Methods of test for soils, Part 6: Determination of shrinkage factors
1972 Edition

The apparatus specified for determining the shrinkage limit includes a glass cup with a flat top, a glass plate fitted with three metal prongs to press the soil pat and remove excess mercury, mercury for volume displacement, an evaporating dish to collect displaced mercury, a precision balance capable of measuring to 0.1 grams, an oven for drying soil samples, molds to shape soil pats roughly 45 mm in diameter and 15 mm in height, and a 425-micron IS sieve for sample preparation. The procedure involves filling the glass cup with mercury, placing the oven-dried soil pat on the mercury surface, pressing it with the pronged glass plate to displace mercury, then weighing the displaced mercury to accurately calculate the soil pat's volume using the mercury displacement method.

The shrinkage limit for undisturbed soil represents the highest water content at which the volume of the soil mass in its natural state no longer decreases with drying. In contrast, the shrinkage limit for remoulded soil refers to the maximum water content at which volume change ceases for soil that has been disturbed, mixed, and passed through a 425-micron sieve. The key distinctions are that undisturbed soil maintains its natural structure, whereas remoulded soil is structurally altered. Typically, remoulded soils exhibit a higher shrinkage limit due to the disruption of soil fabric, allowing greater volume reduction before shrinkage stops.

To measure volumetric shrinkage using mercury displacement, first prepare an oven-dried soil pat approximately 45 mm in diameter and 15 mm in height with rounded edges. Fill a glass cup to overflow with mercury and remove surplus mercury by pressing a glass plate with three prongs firmly on top. Place the soil pat onto the mercury surface and gently press it under the mercury with the pronged glass plate, collecting the displaced mercury. Weigh the displaced mercury with 0.1 g precision. Calculate the volume of the oven-dried soil pat by dividing the weight of displaced mercury by the unit weight of mercury (approximately 13.6 g/cm³). Next, measure the volume of the wet soil pat using a shrinkage dish filled with mercury and calculate the volumetric shrinkage as the percentage difference between the wet and dry soil pat volumes.

For precise shrinkage limit testing, undisturbed soil samples should be trimmed into pats about 45 mm in diameter and 15 mm in height with edges rounded to prevent air entrapment during mercury displacement. Remoulded soil samples require approximately 100 grams of soil that has passed through a 425-micron IS sieve and is well-mixed. When filling the shrinkage dish for undisturbed soil, coat the interior with silicone grease or vaseline to prevent soil sticking, add soil in increments roughly one-third of the dish volume, tapping gently to eliminate air bubbles, and level the surface. During mercury displacement, carefully place the oven-dried soil pat on mercury in a glass cup, press it under mercury avoiding air bubbles, collect and weigh displaced mercury to 0.1 g accuracy, and calculate the soil pat volume accordingly. These procedures ensure reliable volumetric measurements essential for determining shrinkage limits.

The shrinkage ratio (R) is calculated as the ratio of volume change to water content change above the shrinkage limit, expressed mathematically as R = (V_o - V_s) / (W_o - W_s), where V_o is the volume of the oven-dried soil pat, V_s is the volume at the shrinkage limit, W_o is the initial water content, and W_s is the shrinkage limit water content. Volumetric shrinkage (V₃) is determined by multiplying the difference between the initial water content and shrinkage limit (w₁ - w_s) by the shrinkage ratio R, i.e., V₃ = (w₁ - w_s) × R. These calculations quantify the total volume reduction of soil due to drying beyond the shrinkage limit.