The thickness of karst cave roof at the bottom of socketed pile is a significant influence factor on the vertical bearing capacity of the pile. In practice

related engineering standardizations in China request that the thickness of karst cave roof should not be less than 3 times of the pile diameter. Obviously

this method determining the thickness of karst cave roof based on engineering standardizations is empirical and non-specific without consideration of rock-mass quality and karst cave’s size. In this paper

based on punch failure mode of rock mass around the tip of the pile

we develop an approach to determine the critical thickness-diameter ratio of karst cave roof (

the ratio of karst cave roof’s thickness to pile’s diameter) through generalized Hoek-Brown failure criterion and limit analysis method. In addition

the suggested values of critical thickness-diameter ratio for different classifies quality of engineering rock mass are also supplied. The results show that the ultimate tip resistance

hardness degree of rock and rock-mass quality have significant influence on the critical thickness-diameter ratio. The smaller the ultimate resistance

the better the rockmass quality and the harder the rock

the smaller critical thickness-diameter ratio. Besides

this paper suggests that the critical thickness-diameter ratio for the rock-mass with I~IV quality levels are: for I level

for II level

for III level and for IV level.