tailieunhanh - SOIL MECHANICS - CHAPTER 7

Tính thấm 7,1 thấm thử nghiệm dh. ds Trong pháp luật Darcy chương trước cho dòng chảy của chất lỏng thông qua một môi trường xốp đã được xây dựng, trong hình thức đơn giản của nó, như q = k () Điều này có nghĩa là k dẫn thủy lực có thể được xác định nếu việc xả cụ thể q có thể được đo trong một thử nghiệm, trong đó dh Gradient / ds được biết đến. Một ví dụ về một thiết lập kiểm tra được thể hiện trong hình . Nó bao gồm một ống thủy tinh chứa đầy. | Chapter 7 PERMEABILITY Permeability test In the previous chapter Darcy s law for the flow of a fluid through a porous medium has been formulated in its simplest form as q dh kkds- This means that the hydraulic conductivity k can be determined if the specific discharge q can be measured in a test in which the gradient dh ds Ah AL o o Figure Permeability test. is known. An example of a test setup is shown in Figure . It consists of a glass tube filled with soil. The two ends are connected to small reservoirs of water the height of which can be adjusted. In these reservoirs a constant water level can be maintained. Under the influence of a difference in head Ah between the two reservoirs water will flow through the soil. The total discharge Q can be measured by collecting the volume of water in a certain time interval. If the area of the tube is A and the length of the soil sample is AL then Darcy s law gives Q kA AL Because Q qA this formula is in agreement with . Darcy performed tests as shown in Figure to verify his formula . For this purpose he performed tests with various values of Ah and indeed found a linear relation between Q and Ah. The same test is still used very often to determine the hydraulic conductivity coefficient of permeability k. For sand normal values of the hydraulic conductivity k range from 10-6 m s to 10-3 m s. For clay the hydraulic conductivity usually is several orders of magnitude smaller for instance k 10-9 m s or even smaller. This is because the permeability is approximately proportional to the square of the grain size of the material and the particles of clay are about 100 or 1000 times smaller than those of sand. An indication of the hydraulic conductivity of various soils is given in Table . 45 Arnold Verruijt Soil Mechanics 7. PERMEABILITY 46 Type of soil k m s gravel 10-3 - 10-1 sand 10-6 - 10-3 silt 10-8 - 10-6 clay 10-10 - 10-8 Table Hydraulic conductivity k. As mentioned before the .