![]() Essentially, the test involves spreading a soil sample in a cup of standard size, then cutting a groove through the middle. This involves using standardised equipment to ensure consistent measurement. The Liquid Limit is measured by either the Casagrande cup method or a cone penetrometer. The measured moisture content is the Plastic Limit. Once this first crumble has occurred, the sample has its moisture content measured as described earlier in BS 1377-2:1990. The plastic limit is reached when the ‘worm’ first crumbles when reaching 3 mm in diameter. Each time the worm is rolled, the moisture content will be reduced due to loss of water by evaporation. The Plastic Limit Test is performed by rolling a moist clay or silt soil into the shape of a 3 mm diameter ‘worm’ by hand, then remoulding the sample into a ball and repeating. In the UK this is done in accordance with BS 1377-2:1990 and equivalent standards are used throughout the world. So a clayey soil will have a higher PI than a silty soil.Ītterberg Limits are measured by laboratory testing. The smaller the particle size, the more a soil will retain moisture. Pure clay, for example, will have all its solid particles less than 0.002 mm, and pure silt is 0.06 to 0.002 mm (although exact definitions vary from country to country). The main factors that affect how a silt or clay behaves in relation to its moisture content, and hence affect the Atterberg Limits, are the size and distribution of the solid particles within a soil. The Plasticity Index (PI) is the range of moisture contents where the silt or clay remains plastic (PI = LL – PL).įigure 1: Atterberg Limits of a Silt or Clay.The Liquid Limit (LL) is the maximum water content a silt or clay can have before becoming a liquid, i.e.The Plastic Limit (PL) is minimum water content at which a soil is considered to behave in a ‘plastic’ manner, i.e.It is not commonly used and so not discussed further. The Shrinkage Limit (SL) is the water content where further loss of moisture will not result in any more volume reduction.In addition, there is the commonly referred to ‘Plasticity Index’: The Atterberg Limits are: the shrinkage limit, plastic limit, and liquid limit. Atterberg limits are a basic measure of the critical water contents of fine-grained soils, such as silt and clay, as they transition from a solid to a liquid.Ītterberg Limits are an inexpensive and well documented way of predicting the engineering properties of silt and clay soils. ![]()
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