Cohesion and angle of internal friction values of expansive soils
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properly cited.
Abstract
The direct shear test using shear box is commonly recommended by practicing geotechnical engineers to obtain the cohesion and angle of internal friction for granular soils. The clay liners involve sand as a main constituent with added clay of variable proportions. This research aims at investigating the reliability of using the direct shear test for different clay contents and different moisture contents using an adequate shearing strain. These factors were found to affect the bilinear trends of shear force versus horizontal displacement profile as well as vertical displacement versus horizontal displacement curves. The cohesion of the mixture was found to increase consistently with the increase of clay content. Increase in moisture content was found to cause a drop in both cohesion and angle of internal friction. These changes are not independent of the density state of clay-sand mixtures. Standard compaction properties for a range of clay-sand mixtures were investigated. This work provides the general trends expected in direct shear tests for clay-sand mixtures of variable clay and moisture contents.
1. Introduction and Background
Artificial clay-sand mixtures were considered by design geotechnical and environmental engineers for use as hydraulic barriers. Adding clay to the sand helps in achieving low hydraulic conductivity. The term Bentonite Enhanced Sand (BES) was used by many researchers instead of clay sand mixtures, Studds et al. [1], Mollins et al. [2], and Stewart et al. [3]. As implied by the name the material is dominantly granular, and the amount of added clay is not large enough to classify the paste as anything other than sand. However there is a stage at which the paste starts behaving as pure clay when the sand grains are pushed apart, and clay dominates the engineering properties of the mix. The real boundary is not agreed upon as this is largely dependent on the activity and mineralogical composition of the clay. The ASTM standards classify the material as clay when the percentage of material passing sieve no. 200 is greater than 50%, and the consistency tests indicate CL classification. In the British Soil Classification System, detailed in BS 5930 Site Investigation, little above 35% fines (material passing sieve no. 200 63 microns) can lead to classify the material as clay. A bentonite sand mixture of high clay content may no longer be considered as a BES.
This research work considers adding Al-Qatif expansive clays to sand at proportions of 5% to 15% as a first stage. Other percentages and combinations of Al-Qatif clay and commercial bentonite material were also considered as possible alternatives. However the study of the direct shear tests was concentrated on the local clay and sand mixtures. Compressibility and swelling tests were carried out on the mixtures to help understanding the trends and behaviour of the direct shear tests. Falling head permeability tests were conducted for 5 and 10% clay-sand mixtures compacted to variable moisture contents at the maximum dry density.
The direct shear test is based on forcing the sample to fail along a predefined plane while being subjected to normal load. This gives a direct measure of the shear force capacity at specific conditions and enables determination of the angle of internal friction and cohesion. The shear stress in the shear box test is defined as the shear resistance developed within the sliding plane along a known section area of the sample.