whereas discontinuous smear enhanced initial resistance but triggered localized rock shearing at the PRI, ultimately reducing peak capacity. These findings establish the complex mechanisms of representative smear distributions in soft rock sockets and provide a basis for defining the extent of cleaning required for reliable shaft performance.