Similar to the constant head permeameter, there is one more test called falling head permeameter in which fall of head is related to the permeability of the soil sample, Before going to the detail of what the test is we must understand what is permeability?
So by general or lay man terms; permeability may be defined as;
So by general or lay man terms; permeability may be defined as;
Ability of material to allow water to pass through it
Soil in Water 
While technically speaking Permeability may be defined as;
“Rate at which water flows through unit crosssectional area of soil mass under the action of unit hydraulic gradient provided laminar flow conditions prevails”
This test method is used to determine the coefficient of permeability (K) having units of velocity or m/sec
Designation of the test;
This test method has been standardized by American Association for State Highway and Transportation Officials under AASHTO – T – 215.
Significance of the test:
We know that water or moisture is very devastating as far as our pavements and paved structures are concerned. Water within the base and subbase causes a lot of troubles which may ultimately cause failure of the pavement structure. Presence of moisture in the base course causes reduction or loss in shear strength of the soil similarly other problems like frost heave may swell the base and cause cracks on the surface course in the similar manner, freez and thaw effect will be initiated when the present moisture content get freeze due to low temperatures around surroundings.
Coefficient of Permeability:
The amount of water flowing through a certain area can be represented by coefficient of permeability represented by ‘K’ the units of K is meter / seconds. Typical values of K ranges from 0.001 to 0.01 m/s.
Description of constant head permeameter
Description of Constant Head Permeameter 
Sample Preparation
In order to limit consolidation influences during testing, this procedure is limited to disturbed granular soils containing not more than 10 percent (%) soil passing the 75 micron i.e. sieve no. 200.
Total amount of material to be used is 200 kg.
The material gradation is as under;
Total amount of material to be used is 200 kg.
The material gradation is as under;
Sr. NO

Sieve Size

%age Passing

% Retained

Weight of Material (kg)

1

50 mm

83

17

34

2

37.5 mm

65

18

36

3

20 mm

40

25

50

4

10 mm

22

18

36

5

5 mm

13

9

18

6

0.6 mm

2

11

22

7

0.075 mm

0

2

4

Total

200 kg

Test Procedure:
 Place the constant head permeameter on leveled and firm surface.
 The Head difference between upstream and downstream weirs must be within 2040 mm.
 Mix aggregate thoroughly and place in 4 layers in the box.
 Compact the material by using modified AASHTO compacting hammer of 10kg, here no. of blows is not specified time is specified that upto 1 min you will apply blows for each layer while using hammer with normal rate.
 Allow the water to flow from upstream to downstream.
 The vacuum Tap is opened to allow water to pass through sample, on alternate basis the tap is opened and closed to avoid entrapped air.
 Leave the sample over night to allow the sample to get saturated.
Start measuring discharge once sample is saturated after each 15 min interval by using formula of Q = Vol / time; until steady flow of water is observed at downstream end.
Method to determine discharge is simple; you can use some container of known volume and the moment water enters in the container stopwatch is turned on and after the container gets filled stop the stop watch, now you have time and volume you can measure the discharge.
Method to determine discharge is simple; you can use some container of known volume and the moment water enters in the container stopwatch is turned on and after the container gets filled stop the stop watch, now you have time and volume you can measure the discharge.
Calculation
Formula to calculate the permeability;
K = (V x L) / (A h t)
Where
L = length of the specimen which is 1 m.
A is cross sectional area which is 0.3 x 0.3 m2
H is head difference in meters
T is the time for which known volume of V is filled by water.
K = (V x L) / (A h t)
Where
L = length of the specimen which is 1 m.
A is cross sectional area which is 0.3 x 0.3 m2
H is head difference in meters
T is the time for which known volume of V is filled by water.
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