1. LABORATORY TEST FOR SOIL MECHANICS PERMEABILITY TEST
ΑΝΩΤΑΤΗ ΣΧΟΛΗ ΠΑΙΔΑΓΩΓΙΚΗΣ ΚΑΙ ΤΕΧΝΟΛΟΓΙΚΗΣ ΕΚΠΑΙΔΕΥΣΗΣ ΤΜΗΜΑ ΕΚΠΑΙΔΕΥΤΙΚΩΝ ΠΟΛΙΤΙΚΩΝ ΔΟΜΙΚΩΝ ΕΡΓΩΝ
LABORATORY TEST FOR SOIL MECHANICS
PERMEABILITY TEST
Ακαδημαϊκός Υπεύθυνος: Π. Κ. Πελέκης
Εργαστηριακοί Συνεργάτες: Δ. Καββαδία, Γεωλόγος, MSc
Β. Κ. Ξενάκη, Δρ. Πολιτικός Μηχανικός
Ι. Πανταζόπουλος, Δρ. Πολιτικός Μηχανικός
ΙΑΝΟΥΑΡΙΟΣ 2011

2. What is permeability?
A measure of how easily a fluid (e.g., water) can pass through a porous medium (e.g., soils)
water
Loose soil
- easy to flow
- high permeability
Dense soil
- difficult to flow
- low permeability

3. Darcy’s Law
Velocity (v) of flow is proportional to the hydraulic gradient (i) – Darcy (1856)
v = k i
Permeability
or hydraulic conductivity
unit of velocity (cm/s)

4. ONE DIMENSIONAL FLOW –DARCY’S LAWή
Q=k*i*A h1 Δh
Q = quantity of flow (volume / time)
k = coefficient of permeability (distance/ time)
h1-h2= υδραυλικό φορτίο (μήκος)
L = specimen length along the stream line (distance)
Α = cross-sectional area of specimen
i = (h1-h2) / L = hydraulic gradient h2 L

5. PERMEABILITY DEFINITION
The coefficient of permeability, k is defined as the rate of discharge of water at a temperature of 20° C under conditions of laminar flow through a unit cross sectional area of a soil medium under a unit hydraulic gradient.
Types of tests for defining the coefficient of permeability
Directs methods (Constant head test, Falling-head test, δοκιμή άντλησης in the field)
Indirects methods (στοιχεία από δοκιμή στερεοποίησης,Hazen’s equation)

6. CONSTANT HEAD TEST Η δοκιμή εκτελείται με βάση το πρότυπο ASTM D2434
This test is performed by measuring the quantity of water, Q, flowing through the soil specimen, the length of the soil specimen, L, the head of water,h, and the elapsed time, t.
The constant-head test is used principally for coarse-grained soils (clean sands and gravels).

7. CONSTANT HEAD TEST Equipment Permeameter cylinder
A device for maintaining a constant-head water supply
Balance
Oven
Seamless metal containers
Scale, graduated in cm
Manometer board
Thermometer
Graduated cylinder
Timing device

8. CONSTANT HEAD TEST Manometers L Porous stone From distilled water
overflow h
Water
reservoir
Overflow to maintain constant head
Porous stone
Soil
specimen

9. CONSTANT HEAD TEST Procedure
Choose a representative portion of the specimen. The percentage of fine grains must be less than 10%.
Oven-dry the specimen, record the oven-dry weight of material on the data sheet opposite Ws.
Measure the distance, L, between the centers of the piezometer taps and record on the data sheet.
Weigh the specimen and put it in the cylinder. To produce a higher density to the sample compact it in layers.
After the specimen has been placed, weigh the excess material. The specimen weight is the difference between the original weight of sample and the weight of the excess material.
Place a wire screen, with openings small enough to retain the specimen, over a perforated disk near the bottom of the permeameter above the inlet.

