Suction bentonite

More sophisticated suction controlled tests which explore the behaviour of the bentonite under specific stress and suction paths are described in the references given above. Tests on small scale cells involving simultaneous hydration and heating were also performed. They are boundary value problems and their analysis may provide a refined evaluation of constitutive parameters. Some of the research groups participating in the reBEX benchmark test have used this information to their advantage. Their analysis is published elsewhere. 

Lloret, A., Villar, M.V., Sanchez, M., Gens, A., Pintado, X., and Alonso, E.E., Mechanical behavior of heavily compacted bentonite under high suction changes. Geotechnique, 53(1) 27-40. 2003. 

Abstract The Canadian Nuclear Safety Commission (CNSC) used the finite element code FRACON to perform blind predictions of the FEBEX heater experiment. The FRACON code numerically solves the extended equations of Biot s poro-elasticity. The rock was assumed to be linearly elastic, however, the poro-elastic coefficients of variably saturated bentonite were expressed as functions of net stress and void ratio using the state surface equation obtained from suction-controlled oedometer tests. In this paper, we will summarize our approach and predictive results for the Thermo-Hydro-Mechanical response of the bentonite. It is shown that the model correctly predicts drying of the bentonite near the heaters and re-saturation near the rock interface. The evolution of temperature and the heater thermal output were reasonably well predicted by the model. The trends in the total stresses developed in the bentonite were also correctly predicted, however the absolute values were underestimated probably due to the neglect of pore pressure build-up in the rock mass. 

The third term represents water retention due to the unsaturated state of the medium. In this term, w is the gravimetric water content, n is the porosity and is the specific gravity of the solid particles. The water content for the unsaturated FEBEX bentonite can be expressed as an empirical function of suction ... 

The FEBEX T-H-M experiment is a valuable and important project which should lead to an improvement in the understanding of the behaviour of the bentonite barrier around heat-emitting Nuclear Fuel Waste(NFW) containers. Such large field experiments should always be undertaken with the simultaneous development of constitutive and computational models to interpret the experiments. The FEBEX bentonite possesses strong nonlinear behaviour in the unsaturated state. In order to simulate that behaviour, we have adopted a nonlinear poro-elastic approach. In this approach, the coefficients of the poroelastic equations are assumed to be functions of suction and the void ratio. These functions are derived from the state-surface equation which has been experimentally obtained from suction-controlled oedometric tests performed by the Spanish research organizations UPC and CIEMAT. 

The model is fitted to a suction experiment for Febex bentonite and applied to the TH simulation of the bentonite buffer of the Febex in situ test, which is considered in the international Decovalex 3 project. The present approach is to describe the essential features of the TH behaviour of the buffer in a simple ID geometry. The results calculated with FEM are compared to the measurements. 

For the analysis of the FEB EX in situ test, a soil mechanics state-surface model was implemented. This state-surface approach provides a better representation of bentonite behavior under partially saturated conditions than a single effective stress approach. The logarithmic state surface model proposed by Lloret and Alonso (1985) was adopted in this analysis. In their model, void ratio (e) is a function of both net mean stress, (Ora" = Om - Pg. where = total mean stress and Pg is gas pressure) and suction (s = Pg -Pi, where Pg and pi is gas and liquid pressures, repectively). 

The material parameters are as summarised in Table 1. Because the capillary pressures in bentonite are very high (up to 10 ° Pa, as shown in Figure 5), a large suction pressure is created, that... 

In these tests, bentonite was hydrated and loaded controlling the suction applied to the samples by using the axis translation technique. Figure 3 shows the layout of the oedometer cell, which was immersed inside a thermostatic bath (Romero et al. 2001). Suction is applied by changing the pressure of the gas phase in the pores of the sample by injecting nitrogen in the cell to the desired pressure. The bottom of the sample is in contact with water at atmospheric pressure through a cellulose membrane permeable to water but not to air. 

Three tests on bentonite compacted at an initial dry density of 1.7 g/cm with its hygroscopic water content have been performed at different temperatures (20, 40 and 60 °C), following the suction and stress paths indicated in Figure 4. In the first step, under a vertical load of 0.1 MPa, the suction of the sample was equalised to 14 MPa (due to mechanical limitations of the cell, this is the maximum suction that can be controlled). Afterwards, keeping this suction constant, the vertical load was increased up to 5 MPa. Under this vertical load, suction was decreased by steps until... 

In tests with suction reduction, the structure changes due to hydration are more relevant in the subsequent mechanical behaviour of the bentonite than the effects of temperature. 

The space between the drift and the heaters is fdled by blocks of compacted bentonite with a smectite content in the range of 88%-96%. The test is heavily instrumented with measurements of temperatures, relative humidity (equivalent to total suction), pore pressures, displacements, and stresses. 

The initial conditions of the bentonite were as follows dry density 1,7 g/cm and water content 14.4%. This results in an initial degree of saturation of 0.65 and an initial suction of 115 MPa. Both the bentonite (solid phase and interstitial water) and the hydration water were subjected to a full chemical characterization. The thermo-hydro-mechanical parameters were determined in an independent laboratory testing programme. 

Nishimura, T. 2001. Swelling pressure of a compacted bentonite subjected to high suction, pages 109-114. Clay Science for Engineering, Proceedings of the international symposium on suction, swelling, permeability and structure of clays - Is-Shizuoka. Balkema, Rotterdam. 

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