Determination of Coefficient of Permeability by Constant Head Permeability - Assignment Example

Determination of coefficient of permeability by constant head permeability Introduction Permeability can be defined as being a measure of the facility with which water flows through soil and is of importance in seepage, stability and ground water lowering problems( Mlynarek, Z,. 2010). It is defined by Darcy’s Law: Where q represents discharge in unit time; I is hydraulic gradient; A gives the cross-sectional area while k represents coefficient of permeability. In the constant head permeability test, water at a constant head is passed through a soil sample of known dimension and discharge measured. The test is suitable to medium and coarse sands of relatively high permeability. Apparatus 1. Constant head apparatus 2. Stop watch 3. Measuring cylinder and beaker Methodology In experiment two samples which are already placed in permeability cells are supplied. The constant head reservoir was also prepared in advance and it was used in discharging water through the material. The level of water in the two manometer tubes was noted, then the time required to collect a suitable quantity of discharge water was measured and this process was repeated four more times. The stop valve was turned so as to make adjustment to the level of water in manometer tubes and the rate of discharge. The process was repeated so as to have five different discharge rates. Calculations Where k is coefficient of permeability; Q is volume of water that is collected with time t; l is the length of sample between manometer tappings; h is the head difference in the two manometer tubes and A is the cross sectional area of sample Test1 Test2 Test3 Average permeability of the soil k = Discussions Theories that have been established basing on a number of studies give solution for determination of coefficient of permeability where for example basing on the grain size distribution curve or for evaluation of the findings recorded by Khafagi probe but still doubts can be accommodated on whether or not the values that have been determined are appropriate where the measurements made is a replica of nature or give a revelation the k value characteristics for natural permeation. There are local and laboratory measurements of coefficient of permeability that have been done, soil mechanics exploration and identification studies that have been done in the bid to have answers to the questions. It is very important to increase the level of accuracy in the measurement of k factor, due to the fact that the relative error of calculations, processing as well as geometric dimensioning may be of magnitudes smaller than what can be determined as the coefficient of permeability. There is need to have high accuracy since uncertain measurements may result to over dimensioning that is not justifiable in some occasions resulting to unnecessary risks being run into. This makes it very important knowing the true value of k factor so as to avoid this. Conclusion The permeability test is used in measuring the rate of the flow of water through soil. In the test water forced to pass through a soil specimen by a constant pressure with the dimensions of the soil specimen being known and thus making it possible to determine the rate of flow ( Kézdi, Á., 1976). This test find application in the determining how suitable sands and gravel are for drainage application and is made only on remolded samples. This test is restricted to materials with coefficient of permeability of about 300mm/day or higher (Lumb P. ,1966). The constant head test find application on samples that are a representation of materials that are to be used as back fill for abutment to act as permeable materials for under drains and as sand blanket for sand drains areas. References Kézdi, Á. (1976). Soil mechanics, (in Hungarian), Tankönyvkiadó,Budapest Kovács Gy. (1972). Seepage hydraulics, (in Hungarian), Akadémiai Kiadó, Budapest Lumb P. (1966). The variability of natural soils. Canadian Geotechnical Journal Nr 3. Mlynarek, Z. (2010). Quality of in situ and laboratory tests contribution to risk management. Experiment 2 Determination of shear strength by the direct shear box test Introduction This is a direct shear test in which failure is induced on a given plane. The soil is contained in metal box which is split horizontally into two halves. The soil is made to fail by forcing the two halves of the box in opposite directions (Stark, T. D., 1994). Drainage condition cannot be easily controlled and therefore for permeable soils such as sand, the apparatus can only be used for drained test. Apparatus 1. Constant rate of strained shear box and accessories. 2. Stop watch Methodology First there weighing of about 200g of dry sand and then two halves of the box was screwed together. The toothed perforated grid was then placed in the bottom of the box with care being taken to ensure the ridges run perpendicular to the indicated plane of movement. The next step involved sand being placed in the box in five equal layers with each layer being tamped down ten times. The surface of the sand was then leveled and then another toothed perforated grid was placed on top with the ridges such that they were pointing into sample and it was ensured that they were perpendicular to the plane of movement. The pressure pad was placed in position such that the bottom edge of the chamfer was level with the top of the box. The proving ring was then brought into contact with box by use of the adjusting screw. The yoke was then placed on top of the pressure pad after the ball bearing was placed on the pressure pad. Lastly a mass was placed on the hanger so that total mass applied added to 9kg. The next phase involved removing the red screws from the box. First the top half of the box was raised 1mm through adjustment of the silver screws on the box with the vertical dial gauge being set in position on top of the yoke. The motor drive was then engaged and the motor switched on. The proving ring and vertical gauge values were then recorded simultaneous at intervals of 15s, this being continued after failure up to a point when there was no change in stress gauge readings for three consecutive times the readings were being made. The steps up to the point of engaging a motor drive and switching motor on were repeated with total loads of 18kg and 27kg being used. The entire process including the last step was repeated with other sand samples being used. Calculations From the data that was obtained in the experiment shear stress and normal stress is calculated This is converted to  This is converted to  Shear stress Test 1 Peak=114x0.8071=90.0094=0.092Kn Ultimate=83x0.8071=66.989N =0.067Kn From shear stress = Peak shear stress= Ultimate shear stress= Test 2 Peak=222x0.8071=179.1762N =0.179kN Ultimate=166x0.8071=133.9786N 0.134kN From shear stress = Peak shear stress= Ultimate shear stress= Test 3 Peak=250x0.8071=201.775N0.202kN Ultimate=216x0.8071=174.3334N =0.174kN From shear stress = Peak shear stress= Ultimate shear stress= Normal stress calculation Test 1 Normal stress == Test 2 Normal stress == Test 3 Normal stress == Discussion From the graph it can be seen that the angle for shearing resistance for peak stress is higher than that for ultimate stress with respective values of 33.60 and 30.960. The cohesion values are also higher for peak stress than for ultimate stress with values of 14Kn/m2 and 5.8Kn/m2 respectively. Conclusion It is possible to obtain shear strength parameters of unsaturated soil by use of direct shear apparatus, with modification being made so as to apply matric suctions greater than 1 atm to the soil specimen (Bromhead,1992). The direct shear test uses a relatively thin specimen, in comparison with the triaxial test, and can significantly reduce the time required for testing unsaturated soils of low permeability. References Bromhead, E. N. (1992). The stability of slopes; Blackie Academic & Professional, Glasgow, UK, Manual of Soil Laboratory Testing(1994). Permeability, Shear Strength and Compressibility Tests; John Wiley & Sons, Inc., New York, Stark, T. D., Eid, H. T.(1994). Drained residual strength of cohesive soils; Journal of Geotechnical Engineering – ASCE; 120(5), ,pp. 856-871. Read More