Design of anchorage and assessment of the stability of openings in silty, sandy limestone: a case study in Turkey

Abstract This study presents a procedure for the design of anchorage and for the assessment of the stability of openings in silty, sandy limestone through investigating the geotechnical problems of the Hasankeyf historical settlement area in southeastern Turkey, of which a good portion will stay below the reservoir of the proposed Ilisu dam. Rock mass characterization was performed. The RMR and GSI methods were utilized for rock mass classification. The RocLab software was used to determine the shear strength parameters and the geomechanical properties of the rock mass according to the GSI method. The cohesion and angle of internal friction of the rock mass was determined as 257 kPa and 52°, respectively. A back analysis was applied to a structurally controlled planar failure block supporting one of the prominent historical structures in the area to determine the shear strength parameter pairs that satisfy limit equilibrium along the sliding surface of the planar block. A sensitivity analysis of the most likely shear strength pairs satisfying limit equilibrium was performed as a function of reservoir water condition (i.e., rapid drawdown, dry and wet reservoir condition) and the expected earthquake generated peak horizontal ground acceleration coefficient. The results of the back analysis led to a cohesion of 161 kPa and an angle of internal friction of 40° along the failure surface of the planar block. A limit equilibrium sensitivity analysis of the unstable rock block was performed as a function of the water condition of the reservoir, expected earthquake generated peak horizontal ground acceleration coefficient and rock anchor inclination angle. The minimum anchor force required for the stabilization of the planar block was calculated for a rapid drawdown condition as 4000 kN/m. A total of fifty six (56) rock anchors, each with a service load capacity of 1000 kN and spaced at 1 m are recommended for stability. Finite element analysis was utilized to analyze the collapse of some of the roofs of the adjacent man-made cave dwellings carved in rock due to insufficient pillar thicknesses and to determine the minimum pillar thickness required between adjacent caves in the region for stability. Nine different pillar thicknesses ranging from 0.1 to 0.93 m were analyzed. The stages included in the finite element analysis of the caves were: (a) gravity loading due to the thickness of the overburden followed by applying the far field horizontal stresses and inserting the planar block fracture; (b) excavating the caves in stressed rock assuming full face excavation and (c) applying a uniform traction load. The results of the finite element analysis led to a recommendation to apply a steel arch support to one of the adjacent caves located at Hasankeyf in case the wall thickness was less than or equal to 0.5 m.