Backanalysis of thermohydraulic bentonite properties from laboratory tests

The paper presents a general methodology to perform backanalysis of laboratory tests involving thermohydraulic behaviour of bentonite in a systematic manner. The procedure is based on a maximum likelihood approach that defines a probabilistic framework in which error measurements and the reliability of the parameters identified can be estimated. The method is applied to the identification of some thermal and hydraulic properties of a bentonite specimen, using temperature and water content measurements as input data. Three basic cases have been considered: (a) thermal case, identifying the global thermal conductivity, λe, and the global specific heat, ce; (b) hydraulic case, identifying the tortuosity factor, τ, and the exponent of the unsaturated permeability law, n; and (c) thermohydraulic case, in which τ, n, and the saturated thermal conductivity, λsat, have been estimated. A numerical code that solves the coupled thermohydraulic equations has been used as main computational tool. A detailed description of the experimental device and a discussion on the tests results and on the parameters identified are also included. The values obtained are within the normal range of these parameters, but the method provides a systematic and consistent procedure to find the best parameters that reproduce the measurements for the selected models. Also the method gives an insight into the model structure, and allows detecting dependence and coupling between parameters.