Abstract:The key problem of karst foundation is not the foundation strength itself but the foundation stability,especially the stability of the free face of rock mass on the pile tip. Though there are semiquantitative analysis and appraisement,and dead load test for deep well,it is found that there are still some theoretical irrationalities in the current geoengineering. The authors believe that if the structure of the rock mass is comparatively intact and the load of the tip of pile is less than the bearing cpacity of the batholith of the rock mass in the pile tip,the free face will be stable. Owing to this idea,there will be many economical benefits for engineering.