AViewpointonSoilStrengths basedonCSSMResearchin CambridgeTingWenHui IEM
Introduction(1) CharacterizationofSoilPropertiesinsite problemsforapplicationtogeotechnical problems for application to geotechnical design:1. 1 Identification of appropriate property and the ofappropriateproperty andthe specification ofrelevanttestingprogram 2. Assigningvaluetoselectedpropertytocaterfor nonuniformconditions TechniquessuchasLowerBound,Meanand Probability. Probability
Extraneousfactorssuchasshortcomings in testing improved by Characterization process byCharacterizationprocess.
Introduction(2) Presentation herein: CS Model as Theoretical Framework Strength Characterization in site problems. (Model not directly on Stressstrain as in Roscoe & Burland (1968) in Mod. Camclay.) ( ) y) 1) CS Model Critical State Model, launched by Roscoe, Schofield and Wroth (1958) in Cambridge, followed by Original Cam Clay (OCC) and then Mod Camclay etc Mod. etc. 2) Schofield and Wroth (1968) develops fundamentals for Model in the context of the Theory Of Plasticity. 3) Wroth: Rankine Lecture ( ) h ki (1984) f ) focus on Interpretation of In Situ i f i Soil Tests for site deposits within the framework of CSSM. 4) Schofield (2005) presents Frontispiece with single vs. plot, ) ( )p p g p extending (q,p); (, lnp) plots in OCC, covering several aspects of soil properties 5) Schofield (2006) further extends fundamental of Model to Plastic Design with Interlock and Friction Strength concept.
CSSMClassical(Roscoeetal,etc) References Roscoe, K.H., Schofield, A.N. and Wroth, C.P. 1958. On the yielding of soils. G t h i i ldi f il Gotechnique, pp. 9 71 83 9, 7183 Roscoe, K.H. and Schofield, A.N. 1963. Mechanical Behaviour of an idealized wetclay. Proc. of European Conf. SMFE, Wiesbaden. Pp. 4754.
Yielding of soil in Critical State is continuing Plastic Shear Strain without Volume Change and and, Critical State in (q,p) and (,ln p) plots is determined by: q = Mp = + ln p and supported by experimental evidence.
CSSMClassical(Schofield&Wroth) Schofield & Wroth (1968): provides fundamental of theory of plasticity background and refinement to CS Model launched by Roscoe et al in 1958. CS Yield Curves: Fig. 6.1 (Slide 12) Granta Gravel: Rigid Plastic (simpler version); Camclay: Elasticplastic, along line on ln p plane; Fig. 6.4. Undrained Test in Fig. 6.7 ( l d 13) d d (Slide ) Drained Test in Fig. 7.9 (Slide 14)
Camclay (designated as Original Cam Clay) obeys Associated flow rule (Slide 15), with the Yield Locus as potential function for plastic flow; & belongs to Stable St bl material i D k P t l t i li with t i l in Druckers Postulate; in line ith Plastic Theory. (Schofield, 2005) ($5.2 $5.4; pg 91 105).
CSSM OriginalCamClay(OCC) CSSM Original Cam Clay (OCC)
Slide 6 VU5 introduce Burland lecture plot? as slideValued User, 6/13/2009
CamClayPlot Cam Clay Plot
CSSM Mod.CamClay R f References: Roscoe, K.H. and Burland, J.B. (1968). On the generalized stress strain behaviour of wet clay. In: Engineering Plasticity, Cambridge University P C b id U i it Press, ( d ) H (eds) Heyman, J and L ki F A J. d Lockie, F.A. Roscoe, K.H., Schofield, A.N. and Thurairajah, A. (1963). Yielding of Clays in States Wetter than Critical. Geotechnique 13, 211240, 1963. 1963 Burland, J.B. (2005). Soil Mechanics Emma: Elegant, Rigorous and Relevant. Inaugural Lecture (Honorary Fellow, Emmanuel College.
Modified Cam clay: incorporates Camclay: new work equations proposed by Burland (1965) Two new concepts introduced: Yield locus for state paths beneath state boundary surface for shear distortion without plastic vol. change MohrCoulomb criterion applied to 3D stress space
S Stressstrain relation thus f i l i h formulated f l d framework: k Numerical analysis (e.g. FEM) of soil mechanics problems.
CSSM PartlySaturatedSoils CSSM Partly Saturated Soils ( (Burland&Ridley) y) Burland,J.B.&Ridley,A.M.(1996).TheImportance ofSuctioninSoilMechanics.12th SoutheastAsian GeotechnicalConference,KualaLumpur AnintroductiontotheapplicationofCriticalStateConcept toPartlySaturatedSoils. t P tl S t t d S il
Schofield&Wroth(1968) Scope1 Schofield, A., Wroth, P. 1968. Critical State Soil Mechanics. The scope of the book may be portrayed as in: Sect. 6.8: The Critical State Model Concept stated in Roscoe, Schofield and Wroth (1958) Essential Ideas unaltered, but presented in slightly different form.
