Methods of test for soils, Part 17: Laboratory determination of permeability
1986 Edition

IS 2720 Part 17 (1986) addresses laboratory determination of soil permeability for a range of soil types. It is suited for cohesive soils like clays and silts as well as coarse-grained soils such as sands and gravels. The testing applies to saturated soils with particle sizes from fine silts (less than 0.075 mm) up to coarse sands and gravels (greater than 4.75 mm). Falling head tests are generally preferred for fine-grained soils due to their low permeability, while constant head tests are more applicable to coarse soils exhibiting higher permeability.

For undisturbed samples, the soil specimen should be trimmed into a cylindrical shape with diameter not exceeding 85 mm and height matching the mould. The specimen is placed centrally on a porous disc in the drainage base and sealed around the edges with an impervious material such as cement slurry or a mixture of 10% bentonite and 90% fine sand by weight, compacted carefully. A drainage cap is fitted atop the mould. For disturbed samples, soil is remoulded to field dry density or maximum dry density obtained from compaction tests, using optimum or field moisture content. The soil is compacted either statically or dynamically to simulate site conditions, trimmed flush with the mould top, and weighed. Saturation is achieved by head or flow methods for medium to high permeability soils; for low permeability soils, vacuum saturation exceeding 70 cm Hg for at least 15 minutes is used, followed by slow bottom-up saturation with de-aired water while maintaining vacuum.

The falling head test involves a standpipe connected to the soil specimen where water level decreases over time, measuring the time taken for the head to drop from an initial to a final reading. It is suitable for soils with low to medium permeability and involves unsteady flow conditions. The constant head test maintains a steady water level via a reservoir supplying a constant head, measuring the volume of water passing through the soil specimen over a fixed time under steady-state flow, suitable for medium to high permeability soils. Calculation methods differ accordingly, with falling head using logarithmic head ratios and time intervals, and constant head using flow rate, hydraulic gradient, and specimen dimensions.

The standard requires adjusting measured permeability values obtained at test temperature to a reference temperature of 27°C by applying a correction factor based on the kinematic viscosity of water. The formula used is k_27 = k_T × (Y_T / Y_27), where k_T is the permeability at test temperature T, Y_T and Y_27 are viscosity coefficients at temperature T and 27°C respectively. This ensures that permeability values are normalized and comparable, independent of temperature differences during testing.

Key apparatus include a permeameter comprising a cylindrical mould assembly to contain the soil specimen with known diameter and height, a constant head tank for maintaining steady water level during constant head tests, devices for measuring initial and final water heads such as water level gauges, a thermometer for recording water temperature, compaction rammers for specimen preparation, and timing devices like stopwatches for monitoring flow durations. This equipment enables accurate measurement of head loss and flow volume necessary to compute permeability coefficients for soils with permeability ranging from 10^-3 to 10^-7 cm/s and particle sizes up to 9.5 mm.