Coupled flow of water in saturated kaolinite clay under multiple potentials

Abstract

The purpose of this research was to test in the laboratory the validity of a phenomenological equation which combines linearly the forces causing flow of water in continuous clay systems. The equation is based on postulates of irreversible thermodynamics, the validity of which has not been established for clays subjected to combination of hydraulic, thermal and chemical potentials simultaneously. This provided the basis for the development of a new equipment with the objective of obtaining reliable data of water conduction through soil under these multiple potentials. A triaxial test system was developed to simultaneously apply constant hydraulic, thermal, and chemical potentials to a cylindrical soil specimen under confining pressure. The test system had five func- tional units, loading, heating, cooling, flexible wall permeability, and diffusion of chemicals. The various parts of the equipment can withstand large temperature differences (80 C) and chemical environ- ments with pH values ranging from as low as 1 to as high as 12. In addition to its conduction measurement capability, the system is also a conventional triaxial compression testing machine with computer aided data acquisition and real time display of load, displacement and pore pressures. The initial calibration tests showed good repeatabi- lity of data and thus stable functioning of the equipment. The potentials were initially applied separately, and then they were combined simultaneously with either flow allowed through the end boundaries of replicate kaolinite clay specimens. The experimental findings of cumulative flow of water under simultaneous application of the three gradients agreed well with the predictions made for cumulative flow using the superpositioning of the linear relations between the water flux and the forces. The agreement was observed despite the measured changes in the consistency and composition of the soil specimens under the particular magnitude and duration of the potentials applied. This validated the applicability of superpositio- ning to predict coupled flow with the applied hydraulic, osmotic, and thermal potentials. The superpositioning of the forces was not valida- ted when the coupled coefficients were determined from pore pressurmea surements at the potential application sites. In this case, superposi- tioning overestimated the flow since potential induced consolidation