constanthead

Constanthead refers to a laboratory method used to determine the hydraulic conductivity (permeability) of soils or other porous media by maintaining a constant hydraulic head difference across a soil specimen. It is commonly employed in geotechnical engineering and hydrogeology to assess how easily water can flow through a material, and it is typically contrasted with the falling-head method.

The apparatus for a constant-head test generally includes a permeameter or permeability cell containing the soil specimen between two porous discs or filters, a head reservoir on one side to establish a steady water head, and a means to measure the effluent discharge, such as a graduated cylinder or flowmeter. The cross-sectional area of the specimen and its thickness (length) are known, along with the maintained head difference Δh between the inlet and outlet reservoirs.

Procedure follows saturation of the specimen and installation in the cell, followed by establishing and maintaining a constant head difference. The effluent volume is recorded over time to obtain discharge Q. Hydraulic conductivity k is calculated from Darcy’s law: k = (Q × L) / (A × Δh), where L is the length of the specimen, A is its cross-sectional area, and Δh is the hydraulic head difference. Temperature and other environmental factors can influence viscosity and, thus, measured conductivity.

Constant-head tests are especially suited for soils with relatively high permeability, such as sands and gravels. They are less appropriate for low-permeability soils like clays. Advantages include simplicity and rapid results; limitations involve the need for careful head control, accurate discharge measurement, and potential issues with leakage or preferential flow paths.