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  CONSTANT HEAD PERMEABILITY TEST

OBJECTIVE:-To calculate the permeability of sandy soil.  The permeability

test is a measure of the rate of the flow of water through soil.

THEORY:- 

In 1856 the French engineer Henri D’arcy demonstrated by experiment

that it is possible to relate the discharge rate of water flowing from a

soil to the hydraulic or total head gradient in the soil and a property of

the soil which we refer to as the coefficient of permeability or the

hydraulic conductivity.There are two general types of permeability test

methods that are routinely performed in the laboratory: (1) the

constant head test method, and (2) the falling head test method. The

constant head test method is used for permeable soils (k>10-4 cm/s)

and the falling head test is mainly used for less permeable soils (k<10-4

cm/s). ASTM D 2434 - Standard Test Method for Permeability of

Granular Soils (Constant Head).

Q=kiA

The coefficient of permeability(k) is a constant ofproportionality relating to the ease with which a fluid passes through a

porous medium.

1.The two main factors that determine the order of magnitude of the

permeability coefficient are: grain size and cleavage (secondary

interstices).

2.The impact of the factors listed below is inferior, but still not

insignificant:

• grain shape and orientation, 

• quantity and connection of interstices,

• uniformity coefficient, 

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• water content and saturation conditions before seepage begins,

• the properties of the passing liquid (water), 

• hydraulic conditions (hydraulic gradient, Reynolds number etc.),

• transient phenomena (migration, wash-out and wash-in of grains).

SCOPE:- This property is necessary for the calculation of seepage

through earth dams or under sheet pile walls, the calculation of the

seepage rate from waste storage facilities (landfills, ponds, etc.),and the

calculation of the rate of settlement of clayey soil deposits. The test is

limited to materials which have a coefficient of permeability of

approximately 300 mm/day or greater.

APPARATUS:-

Permeameter, Tamper, Balance, Scoop, 1000 mL Graduated

cylinders, Watch (or Stopwatch), Thermometer, Filter paper.

PROCEDURE:- 

A. Preparation:-1) Obtain the mass of the permeameter.

2) Carefully place and compact the dry soil in the permeameter in 3

to5 layers. Level the top surface of the soil by applying a small

 pressure to the porous stone.

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3) Measure the height of the compacted soil. This is equal to L for the

computing the total volume of soil.

4) Measure the distance from the top manometer tube to the top of the

 bottom porous stone. This is the length L for the falling head test.

5) Measure the mass of the permeameter and the dry soil.B. Constant Head Permeability Test:-

1) Assemble the permeameter and attach the manometer tubing to the

side of the permeameter. Attach the tubing from the constant head

supply to the top of the permeameter. Attach the exit tubing to the

 bottom of the permeameter and place the other end in the overflow

flask.

2) Open the valves to the permeameter and slowly add water to the

constant head tank to saturate the soil sample.3) Open the clamps on the manometer tubes.

4) Adjust the rate of flow and allow the flow to reach a stable head

condition, i.e. water levels in the manometer remain constant. Record

the water levels in the manometers as h1 (near the top of the soil) and h2

(near the bottom of the soil).

5) Measure and record the discharge q and the time t.

6) Repeat the steps 3 and 4 two additional times using different values

of h1 and h2 (total head difference), which can be achieved by adjusting

the overflow level of the discharge.

CALCULATION:- Calculate the permeability, using the following

equation: 

=

  

Where:

k = coefficient of permeability at temperature T, cm/sec.

L = length of specimen in centimeterst = time for discharge in seconds

Q = volume of discharge in cm3 (assume 1 mL = 1 cm3)

A = cross-sectional area of permeameter (= D2

4 π, D= inside diameter of the permeameter)

h = hydraulic head difference across length L, in cm of water; or it is

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equal to the vertical distance between the constant funnel head level

and the chamber overflow level.

(h and L are adjusted such that the value of hudraulic gradient equals 1.)

DATA OBSERVED:-

Weight of cylinder=267 gmWt. of cylinder+soil=336 gm

Wt. of soil= 69 gm

ρsoil =69

Π×2.5×2.5×8  =1.757 g/cc

=∆ℎ

l =

8

8 =1 

.

Water discharged time

0 ml 0(2:10 pm)0 ml 30(2:40 pm)

1.3 ml 60(3:10 pm)

3.6 ml 90(3:40 pm)

3.7 ml 120(4:10 pm)

3.6 ml 150(4:40 pm)

So, Q =Average of last two discharges

i.e. 3.65 mlhence, k=4.13 ×10-4 cm/second

CONCLUSION:- 

Silty sand 10-3 to 10-4 

silt is so compactable makes it easier to keep nutrients andmoisture in place for prolonged periods of time. Silt is considereda good compromise soil between clay and sand, since its weight

and density are in between these two other types of soil.