Ứng suất trong một lớp 5,1 Dọc nhấn mạnh Trong nhiều nơi trên trái đất đất bao gồm các lớp thực tế ngang. Nếu một đất như vậy không mang theo một tải trọng bề mặt địa phương, và nếu nước ngầm ở phần còn lại, những căng thẳng theo chiều dọc có thể được xác định trực tiếp từ một xem xét trạng thái cân bằng theo chiều dọc. Thủ tục này được minh họa trong chương này. Một trường hợp đơn giản là một lớp đồng nhất, hoàn toàn bão hòa với nước, xem Hình | Chapter 5 STRESSES IN A LAYER Vertical stresses In many places on earth the soil consists of practically horizontal layers. If such a soil does not carry a local surface load and if the groundwater is at rest the vertical stresses can be determined directly from a consideration of vertical equilibrium. The procedure is illustrated in this chapter. A simple case is a homogeneous layer completely saturated with water see Figure . The pressure in the water is determined by the location of the phreatic surface. This is defined as the plane where the pressure in the groundwater is equal to the atmospheric pressure. Figure Stresses in a homogeneous layer. If the atmospheric pressure is taken as the zero level of pressures as is usual it follows that p 0 at the phreatic surface. If there are no capillary effects in the soil this is also the upper boundary of the water which is denoted as the groundwater table. It is assumed that in the example the phreatic surface coincides with the soil surface see Figure . The volumetric weight of the saturated soil is supposed to be Y 20 kN m3. The vertical normal stress in the soil now increases linearly with depth ơzz Yd. This is a consequence of vertical equilibrium of a column of soil of height d. It has been assumed that there are no shear stresses on the vertical planes bounding the column in horizontal direction. That seems to be a reasonable assumption if the terrain is homogeneous and very large with a single geological history. Often this is assumed even when there are no data. At a depth of 10 m for instance the vertical total stress is 200 kN m2 200 kPa. Because the groundwater is at rest the pressures in the water will be hydrostatic. The soil can be considered to be a container of water of very complex shape bounded by all the particles but that is irrelevant for the actual pressure in the water. This means that the pressure in the water at a depth d will be equal to the weight of the water 31 Arnold .
