ON HYDRAULICS OF JOINTS AND ROCK FALL INITIATION IN YOSEMITE

Water from human sources on top of Glacier Point in the Yosemite National Park was alleged to have been the cause of a series of rock falls involving fatalities at Happy Isles 1996 and Curry Village 1999. Specifically, an assertion was made that water traveled through one particular joint directly from the recharge zone on top of Glacier Point to the zone of failure, hence precipitating the Curry Village failure (Watts and Gilliam 2000). This presentation will address the fundamentals of joint hydraulics and rock slope stability analysis that were examined with respect to this alleged mechanism of rock fall initiation.

A quantitative assessment of the amount of water that can be transmitted through a joint will be presented in order to illustrate that the capacity of the joints to transmit water can easily exceed any amount of potential recharge unless the joints are exceedingly tight. In comparison, local sources, i.e. the rock faces immediately above the rock fall sources can be shown to easily provide the requisite water with a response time and quantity to cause rock falls, if water was in fact the specific cause of the particular failures in question.

Traditional methods of incorporating of joint water pressures in rock block stability analyses (see e.g. Hoek 2007) will then be discussed in order to address some serious misinterpretations of the basic equations that have found their way into the mainstream technical literature and manuals (see e.g. USDA 1994). In particular, the so called “free draining” case (USDA 1994, Watts 2003) will be examined in detail and shown to be physically inadmissible. The implications for the postulated mechanism and causative factors of the specific rock falls will then be addressed.

References:

USDA, Forest Service. Slope Stability Reference Guide for National Forests, Volume II, Report No. EM-7170-13, Washington, D.C., August, 1994.

Hoek, E. Analysis of rockfall hazards, Chapter 9, in Practical Rock Engineering, on line edition, http://www.rocscience.com/hoek/PracticalRockEngineering.asp, 2007.

Watts, C.F., ROCKPACKIII, Software Manuals with Appendices, RockWare Inc., 2003.

Watts, C.F. and Gilliam, D.R. A 2000 Update on Radford University Studies of Glacier Point Rockfalls in Yosemite National Park, California. Institute for Engineering Geosciences, Radford University.