Laboratory investigation of the mechanics of raveling soils

Abstract

Oregon's forestry industry has and will continue to be a vital component of Oregon's economy, with twenty-two percent of Oregon's 1986 gross state product and thirty-nine percent of Oregon's 1986 gross manufacturing product related the forestry goods. An integral component to proper management of Oregon's forestry lands is Sustaining the timber yield from the commercial forest land base. The Oregon Forest Practice Rules and Statue (1985) mandates that commercial forest lands &e subject to reforestation requirements. Forest lands which do not meet these requirements are candidate for deletion from the commercial forest land base. Reforestation of steep skeletal slopes has proven difficult; high seedling mortality has occurred due, in part, to dry raveling of the surface soil. Dry raveling is a form of slope failure on steep slopes generally characterized by surface movement of individual particles, and/or a shallow veneer of material. The extent of seedling mortality due to dry raveling s believed to be substantial in mountainous forest lands in Southwest Oregon. The present study represents a research effort to define the raveling process and the potential for seedling mortality due to dry raveling. Laboratory testing of the raveling process was conducted using a tilting bed constructed to infinite slope criteria. Sixty-one raveling tests were performed on material taken from ravel-prone sites in Southwest Oregon. Tests involved variations in water content, gain size distribution, and profile thickness. Raveling angles, profile mantle failure angles, and the angle of repose were examined, along with the kinematics of the granular flow during mantle failure. The raveling events were found to progress from individual particle movement, to particle assemblage movement, to mantle failure. A dry ravel model was proposed based on the analysis of the literature and test results. The ravel model reflected the dependence to the raveling system on the free surface boundary and momentum transfer to the surface material by the raveling particles. The angle of repose was identified as a fundamental soil property which can be related to a dry raveling potential and therefore used by forest management personal as a slope limit above which progressively greater dry ravel-induced seedling mortality will occur.