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<br /> <br /> s-ts CNIL ENGINEERING REFERENCE MANUAL <br /> <br /> aA <br /> <br /> aR <br /> 7 or S <br /> <br /> <br />aR us <br />rupture line <br />f <br />) <br />. rupture <br />(envelope o <br /> Sus, 2 _-_-_-- -- <br />- <br /> S <br /> us 1 - - <br />. \ <br />_ ~ L <br />_ _ _-- <br />_ _ _ _ _ c 2B 2B a <br /> ~ aR1 aAl aR2 aA2 <br />. <br /> <br />. "A Figure 9.12 Mohr's Circle of Stress <br /> <br />• The plane of failure is inclined at the angle Presence of water in the pores of a sample will not aE- <br /> <br />• Cect these results much if the triaxial test is conducted in <br /> such a manner as to allow pore water pressure to dissi- <br />. B = 95° + z ~ 9.36 pate (i.e., pore water to flow freely). Such triaxial tests <br /> are known as S-tests or consolzdated-drained tests. How- <br /> For slow shear of drained sands and gravels <br />the coke- ever, testing of slow-draining soils may require several <br /> , <br />lion, c, is zero. Therefore, it is possible to draw the weeks time. If the test is peformed quickly so chat the <br />• rupture line with only one test. Typically, c varies Irom Pore pressure does not have a chance to dissipate. the <br /> 200 to 2000 psf for very soft and very stiff clays <br />respec- test is known as an R-test or consolidated-ursdrairsed <br /> , <br />Lively. test. In such a case, much of the axial load will be car- <br />• ried by the pore moisture. In the Q-test (quick test), the <br /> For saturated clays in quick shear, it is commonly as- water content of the specimen is not allowed to change. <br />.: sumed that A = 0. This would be represented as a Such a test is justified only with low permeability (e.g.. <br />• horizontal rupture line. E1r3 cm/sec) soils. <br />• Representative values of ~ are given in table 9.10. ~ is <br />il <br />i <br />h <br /> known as the angle of internal friction.9 gra <br />ns <br />The effective soil pressure is the pressure t <br />at so <br />• exert on each other. This pressure provides the shear <br /> strength of granulaz materials. It can be calculated <br />~~ from equation 9.37, where c' and ra' are the effective <br />•' Sns stress parameters. <br /> 37 <br />= c' + o'tan~' 9 <br />5 <br /> ,,, <br />. <br />~' <br />~, The total pressure also includes the pore pressure, µ. <br /> The pore pressure can be found from the rise in a cap <br /> illary tube, or it can be measured directly in a triaxial <br /> shear test. <br /> 38 <br />o'=o-µ 9 <br /> . <br /> a <br />Figure 9.13 Rupture Lines for Ideal Sand <br />and Saturated Clay <br />' For cohesionless soils, the angle of internal friction is the angle <br />from the horizontal naturally formed by a pile. For example, sand <br />makes a pile with a slope of approximately 30°. Saturated clay, <br />op the ocher hand, ideally behaves like a liquid, with ~ = 0. For <br />most soils, the natural angle of repose will not be the same as the <br />angle of internal fr,ctioo, due to the effects of Cohesion. <br />R-tests are used to determine the effective stress pa- <br />rameters, c' and ~'. In the absence of pore pressure <br />measurements, R-tests can only record the total stress <br />parameters c and x.10 <br />10 I(a soil is always going to be saturated, the total stress pa- <br />rameters cao be used for foundation design. in cases where the <br />soil is not always saturated, only the effective stress parameters <br />should be used. <br />PROFESSIONAL PUBLICATIONS INC. ~ P.O. Box 199. San Carlos, CA 940)0 <br />