Failure Form and Safe Thickness of Pile Karst Roof Considering the Influence of Cracks

In order to study the influence of rock quality on failure form and safe thickness of karst roof, the bearing calculation model of karst roof with rock fracture is established by discrete element software. The mesoscopic mechanism of roof damage fracture failure was revealed and the failure form of karst roof with fractures was clarified. Based on the above, the correction calculation method of safe thickness of karst roof coupling fractures was proposed. The results show that the karst roof of intact rock characterizes typical brittle failure, in which critical displacement for failure is only 60% of more broken rock, and the corresponding failure load is increasing at 67.5% larger. Rock fractures can obviously affect the judgment of the safety redundancy of pile foundations, so the actual project should be fully considered. The karst roof with fracture occurs progressive failure under the pile foundation load and mainly undergoes local compression, micro-fracture closure→new fracture initiation, old fracture expansion→formation of macro-cracks and opening→formation of penetrating failure surface and breaks, rock fragmentation into blocks→development of failure surface, rock fragmentation intensifies→formation of sliding zone, roof overall failure. Because of the complex distribution of rock fracture in karst roof, its spatial failure form is not a regular vertebral platform, nor a rotating body with a specific curve as the busbar, but a spatial irregular failure body derived from fracture. The correction calculation method of roof safe thickness coupling rock fracture and pile load is proposed. Its comparison with the model test results indicates that the correction calculation is reasonable.