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EGB373 (completed)

Permeability

QuestionAnswer
Soil is a... porous material, consisting with solid particles and voids.
Voids in the soil can be... interconnected
Water can flow between... interconnected voids
What is permeability？ A measure of how easily a fluid (e.g. water) can pass through a porous medium (e.g. soil)
Loose Soil - easy to flow - high permeability
Dense Soil - difficult to flow - low permeability
Water flow through soil is termed as... SEEPAGE
Some applications of soil permeability (1) Earth dams • Seepage through dams (or under sheet pile wall) •Stability of dam slopes
Some applications of soil permeability (2) Pore water pressure and slope stability Calculation of rate of settlement of clay soil deposits
Hydraulic Conductivity or Coefficient of Permeability (k) Indicates the flow velocity of water through soils
Factors affect on soil permeability (1) Soil type and particle size distribution Fine grained soils – low permeability Coarse grained soils – high permeability
Factors affect on soil permeability (1(1/2)) Density of soil High density – low permeability Low density – high permeability
Factors affecting soil permeability (2) Stress conditions Higher confining pressure – lower permeability Lower confining pressure – higher permeability
Groundwater flow conditions • Gravitational flow can occur if there is an energy difference (or head difference) • Static Water (no flow)
Groundwater flow conditions - Seepage flow – Steady state condition • Flow properties does not change with time – Transient condition
Note: – 1-D flow – 2-D flow (one-directional flow) (two-directional flow)
Total head (Fluid Mechanics) Total head = Elevation head + Pressure head + Velocity head (Bernoulli’s equation)
Total head in groundwater flow The seepage velocities in soils are normally so small (<1 cm/s) that velocity head can be neglected.
Total head at a point (h) = Pressure head + Elevation head
Head •The value of the head depends on the choice of datum •Differences in total head are required for flow (not pressure/elevation only)
The pressure head/piezometric head is the water height in the standpipe measured from... the point of interest (i.e. the inlet of the standpipe)
The head is the elevation of the... water level in the standpipe (piezometers/manometers) above the datum
Water flow through soil - Darcy found that the flow rate (volume per unit time -q) was •proportional to the head difference, Δh (q ∝ Δh) •proportional to the cross-sectional area, A ( q ∝ A) •inversely proportional to the length of sample, l (q ∝ 1/l)
Determination of coefficient of permeability (k) - Coefficient of permeability (or hydraulic conductivity) [k] of a soil can be determined using one of the following methods Laboratory methods: (a) Constant head permeability test – For coarse grained soils (b) Falling head permeability test – For fined grained soils Indirect methods and empirical equations In-situ (field) methods: Pumping well test
Determination of coefficient of permeability (k) – Laboratory methods - Constant head permeability test – for coarse soils Soil specimen of appropriate density is in a cylinder of cross-sectional area A Prior to running the test, fully saturate the specimen (vacuum the specimen, use de-aired water) Vertical flow of water under a constant total head
Determination of coefficient of permeability (k) – Laboratory methods - Constant head permeability test – for coarse soils (Continued) Once steady state water flow is achieved, measure the volume of water flowing per unit time/flow rate (q) and total head difference Δh Then use Darcy's Law
Falling head permeability test – for fine soil Normally undisturbed specimens are tested and the sampling tube itself is used as the containing cylinder The length of the specimen is L and the cross sectional area is A. The cross sectional area of the standpipe is a
Falling head permeability test – for fine soil (Continued) Prior to running the test, fully saturate the specimen (vacuum the specimen, use de-aired water) The stand pipe is filled with water and the water drains into a reservoir of constant level
Falling head permeability test – for fine soils (2) The fall of water level in the standpipe (relative to the reservoir level) from h_0 to h_1 is measured during time t_1
Falling head permeability test – for fine soils (2) (Continued) For more accurate results, a series of tests should be run using different values of h_0 and h_1 and/or standpipes of different diameters (a)
Sources of Error in Hydraulic Conductivity Testing in Laboratory • Use of non-representative samples • Voids formed during sample preparation • Smear Zones: heterogeneous soil sample • Alteration in Clay Chemistry • Air in Sample
Sources of Error in Hydraulic Conductivity Testing in Laboratory (Continued) • Growth of Microorganisms • Menisci Problems in Capillary Tubes • Temperature • Volume Change Due to Stress Change • Flow Direction
Effect of Temperature The coefficient of permeability varies with viscosity of water, which is temperature dependent
In laboratory, the coefficient of permeability k value is normally specified... at 20°C
The permeability (in contrast the coefficient of permeability or hydraulic conductivity) is independent of the water properties
Soil is a porous medium > presence of interconnected voids > Permeable
No flow: Static Condition
Seepage: Steady-state or transient conditions
1-D Flow: Darcy’s Law Total Head
k = Coefficient of permeability = Hydraulic Conductivity
Determination of coefficient of permeability Laboratory methods a) Constant head method (for coarse grained soils): AS1289.6.7.1 b) Falling head method (for fine grained soils): AS1289.6.7.2
Factors affecting k: temperature correction Indirect methods & Empirical Equations Field tests: Pumping Wells
Determination of coefficient of permeability (k) – Other methods - Indirect method: Permeability of fine grained soils can be determined indirectly using the results of the consolidation test
Determination of coefficient of permeability (k) – Other methods - Empirical methods (based on research finding):
A confined aquifer is an aquifer... below the land surface that is saturated with water. Layers of impermeable material are both above and below the aquifer, causing it to be under pressure so that when the aquifer is penetrated by a well, the water will rise above the top of the aquifer
An unconfined aquifer is an aquifer whose upper water surface (water table) is at... atmospheric pressure, and thus is able to rise and fall. They are usually closer to the Earth's surface than confined aquifers are.
An artesian aquifer is a confined aquifer containing groundwater under... positive pressure. If such a confined aquifer is tapped by a well, water level will rise (until the hydrostatic equilibrium is achieved) above the top of the aquifer and may even flow from the well onto the land surface
Determination of coefficient of permeability (k) – In-situ methods - Pumping well test – suitable for homogeneous coarse soils Pumping well – At least 300 mm in diameter, penetrate to the bottom of the soil stratum under testing condition Pumping at constant rate, q , from the well
Determination of coefficient of permeability (k) – In-situ methods - Pumping well test – suitable for homogeneous coarse soils (2) Steady seepage is established, radially toward the well, resulting in water table being drawn down to form a “cone of depression”
Determination of coefficient of permeability (k) – In-situ methods - Pumping well test – suitable for homogeneous coarse soils (3) Water levels are observed in a number of observation wells spaced on radial lines at various distances from the pumping well
Determination of coefficient of permeability (k) – In-situ methods - For unconfined stratum Assumption – hydraulic gradient, i, at any distance, r, from the centre of the well is constant with the depth and is equal to the slope of water table
Created by: Asher - S
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