Stability of underground excavation in porous limestone: Influence of water content

Abstract

Four different porous limestone lithotypes were collected from the cellar system of Budapest (Hungary). All lithologies have high porosities ranging from 16 to 30%. The laboratory analyses focused on the mechanical properties testing to assess the strength changes due to water saturation. The density, ultrasonic sound wave propagation, uniaxial compressive strength (UCS) and indirect tensile strength (determined by Brazilian test) test were measured on cylindrical specimens according to EN and ASTM standards in the laboratory. Both dry and water-saturated samples were tested. The results indicate that four lithotypes have distinct strength parameters, and both the UCS and the indirect tensile strength decrease with water saturation. The largest decrease in strength was observed at fine-grained porous limestone and at fine-to medium-grained limestone with a decrease in strength of over 50%. On the contrary, the strength loss of coarse porous limestone is in the order of 16 to 14% of UCS and indirect tensile strength, respectively. Comparing the data set of this study with previous works on various limestones, a good correlation was found between density and Brazilian tensile strength. The presented data set was used as the input parameters for calculating the stability of dry and water inundated cellars. The FEM (Finite Element Methods) calculations of cellar stability indicate that the displacement of water-saturated cellars is nearly triple of the dry ones and that the factor of safety reduced from 1.74 in air-dry condition to 1.07 in water-saturated conditions.