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1 9-12 CIVIL ENGINEERING REFERENCE n4ANUAL ' C. MODIFIED PROCTOR TEST G. ATTERBERG LIMIT TESTS (CONSISTEA'CY This test is similar to the standard Proctor test ex- TESTS) ceps that the soil is compacted in 5 Payers with a heav- Clay soils can be either solid, plastic, or liquid depend- , ter hammer falling a greater distance. The result is a ing on the water content. The water contents come- , denser soil which is more representative of compaction sponding to the transitions from solid to plastic or plas- densities available From modem equipment. Table 9.7 tic to liquid are known as the Atterberg limits. These can be used by adding 10 to 20 lbm/fts to the densities transitions are called the pluslic limit (wP) and liquid and taking 3 to 10% from the moisture contents. limit (wi) respectively. . When a soil has a liquid limit of 100, the weight of D. IN-PLACE DENSITY TEST moisture equals the weight of the dry soil (i.e., w = 1). Alternatively, at the liquid limit, the soil is half water . This test. also known as the field density test, starts " " and half solids. A liquid limit of 50 means that the soil by compacting soil in the field and digging a 3 to 5 at the liquid limit is two-thirds soil and one-third water. . deep hole with smooth sides. .41] soil taken from the • hole is saved and weighed before the water content. can Sandy soils have low liquid limits-on the order of 20. change. The hole vollune is determined by filling the In such soils, the test is of little significance in judging hole with sand or a water-filled rubber balloon. The load cazrying capacities. Silts and clays can have sie . required densities aze given by equations 9.8 and 9.10. nificant liquid limits-as high as 100. Most clays, how- ever, have liquid limits between 40 and 60. High liquid . limits indicate high clay content. and low load carrying . E. UNCONFINED COMPRESSIVE capacity. . STRENGTH TEST The plastic limit depends on the clay content. Some A cylinder of cohesive soil (usually clay) is loaded to silt and sand soils have no plastic limit at all. They compressive failure. (Failure of elastic soils is taken as are known as non-plastic soils. The test is of no value . a 20% strain.) The unconfined compressive strength is in judging the relative load carrying capacities of such given by equation 9.18. The ultimate shear strength soils. is taken as one half of the unconfined compressive The difference betx•een the liquid and plastic limits is • strength. known as the plasticity index. S„~ = P/A 9.18 IP = wl - wy 9.21 • Sua = S2c 9.19 The plasticity index gives the range in moisture content • over which the soil is in a plastic condition. A small TIVITY TES ENS S plasticity index shows that a small change in moisture F. S I T content will change the soil from a semisolid to a liquid Clay will become softer as it is worked, and clay condition. Such a soil is sensitive to moisture. A large • soils may rum into viscous liquids during construction. Plasticity index (i.e., greater than 20) shows that con- This tendency is determined by measuring the ultimate siderable water can be added before the soil becomes strength of two unconfined samples, one of which has liquid. • been packed and extruded. Atterberg limits vary with the clay content, type of clay, • undiacurbed Snc and the ions (cations) contained in the clay. • , sensitivity = 9 20 Sn~,remolaea The liquidity index of a clay soil is II = w - wP 9.22 Table 9.8 /n . Sensitivity Classifications The Atterberg liquid limit is found by taking a soil sam- . sensitivity class ple and placing it in a shallow container. The sample . 1-8 natural clays is parted in half with a special grooving tool. The con- 4-8 sensitive tainer is dropped 25 times. At Lhe liquid limit, the 8-15 extra sensitive sample will have rejoined for a length of 2 . > 15 quick The plastic limit test consists of rolling a soil sample PgOF ESSIONAL PUBLICATIONS INC. • P,O. Box 199. San Carlos, CA 96070