Permeability Test of soil Using Constant and Falling Head Method
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1) The document describes laboratory tests to determine the coefficient of permeability of soil samples using the constant head and falling head methods. 2) For the falling head test on a sandy soil sample, the average permeability was found to be 0.00322 cm/sec. 3) For the constant head test on a second sample, the average permeability was determined to be 0.02069 cm/min.
Permeability Test of soil Using Constant and Falling Head Method
- 1. University of Duhok College of Engineering Civil Department Soil Mechanics – Practical Soil Laboratory A Report About : Permeability Test Using Constant & Falling head Method Submitted By : Jameel Masoud Lab. No. : 3 Group Name : ---- Testing Date : 25 / 3 / 2019 Submitting Date : 1 / 4 / 2019 Supervised By : Hussein 2018 – 2019
- 2. Introduction Permeability is the property of soil which permits flow of water through it from points of high to points of low energy, due to existence of interconnected voids. The degree of permeability is characterized by the permeability coefficient k. Two general laboratory methods arc available for determining the coefficient of permeability of a soil directly. These are the constant head method for soils of high permeability (e.g. gravel & sand) and the foiling head method for soils of intermediate and low permeability (e.g. silts and clays). A soil is highly pervious when water can flow through it easily. In an impervious soil, the permeability is very low and water cannot easily flows through it. Purpose: To Introduce the student, the method of determining the Coefficient of permeability of soil. Significant: The permeability of a soil mass is necessary in: (a) Estimating the quantity of underground seepage under various hydraulic conditions. (b) Estimating problems involving pumping of water for underground construction, (c) Determining the rate of settlement (consolidation) of a foundation, (d) Stability analyses of earth dams and earth retaining structures subjected to seepage forces. Supplies and materials for Constant Head Method: 1- Pentameter cell, with loading piston, perforated plates, flow tube connections, p1ezometer connections and air bleed valve. 2- Glass piezometer tubes mounted on a stand with a graduated scale. 3- Graduated cylinder. 4- Inlet water tank with overflow to maintain a constant head. 5- Outlet water tank with overflow to maintain a constant head. 6- Supply of clean water. 7- Small tools funnel tamping rod …. Etc. (Sample preparation) 8-Timer, Thermometer, Ruler (Scale 9- Vacuum device
- 3. Supplies and materials for Falling Head Method: 1-Perneameter device. 2- Standpipe with ring stand and clamp. 3- Distilled water. 4-Timer. 5- Thermometer. 6- Graduated cylinder. 7- Ruler (scale). Test Procedure Constant Head Test : 1) Remove a loading piston of permeameter cell, then measure the inside diameter, height of it and the distance between centers of each piezometer connection points. 2) Place graded gravel filter materials in the bottom of the cell to a depth about 40mm and level the surface, then put wire gauze on it. 3) Place the sample in the pe1meameter cell by one of three following methods, a) Compacting by rod. b) Dry pouring. c) Pouring through water. 4) After placing the sample in the cell measure the length of it at two or three positions and calculate the average length (Ls). 5) Put a wire gauze on the top of the soil sample, then place graded gravel filter materials in the top f the soil sample to a depth about 40mm and level the surface, then assembly the top part of the pentameter cell. 6) Saturated the specimen by immersing in water for water several days (15 min) or by other methods. 7) Connect the top of the cell to the inlet water tank and the base to the outlet water tank; also connect the piezometer to the cell. 8) Turn on the water supply to the constant head device which should be at low level. 9) Allow water to flow through the sample until the conditions appears to be steady and water level in the piezometer tubes remains constant. 10) To start a test run, empty the measuring cylinder and start time at the instant the cylinders placed under the outlet overflow. 11) Record the clock time at which the first run is started.
