Numerical analysis of the mechanical behavior of collapsing earth dams during first reservoir filling

The primary objective of this dissertation is the development of practical procedures to better understand the mechanical behavior of small collapsing dams during the first reservoir filling. To achieve this objective, the present research study was conducted from both experimental and theoretical bases. The influence of the wetting-induced collapse on the mechanical and hydraulic properties for the residual soil compacted as a metastable-structured material was experimentally investigated. Volume changes and the water coefficient of permeability were investigated using the triaxial permeameter system where the stress state variables were independently controlled. As-compacted residual soil specimens were consolidated isotropically after which measurements of total volume change, water content change, and coefficient of permeability were made at specified matric suctions following a wetting stress path. Shear strength was investigated using the modified shear box equipment. As-compacted residual soil specimens were consolidated and sheared at specified stress states. The experimental data were analyzed to define constitutive relationships for the metastable-structured soil. The procedure developed to simulate the mechanical behavior of a metastable soil following a wetting stress path couples stress equilibrium and water flow using a generalized form of the theory of consolidation for unsaturated soils. The modified Mohr-Coulomb failure criterion is utilized to define the failure conditions in soil elements within the dam. The model takes account of the varying permeability of the collapsing soil when following a wetting stress path. Finite element analyses were found to reproduce analytical/numerical solutions for the consolidation of both saturated and unsaturated soils. Finite element analyses were also performed to study the post-filling performance of small dams. These analyses indicate realistic predictions in comparison to field experience and allow a better understanding of the failure mechanisms of small collapsing dams in Northeast Brazil