Predictive permeability model of extensional faults in crystalline and metamorphic rocks; verification by pre-grouting in two sub-sea tunnels, Norway

Abstract

This paper link quantitative fault zone descriptions, qualitative fracture and fault rock properties, and engineering data in the study of the permeability structure of fault zones. Datasets include scan-lines, drill cores and cement pre-grouting from two sub-sea tunnels in gneissic and granitic rocks, from which systematic pre-grouting volumes can be used to analyse the in-site relative permeability both in host rocks and fault zones. Major extensional faults intersected by the tunnels reveal common fault rocks surrounding intensively fractured rock lenses in the core. Fracture frequencies in these lenses can reach 100 fractures/metre (f/m). In the bounding damage zones, networks of fracture sets make up an inner zone of fairly high frequency (20–30 f/m) of fault-parallel, long fractures connected by shorter fractures. An outer zone has lower frequencies (<20 f/m) and more diverse fracture orientations and lengths. There is a general increase in fracture frequency from the background level of the protolith towards the fault core. Tunnel-scale injection of cement reveals patterns that can be ascribed to the impact of faulting; there is an increase in cement injection in fault zones compared to areas with background fracturing away from faults. In detail, there is an innate division of the rock volume into sub-zones characterized by distinct structural style and permeability, with a background level and three fault related sub-zones (fault core, inner damage zone, and outer damage zone). Injection data shows that the background sub-zone commonly can be injected with less than 0.05 m 3 cement per metre tunnel (commonly not injected), whereas the fault core has permeability characteristics nearly as low as the outer damage zone, represented by 0.1–0.2 m 3 cement per metre tunnel, with occasional peaks towards 0.5 m 3. The maximum of cement injection lies in the inner damage zone, marginal to the fault core, with 0.3–0.7 m 3 cement per metre tunnel, locally exceeding 1 m 3. This gives a relative relationship for cement injection of approximately 1:2:1 between fault core, inner damage zone, and outer damage zone of extensional fault zones in crystalline and metamorphic rocks.