- 4. 12) Read the level of water in the piezometer tubes (h1, h2 and hj), and measure the water temperature (TO C) in the outlet tank, when the level in the cylinder reaches a predetermined level. 13) Empty the cylinder and make two to three repeat runs at about (5 min.) intervals. Falling head Test : 1) Measure the inside diameter and height of the permearneter cell. Determine the standpipe area (a) by measuring the volume of water contained in a standpipe section of given height. 2) Put the soil sample on the filter paper in the permeameter (for coarse soil by placing a known weight of the sample to determine its unit weight and for fine soil by compaction inside the permeameter cell (compaction mold), or using undisturbed sample). 3) Put the lilter paper on the top of the soil sample and assembly the top part of the permeameter device. 4) Saturated the specimen by immersing in water for several days ( 15 min). It is important that the specimen be fully saturated; otherwise, the falling head test will give erroneousresults. Then apply a vacuum to remove an entrapped air in the specimen. Prevent to re-enter air to the specimen before testing by closing the clamp. 5) Locate h1 and h2 on the standpipe, and then fill it with distilled water. 6) Commence the flow of water and simultaneously start timing the test. Allow water to flow through the sample until the standpipe is almost empty or to a convenient mark. Simultaneously stop the flow and timing. Obtain the head h2. Take the temperature of the test. 7) Refill the standpipe and repeat the test two additional times. Use the same h1 and h2 values and obtain the corresponding elapsed times. Take the temperature for each run
- 5. Calculation : For Falling head test : L=11.6 cm D= 10.2 cm A = π 4 10.22 = 81.674 cm2 a= 1.71 cm2 For Example For t=54.1 sec , h1=51.1 cm , h2=51.1 cm At T=21O C K at 21O C = 𝑎𝐿 𝐴𝑡 𝑙𝑛 ( ℎ1 ℎ2 ) = 1.71×11.6 81.674×180 𝑙𝑛 ( ℎ1 ℎ2 ) = 0.00334 𝑐𝑚 𝑠𝑒𝑐⁄ K at 20O C =𝑘 𝑎𝑡 21 𝑜 C × 𝜇21 𝜇20 = 0.00334 × 0.976 = 0.003256 𝑐𝑚 𝑠𝑒𝑐⁄ Table: NO. h1 (cm) h2 (cm) T (sec) Q (𝒎 𝟑 ) T (C) Kt at (T) 𝒄𝒎 𝒔𝒆𝒄⁄ 𝜇 𝑡 𝜇20 K at 20C 𝒄𝒎 𝒔𝒆𝒄⁄ 1 51.1 24.3 54.1 45.8 21 0.003337 0.976 0.00326 2 51.1 24.3 54.7 45.8 21 0.0033 0.976 0.00322 3 51.1 24.3 55.3 45.8 21 0.003265 0.976 0.00319 Average 0.00322 For Constant Head Test: L= 20.3 cm , D = 7.62 cm , A = π 4 7.622 = 45.581cm2 , a = 132 cm b = 45 , ∆ℎ = 𝑎 − 𝑏 = 132 − 45 = 87 𝑐𝑚 For Example For t=180 min , Q=775 𝒄𝒎 𝟑 At T=23O C K at 21O C 𝑄𝐿 𝐴∆ℎ𝑡 = 775×20.3 45.581×87×180 = 0.022 𝑐𝑚 𝑚𝑖𝑛⁄ K at 20O C =𝑘 𝑎𝑡 21 𝑜 C × 𝜇23 𝜇20 = 0.022 × 0.931 = 0.02052 𝑐𝑚 𝑚𝑖𝑛⁄
- 6. Table: Discussion and Conclusion: In this test we determine the permeability of two sample of soil using Falling and constant head method, finally we get a result after doing all procedure accurate which is (0.02069 𝑐𝑚 𝑚𝑖𝑛⁄ for sample that used for constant head and 0.00322 𝑐𝑚 𝑠𝑒𝑐⁄ for sample that used for Falling head ), as shown from result we can know the type of soil and degree of permeability for example our sample that used for falling and constant head have a degree of permeability of low and their type is (sand, dirty sand or silty sand). But the result of permeability will be affected by changing the size and shape of particle, void ration, degree of saturation and the important one is fluid viscosity. It’s important to know the degree of permeability depend on the soil that used for project for example soil that used for dam is have a lower permeability than soil used for Normal building. the error that will occur during test is the viscosity of water, it depends on temperature and the specified temperature is (20 C), if changed it must be changed to viscosity of temperature (20 C) using the related formula. In conclusion the purpose of this test is to determine the permeability of soil sample using both falling and constant head method and know is the value of permeability is acceptable with project specification or not. No. Time from start (min) Measured Volume (𝒄𝒎 𝟑 ) Temperature (C) k at Temperature 𝒄𝒎 𝒎𝒊𝒏⁄ 𝝁 𝒕 𝝁 𝟐𝟎 k at 20 C 𝒄𝒎 𝒎𝒊𝒏⁄ 1 180 775 23 0.02204 0.931 0.02052 2 180 772 22 0.021955 0.953 0.020924 3 180 761 22 0.021643 0.953 0.020625 Average 0.02069
- 7. Question for further Discussion : Q1 Neither the constant-head nor the falling-head laboratory provides a reliable value for the coefficient of permeability of a soil? A Falling head have reliable value because constant head from the name it has constant head but in practical we don’t have constant head. Q1 Does the permeability coefficient increases or decreases with water temperature? Why? A Yes, because as the temperature increases, the viscosity of water decreases and the coefficient of permeability increases (the flow rate increases).