# Soil Permeability

Soil permeability is a measure of fluids flowing through the soil. Aside from the soil, ease of flow also depends on the fluid (oil is slower than water etc) but you only need to worry about the soil end of things for the geotech breadth.

### Coefficient of Permeability (K)

The coefficient of permeability is the characteristic that measures how permeable a soil is. The units of K are the same as velocity (ft/s), but theoretically it is volume per area per time ($$\frac{ft^3}{ft^2 t}$$).

Typical values for K are probably in your reference of choice, and probably also in any permeability problem prompts. In the CERM the values are on page 21-3 and are provided for many different unit types.

### Darcy’s Law

Darcy’s Law will calculate the groundwater flow (Q) through a soil given a hydraulic conductivity (K) a hydraulic grade (i, units are $$\frac{in}{in}$$ or $$\frac{ft}{mi}$$ etc.) and an area covered (A). The units of area should match the units for grade.

$$ Q = -KiA$$

Some references show the negative sign, others don’t. I think this is to account for the hydraulic grade and direction of flow. In most cases the point you are measuring the flow at is roughly the lowest point in the area (A). The water table grade/slope from any other point will be negative to end at the point, so to make the flow positive they throw that negative in to reverse the grade negative. Anyway, regardless of the negative, the absolute value for flow doesn’t change and you should be able to tell what direction the flow is going in.

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