CSSM for Dummies

CSSM for Dummies - 2006 Critical-State Soil Mechanics For...

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Unformatted text preview: 2006 Critical-State Soil Mechanics For Dummies Paul W. Mayne, PhD, P.E. Civil & Environmental Engineering Georgia Institute of Technology Atlanta, GA 30332-0355 www.ce.gatech.edu Email: paul.mayne@ce.gatech.edu PROLOGUE Critical-state soil mechanics is an effective stress framework describing mechanical soil response In its simplest form here, we consider only shear- induced loading. We merely tie together two well-known concepts: (1) one-dimensional consolidation behavior, represented by e-log v curves; and (2) shear stress-vs. normal stress ( - v ) from direct shear box or simple shearing. Herein, only the bare essence of CSSM concepts are presented, sufficient to describe strength & compressibility response. Critical State Soil Mechanics (CSSM) Experimental evidence provided by Hvorslev (1936; 1960, ASCE); Henkel (1960, ASCE Boulder) Henkel & Sowa (1961, ASTM STP 361) Mathematics presented elsewhere, including: Schofield & Wroth (1968); Burland (1968); Wood (1990). In basic form: 3 material constants ( ', C c , C s ) plus initial state (e , vo ', OCR) Constitutive Models, include: Original Cam-Clay, Modified Cam Clay, NorSand, Bounding Surface, MIT-E3 (Whittle, 1993) & MIT- S1 (Pestana) and others (Adachi, Oka, Ohta, Dafalias) "Undrained" is just one specific stress path Yet !!! CSSM is missing from most textbooks and undergrad & grad curricula in the USA. One-Dimensional Consolidation Sandy Clay (CL), Surry, VA: Depth = 27 m 0.5 0.6 0.7 0.8 0.9 1.0 1 10 100 1000 10000 Ef f ective Vertical Stress, svo' (kPa) Void Ratio, e C c = 0.38 C r = 0.04 vo ' =300 kPa p ' =900 kPa Overconsolidation Ratio, OCR = 3 C s = swelling index (= C r ) c v = coef. of consolidation D' = constrained modulus C e = coef. secondary compression k hydraulic conductivity v Direct Shear Test Results v Direct Shear Box (DSB) v Direct Simple Shear (DSS) s Slow Direct Shear Tests on Triassic Clay,NC 20 40 60 80 100 120 140 1 2 3 4 5 6 7 8 9 10 Displacement, (mm) Shear Stress, (kPa) n ' (kPa)= 214.5 135.0 45.145....
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CSSM for Dummies - 2006 Critical-State Soil Mechanics For...

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