Permeability, or the ability of soil to let water flow through its pores, is a crucial property for geotechnical engineers. This test helps measure the permeability of soil, especially for less compacted types. The permeability impacts various aspects, such as settlement rates under loads, the design of earth dams, and the stability of retaining structures.
Permeability plays a significant role in the stability of structures and the estimation of underground seepage. Earth structures affected by seepage forces are subject to stability analysis, making understanding soil permeability vital.
Several tools are employed for this test, including a mould assembly conforming to IS: 11209-1985, a compaction hammer, a set of stand pipes of varying diameters, and a constant head tank. Miscellaneous apparatus like sieves, a mixing pan, and a thermometer are also necessary.
Two types of soil samples are prepared: disturbed and undisturbed. Disturbed samples involve taking soil from air-dried or oven-dried material, compacting it in a mould, and ensuring proper saturation for testing. Undisturbed samples require trimming into a cylinder, filling the space with impervious material, and fixing a drainage cap.
The specimen is connected to a stand-pipe, and the time for water level drop from initial to final head is recorded. This is repeated until three successive observations show similar time intervals. Alternatively, specific initial and final heads are chosen, and time intervals for the head to fall within those limits are observed.
Values such as length, diameter, volume of the specimen, water content, stand pipe diameter, area, and specific gravity of solids are recorded. A table for recording observations and calculations includes details like mass of the empty mould, mass of soil, initial and final heads, and time intervals.
Various calculations are performed, including mass of soil, bulk density, dry density, void ratio, and the permeability equation (Equation 1).
Permeability values at different temperatures are reported in units of cm/s. The state of the sample is also detailed, including water content, void ratio, and degree of saturation.
In conclusion, the Variable Head Permeability Test is a comprehensive procedure that involves careful preparation, precise measurements, and thorough calculations to understand the permeability of soil.