From double to triple porosity modelling of bentonite pellet mixtures

The present work analyses the hydro-mechanical behaviour of bentonite pellet mixtures using a macroscopic overlapping continua approach based on a triple porosity model. Two macrostructural functional levels are considered (inter-pellet pore space and void space between the clay aggregates inside the pellets) as well as a microstructural level (corresponding to the space inside the aggregates). This triple porosity approach reveals that, when assuming a double porosity model, the intrinsic permeability, the tortuosity and the degree of saturation of the single macrostructural level under consideration are the average values of the state functions of the adopted triple porosity model, but are not state functions themselves. The analysis also illustrates that, when assuming the same liquid pressure for the two macrostructural levels of the triple porosity model, it is relatively easy to expand upon the calculation procedures initially developed for a double porosity model to include a triple porosity model. When evaluating the applicability of the model, quality results were obtained, comparable to those of a triple porosity model, for both open and closed bentonite pellet mixtures. The results are better than those obtained with a double porosity model, since the state functions have a stronger physic base when using a triple porosity approach. Encouraging results are also obtained when analysing the overall swelling of a bentonite block and a mixture of bentonite pellets, using the same parameters for both materials. Therefore, given its capacity for relatively simple incorporation in a double porosity models, the model offers an interesting tool for researchers working on improving the characterisation of bentonite pellet mixtures.