Civil Engineering Department College of Engineering

Transcription

1 Civil Engineering Department College of Engineering Course: Soil Mechanics (CE 359) Lecturer: Dr. Frederick Owusu-Nimo

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 2

3 Permeability Values Permeability varies widely for different soils (1ms -1 for clean gravels to ms -1 for unfissured clay Soil Type Clean gravel Coarse Sand Fine Sand Silty Clay Clay Permeability (m/sec) <

4 Factors affecting Permeability Permeability values depend on several factors (fluid and soil properties) Fluid viscosity (also dependent on temperature) Size and continuity of pore spaces (depends on size, shape of particles, void ratio, etc) Presence of discontinuities 4

5 Determination of Permeability Laboratory Methods o Indirect Laboratory Methods From Consolidation Tests: k = m v c v γ w From grading characteristics (Hazen formulae) : k = CD 10 2 o Direct Laboratory Methods Constant Head Permeability Test for coarse grained soils Falling Head Permeability Test coarse and fine grained soils Field Methods o Well Pumping Test o Tracer Tests, etc 5

6 Constant Head 6

7 Falling Head 7

8 Example 1 The permeability of a soil sample is determined in the lab using the constant head permeability test. What is the permeability of the soil in m/sec if these test values were obtained/used o Length of specimen =45cm o Area of specimen =23cm 2 o Head difference = 71cm o Amount of water collected in a period of 3min = 354cm 3

9 Example 2 For a falling head permeability test, the following values are given: o Length of specimen = 200mm o Area of soil specimen = 1000mm 2 o Area of standpipe = 40 mm 2 o Head difference at time t= 0 is 500mm o Head difference at time t=180sec is 300mm Determine the permeability of the soil in cm/sec

10 Well Pumping Tests o The reliability of laboratory methods depends on the extent to which the test specimens are representative of the soil mass as a whole. More reliable results can generally be obtained by the in-situ field methods o A well extending the whole thickness of aquifer (water bearing formation) is sunk o As water is pumped, a drawdown, maximum at the well and reducing radially from the well is created o Observation wells are also sunk at radial distances from the well to monitor water levels o When steady state is established, various parameters are measured, and with the help of Darcy s law and Dupuits assumption, the permeability of aquifer is determined

11 Well Pumping Tests Dupuit s assumption- the hydraulic gradient at any point is constant from the top to the bottom of the water bearing stratum and it is equal to slope of the water surface / table i = dh dr h is the height of water table at radius r from the well Aquifer types o Unconfined aquifer impervious layer only at bottom o Confined aquifer impervious layer at both top and bottom

12 Unconfined Aquifer Consider two boreholes located on a radial line at distances r 1 and r 2 from the centre of the well The respective water levels relative to the bottom of the stratum being h 1 and h 2. For any distance r, the area through which seepage takes place is 2πrh, where r and h are variables. Then applying Darcy s law: q Aki q dh 2 rhk d r r2 dr h2 q 2 k hdh r1 r h 1 r q ln k( h2 h1 ) r1 2.3q log( r / r ) k ( h ) h1

13 Confined Aquifer For a confined stratum of thickness H the area through which seepage takes place is 2πrH, where r is variable and H is constant. q Aki The drawdown curve must always be above the confining layer q dh 2 rhk d r r2 dr h2 q 2 Hk dh r1 r h1 r 2 q ln 2 Hk( h2 h1) r1 2.3q log( r2 / r1) k 2 H( h h ) 2 1

14 Assumptions of well pumping formula The test well must penetrate the full thickness of the water bearing formation A steady flow condition must exist (i.e. water levels in test and observation wells become constant) The formation is homogenous, isotropic and extends an infinite distance in all directions The Dupuit s assumption is valid

15 Note Pumping tests lower groundwater unevenly and may cause uneven stresses in soils. Thus pumping tests near existing structures can cause uneven or differential settling.