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Clayton Group Service Paget Standard Operating Procedure SOP -12 - Unified Soil Classification System FINE-GRAINED SOILS Typically, fine-grained soils need to undergo tests for dilatency, dry strength, and toughness, as described below, to distinguish silt (M) from clay ©. These tests also are used to identify the fine-grained portion of coarse-grained soils. The degree of plasticity also is used to classify fine-grained soils. Typically this is estimated in the field, but accurate detennination needs to be made in the laboratory with an Atterberg limits test. Based on these tests, fine-grained soils having a Liquid Limit greater than 50 are modified by the symbol H (MH or CH), and soils with a Liquid of less than 50 have a symbol of L (ML or CL). Fine-grained soils with 20% or more coarse-grained material should be modified by the appropriate descriptive term, such as gravelly clay Procedures for Classifying Fine -Grained Soils in the Field: 1. Dilatancy (Reaction to shaking), Using an approximately 1/2 cubic inch lump of moist soil (if necessary add a small amount of water), place the lump in an open palm and shake horizontally, striking against the other hand several times. If water appears at the surface and the lump has a livery consistency, a positive reaction has occurred. Water disappears from the surface by squeezing the lump, and the soil eventually stiffens and cracks or crumbles. The character of the fines can be described based on how rapidly water appears on the lump surface during shaking and disappearance of water by squeezing. Very fine, clean sand will have a distinct and rapid reaction whereas plastic clays will have no reaction. Silts are intermediate with a fairly rapid reaction. 2. Dry Strength (Crushing Strength) - Prepare a small lump of soil the consistency of putty (if necessary add water). Dry the soil completely either in the sun or in an oven. Break the lump with your fingers. The dry strength increases with increased plasticity, with silts and fine sands having a low dry strength and clays, particularly plastic clays, having a high strength. 3. Toughness (Consistency near the plastic limit) - Add water to an approximately 1/2 cubic inch of soil and make it into the consistency of putty. Roll the soil into a thread approximately 1/8 inch in diameter, fold and reroll the sample repeatedly. With the loss of moisture the soil will become stiff and lose its plasticity. On reaching the plastic limit the sample will crumble. Clays with high plasticity will form a tougher thread near the plastic limit and a stiffer lump when it crumbles. Atterberg Limits - Atterberg limits, including the liquid limit (LL) and plastic limit (PL) need to be determined in the laboratory. Basically, Atterberg limits are used to measure the S:\Ermr\Proj\SOPS\SOP- 12.doc