10. CONSTANT HEAD TEST Procedure
Measure the length of the specimen and the inside diameter of the permeameter. Test the specimen at the estimated natural void ratio or at a series of different void ratios.
Level the top of the specimen, cover with a wire screen similar to that used at the base, and fill the remainder of the permeameter with a filter material. The specimen must fully saturated with water bottom -up.
Open the valve and record the initial piezometer readings after the flow has become stable.
After allowing a few minutes for equilibrium conditions to be reached, measure by means of a graduate the quantity of discharge corresponding to a given time interval. Measure the piezometric heads and the water temperature in the permeameter.
Record the quantity of flow, piezometer readings, water temperature, and the time interval during which the quantity of flow was measured on the data sheet
Repeat steps 10 & 11 several times

11. CONSTANT HEAD TEST Computations
We calculate the coefficient of permeability k, as follows:
(cm/sec)
where: V = quantity of flow, cm3
t = elapsed time, sec
h = loss of head in length, cm
L = length of specimen, cm
A = cross-sectional area of specimen, cm2

12. CONSTANT HEAD TEST Computations
2. Temperature correction factor:
(cm/sec)
where: k20 = coefficient of permeability at 20οC temperature of water
kT = coefficient of permeability measured in our test (water temperature ΤοC)
nT = viscosity of water at temperature ΤοC
n20 = = viscosity of water at temperature 20οC
nT /n20 = Temperature correction factor for viscosity of water obtained from Table 1

13. FALLING-HEAD TEST This test is not standardized at ASTM.
This test is conducted in the same manner as the constant-head test, except that the head of water is not maintained constant but is permitted to fall within the upper part of the specimen container or in a standpipe directly connected to the specimen. The quantity of water flowing through the specimen is determined indirectly by computation.
The falling-head test is generally used for less pervicus soils (fine sands to fat clays)

14. FALLING-HEAD TEST Equipment A permeameter cylinder
Glass tubing, rubber or plastic tubing, stoppers, screw clamps, etc., necessary to make connections
Balance
Oven
Seamless metal containers
Scale graduated in cm
Centigrade thermometer
Timing device

15. FALLING-HEAD TEST h1 h2 Soil specimen L standpipe above
cross-sectional α
Soil specimen
Cross-sectional Α
Porous stone L
valve B
Valve A connects the Water reservoir to the sample

16. FALLING-HEAD TEST Procedure
Put the specimen in the cylinder. Measure and record the height of the specimen, L, and the cross-sectional area of the specimen, A.
Full fill the cylinder slowly with water until the complete saturation of the specimen.
Raise the head of water in the standpipe (cross-sectional a) above the discharge level of the permeameter. Close valves A and B.
Begin the test by opening valve B. Start the timer. As the water flows through the specimen, measure and record the height of water in the standpipe above the discharge level, h1, in cm, at time to, and the height of water above the discharge level, h2, in cm, at time tf.
Repeat the determination of permeability and if the computed values differ by an appreciable amount, repeat the test until consistent values of permeability are obtained.

17. FALLING-HEAD TEST Computations
We calculate the coefficient of permeability as follows:
(cm/sec)
where: α = inside area of standpipe, cm2
A = cross sectional area of specimen, cm2
h1 = height of water in standpipe above at time (t=0), cm
h2 = height of water in standpipe above at the end of the test, cm
L = length of specimen, cm
t = elapsed time, sec
2. Temperature correction factor for viscosity of water at the same way as in the constant head test

18. Permeability and drainage characteristics of soils (Terzaghi et al
Coarse gravels, κροκάλες, λατύπες
Drainage
Soil types
Very good
Good aquifers
Poor aquifers
Practically impervious
Clean gravels
Clean sands
Gravel mixtures
Fine sands
Clayey sands
Silts
“Impervious” soils
modified
by effects of
vegetation &
weathering
Impervious soils e.g., homogeneous clays below
zone of weathering 10 1
10-1
10-2
10-3
10-4
10-5
10-6
10-7
10-8
10-9 k
cm/s