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<br />~ ~ Z^~ ~D~+TtoN 199
<br />ta.,, ,
<br />vction when angu)ar
<br />'specified for use a .~
<br />1 by in8uencin ~
<br />Irate interlocking ~'~
<br />g and _
<br />9Il BCCUIDU18t1On e( ~~~
<br />might be requirty.
<br />~r test on minus Aro
<br />'Y be no more than
<br />d: for example, ex-
<br />;ndisturbed, natural
<br />e, relatively Bat or
<br />n'edominantly hori-
<br />;s soil with rounded
<br />uncommon. Thin
<br />would not be indi- .
<br />oriels, which might
<br />.ion is usuall
<br />y re-
<br />~llowing discussion
<br />of ~ing strength
<br />~' y specimens
<br />IIY aPPlied loading "~
<br />inq strength. Co-
<br />ility of a cube or
<br />!n 8 Aatnral C18y
<br />ed periods of time.
<br />formations appear
<br />apparent cohesion.
<br />uded that strength
<br />e water is crested
<br />J external loading
<br />ensure or effective
<br />lch a cahesienless
<br />ply strength due
<br />gree.that it would
<br />pion of practical
<br />~ of discussion.
<br />!ermine shearing
<br />nternal friction.
<br />y.nt SNEAaING STtENGTM tet
<br />Once this soil chnraeteristic is known, shearing strength is calculated
<br />by means of Eq. (6-6). For many approximate estimates and prelimi-
<br />nary calculations involving the shearing strength of cohesionless soil,
<br />it is unnecessary that shear tests be conducted in orcler to determine
<br />the (riction angle, since its value normally lies within a fairly narrow
<br />range. Thin range is naturally limited by the fact that, with tew excep-
<br />tions, particles of the primary minerals do not differ sufficiently in hard-
<br />ness and other properties to affect triction. The exceptions would in-
<br />ciude shales and weathered residual soils, on the one hand, and unusually
<br />hard materials, such es trap rock, on the other. Furthermore, the sur-
<br />face condition of soil particles except in the upper horizons is not subject
<br />to much alteration. In spite of a fairly generai belief, it is not likely
<br />that wetting results in an effective lubrication of soil particles, since
<br />most soils contain su6cient moisture to forma film around each particle
<br />even in an air-dry condition. Loss of shearing strength in saturated
<br />soil occurs not through any significant change in the friction angle due
<br />to lubrication• but through reduction in intergranular pressure due to
<br />submergence. For cohesionless soils in general, the true friction angle
<br />TABLE t~l
<br />Typical Valves of Friction Angles, Granular Sails
<br /> F rtO•eto0 A0 flee
<br /> $apa aL At Dittman At Peak 9treagth
<br /> Angle °[ Repoas StreOpa t8ed. DeOa Deena
<br />Claeslf/dtlo0 i slops 0 ten Oe ma Oe Lan
<br /> (deg.) Ivert. Lokor.) (def.) • (deg.) 0° Ideg.) 0,
<br /> 28 100 2 28 0.488 26 0.332 70 0.377
<br />SILT (Nenplastldl m m m m
<br /> 70 1001.73 ]0 0.377 J2 0.823 74 11.673
<br /> 26 Ion 2 26 0.488 50 0.517 72 0.673
<br />[I01[Or0 nne m medtom 9AA'D to to Lo to
<br /> 70 Ion 1.73 JO 0.377 14 D.673 38 0.726
<br /> 30 1 m 1.73 70 0.377 34 0.873 38 0.809
<br />Wen-graded SAND to b W to
<br /> J4 100 1.30 7A 0.673 40 0.659 48 1.010
<br /> 72 I w 1.60 32 0.623 78 °.726 40 0.900
<br />sAND 8 CAAV EL to m to w
<br /> 38 1 00 1.40 JB 0.728 42 0.900 4B 3.310
<br />Nora: Within each range, attsign lower values if particles are weu rounded or if
<br />there u eigaificant soft shale or mica content, higher values for hard, angular particles.
<br />l;sc lower values for high normal preseurp than for moderate normal pressure.
<br />=_eldom is less than 26° or greater than 36°, and the possible variation
<br />within this range may be related to particle size, shape, gradation, and
<br />composition, as shown in Table 6-1. Titus, with a reasonably accurate
<br />tte~cription of a cohesionless soil, a true friction angle may be selected
<br />' Certain exceptions to this concept are noted by Lee, Seed, and Dvnlnp, "Effect
<br />of illoisture on the Strength of a Clean Sand;' Jour. Soi! blechanics and Fdrts.
<br />Div., gSCE, Vol. 93, No. SD16, Nov. 1967, p. 17.
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