Soil Mechanic Laboratory Assist Lec Lubna Mohammed Abd
Soil Mechanic Laboratory Assist. Lec. Lubna Mohammed Abd Environmental Engineering Department Al- Mustansiriyah University Permeability Test Sources: Soil Mechanics – Laboratory Manual, B. M. DAS (Chapters 10, 11) Soil Properties, Testing, Measurement, and Evaluation, C. Liu, J. Evett
Class Outlines n Permeability in Soils n Hydraulic Conductivity n Darcy’s Law n Permeability Tests n Constant-Head Test n Falling-Head Test
Soil Permeability Soil Properties Physical Mechanical (Soil Characteristics) Specific Gravity Gradation Atterberg Limits Compaction Permeability Compressibility Moisture Content Unit Weight 1 – Constant-Head Test 2 – Falling-Head Test Strength (Shear)
Permeability in Soils n Permeability is the measure of the soil’s ability to permit water to flow through its pores or voids n It is one of the most important soil properties of interest to geotechnical engineers
Importance of permeability n The following applications illustrate the importance of permeability in geotechnical design: n n Permeability influences the rate of settlement of a saturated soil under load. The design of earth dams is very much based upon the permeability of the soils used. The stability of slopes and retaining structures can be greatly affected by the permeability of the soils involved. Filters made of soils are designed based upon their permeability.
Use of Permeability n Knowledge of the permeability properties of soil is necessary to: Estimating the quantity of underground seepage n Solving problems involving pumping seepage water from construction excavation n Stability analyses of earth structures and earth retaining walls subjected to seepage forces n
Hydraulic Conductivity n Hydraulic Conductivity, k, is a measure of soil permeability n k is determined in the lab using two methods: n n Constant-Head Test Falling-Head Test n K is usually expressed in cm/sec n Hydraulic conductivity is also known as the coefficient or permeability
Hydraulic Conductivity (Cont. ) n Hydraulic conductivity of soils depends on several factors: n n n Fluid viscosity Pore size distribution Grain size distribution Void ratio Degree of soil saturation
Darcy’s Law n The hydraulic conductivity, k, is a product of Darcy’s Law. n In 1856, Darcy established an empirical relationship for the flow of water through porous media known as Darcy’s Law, which states: q = ki. A q = flow rate (cm 3/s) k = coefficient of permeability (cm/s) A = cross-sectional Area (cm 2) i = hydraulic gradient where;
Constant Head Test n The constant head test is used primarily for coarse-grained soils n This test is based on the assumption of laminar flow where k is independent of i (low values of i) n ASTM D 2434 n This test applies a constant head of water to each end of a soil in a “permeameter”
Permeameter Civil Engineering - Texas Tech University
Procedure (Constant head) 1. Setup screens on the permeameter 2. Measurements for permeameter, (D), (L), H 1 3. Take 1000 g passing No. 4 soil (M 1) 4. Take a sample for M. C. 5. Assemble the permeameter – make sure seals are air-tight 6. Fill the mold in several layers and compact it as prescribed. 7. Put top porous stone and measure H 2 8. Weigh remainder of soil (M 2) 9. Complete assembling the permeameter. (keep outlet valve closed) 10. Connect Manometer tubes, but keep the valves closed. 11. Apply vacuum to remove air for 15 minutes (through inlet tube at top) 12. Run the Test (follow instructions in the lab manual) …. . 13. Take readings n n Manometer heads h 1 & h 2 Collect water at the outlet, Q ml at time t 60 sec.
Calculation (Constant head) n Determine the unit weight n Calculate the void ratio of the compacted specimen n Calculate k as n Calculate
Falling Head Test n The falling head test is used both for coarse- grained soils as well as fine-grained soils n Same procedure in constant head test except: Record initial head difference, h 1 at t = 0 n Allow water to flow through the soil specimen n Record the final head difference, h 2 at time t = t 2 n Collect water at the outlet, Q ml at time t 60 sec n
Calculation (Falling head) n Calculate k as n Where; a = inside cross sectional area of the water tank h 1 = distance to bottom of the beaker before the test h 2 = distance to bottom of the beaker after the test n Calculate
Typical Values of k
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