Astm Permeability Falling Head Lab 134616

Experiment

No. 12

COEFFICIENT OF PERMEABILlTY-FALLlNG-HEAD

METHOD

References See Experiment No. 11. This test has not been standardized by ASTM (or AASHTO). Objective To introduce the student to a method of determining the coefficient of permeability of a finegrained soil (such as fine sand, silt, or clay). The test may also be used for coarse-grained soils. Equipment Permeability device Timer Thermometer Ring stand with test-tube clamp or other means to develop a differential head across soil sample Burette to use (with ring stand or other means of ) as a standpipe General Discussion The general discussion of Experiment No. 11 is also applicable to this experiment. The limitations of the constant-head test are inherent in this test, and, in addition, tests of long duration will require some way of controlling evaporation of water in the standpipe (Fig. 12-1).

Figure 12-1

Falling-head permeability test using the standard compaction mold permeameter. Shown are both a disassembled device and a test setup using a IOO-mlburette. A substantial head loss can occur through the thick porous stone in the base. The small water-entry orifice through the cap may produce a sample cavity from local flow conditions. Care is required to produce a watertight system. Use a meterstick to obtain the hydraulic heads hl and hr;..

101

________

.J

Cap with a rubber balloon for long du ration tests. This may be necessary on exit also.

Burette of cross section area a

k

=

aL In AI

!:J. 112

Ring stand

Rubber tube and tube clamp to connect burette to sample

Figure 12-2

Line details of the falling-head test equipment shown in Fig. 12-1.

lL

Collect wa~r to see if qout -

qin

The equation applicable to this experiment can be derived (making reference to Fig. 12-2) but is left as part of the exercise for the student report and will be merely presented: (12-1) where

a

=

A hI h2 L t In

= =

= =

= =

cross-sectional area of burette or other standpipe (Figs. 12-2 or 11-2), cm'' cross-sectional area of soil sample, cm2 hydraulic head across sample at beginning of test (t = 0) hydraulic head across sample at end of test (t = ttest) length of soil sample, cm elapsed time of test, s logarithm to base 2.7182818 . . .

It should be noted that this method of determining the coefficient of permeability k is primarily for economy, since a test to determine k for a fine-grained soil may be of several days' duration. The constant-head test of Experiment No. 11 would consume a rather large quantity of water in the laboratory to maintain a constant head for most setups. For longduration tests and where the quantity of flow through the soil sample is small, one should make some provision to control evaporation of the water in the standpipe and to avoid sample drainage and/or evaporation from the exit tube from the sample. One solution for this problem is to do the test in a controlled-humidity room. Another is to keep the standpipe reservoir covered with a small rubber balloon that has been partially inflated. (Do not plug the standpipe as a vacuum will eventually form, stopping the flow of water.) To control sample drainage and ensure that the exit tube flows full (or to control tailwater evaporation), submerge the exit tube in a container of water. Obtain the tailwater elevation for h2 102

\~----

Experiment

Twelve

according to the various laboratory setups. Use judgment and ingenuity to control any sample leaks. The consolidation test of Experiment No. 13 can also be used to determine the coefficient of permeability. Referring to Fig. 13-2 and using afixed-ring consolidometer with the extreme right piezometer tube connected to the sample base, one may use a graduated burette, as in this experiment, attached to that piezometer and, at the end of primary consolidation for any load, add water to some level and observe the fall and elapsed time. It may, of course, be necessary to cover this reservoir with a rubber balloon to control evaporation. At the end of the test, disconnect (or drain) the burette and continue the consolidation test with the next load increment. Procedure

This Is a Group Project-

1. Build the soil sample, following the instructions given in Experiment No. 11. 2. Fill the burette (or other standpipe) to a convenient height, and measure the hydraulic head across sample to obtain hI. 3. Commence the flow of water and simultaneously start timing the test. Allow water to flow through the sample until the burette (or 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. If it is necessary to obtain the area a of the standpipe, collect the water in a beaker. 4. Refill the burette (or standpipe), and repeat the test two additional times. Use the same hI and h2 values and obtain the corresponding elapsed times. Take the temperature for each run. If it is necessary to compute the area of the standpipe, collect the water for each test run and accumulate it in a graduated cylinder. After the last test run, compute the area a as

This computation may not be necessary if a graduated burette is used. If this test is done with Experiment No. 11 and the constant-head data have not been obtained, take the data for Experiment No. 11 next.! 5.

Each individual should compute the coefficient of permeability at the test temperature kT and for 20°C. Obtain viscosity corrections from Table 6-1 as outlined in Experiment No. 11. Use a data sheet from the data sheet section. Average the results for k (note that a single value can be computed if the temperature does not vary more than 1 or 2°C and you used hI and h2 = constant for all runs, since time can be averaged under these conditions).

Refer to Fig. 12-3 for a typical set of falling-head data. 6.

In a. b. c.

your report: Discuss test limitations (specifically for your test setup). Can you propose a better (and practical) way of doing the test? Compare the k values between Experiments No. 11 and No. 12 (if both are done on the same sample). What could cause any differences between the two values? d. Show the derivation of Eq. (12-1) in sample computations. e. How long will it take for h2 to be zero? 'Either Experiment No. 11 or No. 12 can be done first.

