Permeability Test Lab Report

 OBJECTIVE To determine the coefficient of permeability of a soil using falling head method.

INTRODUCTION Permeability is a measure of the ease in which water can flow through a soil volume. It is one of the most important geotechnical parameters. However, it is probably the most difficult parameter to determine. In large part, it controls the strength and deformation behavior of  soils. It directly affects the following:

•

Quantity of water that will flow to ward an excavation.

•

esign of cutoffs beneath dams on permeable foundations.

•

esign of the clay layer for a landfill liner.

•

Permeability influences the rate of settlement of a saturated soil under load.

!or fine grained soil !alling head permeability test is done, whereas constant head permeability test is done for the coarse grained soil

Darcy’s Law Henry arcy, a !rench scientist, based on his experimental evidences proposed the arcy"s law for ground water flow. In #$%& arcy proposed a simple e'uation ()'uation #* for velocity of water through saturated soils.  + (#*

-here v / ischarge velocity, which is the 'uantity of water flowing in unit time through a unit cross sectional area of soil at right angle to the direction of flow, 0 / 1oefficient of permeability

-ater is allowed to run through the stand pipe of cross sectional area 2a" into the sample of  cross sectional area 23" as shown in !igure #. The times t#, t4, t5 etc. are noted when the dropping water level indicates readings h#, h4, h5 etc

THEORY

In the falling head test a relatively short sample is connected to a standpipe which provides both the head of water and the means of measuring the 'uantity of water flowing through the sample. 6everal standpipes of different diameters are normally available from which can be selected the diameter most suitable for the type of material being tested.

In permeability tests on clays, much higher hydraulic gradients than are normally used with sands can be applied, and are often necessary to induce any measurable flow. The cohesion of clays provides resistance to failure by piping at gradients of up to several hundred, even under 'uite low confining or surcharge pressures. ispersive clays however are very susceptible to erosion at much lower gradient.

The falling head principle can be applied to an undisturbed sample in a sampling tube and to a sample in an odometer consolidation cell. The e'uation used in determines the permeability of fine grained soils is given in )'n (#*.

 Permeability, k  =

aL  A(t 2

− t 1 )

 h1      ………..Eqn (1) h   2  

log e 

The time difference (t4t#* can be expressed as the elapsed time, t (minutes*. The heights h # and h4 and the length, 7 are expressed in millimeters, and the areas 3 and a in s'uare millimeters. )'n (#* then becomes )'n (4*.

 Permeabili ty, k  =

aL  Ax 60t 

 h1      ( mm / s ) ………..Eqn (2) h   2  

log e 

To convert natural logarithms to ordinary (base #8* logarithms, multiply by 4.585. If 0 is epxressed in m9s, the above e'uation becomes )'n (5*.

 Permeabili ty , k  =

2.303aL 1000 xAx 60t 

 h1      ( m / s ) ………..Eqn ()  h2  

log10 

-here: a + area of crosssection of standpipe tube, 3 + area of cross section of sample h# + heights of water above datum in standpipe at time t # h4 + heights of water above datum in standpipe at time t 4 7 + heights of sample t + elapsed time in minutes

E!UI"#ENT$ •

!alling Head 3pparatus.

•

6top watch.

•

earies water.

•

eter scale.

•

;raduated flas0

"ROCEDURE$ #. Prepare the soil sample 4. etermine the bul0 density and moisture content of the soil 5. Place the sample in perspex cylinder with wire mesh and gravel filter above and below ecord the height of water in the glass standpipe at several time intervals $.etermine the average of 0 from the results

RE$ULT$ !alling Head Permeability test

iameter of the sample

+ #88mm

7ength of the sample

+ 4=.5mm

6ample area, 3:

+5#