38_CEG 4012 Notes Fall 2011 CEG 4012 Notes Fall 2011

38_CEG 4012 Notes Fall 2011 CEG 4012 Notes Fall 2011 - l-~...

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p21 OC Magnitude of Consolidation, pc: Over-consolidated (OC) ciay (saturated) I for OC Clay cr'o < cr'c (or Pc) i.e. the present effective overburden pressure, cr'o, is less than the maximum past pressure the soil has experienced, cr'c (or Pc) = preconsolidation pressure Over-consolidation Ratio = OCR = cr'c I cr'o History of OC clay: (classic definition) . " . :::::::::::::::Bejjj~V.~d::::::::::::::::::::: - 77/\\ V77\\ \ p r. Element of soil deposited at P Pressure increases with continued deposition Water is squeezed out Void ratio decreases (e=V,IV,) Overburden is removed External Load Applied T ica e of OC condition: Glaciation Erosion Higher water table (decreases cr'o). Tem orar lower water table (past cr'o > present cr' Void Ratio, e Nature deposits soil (approx 10' yrs process) Slope Cc (= Compression Index) lJfllO<\~ ,4 Slo/l( ell (, al'.l""'PI~Jio" 1")0) v"I"',c1i'J [,f'j(,1 S eo ------~-. ....~~ Log Vertical Effective
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Unformatted text preview: l-~ ~___ Stress, cr'v a'o a'e When an OC soil is loaded, loading occurs back along the unload curve, not the virgin curve, until reaching the preconsolidation pressure. Therefore, an over-consolidated soil is generally a good foundatioo mate.tiaLsince_we can.add.&quot;,-stre.ss.Aq:v. .~ (gJ; -&lt;&gt;'0) before gettingn to the ste(lQer.virgiLL9.u[Ve,(j.J',. ..wJ.WleLCbeogg,in void ratio), _-When building on OC soils there are two possible situations depending on the relative magnitudes of the new effective stress and the preconsolidation pressure. If the new effective streJ;s.is-less.tban-lbapr.e.s;onsolidation then loading occurs ool}Lakmg the reload 'Curve. If the new effective stress exceeds t e preconsolidation then loading &quot;ilLQ.cQur partly along the re oa curve and parj)y aloog.the_virgii1.Curve,---=-. .---&quot;'-----.-...
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