Harry Ferguson’s Theory of Valley Stress Release in Flat-Lying Sedimentary Rocks

Abstract

Harry Ferguson developed his theory of valley stress release in flat-lying sedimentary rocks by observing rock characteristics, behavior, and discontinuities in foundation excavations for navigation locks and dams and flood control dams in the Upper Ohio River Basin in the late 1950s and 1960s. He first presented his theory at the AEG Annual Meeting in 1966, then published it in the AEG Bulletin in 1967. Over the past five decades, this theory has provided a unifying framework for world-wide geologic observations and geotechnical interpretations in flat-lying sedimentary rocks. It is appropriate here to review this theory for an international audience and a new generation of engineering geologists. Essential elements of the theory are: 1. Flat-lying sedimentary rocks near the earth’s surface typically have horizontal stresses greater than vertical stresses corresponding to existing overburden. 2. River (or stream) erosion removes horizontal support from valley walls and vertical support from valley floors. 3. Valley walls deform inward and valley floors deform upward in response to this loss of support. 4. These deformations relieve stresses in rocks of the valley walls and floors. 5. Deformations and stress release produce characteristic patterns and types of fractures and other discontinuities in rocks of valley walls and floors. This theory of valley stress release has both geologic and engineering implications. Geologic implications include a mechanism for on-going valley development independent of tectonic processes; groundwater flow through stress release fractures with associated processes of weathering and alteration plus solution in soluble rocks; mass-wasting processes, e.g., rock slides, rock falls, rock block creep, colluvium development, on valley walls. Engineering implications include layout and interpretation of subsurface exploration programs; foundation depths and treatments; slope and foundation stability; rock excavation and support, both surface and underground; excavation dewatering; dam foundation and abutment grouting.