Aperture measurements and seepage properties of typical single natural fractures

Fractures play an important role in controlling the hydraulic conductivity of rock masses, and the aperture significantly influences the magnitude of fracture seepage. In this study, field measurements and experiments were conducted at a well-exposed granite fracture site in the Beishan area, China. Several types of single natural fractures were selected to remove the weathered surface and expose the fresh fractures. Subsequently, measuring ruler dispersion-tangent middle axis (MRD-TMA) method was developed to measure the fracture aperture and capture fracture geometry. Then, electrical resistivity tomography (ERT) technique was employed to investigate the seepage properties of these fractures. The results reveal that MRD-TMA method achieved good flexibility and accuracy in the current measurement of fracture aperture, and ERT is a useful tool for detecting the seepage properties of fractures in hard rock masses. Combined with field observations, the filling form of fractures can be categorized according to the ERT inversion results, as follows: open-weak filling, open filling, loose filling, and fully cemented-closed form, whose seepage properties decrease as the filling density increases. Generally, the open-weak filling is the main water channel in a fracture network, while the fully cemented-closed type is a water-blocking fracture and typically exhibits a pseudo-fracture with a large surface opening. In summary, the method for obtaining the morphological characteristics of the aperture can provide a low-cost and time-efficient approach for fracture logging in the field, and ERT technique provides a reference for the detection of potential hazards caused by connected water-conducting fractures in rock engineering.