Soils Lab

Topics: Soil mechanics, Triaxial shear test, Geotechnical engineering Pages: 7 (2254 words) Published: January 19, 2013
The stability and life of any structure – a building, an airport, a road, dams, levees – depend on the stability, strength, and deformation of soils.[1] Unfortunately, due to the uncertainties of the world’s natural materials, the study of geotechnical engineering, and soil mechanics in particular, is both challenging and necessary. The following report is a compilation of the results acquired as a result of two laboratories performed by Group 13 at McMaster University on February 1st and 15th, 2007. Four tests; Direct Shear, Vacuum Triaxial Compression, Unconsolidated and Undrained Triaxial and Unconfined Compression were carried out in order to assess the performance of our most ancient and complex engineering materials, sand and clay. The behavior and response of these materials was a result of various applied loads, shearing forces, confinements and drainage conditions. Throughout this report we will describe and define the various analyses used, our expected versus actual results, some of the sources of the inherent inaccuracies and errors, as well as the final shear strength of the soils samples provided.

The four laboratories as outlined above were carried out as per the provided McMaster University 2007 Civil Engineering 3B03 Geotechnical Engineering 2, Lab Procedures Manual, under the guidance of Peter Koudys and Jamie Hamelin.

Direct shear tests are quick and inexpensive test used to determine the corresponding shear strengths and parameters of fine and coarse grained soils. However, a major disadvantage to shear tests can be attributed to the forced horizontal failure plane which is not often the weakest failure in in-situ conditions. We conducted a direct shear test using the direct shear apparatus in order to determine the consolidated-drained shear strength of our sandy soil. Although drainage may not prevented when using a direct shear apparatus, a shear test can be used as a quick estimate of the undrained shear strength of saturated clay samples provided that the loading is completed rapidly enough to ensure no dissipation of pore water. Shear tests are quick and inexpensive

3.1Results and Discussion
The locus of points shown in Figure 1 below, Shear Stress and Vertical Deflection Vs. Horizontal Deflection, clearly shows a peak shear strength at approximately 92 kPa followed by a critical shear in the area of 75 kPa at a constant normal stress of 95 kPa. This is characteristic behavior of dense sand which compacts initially, resulting in a reduction in void ratio and then dilates or expands (reference the decrease in vertical displacement same figure) to the critical shear strength and critical void ratio of the sample. Based on the peak and critical shear values, this corresponds to a peak shear angle of 44 degrees and a critical shear friction angle of 38 degrees. Combined, these values represent a dilation angle of approximately 6 degrees. (See Figure 2, Mohr-Coulomb Failure Envelope). Since this dilation angle is a function of the corresponding vertical force and can not be relied upon, any design subtended in the intermediate region between the peak shear angle and the critical shear friction angle should be avoided in the interest of stability. For illustrative purposes, a fictitious locus of points representing a loose sand sample has been included in order to demonstrate the convergence on a critical shear strength and void ratio.

Figure 1 – Shear Stress and Vertical Deflection Vs. Horizontal Deflection Figure 2 – Mohr-Coulomb Failure Envelope

3.2Sources of Inaccuracies and Errors
A major disadvantage to the direct shear box test is that the strains cannot be calculated since the stresses across the thin predetermined failure plane are non-uniformly distributed. Another common source of error is apparent cohesion which is especially common when studying clayey soils due to some...
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