Constant Head Method

Aim:-

To determine the permeability of soil by contant-head method.
The rate of flow under laminar flow conditions through a unit cross sectional are of porous medium under unit hydraulic gradient is defined as coefficient of permeability. Water flowing through soil exerts considerable seepage force which has direct effect on the safety of hydraulic structures. The rate of settlement of compressible clay layer under load depends on its permeability. The quantity of water escaping through and beneath the earthen dam depends on the permeability of the embankments and its foundations respectively. The rate of discharge through wells and excavated foundation pits depends on the coefficient of permeability of the soils. Shear strength of soils also depends indirectly on permeability of soil, as dissipation of pore pressure is controlled by itspermeability.

Specifications :-

IS 2720-17 (1986): Methods of test for soils, Part 17. This test is used to determine the permeability of granular soils like sands and gravels containing little or no silt.

Theory :-

The knowledge of this property is much useful in solving problems involving yield of water bearing strata, seepage through earthen dams, stability of earthen dams, and embankments of canal bank affected by seepage, settlement etc. Permeability of soil can be determined from Darcy’s Law. The equation to determine the permeability of soil using constant head permeability test is given by:

k = (Q×L)/(A×h×t)

Where,
k = coefficient of permeability
Q = volume of water collected in time t
h = head causing flow
A = cross sectional area of sample
L = length of sample

• A constant head permeameter shown schematically in the figure.

• For a typical setup the following dimensions are used

i. Internal diameter of the mould = 100mm.
ii. Effective height of the mould = 127.3mm.
iii. Detachable collar: 100 mm diameter and 60 mm height.
iv. Drainage base, having a porousdisc.

• Weighing balance, and other accessories.