A Viewpoint on Soil Strengths– based on CSSM Research in 

CambridgeCambridge

Ting Wen Hui – IEM

Introduction (1)• Characterization of Soil Properties in siteproblems for application to geotechnicalproblems for application to geotechnical design:1 Identification of appropriate property and the1. Identification of appropriate property and the 

specification of relevant testing program2. Assigning value to selected property to cater for 

non‐uniform conditions• Techniques such as Lower Bound, Mean and 

ProbabilityProbability.

• Extraneous factors such as short‐comings in testing improved by Characterization processtesting improved by Characterization process.

Introduction (2)• Presentation herein: CS Model as Theoretical Framework –

Strength Characterization in site problems. (Model not directlyon Stress‐strain as in Roscoe & Burland (1968) in Mod. Cam‐clay.)( ) y )

1) CS Model – Critical State Model, launched by Roscoe, Schofieldand Wroth (1958) in Cambridge, followed by Original Cam Clay(OCC) and then Mod Cam‐clay etc(OCC) and then Mod. Cam‐clay etc.

2) Schofield and Wroth (1968) develops fundamentals for Model inthe context of the Theory Of Plasticity.

) h ki ( ) f i f i3) Wroth: Rankine Lecture (1984) focus on Interpretation of In SituSoil Tests ‘ for site deposits within the framework of CSSM’.

4) Schofield (2005) presents Frontispiece with single η vs. ν plot,) ( ) p p g η pextending (q,p); (ν, lnp) plots in OCC, covering several aspects ofsoil properties

5) Schofield (2006) further extends fundamental of Model to Plastic5) Schofield (2006) further extends fundamental of Model to PlasticDesign with Interlock and Friction Strength concept.

CSSM Classical (Roscoe et al, etc)• References

– Roscoe, K.H., Schofield, A.N. and Wroth, C.P. 1958. On thei ldi f il Gé t h i 9 71 83yielding of soils. Géotechnique, pp. 9, 71‐83

– Roscoe, K.H. and Schofield, A.N. 1963. MechanicalBehaviour of an idealized ‘wet‐clay’. Proc. of EuropeanConf. SMFE, Wiesbaden. Pp. 47‐54.

• Yielding of soil in Critical State is continuing PlasticShear Strain without Volume Change andShear Strain without Volume Change and,

• Critical State in (q,p) and (ν,ln p) plots is determinedby:by:– q = Mp

– Γ = ν + λ ln p– and supported by experimental evidence.

CSSM Classical (Schofield & Wroth)• Schofield & Wroth (1968): provides fundamental oftheory of plasticity background and refinement to CSModel launched by Roscoe et al in 1958.

• CS Yield Curves: Fig. 6.1 (Slide 12)Granta Gravel: Rigid Plastic (simpler version);– Granta Gravel: Rigid Plastic (simpler version);

– Cam‐clay: Elastic‐plastic, along κ line on ν ln p plane; Fig.6.4.

d d ( l d )• Undrained Test in Fig. 6.7 (Slide 13)• Drained Test in Fig. 7.9 (Slide 14)

• Cam‐clay (‘designated’ as Original Cam Clay) obeysAssociated flow rule (Slide 15), with the Yield Locus aspotential function for plastic flow; & belongs toSt bl t i l i D k ’ P t l t i li ithStable material in Drucker’s Postulate; in line withPlastic Theory. (Schofield, 2005) ($5.2 ‐ $5.4; pg 91 ‐ 105).

CSSM – Original Cam Clay (OCC)CSSM  Original Cam Clay (OCC)VU5

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CSSM – Details of OCC

Cam Clay PlotCam Clay Plot

CSSM – Mod. Cam ClayR f• References:

• Roscoe, K.H. and Burland, J.B. (1968). On the generalized stress‐strain behaviour of “wet” clay. In: Engineering Plasticity,C b id U i it P ( d ) H J d L ki F ACambridge University Press, (eds) Heyman, J. and Lockie, F.A.

• Roscoe, K.H., Schofield, A.N. and Thurairajah, A. (1963). Yielding ofClays in States Wetter than Critical. Geotechnique 13, 211‐240,19631963.

