The vane shear apparatus makes use of the theory that the summation of the total shear stress is equal to the sum of the shear stress at failure along the cylindrical surface and the total shear stress on the top and the bottom of the surface of the cylinder. The various sizes of the vanes are accounted for by applying the multiplying factor. The value obtained from the medium sized vane is the value of the stress itself, and there is no need to multiply or divide the value. The values obtained from the larger vane is divided by two, since it is harder to rotate the vanes because there is more surfaces that create resistance. Lastly, the value obtained from the smallest vane six is multiplied by two since it is easier to rotate the vanes. From Table 1.0, the values for the undrained shear strength of the soil for the first trial using the medium vane, larger vane, and smaller vane are as follows: 18kN/m3, 17 kN/m3, and 19 kN/m3, respectively. It can be observed that the values are relatively large, this is because the soil sample is fully saturated. However, the results of each test using different vane sizes yielded different values. The differences in the values can be caused by 1) Personal Errors, 2) Overburden Stress 3) The moisture content of the soil and the 4) condition of the soil. Since the whole experiment is done by eye, it is very much susceptible to personal errors; a group member could have misjudged the turning of the bottom part of the vane (which could have caused an insufficient or excess amount of torque applied to the apparatus). The overburden stress is the stress imposed on a layer of soil or rock by the weight of overlying material, this means that the ease at which the vane is being rotated is affected by the depth it is submerged in. In the experiment, the depth was not measured since the soil sample was not that deep, but it is likely that the overburden stress contributed to the slight inconsistency of the results. The...
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