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<br />EM 1110.2-2504
<br />31 Mar 94
<br />
<br />Classification and index (eSIs (wlller conlenl, Atterberg
<br />limits, grain size) should be perfonned on moo or all
<br />samples and shear (eSIs should be performed on selecled
<br />represenla1ive undisturbed samples. Where settlement
<br />of fme-grain foundation malerials is of concern. consoli-
<br />dation (eSIs should also be performed. The Slrength
<br />pammelers . and c are nol intrinsic material properties
<br />001 rather are paramelel'S that depend on the applied
<br />stresses, the degree of consolidation under those
<br />stresses, and the drainage conditions during shear.
<br />Consequenlly, their values musl be based on laboraIol)'
<br />teslS that appropriately model these conditions as
<br />expecled in the field.
<br />
<br />b. Coarst-grain mJJItriDls (cohesionltss). Coarse-
<br />grain IIIlIIeriaIs such as sands, gravels, and nonplastic
<br />sillS are sufficienlly pervious thaI excess pore pressures
<br />do not develop when SlreSS conditions are changed.
<br />Their shear Slrength is characlerized by the angle of
<br />inlemal friction (.) detcnnined from consolidated,
<br />drained (5 or CD) (eSIs. Failure envelopes plotled in
<br />terms of IOIa\ or effective stresses are the same. and
<br />typically exhibil a zero c value and a . value in the
<br />range of 2S 10 45 degrees. The value of . for coarse-
<br />grain soiIs varies depending pedominatcly on the parti_
<br />cle shape. gradation, and relative densily. Becausc of
<br />the difficully of obIaining undisturbed samples of
<br />coarse-grain soils, the . value is usually infClTed from
<br />in silu teslS or conservatively assumed based on material
<br />Iype.
<br />
<br />(1) Table 3-1 shows approximate re1alionships
<br />between the relative densily, slandard penclration resis-
<br />tance (5PTl, angle of inlemal friction, and unil weighl
<br />of granular soils. Figure 3-1 shows another corre1alion
<br />between ., relative densily, and unil weighl for various
<br />types of coarse-grain soils. Where sile-speciflC correla-
<br />tions are desired for importanl slruCIures. laboratory
<br />teslS may be performed on samples recompacled 10
<br />simulate field densily.
<br />
<br />(2) The wall friction angle, ~, is usually expressed
<br />as a fraction of the angle of internal friction, ,.
<br />Table 3-2 shows the smallest ratios between l) and .
<br />detcnnined in an extensive series of (eSIs by Potyondy
<br />(1961). Table 3-3 shows angle of wall friction for
<br />various soi1s against steel and concrete sheet pile walls.
<br />
<br />c. Fint-grain mattrials (cohesive soils). The shear
<br />strength of fmc-grain materials, such as clays and plastic
<br />sillS, is considerably more complex than coarse-grain
<br />soils be<'ause of their significantly lower permeabilily,
<br />
<br />3-2
<br />
<br />higher void ratios, and the inleraction between the pore
<br />waler and the soil particles.
<br />
<br />.
<br />
<br />(1) Fine-grain soils subjecled 10 Slress changes
<br />develop excess (either positive or negative) pore pres-
<br />sures because !heir low permeabilily precludes an
<br />inslanlaneous waler conlenl change, an apparenl . = 0
<br />condition in terms of loW stresses. Thus. !heir behavior
<br />is time dependenl due 10 !heir low penneabilily, resull-
<br />ing in differenl behavior under short-Ienn (undrained)
<br />and long-Ierm (drained) loading conditions. The condi-
<br />tion of . = 0 occurs only in nonna1ly consolidaled soils.
<br />Overconsolidated clays "remember" the past effective
<br />stress and exhibil !he shear Slrenglh corresponding 10 a
<br />stress level closer 10 !he preconsolidation pressure ralher
<br />than !he currenl slress; aI higher Slresses, above !he
<br />preconsolidation pressure, !hey behave like normally
<br />consolidaled clays.
<br />
<br />"
<br />
<br />(2) The second faclor, higher void ratio, generally
<br />means lower shear Slrength (and more difficull designs).
<br />Bul in addition, il creates other problems. In some
<br />(sensitive) clays !he loose slruclUre of Ihe clay may be
<br />disturbed by construction operations leading 10 a much
<br />lower strenglh and even a liquid SIaIe.
<br />
<br />(3) The Ihird faclor, Ihe inleraction between clay
<br />particles and wa/er (a1 microscopic scale). is Ihe main
<br />cause of !he "different" behavior of clays. The fust two
<br />faclors, in facl, can be allribuled 10 this (Lambe and
<br />Whitman 1969). OIher aspects of "peculiar" clay behav-
<br />ior, such as sensitivily, swelling (expansive soils), and
<br />low, effective-+ angles are also explainable by this
<br />factor.
<br />
<br />e)
<br />
<br />(4) In practice, !he overall effects of !hesc factors
<br />are indireclly expressed with the index properties such
<br />as U (liquid limil), PL (plastic Iimil), w (waler con-
<br />1CII1), and t (void ratio), A high LL or F'L in a soil is
<br />indicative of a more "clay-like" or "plastic" behavior.
<br />In general, if the nallU'81 wa/er conlen!, w. is closer 10
<br />PL. Ihe clay may be expected 10 be stiff, overcon-
<br />solidated, and have a high undrained shear Slreng!h; this
<br />usually (but not always) means that Ihe drained condi.
<br />tion may be more critical (wi!h respect 10 !he overall
<br />stability and the' passive resistance of the bearing stra-
<br />tum in a sheel pile problem). On !he other hand, if w is
<br />closer 10 LL, the clay may be expected 10 be soft
<br />(Table 3-4), nonnal1y consolidated, and have a low,
<br />undrained shear strength; and this usually means that !he
<br />undrained condition will be more critical.
<br />
<br />e)
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