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~ ~~ ~ SOILS s-ls - Line m-m' is a rebound curve. Line m'-r is known as "" l (In equation 9.32, wl is a whole number, not a decimal reloading curve. Such curves resu t when a normally percentage.) _ded clay is relaxed and restressed. C° ~ o.oos(wl - 10) 9.32 . Notice that point m' can only be reached by loading the The reconsolidation index and swelling index can be soil to a pressure of p'° and then removing [he pressure. found from the (negative of Che) ]ogazithmic slopes of l i l b d di d This clay is said to have been prelouded or overconsoli- oun ng curves, respect ve y. the re an re oa dated.s Although the pressure of the clay is essentially the same as when it started, its void ratio has been re- duced. The overconsol:dation ratio is defined by aqua- . Lion 9.30. . R° = P°lP~v 9.30 I . The overconsofidation pressure, p'°, can be estimated by eye as a point slightly above the point of maximum . curvature. Graphical means are also used. . The shape of the e-log p curve will depend on the degree • of previous overconsofidation, as shown in figure 9.10. e undisturbed field il li • so ne . remolded Figure 9.11 Casagrande Method J. TRIAXIAL STRESS TESTS P In a triaxia] stress test, a cylindrical sample is loaded on Figure 9.10 Consolidation of Various Soils both ends and all around its surface. Usually, the radial stress (vR) is kept constant and the axial stress (oq) is varied. The normal and shear st.recses on a plane of any • Laboratory results can be used to find the preconsolida- angle can be found from the combined stress equations. Lion pressure (point m in figure 9.9). This is illustrated (Consider compression positive.) • in the following procedure. v ~ 1 e=-y (an+oR)+z(oq-oR)cos2B 9.33 Draw 2 lines-a tangent line and a horizontal line- 1 re = +Z (oq - oR) sin 2B 9.34 • through the point of maximum curvature. Bisect the resulting angle. Draw a tangent to the tai] of the These equations represent points on 1,9ohr's circle, field soil line. The intersection of this tangent and which can easily be constructed once o,l and oR are • the bisection line defines e° and p°.a known. (Gaze must be taken when plotting this graph. • Line k can be used to predict consolidation of the soil The sample is usually exposed t.o a pressure pR over a]] lmder various loadings. The compression index is the of its surface, including the ends. Thus, pR is equal to (negative of the) logarithmic slope of line k and is given oR. The pressure applied to the ends, pA, is in addition • by equation 9.31, where points 1 and 2 correspond to to radial pressure. Therefore, oq = pR + pA.) Test re- any two points on line k. suits aze shown in figure 9.12 for two different samples • which were both tested to failure. The ultimate shear • _ e2 - el - e° - el 9.31 ° strength, S,,,, can be read directly from the y-axis.' log10(pl/ps) IOg'•°(pl/p°) The equation for the rupture line (also known as the envelope of rupture) is given by Coulomb's equations • If the clay is soft and near its liquid limit, the com- • Pression index can be approximated by equation 9.32. r = Sa, = c + o(tan~) 9.35 ' f Prdo°dei clay is also known as prnvnsolutatrd cky, as well as _ i • Kreonrolidattd clay. The ultimate shear strength ie given the symbol a io most soils ~ • B This method of finding the pre<onsolidatinn pressure is known books. . ~.. ~ I • as the Cmagrnride meNod g Equation 9.35 is also known as the Mohr Coulomb cpuadon i I' I • PROFESSIONAL PUBLICATIONS INC ;. . ~ P.O. Box 199, aen Carlos, CA 94070 I' • .