Roughness indices determination for fracture surfaces in Luxemburg Sandstone

This study is a computation of fracture roughness indices for a series of subvertical joints and faults in 'Luxemburg Sandstone' rock mass within a well-identified geological context complemented by general description of the sandstone, combining on-site work and rock mechanics lab testing. The selected fractures in this Lower Lias, Hettangian-aged rock were all representative of the general tectonic system affecting the rock mass as observed in central Luxemburg. For comparison, identical roughness computation methods were conducted on lab-generated fractures in cylindrical samples of the sandstone. Fracture roughness was computed from laser surface topography acquisition every 172μm. Large sets of natural and lab-generated surfaces cover both shear and tensile modes. The roughness indices computed are: average asperities height (Ra and associated σa), Z2 root-mean-square (RMS) value, fractal dimensions Dvar (semi-variogram) and Dyard (yardstick rule). This was complemented by comprehensive background information issued from field work - taking into consideration the main structural directions and including key petrophysical properties. The Luxemburg Sandstone presents fair porosity (18%), matrix permeability of 17mD with intense fracture networks, combined to a moderate to strong mechanical behaviour (56MPa unconfined compressive strength). Due to a coastal-type depositional environment, the sandstone reveals compositional differences between calcarenite and sandstone sensus stricto. Studied natural samples present computed roughness indices as follow: Ra= 0.020mm (σa= 1.222), Z2= 0.46, Dvar= 1.36, Dyard= 1.0080. Lab-generated fractures give the following indices: Ra= -0.039mm (σa= 0.818), Z2= 0.34, Dvar= 1.37, Dyard= 1.0047. Natural surfaces present higher computed values than lab-generated fractures, whether sheared or produced in tensile mode.