工程科学与技术 (Mar 2025)
Study on the Effect of Rock Mass Wetting and Drying Cycle on the Mechanical Behavior of Rock-socketed Piles on Slopes
Abstract
Objective This study addresses the critical issue of mechanical property degradation in rock-socketed piles under cyclic wetting-drying conditions during reservoir operation. The research aims to quantitatively evaluate the weakening mechanisms of sandy shale and systematically reveal the evolution characteristics of pile bearing behavior under hydraulic cycling effects.Methods Cyclic wetting-drying tests were conducted on sandy shale specimens from the Luogu Bridge slope in Lianghekou reservoir. The generalized Hoek–Brown criterion was applied to characterize rock mass degradation, while finite difference numerical modeling incorporating Mohr-Coulomb constitutive relationships simulated pile behavior. Microstructural analysis using scanning electron microscopy (SEM) and X-ray diffraction (XRD) revealed mineralogical alterations. A computational model was established considering reservoir water level fluctuations between 2785 m (dead water level) and 2865 m (storage level), with 35 m-long rock-socketed piles (Pile 1-1 and 1-2) analyzed under 100 wetting and drying cycles.Results and Discussions The findings reveal that after 28 wetting-drying cycles, the uniaxial compressive strength, Young’s modulus, cohesion, and internal friction angle of the rock gradually decrease by 20.82%, 25.97%, 14.83%, and 28.67%, respectively. Simultaneously, Poisson’s ratio gradually increases, with a degree of deterioration of 44.13%. The mechanical parameters demonstrate strong correlations with the number of cycles through power and logarithmic functions. Microstructural analysis of the rocks indicates that after 28 cycles, hydration-prone minerals decrease, with mica content reducing from 35.1% to 27.5%, chlorite content from 11.4% to 8.4%, and anorthose content from 25.3% to 14.5%. Scanning electron microscope results highlight significant damage to the rock specimens’ microstructure after 28 cycles, including an increase in cracks and pores and an expansion of the dissolution area. The loss of hydration-prone minerals and the accumulation of microscopic cracks are the primary causes of the weakening of the rock’s mechanical indices during the wetting and drying cycles. The computational model for the Luogu Bridge slope rock-socketed pile is developed using finite difference methodology, incorporating the Mohr-Coulomb constitutive model for the numerical simulation of the rock mass. The design dead water level for the slope of the rock-socketed pile is established at 2785 m, with the storage water level set to 2865 m. The rock mass within this elevation range is identified as the zone affected by wetting and drying cycles. The rock-socketed pile, extending to a length of 35 m, includes one beneath the bearing platform of the bridge abutment, designated as Pile 1-1, and another near the river valley direction, named Pile 1-2. This study simulates the mechanical behavior of rock-socketed Piles 1-1 and 1-2 after 100 cycles of wetting and drying. The computational outcomes suggested that wetting and drying cycles significantly impact the mechanical behavior of rock-socketed piles. After a certain number of cycles, the axial force of Pile 1-1 initially increases with depth, then gradually decreases. In contrast, the axial force of Pile 1-2 first decreases with depth, then gradually increases, and finally trends downward again. As the number of wetting and drying cycles increases, the axial force at the same depth of the rock-socketed pile progressively rises, with a more noticeable increase in the central region of the pile. The increments at the top and bottom of the pile are comparatively modest. The variation curve of lateral frictional resistance with depth reveals distinct patterns for two piles. The curve for Pile 1-1 displays a “single peak” pattern, where the resistance stabilizes after reaching a burial depth of 25 m. In contrast, the curve for Pile 1-2 follows an “R” shape distribution, with the resistance peaking at a depth of 15 m, then continuously decreasing, and finally increasing towards the end of the pile. This phenomenon may be attributed to the bending moment exerted by the bearing platform above the pile. The attenuation of shear strength in the rock mass surrounding the rock-socketed pile during wetting and drying cycles significantly reduces friction between the pile and the rock material. As a result, the total lateral frictional resistance of the rock-socketed pile under bridge loading tends to decrease with an increasing number of wetting and drying cycles, leading to a similar decrease in the lateral frictional resistance of the pile foundation at the same depth. Deformation measurements after varying numbers of cycles indicate that the horizontal displacement at each depth and the settlement at the top and bottom of the pile exhibit an increasing trend with more cycles. For rock-socketed Pile 1-1, the horizontal displacement is larger after 100 wetting and drying cycles within the depth range of 5～25 m, with the maximum horizontal displacement increasing from 0.316 to 1.029 mm, whereas the horizontal displacements at the top and bottom of the pile are relatively smaller. For rock-socketed Piles 1-2, the horizontal displacements at the top and bottom change significantly under wetting and drying cycles, while changes in the middle are relatively minor. After 100 cycles, the displacement at the top of the piles is 0.656 mm, and at the bottom, it is 0.518 mm.Conclusions The study demonstrates that wetting and drying cycles significantly impact the mechanical behavior of rock-socketed piles, with both axial force and lateral frictional resistance decreasing over time. The accumulation of microcracks and loss of hydration-prone minerals in the surrounding rock are the primary causes of the observed weakening. These findings highlight the necessity of considering the effects of wetting and drying cycles when designing and constructing rock-socketed piles in reservoir embankments, in order to prevent potential reductions in load-bearing capacity.
