Constant-Head Permeability Test free essay sample

In the design of engineering projects, one of the most important soil properties of interest to the soils engineer is permeability. To some degree, permeability will play a role in the design of almost any structure. For example, the durability of concrete is related to its permeability. In designs that make use of earthen materials (soils and rock, etc. ) the permeability of these materials will usually be of great importance. Soils are permeable (water may flow through them) because they consist not only of solid particles, but a network of interconnected pores. The degree to which soils are permeable depends upon a number of factors, such as soil type, grain size distribution and soil history. This degree of permeability is characterized by the coefficient of permeability. A number of different methods for determining the coefficient of permeability for soils exist, including in-situ methods and laboratory methods. In the laboratory, two common tests are generally used to determine this soil property. We will write a custom essay sample on Constant-Head Permeability Test or any similar topic specifically for you Do Not WasteYour Time HIRE WRITER Only 13.90 / page These two tests are the falling head permeability test and the constant head permeability test. Which test is used depends upon the type of soil to be tested. For soils of high permeability (sands and gravels) a constant head test is used. For soils of intermediate to low permeability, a falling head test is used. As we were testing sand we used a constant head permeability test. By carrying out the constant head permeability test we can determine the coefficients of permeability of given sand over a range of unit weights. During the test we can also observe the phenomenon of piping. Initially adjust to a difference of 20 mms. Allow to stand for a few minutes and adjust if necessary. Using a measuring cylinder and a stop watch determine the rate of flow then accurately measure the head difference in the stand pipes and hence determine the hydraulic gradient. Increase the flow to give a head difference of about 40mms and repeat above steps. Keep increasing by increments of about 20mms till piping occurs. When piping occurs note critical value. Compact sample to produce a dense packing then repeat experiment. At best the constant head permeability test gives only a guide to the coefficient of permeability of the soil under test. The reasons are as follows. It is difficult to ensure that there has been no sample disturbance. This is particularly so with course grained soils where, because it is almost impossible to take undisturbed samples, the sample is compacted into the peraeameter. This in turn meaning that although we can gain accurate readings and results on the sand we tested in the lab, in practise the tested subjects would behave differently on site. The behaviour and properties of soils over a large area vary from those of a test sample. Within a large mass of soil there are permeability variations, possibly in every direction, over quite small plan areas. Consequently it is necessary to test representative samples in order to establish valid permeability characteristics. Where the direction of seepage is predominantly horizontal or at an inclined angle, observation of the permeability characteristics in a vertical direction is of little practical value.