|THE COOPER UNION
Albert Nerken School of Engineering
Soil Mechanics Laboratory
Experiment No. 7 - Variable Head Permeability Test
|To measure the coefficient of permeability of several cohesive soil samples; and
to study the relationship between the coefficient of permeability and voids ratio.
|1) 'Soil Testing for Engineers' by T. W. Lambe; Chapter VI.
2) "Engineering Properties of Soils and Their Measurements", 4th ed. by Joseph E. Bowles; Experiment No. 9.
3) A.S.T.M. Standards, 1978, Part 19; Designation D698-70, D1557-70.
|1) The -No. 4 material from Experiment No. 5 will be used to
prepare the samples for this test.
2) The specific gravity of this soil is Gs @ 20†C, and was obtained in Experiment No. 2.
3) Instead of the cast iron mold used in Experiment No. 5, a plastic mold will be used. The diameter should be measured in cm., and from this determine the cross-sectional area. The height should also be measured in cm, and will eventually be the height of the sample. From the area and height, determine the volume of the sample. Record these values on Data Sheet (A).
4) Take the bottom of one of the cast iron molds used in Experiment #4 and weigh it together with the plastic mold to the nearest 0.5 gm. Record this as the weight of permeameter, W1, on Data Sheet (A).
5) Place the -No. 4 material into a stainless steel pan, and add the appropriate amount of de-ionized water necessary to bring the water content of the soil to the optimum water content determined for this sample in Experiment No. 5. If the sample is completely dry, the chances of eventual total saturation are small and very time-consuming. If too much water is added, good compaction is difficult.
6) Add water by using a spray bottle, with a slight mist while mixing the soil with a spatula.
7) When water is added, take a sample for a water content detrmination. Then cover the soil with wet paer towels to prevent the escape of moisture.
8) Place the soil in the mold in three layers, each layer having the same thickness when compacted. Each layer is compacted with a 5.5 lb. hammer, using a 12" drop and 25 blows evenly distributed over the cross-sectional area of the layer. Before placing the next layer, scour the top of the last layer to assure bonding between layers (homogenous). Also make sure that last layer is above the crack between the mold and the collar. Remove the collar and level off the top of the sample.
9) Take the weight of the bottom of the cast iron mold and the plastic mold, filled with the wet soil, to the nearest 0.5g. Record this as the weight of the permeameter and sample on Data Sheet (A).
10) Determine the weight of the wet soil, WT.
11) Remove the bottom of the mold and place the mold and wet soil in the permeameter. The bottom of the apparatus should have de-aired, distilled water in it, and a porous stone, which has been soaking. Ensure that no air is trapped between the water and the stone. The mold and wet soil is placed upon the porous stone. When the mold and soil are in place, the top of the permeameter, which also contains a porous stone that has been soaking, is carefully put into place. Make sure that a tight fit is secured with the gaskets so that no leaks develop.
12) When the sample is ready, place it into a large brass pan filled with water, and back-saturate the sample by connecting the plastic tubing coming from the plastic reservoir filled with water. Indication of saturation is when water is draining from the top of the sample.
13) Repeat steps 3 to 12 to prepare three additional samples.
14) Fill the reservoir on the variable head permeability apparatus until it is full of water. This will be known when water begins to come out of the pit-cock at the top of the reservoir. This pit-cock has been open during the entire filling process; now close the pit-cock.
15) Choose the entrance tube with the same diameter as the spigot at the top of the top of the permeameter which is to be used.
16) Open the valve on the variable head reservoir for the entrance tube chosen, and water will pass from the reservoir into the entrance tube. The water will rise to the of the water in the constant head tank.
17) Connect plastic tubing from the entrance tube chosen to the top of the permeameter, while disconnecting the plastic tubing from the bottom of the permeameter, which is coming from the plastic back-saturating reservoir. The test is ready to begin.
18) Determine h0, which is the distance, in cm, from the tail water in the large brass pan to the water level in the entrance tube at t=0. Note: Behind the entrance tube is a sheet of graph paper with a scale indicated on it. However, the distance from the tail water to 0 must be added to the value indicated by the scale to obtain the head. Therefore, h0 = scale + x. Use x = 62.5cm. Record h0 on Data Sheet (B).
19) Set the time increment at which readings are to be taken.
20) Close the valve for the entrance tube and the test is ready to begin. The water level in the entrance tube will begin to fall.
21) Determine h1, which is the distance in cm from the tail water in the large brass pan to the water level in the entrance tube at t = t > 0. The h1 reading is taken at selected time increments, and h1 = scale + x. Record the time and h1on Data Sheet (B).
22) Place a thermometer in the brass pan and determine the temperature of the water, T1, in †C, and record it on Data Sheet (B).
23) Determine the ratio h0/h1 and record it on Data Sheet (B).
24) Convert the time at temperature T1 to the time at temperature T2 = 20†C, which is the standard temperature, using: time@T1 / time@T2 = (µ@T1 / µ@T2)(g@T2 / g@T1), where µ = absolute viscosity of water, and g is the unit weight of water. Record the time on Data Sheet (B).
|1) Compute the dry unit weight, void ratio, and porosity.
2) Compute the permeability at 20†C.
3) Compute the absolute coefficient of permeability, in cm2.
|1) Make a plot of h0/h1 as the ordinate,
versus the time at T2=20†C, in seconds, as the abcissa. Note that h0/h1
is plotted on a natural log scale [ ln(h0/h1) ].
|1) What is the degree of permeability according to the Terzaghi
and Peck table?