Lab 5 - Constant Head Permeability Test PDF

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Constant Head Permeability Test Lab 5 CIVE 334 Dr. Song Due February 27th, 2018

Purpose The purpose of this lab is to determine the coefficient of permeability for sand in the laboratory via the Constant Head test for permeability. This test is an easy, cost effective way to determine permeability in a laboratory setting.

Equipment Used

Figure 1. Constant Head Permeameter

Other Equipment : -

Sand Balance Distilled Water

-

Rubber Tubing Stopwatch Graduated Cylinder

Test Procedure 1. Determine mass of the plastic specimen tube, the porous stones, the spring and the two rubber stoppers of the permeameter, then assemble permeameter 2. Pour sand into the specimen tube, creating small compacted layers as we poured, placing the top porous stone at about two thirds of the height of the tube 3. Determine the mass of the assembly, M2 , including the spring and the top rubber stopper 4. Run water into the funnel from a sink via the rubber tubing, and in to the specimen chamber, adjusting the water supply to make sure that the water level remains constant in the chamber 5. Record the flow rate of the water, Q, flowing from the chamber to the graduated cylinder, record time with a stopwatch 6. Repeat step 5 two more times, using the same time value for collection, and take the average 7. Change the head difference, h, and repeat steps 11 – 13 two more times, and record the temperature of the water, T

Results Data Table Table 1. Constant – Head Test data

Item

1

Test No. 2

3

1. Average Flow, Q (cm3)

366

310

234

2. Time of collection, t (s)

60

60

60

22

22

22

47

37

28

6.35

6.35

6.35

6. Length of specimen, L (cm)

10

10

10

7. Area of specimen, A = π/4*D2 (cm2)

31.67

31.67

31.67

3. Temperature of water, T (°C) 4. Head Difference, h (cm) 5. Diameter of specimen, D (cm)

8. k = QL/Aht (cm/s)

0.1118

0.1203

0.1199

Average k =

0.1118 + 0.1203 + 0.1199 =0.1173 cm / s 3

k 20 ℃= k T ℃

ηT ℃ =0.1173∗ 0.953=0.1118 cm / s η20℃

Table 2. Void Ratio Data

1. Volume of specimen, V = π/4 D2L (cm3) 2. Specific Gravity of soil solids, Gs

316.69 2.66

3. Mass of specimen tube with fittings, M1 (g) 4. Mass of tube with fittings and specimen, M2 (g)

2640.01 3233.14

5. Dry density of specimen, ρd = (M2 - M1)/V (g/cm3)

1.872904

6. Void ratio, e = Gsρw/ρd - 1

0.420254

Sample Calculations

Dry Density –

ρd =

M 2−M 1 3233.41 g−2640.01 g = =1.873 g /cm3 2 π π 2 ∗D ∗L ∗( 6.35 cm) ∗10 cm 4 4

Void Ratio –

2.66∗1 g G ρ cm3 e= s w −1= −1 =0.4203 4 ρd g 1.873 3 cm k, Hydraulic Conductivity – k=

QL 366 cm 3∗10 cm =0.1118cm / s = Aht 31.67 cm2∗47 cm∗60 sec

K at 20°C – k 20 ℃= k T ℃

ηT ℃ =0.1173∗ 0.953=0.1118 cm / s η20℃

Discussion The exact D10 was not available, so the calculation of the k using equations 13.5 and 13.6 and Fig 13-3 were not possible. However, looking up a possible value for the D10 of 49 mm yields a conductivity of 0.07498, meaning that in equation 13.6 the actual value for the D10 of our soil would be higher than 49 mm. This can be concluded because the e value used in the equation was the e value we found with our soil. Though the assumption that the void ratio for a smaller diameter soil would be the same is not correct, it provides some insight about how our soil used likely had a higher diameter because of the difference in the hydraulic gradient.

Sources of Error A number of potential errors could have taken place. These include improper assembly of the permeameter which could lead to inaccurate numbers for flow through the device. Also, using a stopwatch or timer that was not correctly calibrated would yield inaccurate times measuring, leading to data that would deviate from its true value. By using a general temperature for the water, such as 20 °C instead of an accurate, measured value would also yield inaccurate data values. Another potential source of error would be not using the same scale for all measurements, including when taking the different measurements of the permeameter. Some of the scales may not be completely accurate and may be biased. By using the same scale for every measurement, the bias can be eliminated....