Rock Fracture Spacings, Openings, and Porosities

The amount of grout to inject into a given volume of fractured foundation rock depends on the fracture porosity and on the spacing and sizes of fracture openings. These properties cannot be measured directly but can be calculated from water-pressure tests if several simplifying assumptions are made. The method is valid for fractured rock masses whose intergranular permeability is very small compared to the fracture permeability, and if solution cavities or pervious interbeds are absent. Studies of 35 dam sites indicate that maximum fracture porosities are about 0.05% near the surface, decreasing to about 0.005% at the 200-ft depth. The volume of grout required for impregnation is correspondingly small. Fracture openings decrease from about 100 microns to 50 microns in the same depth interval. Cement grout penetration is accordingly limited to a small proportion, only the largest, of fractures. The minimum spacing of open fractures increases from 4 ft to 14 ft. All rock types appear to be similar in fracture properties.