• Burland, J.B. (2005). Soil Mechanics Emma: Elegant, Rigorous andRelevant. Inaugural Lecture (Honorary Fellow, Emmanuel College.

• Modified Cam clay: incorporates• Modified Cam‐clay: incorporates– ‘new’ work equations proposed by Burland (1965)– Two ‘new’ concepts introduced:

• Yield locus for state paths beneath state boundary surface forshear distortion without plastic vol. change

• Mohr‐Coulomb criterion applied to 3D stress space

S i l i h f l d f k• Stress‐strain relation thus formulated framework:– Numerical analysis (e.g. FEM) of soil mechanics problems.

CSSM Partly Saturated SoilsCSSM – Partly Saturated Soils(Burland & Ridley)( y)

• Burland, J.B. & Ridley, A.M. (1996). “The Importance of Suction in Soil Mechanics”. 12th Southeast Asian Geotechnical Conference, Kuala Lumpur– An introduction to the application of Critical State Concept t P tl S t t d S ilto Partly Saturated Soils.

Schofield & Wroth (1968) – Scope1

• Schofield, A., Wroth, P. 1968. Critical State SoilMechanics.

• The scope of the book may be portrayed as in: ‐

• Sect. 6.8: The Critical State ModelSect. 6.8: The Critical State Model– Concept stated in Roscoe, Schofield and Wroth (1958)

• ‘Essential Ideas unaltered’, but presented in ‘slightly differentform’.

– Two Hypothesis:• ‘Yielding of Soil through progressively severe distortion’Yielding of Soil through progressively severe distortion

• ‘Critical States’ approached after severe distortion

– Plasticity fundamentals established ‐ Associated Flow Rulef li d lof as applied to Cam‐clay• Clarified in Fig. 6.9 (Slide 14)

Schofield & Wroth (1968) ‐ Scope2

– CS (q p) theoretical curve ‐ Eq 6 17 (Slide 5):– CS (q,p) theoretical curve ‐ Eq. 6.17 (Slide 5): 

– Appendix C• ‘A yield function and plastic potential for soil under generalA yield function and plastic potential for soil under general principal stresses

– Provides the foundation to Schofield (2005, 2006)

Schofield & Wroth (1968)Th f Pl ti itTheory of Plasticity 

• Yield Functions (Failure Criterion):– Tresca and Von Mises; & latter preferred by Schofield & Wroth (1968)

after,– Wroth (1984) advocates Matsuoka’s Criterion as better fit to test dataPl ti B h i i t t• Plastic Behaviour in two parts: ‐– The Yield Function is a Potential Function– ‘Plastic Strain‐increments are Gradients of a Potential Function – and is

N l t th F ti ’ ‘A i t d Fl R l f Th f Pl ti itNormal to the Function’. ‘Associated Flow Rule of Theory of Plasticityobeys the Normality condition’.

• Isotropic Hardening and StabilityStrain increment Vectors are normal to the Hardening f nctions– Strain‐increment Vectors are normal to the Hardening functions

• Drucker’s Stability Criterion (provides quantitative relation)– Stability Postulate: Plastic material are stable only if they yield such

that following is obeyedthat following is obeyed:– Vector product, of stress increment vector, and associated strain‐

increment vector ,will be positive or zero. (quantitative)

Schofield & Wroth (1968)Fi 6 1 Yi ld C (G t G l/C l )Fig. 6.1: Yield Curves (Granta Gravel/Cam‐clay)

Schofield & Wroth (1968)Fig. 6.7 – Undrained Test

Schofield & Wroth (1968)Fig. 7.9 – Drained Test

Schofield & Wroth (1968)Fig. 6.9 – Associated Flow Rule

Schofield & Wroth (1968) Fig. 7.17 – η vs νλ introduced

Schofield & Wroth (1968)Fig. 7.18 – Loudon (η,νλ) Test Path

Soil Strength – Obs./CharacterizedSh St th tit i l d• Shear Strength quantity variously expressed as:– s (SB), su (Wroth, 1984), su (Schofield, 2006), cu = su = 1/2Unconfined Compr. Strength (Schofield & Wroth,1968)p g ( )

• Terminology (of strength parameters) is issue:– ‘true’ cohesion (c) due to ‘adhesion’ ascribed to Terzaghi,

(S h fi ld 2005 2006) h ‘ ’ h i i– (Schofield, 2005, 2006) asserts that ‘true’ cohesion 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 orbonding of soil grains ”bonding of soil grains.

