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PERMEABILITY OF SOILS
DEFINITION
Permeability is the property of soil which permits flow of water through it from points of high energy to points of low energy, due to existence of interconnected voids. INTRODUCTION
The degree of permeability is characterized by the permeability coefficient k. Two general laboratory methods are available for determining the coefficient of permeability of a soil directly. These are the constant head method for soils of high permeability (e.g. gravel & sand) and the falling head method for soils of intermediate and low permeable (e.g. silts and clays). A soil is highly pervious when water can flow through it easily. In an impervious soil, the permeability is very low and water cannot easily flows through it. DARCY’S LAW
In 1856 Darcy defined a coefficient of permeability, from a study of water passing through a saturated porous medium. He demonstrated experimentally that for laminar flow in a homogeneous soil, the velocity of flow ( v ) is given by: v = k i If the quantity of water (discharge) flowing through the soil in unit times is q , then: q = v A = k i A Where: q = quantity of fluid flowing through the soil in unit time (volume/ time). k = coefficient of permeability (length/ time).
i = hydraulic gradient. i = Lh
A = cross sectional area of soil mass includes both the solids and the voids. Coefficient of permeability is generally expressed in cm/sec or m/sec in SI units and in ft/min or ft/ day in English units.
The value of ( k ) varies widely for different soils. Some typical values for saturated soils are given in table (1). The coefficient of permeability for unsaturated soils is lower and increases rapidly with the degree of saturation.
Table (1): gives the typical values of the coefficient of permeability of different soils.
Soil type k (cm/s) Degree of permeability Gravel Over 10-^1 High Sandy gravel, clean sand, fine sand 10 -^1 to 10-^3 Medium Sand, dirty sand, silty sand 10 -^3 to 10-^5 Low Silt, silty clay 10 -^5 to 10-^7 Very low Clay Less than 10-^7 Practically Impermeable
Fluid viscosity: The coefficient of permeability is standardized at 20°C, and the coefficient of permeability at any temperature T is related to k 20 :
k 20 = k T
T
The variation of 20
^ T with the test temperature is given in table (2).
Table (2): Variation of
^ T.
Temperature, T
(°C) 20
T Temperature, T
(°C) 20
T
10 1.301 21 0. 11 1.265 22 0. 12 1.230 23 0. 13 1.197 24 0.91 0 14 1.165 25 0. 15 1.135 26 0. 16 1.106 27 0.85 0 17 1.077 28 0. 18 1.051 29 0. 19 1.025 30 0. 20 1.
11-Vacuum device.
PROCEDURE
1- Remove a loading piston of permeameter cell, then measure the inside diameter, height of it and the distance between centers of each piezometer connection points.
2- Place graded gravel filter materials in the bottom of the cell to a depth about 40mm and level the surface, then put a wire gauze on it.
3- Place the sample in the permeameter cell by one of three following methods,
a) Compacting by rod. b) Dry pouring. c) Pouring through water.
4- After placing the sample in the cell measure the length of it at two or three positions and calculate the average length (Ls).
5- Put a wire gauze on the top of the soil sample, then place graded gravel filter materials in the top of the soil sample to a depth about 40mm and level the surface, then assembly the top part of the permeameter cell.
6- Saturated the specimen by immersing in water for water (15 min) or by other methods.
7- Connect the top of the cell to the inlet water tank and the base to the outlet water tank; also connect the piezometer to the cell.
8- Turn on the water supply to the constant head device which should be at low level.
9- Allow water to flow through the sample until the conditions appears to be steady and water level in the piezometer tubes remains constant.
10- To start a test run, empty the measuring cylinder and start time at the instant the cylinder is placed under the outlet overflow.
11- Record the clock time at which the first run is started.
12- Read the level of water in the piezometer tubes (h1, h2 and h3), and measure the water temperature (T) in the outlet tank, when the level in the cylinder reaches a predetermined level.
13- Empty the cylinder and make two to three repeat runs at about (5 min.) intervals.
CALCULATION
1- Calculate
i = Δh/ L
k 20 = k T
T
Where:
Q = volume of water collected.
A = area of cross section of the specimen.
Δ h = the difference between elevation of water level inside piezometer tubes.
L = distance between piezometer connections.
t = duration of water collection.
i = hydraulic gradient.
2- Calculate unit weight of the soil.
Fig (1): Principle of constant head test.
Q
L
h
A
kT = AQL ht
c b a
Air valve Loading piston Perforated plate
Wire gauze
CONSTANT HEAD TEST DATA SHEET
Name: Class: Group No.
Weight of the sample = 1500 gm
Specimen height H = 18 cm Specimen diameter D = 7.5 cm
Height above Datum: Piezometer a ……………cm a ……………cm a ……………cm a ……………cm
b …………… cm b …………… cm b …………… cm b …………… cm c …………… cm c …………… cm c …………… cm c …………… cm
Distance between a & b = b & c = a & c =
Hydraulic gradient between a and b =
a and c =
No.
Time for each Measured flow (min.) t
Measured flow (cm)^3 Q
Temperature (Co^ ) 1 2 3 4 5
kT = AQL ht
Signature: Date: 25 /01/ 2017
