Permeability Tests. Geotechnical Engineering, Laboratory 7

Transcription

1 1. General Notes Permeability Tests Geotechnical Engineering, Laboratory 7 Be prepared to get wet. All stations, benches, and tables must be cleaned before your group leaves the lab. Please clean all glassware and graduated cylinders. The tubing has an inner diameter of ½ in. 2. Sieve Analysis 2.1. Form a subgroup consisting of one member from each group to perform a sieve analysis on the soil to be used in the constant and falling head tests 2.2. Select a series of sieves appropriate for the particles sizes of the sand to be tested in the Constant Head setup Perform a sieve analysis on a dry sample of the soil. All groups will use the results of this one sieve analysis. Following the procedures from the sieve analysis laboratory session. 3. Constant Head Test 3.1 Enter names onto the data sheet. 3.2 Find the empty permeameter cell on the bench The cell needs to be prepared Measure the height and diameter of the inside of the cylinder Place the provided soil into the cell in thin lifts. After placing each loose lift, lightly tamp the soil to densify it slightly Fill with soil until the top level of the soil is flush with the top of the permeameter Screw the top plate onto the top of the cell until it is tight Pour water through the funnel into the soil to saturate the sample in the cell. It may take a while to get all the air bubbles out of the system. 3.3 Fill a glass beaker with distilled water. 3.4 Have someone stand on a chair to pour water in the funnel to the appropriate mark. 3.5 Hold a graduated cylinder beneath the outlet tube to catch the water that flows through the cell. 3.6 Open the valve and start the timer. 3.7 Let water drain into the graduated cylinder until a volume of about 100 ml has been collected. The person adding water should ensure that sufficient water is added to keep water at the specified elevation (constant head) Bret N. Lingwall and Evert C. Lawton Page 1 of 5

2 3.8 Record the volume of water gathered and the time required to collect the water. 3.9 Repeat the test three more times with collected volumes of water of approximately 250, 500, and 1000 ml Measure the height of the soil specimen and difference in heights of constant head water levels (total head loss for the system). 4. Falling Head Test 4.1. Enter names onto the data sheet Prepare the soil specimen in the same manner as for the constant head test Fill a glass beaker with distilled water Have someone stand on a chair to pour water into the funnel. Fill to the maximum height mark. This is Head Hold another beaker or other type of container below the outlet to the cell to catch the water that flows through the system Open the valve and start the timer. Hold the ruler next to the tubing to measure heads Let water drain into the glass beaker or other container until the head has dropped to the minimum mark. This is Head Record the time it takes for the water to fall from Head 1 to Head 2. Let water run out until is has reached the minimum mark, then immediately close the outlet valve Repeat the test a second time. Do not remove the soil from the cell and recompact between tests After completion of test, place the soil back in the pan and clean all parts of the permeameter. 5. Calculations Calculate the coefficient of permeability (hydraulic conductivity) of the soil in units of cm/sec from measurements obtained in each of the Constant Head tests. Calculate the coefficient of permeability of the soil in units of cm/sec from measurements obtained in each of the Falling Head tests. Determine the % passing each of the sieves in the sieve analysis. Plot the results and determine D 10 for the soil. Estimate the coefficient of permeability of the soil in units of cm/sec using Hazen s equation (Eq on p. 281 of HKS) Bret N. Lingwall and Evert C. Lawton Page 2 of 5

3 6. Questions Permeability Tests Were the values of coefficient of permeability the same for each of the four trials of the Constant Head test? If not, why not? Compare values of coefficient of permeability obtained from the two types of tests (Constant Head test and Falling Head test). Are the relative values consistent with the types of soils tested? Does Hazen s equation provide a reasonable estimate of the coefficient of permeability for the sand tested in the Constant Head tests? Does this soil meet the % fines and D 10 requirements for proper use of Hazen s equation? Discuss the advantages and disadvantages of the two types of tests. Which of the two types of test would be better to use for a silt? Why? Why do engineers place soils with high permeability beneath roads and foundations? How would the permeability of this soil be affected by a change in void ratio? Bret N. Lingwall and Evert C. Lawton Page 3 of 5

4 Constant Head Permeability Data Sheet Group Member Names: Test Date: Height of Soil : Head 1 = Soil Description : Trial 1 Trial Trial 3 Trial Trial 5 Trial Bret N. Lingwall and Evert C. Lawton Page 4 of 5

5 Falling Head Permeability Data Sheet Group Member Names: Test Date: Height of Soil : Head 1 = Soil Description : Head 2 = Trial 1 Trial 1 Time (s) Head Time (s) Head Bret N. Lingwall and Evert C. Lawton Page 5 of 5