– Randolph’s Foreword to Schofield (2005) is relevant.• Characterized In‐situ Soil Strength as interpreted byCSSM and applied, is also presented in:– (Ting, 2003): “CS Strength – (Twh Ref.)” in Slide 44.

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CS Strength (Wroth) – Ref.CS Strength (Wroth)  Ref.

• ReferencesReferences– Wroth, C.P. 1984. The interpretation of in situ soiltests Rankine Lecture Géotechnique 34 No 4tests. Rankine Lecture, Géotechnique 34, No. 4,pp. 449‐489

– Loudon, P.A. 1967. Some deformationLoudon, P.A. 1967. Some deformationcharacteristics of kaolin. Ph.D thesis, University ofCambridge. Fig. 6. Effective stress paths forundrained triaxial compression tests on kaolin(that include Oc soils)

CS Strength (Wroth,1984) – Formulation 

• su (Wroth, 1984) = Srupo’ = M/2(R/r)Λu ( , ) rupo / ( / )– where su is the undrained shear strength equal to half thedeviator strength,S (in Eqn (1)) is the undrained strength ratio that has a value– Sru (in Eqn. (1)) is the undrained strength ratio that has a valuedepending on the over‐consolidation ratio (= 0.2‐0.25 for Ncsoils) 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 & Λ are over‐consolidation, spacing ratio& plastic volume strain ratio in the Cam‐clay Model& plastic volume strain ratio, in the Cam clay Model.

• The above formulation may be usefully applied tonatural site soil strength profiles.g p

• Theoretical Expressions for Undrained Strength follows:

CS Strength (Wroth)h d‐ Twh data

• Twh Observation

CS Strength (Wroth,1984) ‐ Loudon

CS Strength (Wroth,1984)U d i d S h R i‐ Undrained Strength Ratio

Schofield (2005) – Inside Cover

Schofield (2005) – FrontispieceStress  obliquity (η) vs. CS Specific Volume  (νλ)

Schofield (2005) ‐ (η,ν)i ifi f i iSignificance of Frontispiece

Schofield (2005) – (η,ν)( ) (η, )Definition: νλ, νκ

Schofield (2005) ‐ (η,ν)OCC Constants (Fig. 61)

Schofield (2005) – (η,ν)Test data (Fig. 62)   

Schofield (2005) – (η,ν)Theoretical Equations

• q/p (η) = M/(λ-κ).(Γ +(λ-κ)-ν -λ ln p)– Eqn. 6.19, pg 142; derived from State Boundary q , pg ; y

theoretical equation (presented in Slide 5)

• Mλκ = Μ/(λ−κ) − definitionλκ ( )• νλ = v+λ ln p - definition• Then q/p (η) = M Γ+ Μ − M ν• Then q/p (η) = Mλκ Γ+ Μ − Mλκ .νλ

– providing theoretical linear relationship between q/p (η) and νλλ

Schofield (2005) ‐ (η,ν): Observation• q vs. ν plot

– Denotes physically the effect of shearing onl hvolume change

• Particularly vis‐à‐vis dilatancy, dense/oc (dry) andloose/nc (wet) soils( )

• q/p (η) vs. vλ :– Avails 3‐D: OCC (q,p,ν) on ‘condensed’ 2‐D plot ,

• that affords normalization procedure for q/p,– Uniqueness in the η vs. vλ relationship and

• that necessitates Affirmation by Observation• that necessitates Affirmation by Observation.

• Thus (η,ν) is add‐on to OCC that extends theModel range to 2 types of liquefaction with,Model range to 2 types of liquefaction with,– OCC (q,p,ν) in (q,p) & (p,ln ν): better visualization?

Schofield (2005) for Debris Flow1– “  while grains like those used in an hour glass are required for 

critical state flow, in a debris flow there is continual change of grain shape and size; an ever changing debris is never in a well defined state..”

