HYDRAULIC FRACTURING OF SOIL DURING LABORATORY EXPERIMENTS .2. PROPAGATION

The propagation of hydraulic fractures in soil was studied in the laboratory by glycerine injected at a constant rate into rectangular samples of silty clay confined in a triaxial cell. During propagation, the trace of a hydraulic fracture appeared on the side' of the cell as a nearly straight, narrow line that tapered from 0.05 mm to negligible aperture at the tip. Pressure of the glycerine was recorded as a function of time and is characterized by a period of nearly linear increase, a break in slope followed by a period of increasing pressure but decreasing slope, a maximum, and then a period of decreasing pressure. Tests terminated early in that sequence of events indicated that the break in slope marks the onset of hydraulic fracturing. The pressure at the onset of fracturing was predicted from a determination of the fracture toughness of the silty clay. The forms of the pressure records were markedly affected by the water content of the soil:increasing water content decreased the pressure required to initiate a hydraulic fracture (and decreased fracture toughness) and flattened the pressure record during propagation.