Stability assessment of shallow limestone caves through an empirical approach: application of the stability graph method to the Castañar Cave study site (Spain)

Abstract

The Castañar Cave (central W Spain) was formed within mixed carbonate-siliciclastic rocks of the Neoproterozoic age. The host rock is finely bedded and presents a complex network of folds and fractures, with a prevalent N150E strike. This structure controlled the development and the maze-pattern of the cave, as well as its main water routes. The cave was formed more than 350,000 years ago as a result of the dissolution of interbedded carbonates along with weathering of siliciclastic beds, which also promoted the collapse of the overlying host rock. Currently, it is a vadose cave, but its initial development could have been phreatic. The cave is well known for the outstanding quality of its aragonite speleothems. At present, the cave only receives restricted scientific and educational visits, and therefore it is advisable to perform a preliminary stability assessment through the application of internationally accepted engineering criteria for the evaluation of underground space stability. The aim of this study is to apply engineering rock mass classifications and an empirical approach for tunneling design to the preliminary assessment of karstic caves. The stability of some of the rooms was assessed by the stability graph method, widely used to analyze polyhedral mining rooms, but there are no existing references for its application to karstic caves. The stability of karstic caves can be assessed similarly to man-made excavations, but due to its geological heritage, recognition must always be non destructive. Geotechnical observation points are useful tools fulfill this requisite, and have been applied to the Castañar Cave. The Q index and the stability graph method have both proven to be useful, but due to the polyhedral shape of the cave, the Stability Graph technique has presented more realistic results.