Study On The Roof Stability Of Concealed Karst Cave Under Pile Foundation Loading In Karst Areas

The roof stability of karst cave is an important engineering geology problem in karst areas. Based on the engineering of the concealed karst cave under the pile foundation of a bridge in karst areas of Guizhou province, the roof stability of karst cave is studied synthetically. Safe thickness of karst cave roof that break the norm is dissolved by effective evaluation methods and, the roof stability is monitored during construction process. The main original work and results are as follows:1. The factor that influences the roof stability of karst cave mainly contains two kinds, i.e. the intrinsic factor and the exopathic factor. The exopathic factor is the primary one. Recently, many evaluation methods of the roof stability that tend to be perfect have been presented. However, exploration technique to karst site has become the practicable confinement factor to evaluating roof stability of karst cave.2. Uniaxial compression test and triaxial compression test are performed on carbonatite sample in the region of the engineering site. The test results indicate that the elastoplastic mechanical property is exhibited in carbonatite sample. Then, the rock mechanical parameters are obtained by analyzing the test results with Z.T. Bieniawski's CSIR, Hoek-Brown's and Mohr-Column'rock strength criterion.3. The engineering of roof stability of concealed karst cave is introduced firstly. Then the safe thickness of the karst cave roof under the pile foundation loading, which is greater than 7.5 meter, is ascertained by qualitative evaluating method and half-quantitative evaluating method and finite element method (FEM).4. The vibrating-wire multipoint-displacement metre and its installation technique are improved, and are successfully used to monitor the roof stability of concealed karst cave during construction. The monitoring results indicate that the displacement of karst cave roof accounts for 2/3 of total displacement and the roof is stable under the existing loading.5. The measured results of settlement are compared with those obtained by FEM. It is shown that the latter is greater than the former and they have the same tendency along the test hole depth. Therefore, the reliability of the evaluating method and monitoring technique of the roof stability of karst cave are validated.