Cơ học đất là một nhánh liên ngành của cơ học ứng dụng, địa chất công trình nghiên cứu các tính chất vật lý, cơ học của đất để áp dụng vào mục đích xây dựng, các nguyên nhân quyết định các đặc trưng đó, nghiên cứu trạng thái ứng suất - biến dạng của đất. | 36 va ki -k where the constant k is the coefficient of permeability sometimes called the hydraulic conductivity and has the dimensions of a velocity. Although the value of k is constant for a particular soil at a particular density it varies to a minor extent with viscosity and temperature of the water and to a major extent with pore size. For instance for a coarse sand k may be as large as cm s 3x105 ft yr and for clay particles of micron size k may be as small as 3x10-8 cm s 3x10-2 ft yr. This factor of 107 is of great significance in soil mechanics and is linked with a large difference between the mechanical behaviours of clay and sand soils. Fig. Results of Permeability Test on Leighton Buzzard Sand Typical results of permeability tests on a sample of Leighton Buzzard sand between Nos. 14 and 25 . Sieves for a water temperature of 20 C are shown in Fig. . Initially the specimen was set up in a dense state achieved by tamping thin layers of the sand. The results give the lower straight line OC which terminates at C. Under this hydraulic gradient ic the upward drag on the particles imparted by the water is sufficient to lift their submerged weight so that the particles float as a suspension and become a fluidized bed. This quicksand condition is also known as piping or boiling. At this critical condition the total drag upwards on the sand between the levels of A and B will be At õu Ảtỵwbh and this must exactly balance the submerged weight of this part of the sample namely Atỵ ỏs. Hence the fluidizing hydraulic gradient should be given by i - it Y Gzl. ds Yw 1 e For the sand in question with e obtained by measuring the weight and overall volume of the sample and Gs eq. gives a critical hydraulic gradient of which is an underestimate of the observed value of which included friction due to the lateral stresses induced in the sand sample in its preparation . If during piping the supply of water is rapidly stopped the .
