The existing formulae of roof bearing layer thickness for punch damage and shear damage are modified by reasonably simplifying the model for bearing layer of karst cave roof. A formula of roof bearing layer thickness under tensile and bending damage conditions is proposed with considering the deadweight of roof. With clear physical significance

the proposed formula can well address the effects of the effective width and the deadweight in one-way slab and two-way slab models

and it is straightforward to be applied. Based on the observed results in practice

the range of parameter variation is determined for various failure modes such as punching

and it is concluded that the net thickness of roof bearing layer is more than 2.5d-3.5d for punching and shearing (where d is pile diameter)

while the total thickness is more than 5.0d-5.5d for bending

so that the design thickness can meet the design requirements. The results can provide reference for design and construction of bridge piles in karst area.