The stability of Bet Guvrin caverns-Integrated analysis in weak, anisotropic, and discontinuous chalk

An integrated geotechnical investigation of the ancient bell-shaped caverns at Bet Guvrin National Park is presented. The stability of the caverns is studied using geotechnical mapping, rock testing, back analyses of key-block failures, numerical analyses of stress distributions, block theory limit equilibrium and removability analyses, and monitoring of key-block displacements. The rock is described as an elastic, nonlinear, and transversely isotropic solid. Uniaxial compressive strength values are: σ_c = 5.5-6 MPa normal to bedding, σ_c = 8.5-9 MPa parallel to bedding. Tensile strength is σ_T = 1 MPa and σ_T = 0.05 MPa parallel and normal to bedding, respectively. Four persistent, subvertical joint sets comprise the structure of the rock mass. Two major failure scenarios are analyzed and discussed: (1) Vertical compressive stresses, exceeding the uniaxial strength normal to bedding, are developed at the common sidewalls of neighboring openings. Compressive (shear) failure of common sidewalls leads to formation of a large-span, unsupported opening. Horizontal tensile stresses, exceeding the tensile strength parallel to bedding are developed at the roof and around the newly formed larger opening. Consequently, propagation of vertical tensile fractures may lead to collapse of the roof. (2) Abutment relaxation due to block sliding along pre-existing discontinuities and subsequent reduction of arching stresses leads to unlocking of originally removable key blocks. This process may eventually lead to progressive failure. Monitoring of rock mass displacements over a period of 30 months shows two modes of deformation: reversible, climatically-controlled thermal response of the rock mass; and nonreversible, local and episodic deformations in the rock mass. The nonreversible deformations agree with the proposed modes of failure and imply current instability.