Methods of test for soils, Part 18: Determination of field moisture equivalent
1992 Edition

Field Moisture Equivalent (FME) represents the moisture content at which a drop of water placed on a smooth soil surface spreads evenly without being absorbed immediately, producing a glossy appearance. In fine-grained soils, it indicates the moisture level at which air in soil pores becomes sealed by water films, halting capillary suction. For coarse-grained soils, it signifies that all void spaces are saturated with water. When FME equals or exceeds centrifuge equivalent moisture, it may indicate the presence of harmful organic substances. This measurement is crucial for evaluating soil moisture retention and drainage, which directly impacts foundation and earthwork design.

Soil samples should first be air-dried at room temperature until completely dry. Aggregates are then gently broken using a mortar with a rubber-covered pestle or softwood pestle. The dry soil is sieved through a 2 mm IS sieve, and the material retained is ground to break down aggregates further. This ground soil is resieved through the 2 mm sieve. The fraction passing the 2 mm sieve is subsequently sieved through a 425 micron IS sieve, discarding the material retained on it. Approximately 30 grams of the soil fraction passing the 425 micron sieve is taken for the moisture equivalent test, ensuring a uniform and fine soil sample for consistent results.

The test requires a 425 micron IS sieve to obtain the fine soil fraction, an evaporating dish to hold and mix the soil sample with water, and a spatula for smoothing the soil surface. Distilled water is used for moistening the soil. Pre-weighed containers (mass M1) are needed to hold the wetted soil sample. An oven capable of maintaining 105 to 110°C is essential for drying the soil to a constant weight. Accurate weighing scales or beam balances conforming to IS 1433:1965 are required to measure the masses of the container, wet soil, and oven-dried soil (M1, M2, and M3 respectively).

After preparing approximately 30 g of air-dried soil passing the 425 micron sieve and moistening it until it forms cohesive balls, the soil surface is smoothed and a drop of water is placed on it. If the water drop disappears in less than 30 seconds, more water is added and the process repeated. When the drop spreads and leaves a shiny surface without disappearing within 30 seconds, the sample is ready for weighing. The container mass (M1), wet soil plus container mass (M2), and oven-dried soil plus container mass (M3) are recorded. Field Moisture Equivalent (FME) is calculated using the formula: FME = ((M2 - M3) / (M3 - M1)) × 100. The result is reported as a percentage moisture content, rounded to two significant figures according to IS 2:1960.

A high Field Moisture Equivalent reflects that fine-grained soils retain moisture at a level where air in the pores is sealed by moisture films, preventing capillary suction. In coarse-grained soils, it means all voids are saturated with water. Additionally, when the Field Moisture Equivalent is equal to or greater than the centrifuge equivalent moisture content, it suggests the presence of detrimental organic matter within the soil. This information is critical for understanding moisture retention behavior and influences the design considerations for foundations and earthworks.