Object: To determine the coefficient of permeability of fine-grained soil by Falling Head Method
Standard:
AASHTOT215-86
ASTMD234-70 BS1377
Apparatus:
Combination Permeameter assembly, Stopwatch, Graduated cylinder (250 or 500 ml), Balance sensitive to 0.01 g, Moisture cans, Oven, Thermometer etc.
Need and Scope:
The knowledge of this property is much useful in solving problems involving yield of water bearing strata, seepage through earthen dams, stability of earthen dams, and embankments of canal bank affected by seepage, settlement etc.
Falling Head Permeability Test:
The falling head permeability test is a common laboratory testing method used to determine the permeability of fine-grained soils with intermediate and low permeability such as silts and clays. This testing method can be applied to an undisturbed sample.
Description and Procedure:
The falling head permeability test involves flow of water through a relatively short soil sample connected to a standpipe which provides the water head and allows measuring the volume of water passing through the sample. The diameter of the standpipe depends on the permeability of the tested soil. The test can be carried out in a Falling Head permeability cell or in an oedometer cell.
Before starting the flow measurements, the soil sample is saturated, and the standpipes are filled with de-aired water to a given level. The test then starts by allowing water to flow through the sample until the water in the standpipe reaches a given lower limit. The time required for the water in the standpipe to drop from the upper to the lower level is recorded. Often, the standpipe is refilled, and the test is repeated for couple of times. The recorded time should be the same for each test within an allowable variation of about 10%.The equation used to determine the coefficient of permeability of fine grained soils is:
𝐶𝑜 − 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡 𝑜𝑓 𝑝𝑒𝑟𝑚𝑒𝑎𝑏𝑖𝑙𝑖𝑡𝑦, (𝑘) = 2.303𝑎𝐿
logℎ1 (m/sec)
1000∗𝐴∗60𝑡
10 ℎ2
Where,
a= Area of cross section of standpipe A= area of cross section of sample L=height of sample
t= elapse time in minutes
h1=initial height of water above datum
h2=final height of water above datum Observations and Calculations
Determination No. | Initial Head (h1) | Final Head (h2) | Time (t) | Coefficient of Permeability (k) |
Results
The coefficient of permeability of given soil, k =mm/sec
