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Experimental study of mechanical behaviour of compacted Czech
Bentonite 75
Haiquan Sun, David Mašín, Jan Boháč
Institute of Hydrogeology, Engineering geology and Applied Geophysics, Faculty of Science, Charles University in Prague
Abstract
Compacted bentonite is often planned as an engineered barrier material between host rock and canister in nuclear waste disposal repositories thanks to its favourable swelling characteristics and low permeability. This study deals with the effects of initial dry density and vertical stress on the swelling behaviour of compacted bentonite under distill water infiltration. Conventional oedometer tests were carried out on bentonite from Czech Republic named Czech bentonite 75 (B75). One-dimensional swelling tests were performed on compacted bentonite with various initial dry densities to explore the influence of vertical stress and initial dry density on swelling strain. Results show that, for all the swelling deformation tests, the swelling strain decreases with increase of vertical load and decrease of initial dry density. The coefficient of primary and secondary swelling decrease with the increase of vertical stress. The swelling pressure determined by the swell-consolidation method is higher than that by constant volume method.
Introduction
The commercial Czech B75 bentonite extracted from the Cerny vrch deposit (north-western region of the Czech Republic), was used in this study. It is a calcium magnesium bentonite with a montmorillonite content around 60% and initial water content about 10%. Table 1 lists its physical parameters. The plastic limit, liquid limit, specific density of solid soils is 65%, 229%, and 2.87, respectively.
Table 1: The physical parameters of B75 (Stastka et al., 2015)[1] Property Description Montmorillonite (%) 60 Liquid limit (%) 229 Plastic limit (%) 65 Plasticity index Ip 164 Particle density ρs (g/cm3) 2.87 Methods
The conventional oedometer apparatus was used for measuring the swelling deformation of Czech bentonite 75. The compacted bentonite was performed in the standard fixed stainless steel ring, 50 mm inside diameter and 20 mm height. Silicone grease was applied to the inner wall of the stainless steel ring to reduce friction between the specimen and the wall. The filter papers were placed at top and bottom of specimen, and then following the porous stone. Once the compacted bentonite was introduced in the stainless steel ring, the prescribed vertical stress was applied immediately last for several hours until the deformation changed no more than 0.001mm in two hours. Then the distilled water was supplied to the specimen. The vertical deformation and time required from the start of water supplied were recorded. For saturated oedometer test, the loading and unloading test continued after the swelling deformation tests. Eventually, the water content of the specimen was measured. The ASTM D2435/D2435M [2] standard recommends that the oedometer test data should be corrected for oedometer apparatus compressibility. The deflection of the apparatus was measured by substituting a smooth hard steel disk for the soil specimen before the experiment for all the test conditions. The detailed stress paths performed in the experiments presented in Fig.1.
2nd Petrus-OPERA Conference on Radioactive Waste Management and Geological Disposal
82
Figure 1: Stress path of various experimental methods used in this research
Results/Discussion
The swelling strain is defined by 0
100%
H
H
Where ɛ is the swelling strain(%) under prescribed load, ∆H is the measured vertical swelling deformation and H0 is initial height of specimen before distilled water soaked.
Fig. 2 followed by the method (Sridharan and Gurtug (2004))[3] presents the swelling strain vs. log of time for compacted Czech B75 with initial dry density of 1.25g/cm3. For all the tests, it can be observed the primary swelling phase starts after about 0.1-0.2h of distill water infiltration. It can be seen that the swelling strain increased rapidly at the initial stages, and it reached the asymptotic level eventually. For dry density ρd = 1.25 g/cm3 with 0.98KPa vertical load (load only with top plate nearly equals to free swell), it will take about 100 hours to reach equilibrium. For 50 KPa, it will need 50 hours. The higher the prescribed load, the shorter is the time required to the equilibrium state.
2nd Petrus-OPERA Conference on Radioactive Waste Management and Geological Disposal
83
Figure 2: Swelling strain vs. log of time for compacted Czech B75 with initial dry density of 1.25g/cm3
Conclusion
(1) The swelling strain decreases with increase of vertical load, and increases with increase of initial dry density. The relationship of maximum swelling strain and applied pressure shows a linear function, which position depends on the initial dry density.
(2) The swelling process of an initial stage, primary swelling, and a small secondary stage were observed for all the tests. The coefficient of primary and secondary swelling decrease with the increase of vertical stress.
Acknowledgments
Financial support by the research grant No. 846216 of Charles University Grant Agency is greatly appreciated.
References and Citations
[1] Jan Smutek, 2015, Experimental works with bentonite pellets at the CEG, LUCOEX conference and workshop
[2] ASTM D2435/D2435M-11. 2011. “Standard Test Methods for One-Dimensional Consolidation Properties of Soils using Incremental Loading.” ASTM International: West Conshohocken, PA.
[3] A. Sridharan, Y. Gurtug Swelling behaviour of compacted fine-grained soils Eng. Geol., 72 (1) (2004), pp. 9–18
