The objective of the permeability test is obviously to determine the permeability of the soil.  Soil permeability is used to calculate drainage.  For example, perimeter drains around a building and infiltration through a dam or landfill structure.

The rate at which water flows through a soil is proportional to the hydraulic gradient and is expressed by Darcy's Law as:

Darcy's Law

The coefficient of permeability varies with the type of soil and conditions.  It is influenced by:

1. Size and shape of the soil particles
2. Void ratio
3. Temperature
4. Degree of saturation

Empirical methods to estimate permeability using Hazen equations:

Hazen equations

for sands:

k = CD₁₀²
k = coefficient of permeability (cm/sec)
C = 0.4 to 1.2, typically 1.0
D₁₀= grain size of 10% passing (mm)

for estimating flow through graded sand filters (clean sands and gravels):

k = D₆₀D₁₀
k = coefficient of permeability (cm/sec)
D₆₀= grain size of 60% passing (mm)
D₁₀= grain size of 10% passing (mm)

Several different laboratory testing methods can be performed to more accurately determine permeability, including constant-head and falling-head types.   For our purposes, a falling-head test will be performed.  The falling-head test essentially consists of measuring change in head and quantity of flow over time as follows:

Falling head test

Error considerations

Several errors could have affected the test results:

• air trapped in sample or sample not 100% saturated;
• soil was washed from the sample;
• some of the head loss occurred in the apparatus rather than in the sample;
• not starting and stopping stop watch at correct point;
• sample settling during test;
• sample disturbed by flowing water at inlet;
• difficulty of accurately measuring heads relative to tail water and significant figures