CSSMforDummies临界状态土力学的讲座.pptx

PROLOGUEq Critical-state soil mechanics is an effective stress framework describing mechanical soil responseqIn its simplest form here,we consider only shear-induced loading.qWe merely tie together two well-known concepts:(1)one-dimensional consolidation behavior,represented by e-logs sv curves;and(2)shear stress-vs.normal stress(t-st-sv)from direct shear box or simple shearing.qHerein,only the bare essence of CSSM concepts are presented,sufficient to describe strength&compressibility response.Critical State Soil Mechanics(CSSM)q Experimental evidence provided by Hvorslev(1936;1960,ASCE);Henkel(1960,ASCE Boulder)Henkel&Sowa(1961,ASTM STP 361)qMathematics presented elsewhere,including:Schofield&Wroth(1968);Burland(1968);Wood(1990).qIn basic form:3 material constants(f f,Cc,Cs)plus initial state(e0,s svo,OCR)qConstitutive Models,include:Original Cam-Clay,Modified Cam Clay,NorSand,Bounding Surface,MIT-E3(Whittle,1993)&MIT-S1(Pestana)and others(Adachi,Oka,Ohta,Dafalias)q Undrained is just one specific stress pathq Yet!CSSM is missing from most textbooks and undergrad&grad curricula in the USA.One-Dimensional Consolidation Cc=0.38Cr=0.04s svo=300 kPas sp=900 kPaOverconsolidation Ratio,OCR=3Cs=swelling index(=Cr)cv=coef.of consolidationD=constrained modulusCa ae=coef.secondary compressionk hydraulic conductivitys svDirect Shear Test ResultssvtDirect Shear Box(DSB)svtDirect Simple Shear(DSS)tdtg gsCSSM for DummiesLog s svEffective stress s svShear stress t tVoid Ratio,eNCCCtanf fCSLEffective stress s svVoid Ratio,eNCCSLCSSM Premise:“All stress paths fail on the critical state line(CSL)”CSLfc=0CSSM for DummiesLog s svEffective stress s svShear stress t tVoid Ratio,eVoid Ratio,eNCNCCCtanf fCSLCSLCSLSTRESS PATH No.1NC Drained Soil Given:e0,s svo,NC(OCR=1)e0svosvoDrained Path:D Du=0tmax=c+s tanfefDeVolume Change is Contractive:e evol=D De/(1+e0)0 svoe0svoEffective stress s svCritical state soil mechanicsInitial state:e0,svo,and OCR=sp/svoSoil constants:f,Cc,and Cs (L=1-Cs/Cc)For NC soil(OCR=1):qUndrained(evol=0):+Du and tmax=su=cuqDrained (Du=0)and contractive(decrease evol)For OC soil:qUndrained(evol=0):-Du and tmax=su=cuqDrained (Du=0)and dilative(Increase evol)Theres more!Semi-drained,Partly undrained,Cyclic.Equivalent Stress ConceptLog s svStress s svShear stress t tVoid Ratio,eNCNCCCtanf fCSLCSLCSLCSOC1.OC State(eo,s svo,s sp)svo2.Project OC state to NC line for equivalent stress,s se3.s se=s svo OCR1-Cs/Ccsvoe0Effective stress s svVoid Ratio,espspsesesusvfepDe=Cs log(sp/svo)De=Cc log(se/sp)Deat s sesuOC=suNCCritical state soil mechanicsPreviously:su/svo=constant for NC soilOn the virgin compression line:svo=seThus:su/se=constant for all soil(NC&OC)For simple shear:su/se=sin fEquivalent stress:Normalized Undrained Shear Strength:su/s svo=sinf f OCRL L where L L=(1-Cs/Cc)s se=s svo OCR1-Cs/CcUndrained Shear Strength from CSSMUndrained Shear Strength from CSSMPorewater Pressure Response from CSSMYield SurfacesLog s svNormal stress s svShear stress t tVoid Ratio,eNCNCCSLCSLCSLOCNormal stress s svVoid Ratio,espspOCq Yield surface represents 3-d preconsolidationq Quasi-elastic behavior within the yield surfacePort of Anchorage,AlaskaCavity Expansion Critical State Model for Evaluating OCR in Clays from Piezocone Tests where M=6 sinf f/(3-sinf f)and L L=1 Cs/Cc 0.8qcfsub q qT TCritical state soil mechanicsInitial state:e0,svo,and OCR=sp/svoSoil constants:f,Cc,and Cs (L=1-Cs/Cc)Using effective stresses,CSSM addresses:qNC and OC behaviorqUndrained vs.Drained(and other paths)qPositive vs.negative porewater pressuresqVolume changes(contractive vs.dilative)qsu/svo=sinf OCRL where L=1-Cs/CcqYield surface represents 3-d preconsolidation