parmiability and seepage.pptx

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 permeability The 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 Law Water 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 type

K sand > k clay Void ratio K 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 Test Head difference constant. Apply Darcy’s law: Q = Av V/t = Akh/L `

k = VL/Aht

where  V = volume of water collected in time = t  h = constant head difference  A = x-sectional area of soil specimen  L = length of soil specimen

Falling Head Test Head 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 dams Failure 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: Discharge velocity VT = total volume of sample Vv = volume of voids within sample By Definition, Vv / VT = n, the soil porosity Thus Vs = V/ n 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 from upstream to downstream 2. Equipotential lines: lines of constant total head piezometers reach same level

Conditions:

1. Lines intersect at right angles 2. Square elements 3. Flow lines don’t intersect 4. 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