– “..One of the early results that I described in my 1980 Rankine lecture was that I saw rapid failure and transformation oflecture was that I saw rapid failure and transformation of initially solid ground into a debris flow as a process that began with cracking; page 43 of my new describes a type of quick clay flow slide where “small initial cracks in the level ground surfaceflow slide where  small initial cracks in the level ground surface can be detected when pools of water on the surface are seen to drain into them …… These cracks open up. Large blocks of ground then slip taking trees and buildings with them In theground then slip, taking trees and buildings with them. In the flow slide large blocks crumble, and as rubble flows down a valley, it turns into fluidized silt with zero effective stress.  A visitor to an area that is well known for liquefaction slides willvisitor to an area that is well known for liquefaction slides will be surprised to find how strong the undisturbed soil is, with steep almost vertical escarpments beside small streams.”

Schofield (2005) for Debris Flow2• Schofield on Debris Flow within (η ν) plot:• Schofield on Debris Flow within (η,ν) plot:

– “I have several series of photo‐slides of this breaking up of large undisturbed bodies of lightly cemented “loess‐like” silt‐sized soil, and I showed one series in my Rankine Lecture from a visit north‐east of Oslo with Bjerrum and Eide to a slide that took away a farm in Hedmark.”

– “A Casagrande quotation describing a type of ground that lies in a high Alpine valley is another example. A mountain stream flows across the intact ground; people lower down the valley take warning from any stoppage of the stream flow as indicating that the intact body of ground has started to crack and crumble and water that gets into the resulting body of rubble will g g ysuddenly cause a catastrophic flow slide. Among other Canadian slides one series shows a housing development on a level area of undisturbed ground above a vertical escarpment of perhaps g p p p12m height; pools of water after heavy rain drained into the cracks that preceded the flow slide; it destroyed many houses. “

Schofield (2005) for Debris Flow3– “Another series followed the rubble down the path of the slide 

and showed how intact blocks broke up and were reduced to slurry.”

– “The US Army supported a student here from Toronto (Deborah Goodings) for centrifuge model tests on models made from intact Canadian quick clay; I think those slides came from herintact Canadian quick clay; I think those slides came from her. Other (Scandanavian) slides show trees and farm buildings on rafts of intact ground surface which float away on the river of slurry In such flow a weak rubble or friable debris is not in aslurry. In such flow a weak rubble or friable debris is not in a steady state; as the rubble moves the grading curve changes. Such debris flow of friable blocks can not be explained in terms of critical states but in a rock debris flow where large blocksof critical states but in a rock debris flow where large blocks remain strong enough the debris will interlock and dilate. Fig 64 shows interlocking triangular sectors in a cracked‐slab mechanism You probably have many slides showing your debrismechanism. You probably have many slides showing your debris flows.”

CS Strength (Schofield) – Ref.• References

– Schofield, A.N. 1980. Cambridge Geotechnical CentrifugeO ti R ki L t G t h i 30 225 268Operations. Rankine Lecture. Geotechnique, 30, 225‐268.

– Schofield, A.N. 1998. Mohr‐Coulomb error correction.Ground Engineering, August, 1998.

– Schofield, A.N. 2000. Rankine’s earth pressure fallacy.Proceedings of the Geotech‐Year 2000, Developments inGeotechnical Engineering AIT Bangkok: 1‐2Geotechnical Engineering, AIT, Bangkok: 1 2.

– Schofield, A. N. 2001. Re‐appraisal of Terzaghi’s soilmechanics. Special lecture. 15th ISSMGEC, Istanbul.S h fi ld A N 2005 Di t b d il ti Th– Schofield, A. N. 2005. Disturbed soil properties. ThomasTelford. (with Forward by Randolph MF)

– Schofield, A. N. 2006. Interlocking, and peak and designstrengths. Letter To The Editor. Geotechnique, 56, No. 5,357–358

Schofield – cohesion?Schofield  cohesion?

