Chapter 6: Hydraulic Conductivity

The hydraulic gradient

is dimensionless AND is a ratio of head loss between two points to the distance between these points.

The discharge velocity of water

is the quantity of water flowing in unit time through a unit gross cross-sectional area of the soil at right angles to the direction of flow. AND has the SI unit as m/s.

The hydraulic conductivity of soil does not depend on

unit weight of soil solids.

Darcy's law relates the discharge velocity (v) to the hydraulic gradient (i) and the hydraulic conductivity (k) as

v = ki

The hydraulic conductivity of fine sand typically ranges from

0.01 to 0.001 cm/s

If the hydraulic conductivity of a sand at a void ratio of 0.6 is 0.03 cm/s, its value at the void ratio of 0.75 will be

0.05

If a soil deposit consists of two layers having thickness of 1 m and 1.5 m with hydraulic conductivity values of 3 × 10-3 m/s and 5 × 10-4 m/s, respectively, the equivalent hydraulic conductivity of the soil deposit in the horizontal direction will be

1.5 × 10-3 m/s

If discharge velocity of water through a soil is 24 cm/hr and soil porosity is 30%, the seepage velocity will be

80 cm/hr

True Statements

Elevation head at a given point is its vertical distance above or below a datum plane. Pressure head at a given point is the water pressure at this point divided by the unit weight of water. The SI unit of head is m.

For the flow of water through the void spaces of most saturated soils,

discharge velocity is proportional to the hydraulic gradient.

The hydraulic conductivity k of soil is related to its void ratio e as

e is e^3/(1+e)

For the flow of water through a porous soil medium, the total head at any point can be adequately represented by the sum of

elevation head and pressure head.

The laboratory test used for determining permeability of fine-grained soils is

falling head permeability test

For fairly uniform sand, the hydraulic conductivity k (in cm/s) is related to the effective size D10 (in mm) of the soil particles as

k = cD210

In which of the following flow zones, the discharge velocity bears a linear relationship to the hydraulic gradient?

laminar flow zone

The SI unit of hydraulic conductivity is

m/s

The absolute permeability of soil is expressed in

m^2

The hydraulic conductivity of soil is

proportional to the unit weight of water flowing through the soil. AND inversely proportional to the viscosity of water flowing through the soil.

The actual velocity of water through the void spaces of soil is called

seepage velocity.