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SOIL MECHANICS - CHAPTER 21

Sự thất bại của một mẫu đất theo cắt có thể có lẽ tốt nhất được điều tra trong một thử nghiệm phòng thí nghiệm, trong đó mẫu là bị distorsion tinh khiết, khối lượng không đổi. Khối lượng này có thể được giữ không đổi bằng cách chăm sóc căng thẳng đẳng hướng σ0 = 1 (σ1 + σ2 + σ3) RE3 nguồn điện liên tục trong thời gian thử nghiệm, hoặc tốt hơn, bằng cách sử dụng một thiết lập thử nghiệm mà trong đó sự thay đổi khối lượng có thể được đo và kiểm soát rất. | Chapter 21 TRIAXIAL TEST The failure of a soil sample under shear could perhaps best be investigated in a laboratory test in which the sample is subjected to pure distorsion at constant volume. The volume could be kept constant by taking care that the isotropic stress ƠQ 3 Ơ1 ơ2 Ơ3 remains constant during the test or better still by using a test setup in which the volume change can be measured and controlled very Figure 21.1 Triaxial test. accurately so that the volume change can be zero. In principle such a test is possible but it is much simpler to perform a test in which the lateral stress is kept constant the triaxial test see Figure 21.1. In order to avoid the complications caused by pore pressure generation it will first be assumed that the soil is dry sand. The influence of pore water pressures will be considered later. In the triaxial test a cylindrical soil sample is placed in a glass or plastic cell with the sample being enclosed in a rubber membrane. The membrane is connected to circular plates at the top and the bottom of the sample with two o-rings ensuring a water tight connection. The cell is filled with water with the pressure in the water the cell pressure being controlled by a pressure unit usually by a connection to a tank in which the pressure can be controlled. Because the sample is completely surrounded by water at its cylindrical surface and at the top a pressure equal to the cell pressure is generated in the sample. The usual and simplest test procedure is to keep the cell pressure constant during the test. In addition to the lateral and vertical loading by the cell pressure the sample can also be loaded by a vertical force by means of a steel rod that passes through the top cap of the cell. The usual procedure is that in the second stage of the test the rod is being pushed down at a constant rate by an electric motor. This means that the vertical deformation rate is constant and that the force on the sample gradually increases. The force .