Fluid Flow in Soils

Permeability

The measure of a soil's ability to permit water to flow through its interconnected pores or voids.

Laminar Flow

The type of flow where the relationship between velocity (v) and hydraulic gradient (i) is linear .

Bernoulli’s Equation

The specific equation used to calculate total head (h) in fluid mechanics, consisting of pressure, velocity, and elevation heads.

Velocity Head

The specific head component that is typically considered negligible when applying Bernoulli’s equation to water flow in soil.

Hydraulic Gradient

The mathematical ratio of the change in head to the length of flow (L).

Darcy’s Law

The name of the law formulated in 1856 that states v = ki

Hydraulic Conductivity

The term for "k" in the equation v = ki.

Fluid Viscosity

A factor that influences hydraulic conductivity related to the thickness or "stickiness" of the fluid.

Degree of Soil Saturation

The state of soil where all voids are filled with water, which influences permeability.

Cross-sectional Area

The discharge velocity is defined as the quantity of water in unit time through a unit of _____.

Right Angles

The dimension of the unit cross-sectional area in Darcy's Law relative to the direction of flow.

Underground Seepage

A primary application of permeability data used to estimate the quantity of water moving under a structure.

Reynolds Number

The upper boundary for the validity of Darcy’s Law is based on this dimensionless number being less than 10.

Clay

The specific soil type that typically has a hydraulic conductivity (k) of less than 1x10^{-6} cm/sec.

Elevation Head

The component of Bernoulli’s equation represented by the variable Z

Water

The unit weight of _______ is required to calculate pressure head

Seepage Velocity

The actual velocity of water as it moves specifically through the soil voids.

Void Ratio

The ratio of the volume of voids to the volume of solids, denoted as e.

ASTM D2434

The ASTM designation for the Constant Head Permeability test.

Constant Head Test

The laboratory test most suitable for clean sands and gravels.

Standpipe

The variable "a" represents the cross-sectional area of this component in a falling head test.

D10

The "Effective Size" of soil particles used in the Hazen formula.

Falling Head Test

The laboratory test that typically has no ASTM designation and is used for fine-grained soils.

Porous Stone

The material used at the top and bottom of a soil specimen in a permeameter to allow water flow while retaining soil.

Homogenous

The property of a soil having the same material properties throughout.

Isotropic

The property of a soil having the same permeability in both the x and y directions.

Anisotropic

The term for soil that has different permeabilities in different directions.

Heterogenous

The term for a soil deposit made of different materials or layers.

Falling Head Permeameter

According to Casagrande and Fadum, this test is "unreliable" for practically impervious soils.

Horizontal Direction

The direction of flow where the hydraulic gradient (i) is the same for all layers.

Vertical Direction

The direction of flow where the discharge velocity (v) is the same for all layers.

Pumping Tests

The method used to determine the average hydraulic conductivity in the field by extracting water.

Test Well

The specific type of well from which water is extracted at a constant rate during a field test.

Steady State

The state reached when the water level in observation wells becomes constant.

Draw-down Curve

The curve formed by the water table during pumping in an unconfined aquifer.

Impermeable Layer

The layer of soil that does not allow water to pass, often found underlying a permeable layer.

Confined Aquifer

An aquifer that is trapped between two impermeable layers.

Radial Distance

The variable "r" represents this measurement from the center of the test well.

Perforated Casing

The perforated tube or pipe used in a test well to allow water entry

Clean Sands

This specific type of sand mixture is considered to have "good" drainage.

Piezometric Level

The name of the level of water in a confined aquifer before pumping begins.

Logarithm

The constant value (approx. 2.303 or 2.727) used in field permeability equations accounts for the conversion of this mathematical function.

Total Head/Thickness

The total thickness of the permeable layers in a stratified soil system.

q

The symbol used to denote the rate of groundwater flow into a well.

Flow Net

A graphical construction used to calculate groundwater flow through soil.

Flow Line

A line along which a water particle moves through a permeable medium

Equipotential Line

A line along which the potential head at all points is equal.

Flow Channel

The strip or area located between any two adjacent flow lines.

Right Angles

The angle at which flow lines and equipotential lines must always intersect.

Potential Drop

The difference in total head between two adjacent equipotential lines.

Number of Flow Channels

The symbol N_f represents this in a flow net calculation.

Number of Potential Drops

The symbol N_d represents this in a flow net calculation

Square

The geometric shape that elements in a flow net are intended to approximate.

Sheet Pile Wall

An impervious barrier driven into the ground, often used in examples of flow nets.

Flow Line

The surface of an impervious layer or sheet pile acts as this type of line in a flow net.

Equipotential Lines

Upstream and downstream soil surfaces are considered to be this type of line

z

The vertical direction in a 2D Laplace equation is typically represented by this letter.

Impermeable Dam

A structure used to retain water where seepage occurs underneath through the foundation soil.