Experiment

Twelve

103

Table 12-1 Viscosity Corrections

104

for ..,.,T/rl2o

'C

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

O.B

0.9

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 2,6 27 28 29 30 31 32 33 34 35

1.3012 1.2650 ·1.2301 1.1968 1.1651 1.1347 1.1056 1.0774 1.0507 1.0248 1.0000 0.9761 0.9531 0.9311 0.9097 0.8893 0.8694 0.8502 0.8318 0.8139 0.7967 0.7801 0.7641 0.7486 0.7334 0.7189

1.2976 1.2615 1.2268 1.1936 1.1621 1.1318 1.1028 1.0747 1.0480 1.0223 0.9976 0.9738 0.9509 0.9290 0.9077 0.8873 0.8675 0.8484 0.8300 0.8122 0.7950 0.7785 0.7626 0.7471 0.7320 0.7175

1.2940 1.2580 1.2234 1.1905 1.1590 1.1289 1.0999 1.0720 1.0454 1.0198 0.9952 0.9715 0.9487 0.9268 0.9056 0.8853 0.8656 0.8465 0.8282 0.8105 0.7934 0.7769 0.7610 0.7456 0.7305 0.7161

1.2903 1.2545 1.2201 1.1873 1.1560 1.1260 1.0971 1.0693 1.0429 1.0174 0.9928 0.9692 0.9465 0.9247 0.9036 0.8833 0.8636 0.8447 0.8264 0.8087 0.7917 0.7753 0.7595 0.7440 0.7291 0.7147

1.2867 1.2510 1.2168 1.1841 1.1529 1.1231 1.0943 1.0667 1.0403 1.0149 0.9904 0.9669 0.9443 0.9225 0.9015 0.8813 0.8617 0.8428 0.8246 0.8070 0.7901 0.7737 0.7579 0.7425 0.7276 0.7133

1.2831 1.2476 1.2135 1.1810 1.1499 1.1202 1.0915 1.0640 1.0377 1.0124 0.9881 0.9646 0.9421 0.9204 0.8995 0.8794 0.8598 0.8410 0.8229 0.8053 0.7884 0.7721 0.7564 0.7410 0.7262 0.7120

1.2795 1.2441 1.2101 1.1777 1.1469 1.1172 1.0887 1.0613 1.0351 1.0099 0.9857 0.9623 0.9399 (UJl83 0.8975 0.8774 0.8579 0.8392 0.8211 0.8036 0.7867 0.7705 0.7548 0.7395 0.7247 0.7106

1.2759 1.2406 1.2068 1.1746 1.1438 1.1143 1.0859 1.0586 1.0325 1.0074 0.9833 0.9600 0.9377 0.9161 0.8954 0.8754 0.8560 0.8373 0.8193 0.8019 0.7851 0.7689 0.7533 0.7380 0.7233 0.7092

1.2722 1.2371 1.2035 1.1714 1.1408 1.1114 1.0803 1.0560 1.0300 1.0050 0.9809 0.9577 0.9355 0.9140 0.8934 0.8734 0.8540 .....,.;.;--.- ......... 0.8355 0.8175 0.8001 0.7834 0.7673 0.7517 0.7364 0.7218 0.7078

1.2686 1.2336 1.2001 1.1683 1.1377 1.1085 1.0802 1.0533 1.0274 1.0025 0.978':: 0.9554 0.9333 0.9118 0.9813 0.8714 0.8521 0.8336 0.8157 0.7984 0.7818 0.7657 0.7502 0.7349 0.7204 0.7064

Experiment

Twelve

~

COEFFICIENT OF PERMEABILITY (Constant Head, Failing H.ad)

-r~.!

;:4#1119 ,J(~4J

Project

Description of Soil Tested by

L (10t Brown, J£8

Sample Dimensions: Diam.

10· V

I S- t/ CJ

Constant h=

I

8

(3fJ.

r-Y

Fln~

{j/1iferrn

--

Co,,"P. mo/d)

•

5Q.nd

Date of Testing _-'7'-------_9L---'}(~)(~ cm;

Wt. soil + pan Init. ----.: •••. .".1.l'.•... l'-!1'--!+L..'---9'------ __ Wt.soil + pan Final 15'2.6". I Wt. of Sample

Job No.

La. bo ro.flJ

Location of ProjectSoil

Data Sheet 13

81.1

Area

ern";

g g

Vol. Unit

g

Ht.

_

//.,

cm

9'/1/.0

cm3

_~c......:.....:.....:::~ __

wt.----'I'--'~•....:c/-"'O'------

__

Head

cm

Test data

Test data used t.s

Test No.

T.oC

a. cm'

t.s

Test No.

T. CC

a.cm'

1

2 3 4 Average"

r

kT=QLlAht=

_ ________

cm/s

Falling Head Standpipe = [burette. other (specify)] Area of standpipe. a

=

T/T/T/2!J.

,aO

/. ?/

k2•

tnl.

= --------

= ktT/T/T/20 =

cm/s

hVY4 fIt!

ern-

Test data"

Test data used

Test

n,

no.

cm

h,. cm

0,a , t. S

cm"

T.

Test

n,

cm,3

°C

no.

cm

2.1

OQltt.

,

SI./

Z#·l

"11./

.lfS·S

2

SI./

l..f/>.3

.5"".7

"

3

.51.'

2.('.3

.$S.3

.,

h,. cm

T.

t.s

°C

1/

II

4 Q..-.:

~s."

I

-:I.?I(!~&

Average

Z¥ ..,3

.:T/./

s:-f'. ?

.s. 3L "I.-~ ~'

2. -¥ kiD

-.I

"Use averaged values only if there is a small difference in test temperature. say. 1-2°C. "This test can be considerably simplified by using the same values of h, and h" each time. otherwise average these values regardless

ZI

cm/s cm/s

you cannot

of T.

12-3 Data from a falling-head permeability test using equipment of Fig. 12-1 or of Fig. 11-1. Figure

105