The purpose of this paper is to present a brief, practical overview of valley stress relief phenomena in flat-lying sedimentary rocks with emphasis on fundamental geologic and engineering implications.
The writers' experience with valley stress relief in flat-lying sedimentary rocks, which typically include weak strata or strata with weak bedding contacts, is derived largely from observations in the Appalachian Plateau of the eastern United States, primarily in excavations for foundations and abutments of dams (Ferguson 1967, 1974). Recent experience at dams and other engineering projects elsewhere, plus extensive literature review, indicates that similar valley stress relief features, mainly rock discontinuities of various types, exist throughout the world in flat-lying sedimentary rocks.
Stress relief features in flat-lying sedimentary rocks have been observed for many years (Van Horn 1909, Grice 1968, Deere & Patton 1971, Pasek 1974, Patton & Hendron 1974, Huntoon & Elston 1980). With the development of rock mechanics over the past two decades, there has been increased interest in rock stresses and stress release ac- Companying valley erosion and man-made excavations (Coates 1963, Uriel 1966, Lee 1978, Agapito et al. 1980, Kulhawy & O'Rourke 1981). Studies of stiff to hard argillaceous Soils or weak argillaceous rocks (Matheson & Thomson 1973, Palmer & Rice 1973, Burland et al. 1977) have clarified certain aspects of stress relief phenomena and related soil or rock discontinuities. Processes of stress release due to unloading. are still poorly understood (Nichols 1980) though progress has been made in recognizing stress relief features in flat-lying sedimentary rocks and dealing with these features in design and construction.
Rock mechanics investigations(Agapito etal.1980,Nichols 1980,Kulhawy&O'Rourke 1981) have shown that many flat-lying sedimentary rocks near the earth's surface have horizontal stresses equal to or greater than vertical stresses corresponding to existing overburden. These high horizontal stresses are presumably related to previous overburden removed by erosion.
River erosion removes horizontal support from valley walls and vertical support from the valley floor. Valley walls tend to deform inward and the valley floor tends to deform upward in response to this loss of support. The mechanics of this deformational response are well understood in concept, e.g., from elastic theory, but details of this deformational response are often complex, depending, e.g., on time dependent phenomena and stratigraphic sequences. Deformational response due to valley erosion in flat-lying sedimentary rocks will be enhanced by high horizontal stresses in the rocks.
Deformational response and rock discontinuities due to val1ey erosion and stress relief in flat-lying sedimentary rocks are shown schematically in Fig. 1 and on a typical valley cross-section in Fig. 2. The valley walls are zones of extension and the valley floor is a zone of compression. Rock discontinuities in the valley walls and floor reflect these deformation conditions and also the stratigraphy, particularly the strength and stiffness, of individual beds.
Inward movement of valley walls concentrates in the weaker, more deformable beds which sometimes develop diagonal to curved shear joints and commonly develop shear zones or mylonite seams at contacts with stronger, Stiffer beds (Fig. 3).