CS Strength (Schofield,2006)Guide to comprehension

• Notwithstanding: ‘c’ word is discussed at length inBook (2005) and Letter to Geotechnique (2006).( i Slid 39 40)(note comments in Slides 39, 40)

• Schofield Book:– Disturbed soil properties and geotechnical design

• Concept of Stress Obliquity/Vol. Strain

• References (provided to Twh):• References (provided to Twh):– My new book (Folder: Technical Paper/Properties/Schofield)

– Frontispiece (Folder: Technical Paper/Properties/Schofield)

– ting (Folder: Technical Paper/Properties/Schofield) ; commentthat debris flow is not in a definitive state and then proceed

CS Strength (Schofield,2006)‐ Two questions 

• Two questions posed for discussion in LETTERTO THE EDITIOR:– Is the peak strength or remoulded reconsolidatedfine‐grained soil: (i) Terzaghi’s sum of ‘truecohesion and ‘true’ friction (Fig. 1(a): or is it (ii)Taylor’s sum of interlocking and ultimate criticalt t d i d f i ti (Fi 1(b))?states drained friction (Fig. 1(b))?

– Can interlocking be included in plastic designstrength?strength?

CS Strength (Schofield,2006)‐ Fig. 1: Terzaghi (a), Taylor (b)

CS Strength (Schofield,2006)‐ Fig. 2: CS Framework

CS Strength (Schofield,2006)Concept‐ Concept

• su is defined as ultimate shear strength (detailed below)• Schofield (2006) in further comments in Letter to the Editor:• Schofield (2006) – in further comments in Letter to the Editor:

Geotechnique (ref. Figs. 1 & 2).– Terzaghi/Hvorslev Model: “His (Hvorslev) seminal study in 1937

i t t d th k t th d t (f i ith th tinterpreted the peak strength data (for specimens with the same watercontent at failure) as the sum of ‘true’ cohesion c and ‘true’ friction onthe line BC (Fig. 1(a)).....and did not realize that apparent cohesion

ld l f h l h ”could result from a change in volume as water content changes.”– CS Strength Model: “In CS soil mechanics (Schofield and Wroth, 1968)

the ultimate shear strength su in Fig. 2 is the soil property often calledcohesion on the wet side of CS. It is the constant strength of soil pasteat C flowing in undrained plastic deformation, with work beingdissipated by CS internal friction and not by cohesive or adhesive bondsbetween disturbed soil grains.”

CS Strength – (Schofield,2006)‐ Randolph Foreword 

• Randolph Foreword to Schofield (2005) –Randolph Foreword to Schofield (2005)extract

• “A modest ambition for the present book mightp gbe to see the words ‘cohesion’ and ‘adhesion’excised from our soil mechanics vocabulary,replacing them with respectively. ‘shearstrength’ (at a given water content andeffective stress level) and on the rather rareeffective stress level), and, on the rather rareoccasions where it is appropriate,‘cementation’.”cementation .

• Randolph as interlocutor.

CS Strength (Schofield,2006)g ( , )‐ Fig. 10 – Interlocking component

CS Strength (Schofield,2006)Fig. 11 – Interlocking vs. Pressure

CS Strength – (Twh Ref.)  g ( )

• References:References:– Ting, W.H. (1999). “The Failure Behaviour of Over‐Consolidated Soils.” Journal of the Southeast AsianGeotechnical Society, Volume 30, Number 3, December1999.

Ting WH (2000) “Characterisation of Components of– Ting, W.H. (2000). Characterisation of Components ofShear Strength.” Journal of the Southeast AsianGeotechnical Society, Vol. 31, No. 3, December 2000.

– Ting, W.H. (2003). “Characterization of Strength of In‐SituSoils.” A.A. Balkema, the Netherlands, Oxford & IBHPublishing Co Pvt Ltd New Delhi IndiaPublishing Co. Pvt.Ltd., New Delhi, India.

‘Reinforcement’ – Obs. 1 (Twh)F R i f d S d l ‘I l k’Fg. Reinforced Sand – relate to ‘Interlock’

‘Reinforcement’ – Obs. 2 (Twh)• ‘Reinforcement’ mechanism concept exists:

– typified by Over‐consolidation (adhesion/bond etc.) effectsin:in:

• Schofield model soil with ‘interlocking’ strength that:• When sheared passes to plastic mechanism stage after largestrains; to reach post‐peak residual stress: and may then bestrains; to reach post peak residual stress: and may then bedepicted on Schofield framework;

• The Interlocking followed by plastic mechanism also applies toshearing with other types of ‘reinforcement’ as follows:g yp

– Suction of Partly Saturated natural deposits being• Residue of Weathered formation (ranges from partial to fullsaturation as natural condition))

– Compaction (man‐made) of partial saturated fill increasespacking due to reduction of air voids.

