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Như đã đề cập trước đây, một trong những đặc điểm chính của đất là các biến dạng trượt tăng dần khi cắt những căng thẳng gia tăng, và đủ lớn cắt nhấn mạnh đất cuối cùng có thể thất bại. Trong tự nhiên, hoặc trong thực hành kỹ thuật, đập, đê, kè cho đường sắt hoặc đường cao tốc có thể thất bại một phần của khối lượng đất trượt trên lớp đất bên dưới nó. Ví dụ, hình 20,1 cho thấy sự thất bại của một độ dốc nhẹ nhàng ở Na Uy, trong một đất sét. Nó. | Chapter 20 SHEAR STRENGTH Figure 20.1 Landslide Hekseberg. As mentioned before one of the main characteristics of soils is that the shear deformations increase progressively when the shear stresses increase and that for sufficiently large shear stresses the soil may eventually fail. In nature or in engineering practice dams dikes or embankments for railroads or highways may fail by part of the soil mass sliding over the soil below it. As an example Figure 20.1 shows the failure of a gentle slope in Norway in a clay soil. It appears that the strength of the soil was not sufficient to carry the weight of the soil layers above it. In many cases a very small cause such as a small local excavation may be the cause of a large landslide. Other important effects may be the load on the structure such as the water pressure against a dam or a dike or the groundwater level in the dam. In this chapter the states of stresses causing such failures of the soil are described. In later chapters the laboratory tests to determine the shear strength of soils will be presented. 20.1 Coulomb It seems reasonable to assume that a sliding failure of a soil will occur if on a certain plane the shear stress is too large compared to the normal W Figure 20.2 Block on slope. stress. On other planes the shear stress is sufficiently small compared to the normal stress to prevent sliding failure. It may be illustrative to compare the analogous situation of a rigid block on a slope see Figure 20.2. Equilibrium of forces shows that the shear force in the plane of the slope is T W sin a and that the normal force acting on the slope is N W cos a where W is the weight of the block. The ratio of shear force to normal force is T N tan a. As long as this is smaller than a certain critical value the friction coefficient f the block will remain in equilibrium. However if the slope angle a becomes so large that tan a f the block will slide down the slope. On steeper slopes the block can never be in .
