Comparative Experimental Study on Bearing Capacity of Roof Beneath Single Piles and Pile Groups in Karst Area

By designing and conducting indoor model tests on single piles and 2 × 2 pile groups under four working conditions—underlying cave, overlying cave, pile penetrating underlying cave, and beaded caves—the failure mode of the roof was explored, and its ultimate bearing capacity was analyzed and compared. Unlike previous studies that focused on single piles in isolation, this paper combines scaled laboratory tests with validated 3D finite element analysis to systematically compare the bearing behaviors of single piles and pile groups under different karst roof conditions. After verifying the model in ABAQUS using the experimental parameters, the researchers established a 3D model of single piles and 2 × 2 pile groups, considering different roof thicknesses, roof spans, cave heights, pile-hole eccentricities, and roof inclination angles. The bearing capacities of single piles and pile groups were analyzed and compared, and the bearing-capacity-improvement factors of pile groups under various working conditions were proposed. The research results show that increasing roof thickness enhances the bearing capacity of both single piles and pile groups, with single piles experiencing more significant improvements. Pile groups boost roof bearing capacity by 455.6% compared to single piles. Conversely, larger roof spans reduce the bearing capacity of both pile types, though pile groups show greater vulnerability to span-related reductions. Notably, increasing pile-hole eccentricity significantly improves the bearing capacity of both configurations. When roof inclination increases, bearing capacity decreases for both systems, with single piles demonstrating greater susceptibility to inclination-induced weakening. These findings reveal distinct mechanical behaviors: pile groups offer stability advantages against span and inclination changes, while single piles benefit more from thickness increases. This divergence highlights the importance of aligning pile configuration with specific structural parameter requirements in engineering design; under different roof sensitivity factors, the bearing capacity improvement of 2 × 2 pile groups relative to single piles is different, and several pile group bearing-capacity-improvement coefficients for five roof sensitivity factors are summarized, and the corresponding and interpolated pile-group bearing-capacity-improvement coefficients are selected when selecting; the influence of the five sensitivity factors is ranked as follows: pile-hole eccentricity > roof thickness > inclination angle > roof span > cave height.