– Structural: steel & polymer reinforcement in soil– Structural: steel & polymer reinforcement in soil

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‘Reinforcement’ – Obs. 3 (Twh)P ( ‘D ’) i ‘ i f t’• Permanency (or ‘Decrease’) in ‘reinforcement’(interlock) strength is issue:– Over‐consolidated SoilsOver consolidated Soils

• ‘Interlocking’ as proposed by Schofield affected by shearing movement.For e.g. in seismic prone areas ground motion may impair integrity ofStrength, but considered as mechanism in Schofield model notgclassified as decrease in ‘reinforcement strength unlike:

– Partially Saturated natural deposit affected by• Saturation due to rainfall – surface & sub‐surface infiltration• Stress relief by excavation of natural deposits

– Compacted man‐made fill affected by• Stress relief by excavation or lack of confinement of fill reduces‘reinforcement effects.

– Manufactured (steel or polymers)• Depends on ‘Life’ of reinforcement subject to ‘chemical’ and ‘physical’agency. E.g. corrosion for steel and toxic chemicals and infra red orultraviolet rays for polymers. Generally long‐term effects.

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CS Strength – Comments1 (Twh)• Schofield:

– Acknowledges:g• Existence of ‘cohesion’ (c); but not as interpreted byTerzaghi (with c being σ’ independent and φ‘ being σ’dependent) but as ‘Interlocking’ strength and CSdependent) but as Interlocking strength and CS‘Friction’ as in Fig. – Terzaghi/Taylor. (next Slide).

– Asserts:• During continuing shearing process, a soil passes from‘Interlock’ to CS ‘Friction’ mechanism and though,

• ‘cohesion’ as above exists when ‘peak’ stress is plotted;cohesion as above exists when peak stress is plotted;it is nonetheless recommended to be denominated asInterlock with the value as given in the Fig., and not

d d t b li d i l ti d i (B t ‘recommended to be applied in plastic design. (But ‘maybe considered if experience justifies the application’.}

CS Strength – Fig. Schofield Model /(Terzaghi/Taylor) (Twh)

CS Strength – Comments2 (Twh)• Schofield (cont.):

– Interlock strength should not be applied (citing Coulomb, etc) because of:etc) because of:

– ‘Unstable’ (brittle and non‐ductile)nature of interlock component and is portrayed by Schofield in Section 2.6 of his book under the title: Failure at low effective stress which can result in: 

– Fluidize soil rubble, that has crumbled into rubble in the ,presence of high hydraulic gradients (Herrick’s liquefaction), and a range of similar failures such as hydraulic fracture of dams sand boils behind flood leveeshydraulic fracture of dams, sand boils behind flood levees, upward gradient fluidizing sand at the bottom of a cofferdam, and fluidization of trench fill due to pressure f b k l dfrom broken piles and,

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CS Strength – Comments3 (Twh)• Schofield (cont.)

– Generally soil on the ‘DRY’ side in the Frontispiece(Slide 19) tend towards an unstable state that canfinally lead to Herrick‐type liquefaction,fl idi ifluidization, etc.

– Ignores (‘Twh’):• Occurrence of ‘Reinforcement’: ‘cementing’ ‘bonding’Occurrence of Reinforcement : cementing , bonding ,‘suction’ actions and,

• Pre‐Existing States before Shearing Mechanism

CS Strength – Comments4 (Twh)CS Strength  Comments4 (Twh)

• Schofield (cont )Schofield (cont.)– Begging the question (‘Twh’) reference conditions:

• For site problems involving states before peak andFor site problems involving states before peak and,

• When large strain processes does not easily take place;as all Schofield arguments on Strength are based on‘large’ movements.

