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SOIL MECHANICS - CHAPTER 22

Quan điểm cho rằng thất bại của đất xảy ra bằng cách trượt dọc theo một mặt phẳng mà trên đó sự căng thẳng cắt đạt đến một giá trị tối đa nhất định đã dẫn đến sự phát triển của các bài kiểm tra cắt. Trong các thử nghiệm mẫu được tải rằng nó sẽ là một phần của mẫu trượt trên một phần khác, cùng một chiếc máy bay trượt cho. Nó thường được giả định rằng chiếc máy bay trượt là cố định và được đưa ra bởi loại hình học của các thiết bị được sử. | Chapter 22 SHEAR TEST The notion that failure of a soil occurs by sliding along a plane on which the shear stress reaches a certain maximum value has lead to the development of shear tests. In such tests a sample is loaded such that it is expected that one part of the sample slides over another part along a given sliding plane. It is often assumed that the sliding plane is fixed and given by the geometry of the equipment used but it will appear that the deformation mode may be more complicated. 22.1 Direct shear test The simplest apparatus is shown in Figure 22.1. It consists of a box the shear box of which the upper half can be moved with respect to the lower half by means of a motor which pushes the lower part away from the upper part which is fixed in horizontal direction. N T T Figure 22.1 Direct shear test. by means of a motor which pushes the lower part away from the upper part which is fixed in horizontal direction. The cross section of the container usually is rectangular but circular versions also exist. The soil sample is loaded initially by a vertical force only applied by the dead weight of a loading plate and some additional weights on it through the intermediary of a small steel plate on top of the sample. Because of this plate the sample is free to deform in vertical direction during the test. The actual consists of the lateral movement of the lower half of the box with respect to the upper half at a constant small speed with a horizontal force acting in the plane between the two halves. This force gradually increases as the box moves and is measured by a pressure ring or a strain gauge. The horizontal force reaches a maximum value after some time and the force remains more or less constant afterwards or it may slowly increase or decrease. It seems logical to assume that the maximum value of the horizontal force Tf is related to the vertical force N by a relation of the form Tf cA N tan Ộ 22.1 where A is the area of the sample c is the cohesion of .