soil Permeability and seepage

Fardous Rababah

Permeability is the measure of the soil’s ability to permit water to flow through its pores or voids.

Importance of permeabilityThe following applications illustrate the importance of

permeability in geotechnical design: – 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.

Darcy’s LawWater moves through soil with discharge Q, and velocity

v.

Coefficient of Permeability “k”Also called “hydraulic conductivity” k=v/i Define k: “the velocity of water flowing through a soil

medium under a unit hydraulic gradient”.Note:flow of water through soil is governed by:1. Head difference (i=h/l)2. Soil permeability (k)

Factors affecting “k”Soil typeK sand > k clayVoid ratioK loose sand > k dense sand.

Temperature

Laboratory determination of “k”1. Constant head test( for coarse-grained soils)2. Falling head test (for fine-grained soils).

Constant Head TestHead difference constant.Apply Darcy’s law:Q = AvV/t = Akh/L k = VL/Ahtwhere V = volume of water collected in time = t h = constant head difference A = x-sectional area of soil specimen L = length of soil specimen

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Falling Head TestHead is variable Coefficient of permeability (k) can be calculated using the

following relationship:

Typical Values of “k”

seepage seepage flow refers to the flow of a fluid (water) in

permeable soil layers such as sand. The fluid fills the pores in the unsaturated bottom layer

and moves into the deeper layers as a result of the effect of gravity.

The effect of seepage flow when: flowing through dams. flowing around structures in the water .

Seepage and damsFailure of the Teton Dam (USA) , in 1976. 

this is the highest dam that has ever failed.  The cost of the damage was just about $1 billion.

Reasons: • Differential settlement and cracking• Seepage through rock openings.

Seepage velocity Vs Discharge velocity (v): velocity of flow through entire

cross-section.Q = Av (Can be measured)The Discharge velocity is not the actual velocity through

soil pores.Seepage velocity (Vs): velocity of flow through voids.Q = Av.Vs (Can’t be measured, only calculated, how?)

Therefore: Vs = V ( A/Av)Multiplying both areas (A and Av) by the length of the medium

(L)Vs = V ( AL / AvL ) = V ( VT / Vv )where:VT = total volume of sampleVv = volume of voids within sampleBy Definition, Vv / VT = n, the soil porosityThus Vs = V/ n

Discharge velocity

seepage velocity

Seepage velocity is significantly greater than the Darcy’s discharge velocity.

Because total area more than area of voids.

Two Dimensional Flow

Flow net:Two orthogonal families of curves:1. Flow lines: along which a water particle travels fromupstream to downstream2. Equipotential lines: lines of constant total headpiezometers reach same level

Conditions:

1. Lines intersect at right angles2. Square elements3. Flow lines don’t intersect4. Equipotential lines don’t intersect Need to draw to scale

Rate of seepage or flow “Qt”:Nf = number of flow channels (ex: 5) Nd = number of head drops (ex: 11)For a single flow channel:

The "Quick" Condition and the Critical Hydraulic Gradient

The "quick" condition may occur in sandy soils where there is an upward flow of water. 

The soil appears to be boiling and loses its shear strength.In this case the surface of the soil crack and water will

flow up through the cracks.The soil state necessary for the "quick" condition is for

the effective stress to equal zero and the hydraulic gradient to be a critical value.

Boiling condition in soils occur when the maximum exit hydraulic gradient ie is equal or grater than the critical hydraulic gradient.

The max exit hydraulic gradient is determined from the flow net of the problem.:

ie =

: is the drop in total head. : is the shortest seeping distance in flow net.

The factor of safety against boiling condition F

In the shown example, if F.S.against piping<1.2 :Increase penetration depth of sheet pile

Any Question ???

Thank you