– And if site stress level below ‘residual’ stress, site strainsload/unload may be ‘elastic’ (e.g. as in Pavements)y ( g )

• If Interlock were to be applied at all, it would be inSchofield model (vis‐à‐vis Taylor) and not as Terzaghicohesion as illustrated in Slide 48cohesion as illustrated in Slide 48

CS Strength – Comments5 (Twh)g ( )

• ‘Reinforcement’ or Interlock Strength as ‘ ’ h b i d i‘temporary’ strength cannot be ignored in practice as:

It d fi ‘ t d ’ ti f t ti– It defines ‘stand‐up’ time of temporary excavation relevant to hand‐dug caissons and soldier pile walls tunnel support before permanent liningwalls, tunnel support before permanent lining constructed, etc.

– The temporary nature could be of duration p ysufficient for construction purposes.

– Its contribution can however be evaluated in line h hwith CS Strength concepts.

Concluding Comments (1)Concluding Comments (1)• Schofield has ‘created’ a single model that providef ll th ll k t h i l ffor, all the well known geotechnical processes of‘disturbed’ soils, and his Frontispiece (according toRandolph) is the lynchpin of the modelRandolph), is the lynchpin of the model.

• Randolph’s Forward to Schofield (2005) is key tointerpretation of the book.interpretation of the book.

• Interlocking strength (as related to ‘cohesion’) exists,and may be applied notwithstanding thea d ay be app ed ot t sta d g t e‘reservations’ of Schofield; bearing in mind thataccording to,

Concluding Comments (2)• Randolph: “In defence of his (c,φ) strength model, Terzaghi did advocate that clay should be testedTerzaghi did advocate that clay should be tested ‘under conditions of pressure and drainage similar to those under which the shear failure is likely to occur in the field’. However, that caveat seems to have been overlooked and, even today the c‐φ strength 

d l h d l d d l ”model is taught widely and used in‐appropriately.” Failure at the teaching level?

If T hi’ t i f ll d ti f th i d• If Terzaghi’s caveat is followed, meeting of the minds: Terzaghi/Schofield is possible bearing in mind,

Concluding Comments (3)• If Interlock strength is to be applied, the Schofieldstricture re the ‘brittle’ nature and liquefactionpotential in that state of ‘low effective stress’ for soilpotential in that state of low effective stress for soilon the ‘dry side’ (Frontispiece) to be considered.

• In site problems consider ‘permanency’ of interlock:In site problems consider permanency of interlock:– The interlock in partially weathered formation may besustained by suction which may be reduced by saturationor stress releaseor stress release.

– In dam stability, residual soil strength is mobilized withcontinuing strain notwithstanding the high strengthsavailable due to compaction.

• Shear Strength to be applied in site problems as inOcc (Slide 5) and then further developed (Slide 42)Occ (Slide 5), and then further developed (Slide 42)to consider ‘Interlocking’ component.

VU3

Slide 60

VU3 on reinforcement & interlockingValued User, 4/12/2009

Concluding Comments (4)• Notable also (and as shown in Frontispiece) are thetwo concepts of Liquefaction: Herrick’s and Hazen’s.

H i k’ d b l fi i d H ’– Herrick’s governed by low confining pressure; and Hazen’sby loose packing.

– With Herrick’s, Schofield (2000) offered an ‘alternative’explanation of the observed liquefaction that lead to thecollapse of the Teton Dam in the US in 1976.

• FINAL COMMENTS: The CS Model provides anFINAL COMMENTS: The CS Model provides anidealized, & necessary Theoretical Framework forCharacterization of Strength in site problems so that:g p– Strength parameters may be conceived as an Integralentity; rather than as the separate values obtained bytesting with various paths followed in the site problemtesting with various paths followed in the site problem.

– Recent Models have room for further Research.

Thank youThank you

Presentation Organization

Soil Strengths

CSSM Mohr‐Coulomb: Terzaghi caveat

Partially Saturated Soils

Burland

OCCMod. Cam‐clay

Roscoe & Burland

Test Path in Lab similar to site 

hBurland

Schofield

η vs νWroth ‐

Ranking Lecture

Schofield

Interlocking strength

Burland

Cam

Stress‐strain h

test path

η vs ν

Characterized In‐l h

strengththeory

situ Soil Strength

‘Reinforcement’/Interlock (Twh)Reinforcement /Interlock (Twh)