Shear strength of unsaturated bentonite buffer material
There are two widely accepted approaches to determine the shear strength of unsaturated soils: the effective stress approach (Bishop, 1959) and the independent stress state variables approach (Fredlund et al., 1978). The main difference between these two approaches lies in how to reduce the effect of increasing matric suction. Bishop’s coefficient χ can be calculated using ϕb (χ = tan ϕb / tan ϕ’) and vice versa. However, Bishop’s approach with the coefficient χ “of the matric suction efficiency” seems to be closer to the reality since there is a difference between decreasing ϕb and the experimental results, which have proved that the effective friction angle slightly increases with increasing matric suction. The coefficient χ decreases with increasing suction as a result of decreasing both the total area of water-solid contacts and number of particles connected by water menisci. This explains why the parameter χ decreases with increasing porosity. Thus, the maximum effect of matric suction can be reached with higher water content as a result of two opposite influences: decreasing matric suction and increasing number of particles connected by water menisci (matric suction efficiency χ).
The analysis of this problem is based on the experimental programme of the bentonite buffer material under unsaturated conditions. Pure bentonite as well as bentonite mixtures with siliceous sand and graphite were tested. Samples with extremely different porosities were prepared under compaction pressures ranging from 300 kPa to 100 MPa and then tested in the triaxial apparatus.
KeywordsShear Strength Friction Angle Unsaturated Soil Siliceous Sand Matric Suction
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