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

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

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)

ONE DIMENSIONAL FLOW –DARCY’S LAW ή 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

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)

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).

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

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

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.

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

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

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

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)

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

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

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.

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

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