Effect of chemical transport on seepage and stability of geotechnical structures.

Abstract

Soil properties such as stress-strain response and hydraulic conductivity are affected by the transport of chemicals in soils. For stress-deformation and stability analysis of geotechnical structures, it is necessary to consider the effect of chemicals on the stress-strain-strength behavior. In this study, the residual flow procedure is modified to incorporate the chemical effects on the hydraulic conductivity to solve interface between fresh water and salt water in intrusion problems. The results are compared with previous analysis that did not consider chemical effect under the interface. The residual flow procedure of free surface problem is also modified to involve the capillary zone that experiences influences in soil strength parameters. In this study, the chemical considered is sodium chloride. Laboratory model tests are performed to study the changes in conductivity and free surface seepage characteristics of sand-bentonite mixtures with different concentrations of sodium chloride. A series of laboratory triaxial tests are performed on the cylindrical specimens of sand-bentonite mixture with different (5, 10, 15%) sodium chloride contents. Deformation (elastic modulus, E) and strength (cohesion, c, and angle of friction, φ) parameters are obtained from the triaxial tests as functions of confining pressure and sodium chloride concentrations. The stress-strain-strength behavior based on the above parameters is introduced in a finite element procedure with the residual flow procedure (RFP) as explained above. The latter allows consideration of free surface seepage with locations of salt water fronts in dams/slopes subjected to transient (rise - steady state - draw down) fluctuations in the upstream heads. By integrating a slope stability (limit equilibrium) procedure in the finite element method, factors of safety with time are computed. The analyses are performed with fresh water and salt water, and the two results are compared to identify the effect of salt water on stability and deformation in a typical dam configuration. Overall, the study presents an integrated procedure in which constitutive response as affected by salt water is introduced in a finite element procedure. The methodology can permit stress-deformation and stability analyses of geotechnical structures once the stress-strain models, including the effect of chemicals, are established based on appropriate laboratory tests.