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<br />e <br /> <br />e <br /> <br />e <br /> <br />3-29 <br /> <br />Specific gravity. - - When soils are composed of particles having 95 <br />percent or more (by dry weight) smaller than the No.4 sieve (4.76 <br />mm.) and when compaction or shear tests are required, specific gravity <br />tests should be performed on the minus No. 4 fraction. <br /> <br />When soils contain five percent or more (by dry weight) of hard par- <br />ticles larger than the No. 4 sieve and when compaction or shear tests <br />are required, absorption and bulk specific gravity (saturated, surface <br />dry) tests should be performed on the coarse materials as well as on <br />the fraction smaller than the No. 4 sieve; . <br /> <br />Results of specific gravity tests are needed for computation of poros- <br />ity, void ratio, and associated air~ater-solid relationships; also <br />results are needed for adjustment of moisture-density relationships <br />for compacted materials having five percent or more larger than the <br />No.4 sieve (by dry weight). <br /> <br />Strength considerations. - - Some measure of inter-particle attraction <br />may prove helpful in evaluating erosional stability of channels on <br />the basis of the tractive force theory. Although shear strength <br />obtained by various tests has been used by some investigators, it <br />may not be a true indicator of inter-particle attraction. <br /> <br />Strength values for use in slope stability analyses of banks should <br />be governed by site conditions and the method of analysis to be used. <br />Therefore, some latitude is needed by the engineer in the selection <br />of appropriate tests to be performed at a given site for determination <br />of shear strength parameters. <br /> <br />I. Unconfined compression test. This test is commonly called a qu <br />test and may be performed on saturated, undisturbed specimens. <br />The test may be made for two purposes: (l) to obtain unconfined <br />compressive strength values for use in estimating erosional <br />stability (by the tractive power method) of those soils in which <br />individual particles cohere under natural field conditions, and <br />(2) to obtain values of cohesion (no load strength) for use in <br />analyzing the stability of banks from a strength standpoint. In <br />the latter case, the test is generally applicable to highly <br />plastic soils, but it may be used for other soils where the in- <br />place strength will simulate ultimate field conditions. <br /> <br />The qu test should not be performed on those soils in which natural <br />planes of weakness are an inherent characteristic of the soil, <br />as exemplified by stiff, fissured clay or soils having blocky <br />or columnar structural development. <br />