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 di scharge 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 c onstant of proportionality 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,
• 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.
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 h 1 (near the top of the soil) and h 2 (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 centimeters t = 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
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 gm Wt. of cylinder+soil=336 gm Wt. of soil= 69 gm 69
ρsoil =
= .
Π×2.5×2.5×8 ∆ℎ 8 l
=
8
=1.757 g/cc
=1
Water discharged 0 ml 0 ml 1.3 ml 3.6 ml 3.7 ml 3.6 ml
time 0(2:10 pm) 30(2:40 pm) 60(3:10 pm) 90(3:40 pm) 120(4:10 pm) 150(4:40 pm)
So, Q =Average of last two discharges i.e. 3.65 ml hence, 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 and moisture in place for prolonged periods of time. Silt is considered a good compromise soil between clay and sand, since its weight and density are in between these two other types of soil.
