Seepage Analysis of Bentonite

As described previously, the viscosity of water in the interlayer space of smectite clay stacks is strictly influenced by the processes at the clay surface. If the viscosity is location-dependent, the seepage problem is usually not solvable through an analytical technique. In this case, we solve the microscale equations (8.49) and (8.53) using a finite element method, and determine the HA-permeability Kij and C-permeability Kfj. 

The bentonite analysed here is Kunigel VI , which is a candidate buffer material for the Japanese high-level nuclear waste management program. This bentonite is made up of 50% smectite minerals by weight, while the remainder is mainly grains of quartz. The intrinsic mass density of a clay mineral is almost the same as that for quartz, which is about 2.7 Mg/m, and the porosity of Kunigel VI at a dry density of 1.8Mg/m is approximately one-third (i.e., the pore ratio e = 0.5). 

From the measured data of the quartz grains, we can see that the diameter varies from about 5-100/rm. Based on the peak value of the data and the fact that the permeability characteristics are mainly controlled by smaller grains, we assume 

One crystal of the smectite clay is a platelet of about 100 x 100 x 1 nm, as mentioned previously, and several crystals form a stack. Here we assume that six crystals form a stack. The interlayer distance is 0.56 nm, which corresponds to the state of hydration by two water layers. The distance between stacks, shown as X in Fig. 8.22, is determined by the given saturated density of the bentonite, and if the distance X becomes smaller than 0.56 nm, we assume that both X and the interlayer distance of the stack are equally reduced. Since the smectite mineral is very thin, one of the micro-domains is pseudo-one-dimensional, which represents 

In Fig. 8.23 we show the results for C-permeabiUty using the distribution of viscosity obtained by MD (Fig. 8.6), for pure water and for salt water, as a function of the pore ratio e. We observe that by increasing the density (i.e., decreasing e), the permeability decreases rapidly, and the permeability for salt water is larger than that for pure water. 

---