Two Hypothesis: Yielding of Soil through progressively severe distortion Yielding distortion Critical States approached after severe distortion
Plasticity fundamentals established Associated Flow Rule of as applied to Camclay f li d l Clarified in Fig. 6.9 (Slide 14)
Schofield&Wroth(1968) Scope2 CS (q p) theoretical curve Eq 6 17 (Slide 5): CS(q,p)theoreticalcurve Eq.6.17(Slide5):
AppendixC A yield function and plastic potential for soil under general Ayieldfunctionandplasticpotentialforsoilundergeneral principalstresses
Schofield&Wroth(1968) TheoryofPlasticity Th f Pl ti it Yield Functions (Failure Criterion): Tresca and Von Mises; & latter preferred by Schofield & Wroth (1968) after, Wroth (1984) advocates Matsuokas Criterion as better fit to test data
Pl ti B h i Plastic Behaviour i t parts: in two t The Yield Function is a Potential Function Plastic Strainincrements are Gradients of a Potential Function and is Normal t th F ti A N l to the Function. Associated Fl i t d Flow R l of Th Rule f Theory of Pl ti it f Plasticity obeys the Normality condition.
Isotropic Hardening and Stability Strain increment Vectors are normal to the Hardening f nctions Strainincrement functions
Druckers Stability Criterion (provides quantitative relation) Stability Postulate: Plastic material are stable only if they yield such that following is obeyed obeyed: Vector product, of stress increment vector, and associated strain increment vector ,will be positive or zero. (quantitative)
Schofield&Wroth(1968)Fig.6.1:YieldCurves(GrantaGravel/Camclay) Fi 6 1 Yi ld C (G t G l/C l )
Schofield&Wroth(1968) Fig.6.9 AssociatedFlowRule
Schofield&Wroth(1968)Fig.7.17 vs introduced
SoilStrength Obs./Characterized Sh Shear St Strength quantity variously expressed as: th tit i l d s (SB), su (Wroth, 1984), su (Schofield, 2006), cu = su = 1/2 Unconfined Compr. Strength (Schofield & Wroth,1968) p g ( )VU6 Terminology (of strength parameters) is issue:
true cohesion (c) due to adhesion ascribed to Terzaghi, (S h fi ld 2005 2006) asserts that (Schofield, 2005, h true cohesion i h i is apparent cohesion due to internal friction. interlocking proposed by Schofield (2006) in relation to peak strength, and Terzaghi true cohesion and friction. Schofield (2001): ..peak strength includes cementing or bonding of soil grains grains. Randolphs Foreword to Schofield (2005) is relevant.
Characterized Insitu Soil Strength as interpreted by CSSM and applied, is also presented in: (Ting, 2003): CS Strength (Twh Ref.) in Slide 44.
Slide 20 VU6 in particular issue of cementing and bond as peak strength and thus interlock; but physically different in natureValued User, 6/14/2009
CSStrength(Wroth) CS Strength (Wroth) Ref. References Wroth, C.P. 1984. The interpretation of in situ soil tests. tests Rankine Lecture Gotechnique 34 No 4 Lecture, 34, No. 4, pp. 449489 Loudon, P.A. 1967. Some deformation characteristics of kaolin. Ph.D thesis, University of Cambridge. Fig. 6. Effective stress paths for undrained triaxial compression tests on kaolin (that include Oc soils)
CSStrength(Wroth,1984) Formulation su (Wroth, 1984) = Srupo = M/2(R/r) ( , ) / ( /) where su is the undrained shear strength equal to half the deviator strength, Sru (in Eqn (1)) is the undrained strength ratio that has a value Eqn. depending on the overconsolidation ratio (= 0.20.25 for Nc soils) and nature of the deposit of the sample. po is the effective vertical stress on the test sample, M is q/p at CS ; R, r & are overconsolidation, spacing ratio & plastic volume strain ratio in the Camclay Model. ratio, Cam clay
The above formulation may be usefully applied to natural site soil strength profiles. g p Theoretical Expressions for Undrained Strength follows:
CSStrength(Wroth) Twhdata hd
CSStrength(Wroth,1984)U d i dS UndrainedStrengthRatio hR i
Schofield(2005) FrontispieceStressobliquity() vs.CSSpecificVolume()
Schofield(2005) (,) SignificanceofFrontispiece i ifi f i i
Schofield(2005) (,) ( ) (, ) Definition:,
Schofield(2005) (,) OCCConstants(Fig.61)
Schofield(2005) (,) Testdata(Fig.62)
Schofield(2005) (,) TheoreticalEquations q/p () = M/(-).( +(-)- - ln p) Eqn. 6.19, pg 142; derived from State Boundary q , ; ytheoretical equation (presented in Slide 5)
M = /() definition ( ) = v+ ln p - definition Then q/p () = M + M . providing theoretical linear relationship between q/p () and
Schofield(2005) (,):Observation q vs. plot Denotes physically the effect of shearing on volume change l h Particularly visvis dilatancy, dense/oc (dry) and loose/nc (wet) soils ( )
q/p () vs. v :
Avails 3D: OCC (q,p,) on condensed 2D plot , Uniqueness in the vs. v relationship and that necessitates Affirmation by Observation Observation. that affords normalization procedure