Hydraulic fracturing in zoned earth and rockfill dams
Nobari, E. S.; Lee, Kenneth L. (Kenneth Lester), 1931-1978; Duncan, J. M. (James Michael)
Contract ReportAbstract: The investigation described in this report involved two parts: first, an experimental investigation to study the phenomenon of hydraulic fracturing under carefully controlled laboratory conditions; and second, an analytical investigation to determine the conditions under which the stresses in the cores of zoned dams may be reduced sufficiently by arching so that hydraulic fracturing can occur. Analyses were also performed to examine the effectiveness of various countermeasures which can reduce the arching and the likelihood of hydraulic fracturing. The experimental study showed that hydraulic fracturing involves tension failure of the soil, with formation of a crack or fracture on the plane of maximum tensile stress. The tensile strength of the compacted clay used in this study, and the tensile strengths of other clays and silts which might be used as core materials, are so small that they may be disregarded without significant inaccuracy for purposes of investigating the likelihood of hydraulic fracturing. The experiments also showed that, under conditions of nonuniform stress, hydraulic fracturing can occur progressively rather than suddenly. Initiation of hydraulic fracturing at the upstream face of the core in a dam thus does not necessarily mean that the fracture will propagate entirely across the core. The analytical studies showed that, all other things being equal, hydraulic fracturing is more likely in a high dam than in a low dam. The studies indicate that the likelihood of hydraulic fracturing may be reduced by making the core wider, compacting the core at a lower water content, using transition zones which are less stiff than the shells, and using a sloping core rather than a central core. Several factors, such as three-dimensional effects, changes in stress during consolidation, and nonhomogeneous conditions in the core, may make precise evaluation of the core stresses difficult. In critical cases, therefore, it is probably desirable to investigate the likelihood of hydraulic fracturing in the field, after the dam has been built, by using hydraulic fracturing tests performed in piezometers installed in the core as suggested by Vaughan (1970).
U.S. Army Engineer Waterways Experiment Station.Engineer Research and Development Center (U.S.)
Arching in soils; Cores (dams); Earth dams; Hydraulic fracturing; Dam failures; Rockfill dams; Soil stresses; Soil mechanics
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