A FRACTURE PERMEAMETER FOR EXAMINING THE ROLE OF CLEFT WATER PRESSURE IN TRIGGERING ROCK SLIDES AND A COMPARISON WITH WATER-TRIGGERED DEBRIS FLOWS

Equations were developed decades ago by Evert Hoek for estimating cleft water pressures and their corresponding “uplift” forces within rock mass discontinuities, including fractures, so that water effects could be accounted for in rock slope stability safety factor calculations. Confusion persists over the uses of such terms as “ice dammed toe,” “free draining toe,” and “self draining slope.” In particular, the equation for estimating water pressure in the “free draining toe” case is empirical and has recently been called into question. A fracture permeameter was constructed in order to experimentally assess relationships between cleft water pressure, aperture size, fracture flow, and the effects of natural infilling materials including weathering products like sand, silt, and clay. Fractures are simulated in the permeameter using two parallel Plexiglas plates with a variable aperture between them, mounted on a hinged framework that allows for changing discontinuity dip angle. The uppermost plate contains piezometer tubes arranged in a grid pattern so that water pressures can be measured across the entire surface under a variety of simulated natural conditions. The resulting data are contoured to produce water pressure maps from which uplift forces may be calculated. Based on numerous experiments with the fracture permeameter, the basic Hoek equation is being updated and expanded into a matrix of at least 12 equations suitable for a variety of rock slope configurations and flow conditions. The data reveal that the Hoek equations have long provided reasonable and useful estimates of cleft water pressure for instances in which seepage is observed yet actual internal conditions are unknown. The fracture permeameter demonstrates beyond any doubt that conditions commonly and easily exist within rock fractures that allow critical water pressures to develop even when water exits from the toe at the same rate it enters from above despite recent concerns to the contrary. Importantly, the fracture permeameter further demonstrates that extremely large rockslides can be triggered by relatively small quantities of water flowing through a sensitive location compared to the large amounts of water generally needed to trigger large storm-generated debris flows.