PREDICTION OF POREWATER PRESSURES IN THE CORE OF EARTH DAMS DURING CONSTRUCTION

摘要

The stability of an earth dam with respect to a slope or foundation failure depends to a large extent on the pore pressures. During construction, it has been recognized that the positive and sometimes alarmingly high porewater pressures, associated with progressive consolidation, develop in impervious sections of some earth dams before there is any water in the reservoir to cause seepage pressures. The intensity and distribution of the pressures depend not only on the consolidation characteristics of the materials, which are changing as the dam is being constructed, and on the drainage conditions, but also to a large extent on the rate of construction. The pressures, which may develop at a given dam, can be predicted by analysis, and can be observed by piezometers during construction. The prediction is based on the adaptation of the theories of consolidation and the flow of water through porous media subject to the internal and external boundary conditions, and on a knowledge of the stress-deformation pore-pressure characteristics of the materials. A numerical technique was developed and a computer program was written to model the generation and dissipation of porewater pressures during construction. In this technique, a zoned dam and its foundation are discretized into elements. Each element is characterized by its geometry, total unit weight, cohesion and friction angle, horizontal and vertical hydraulic conductivity, maximum shear and bulk moduli, and initial degree of saturation and porosity. For a given time history of fill elevation, the analysis provides variations of pore pressures with time in the core of a dam. At each time step, the program calculates the total stresses as well as generation and dissipation of porewater pressures. The calculation cycle is repeated for the entire history of fill elevation. In this repetitive manner, the calculation marches with time providing deformations and movemnents of assemblage of elements in two-dimensional space. As time proceeds, static equilibrium of the dam is developed naturally. Analysis results are given for the West Dam of Diamond Valley Lake, California, to demonstrate the intensity and distribution of the porewater pressures during construction and compare them with the observed values from vibrating wire piezometers.