Research In Phase Report No. 1 Earth Physics Part I STRESS .
Phase Report No. 1Research inEarth PhysicsPartISTRESS-STRAIN BEHAVIOR OF SATURATED CLAYAND BASIC STRENGTH PRINCIPLESbyCharles C. LaddCosponsoredArmy Materiel CommandProjects Nos. 1-A-0-1101-B-021-30and 1-T-0-21701-A-046-05andAdvanced Research Projects AgencyARPA Order No. 400Contract No. DA-22-079-eng-330withU.S. Army Engineer Waterways Experiment StationResearch Report R64-17Soil Mechanics DivisionDepartment of Civil EngineeringMassachusetts Institute of TechnologyApril, 1964
ABSTRACTThis report presents a set of basic principles developed todescribe the strength behavior of saturated clay and illustratesstress-strain properties via detailed data on the hypothetical "SimpleClay. I" The three basic principles are: the relationship betweenstrength and effective stress at failure; the relationship among watercontent, shear stressand effective stress during shear; and therelationship among strength, water content at failureand effectivestress at failure as expressed by the Hvorslev parameters.Stressvs. strain, water content vs. log stress and effective stress pathdata are presented on the influence of type of triaxial test, over-consolidation ratio, stress path, stress ratio at consolidation, andvalue of the intermediate principal stress.These principles anddata, which represent a simplified picture of the behavior of realclays, can be used as a framework with which to study the propertiesof actual clays in terms of deviations from this idealized picture,rather than as a set of isolated facts.i
PREFACE"The work described in this report was performed under ContractNo. DA-22-079-eng-330 entitled 'Research Studies in the Field of EarthPhysics' between the U. S. Army Engineer Waterways Experiment Stationand the Massachusetts Institute of Technology. The research is cosponsored by the U. S. Army Materiel Command under DA Projects1-A-0-1101-B-021-30, 'Earth Physics (Terrain Analysis), ' and1-T-0-21701-A-046-05,'Mobility Engineering Support,' and by theAdvanced Research Projects Agency, ARPA Order No. 400".The general objective of the research is the development of afundamental understanding of the behavior of particulate systems,es-pecially cohesive soils, under varying conditions of stress and environment. Work on the project, initiated in May, 1962, has been carriedout in the Soil Mechanics Division (headed by Dr. T. William Lambe)of the Department of Civil Engineering under the supervision ofDr. Charles C. Ladd, Assistant Professor of Civil Engineering. Thisreport represents only one phase of the overall research being conducted under the contract.Part II of this report will pr esent detailed strength data on normally consolidated Boston Blue Clay. Major topics of investigation, asmeasured by consolidated-undrained triaxial tests with pore pressuremeasurements, include:ii
(1) Effects of anisotropic consolidation(2) Effects of the intermediate principal stress(3) Effects of rotation of principal planesThese data are analyzed in terms of deviations from the idealizedtheoretical framework presented in this report. In particular, commonmethods for selecting undrained strengths for total stress ( 0)stability analyses are examined critically and alternate methodssuggested.In essence, Part I of this report represents the background materialrequired for the presentation and analysis of the experimental datapresented in Part II.iii
TABLE OF CONTENTSPREFACEI.II.INTRODUCTIONPage e No.A.Purpose of Report1B.Concept of the "Simple Clay"2C.Types of Shear Tests3D.Statement of Strength Principles7E.Variables Considered7F.Consolidation Curves for Simple Clay8STRENGTH BEHAVIOR OF NORMALLY CONSOLIDATED 10SIMPLE CLAY (Compression tests only)A.CID Tests101.Stress-Strain2.Mohr Coulomb Strength Criteria3.Effect of Different Stress Paths4.Stress-Strain Data for Different Stress 1,4DataStress Paths10and1113Paths and the Hyperbolic Stress-Strain RelationshipB.5.Water Content versus Log Stress166.Review17CIU Tests181.Stress-StrainData182.Total and Effective Stress Paths203.Effective Stress Path and Axial Strain 234.Water Content versus Lg Stress235.Review23Relationshipiv
C.Unique Relationship Among Water Content,Shear Stress and Effective Stress1.2.3.RelationshipPage No.24Between CIU and CID Tests'Effect of Anisotropic Consolidation2426Review29D.UU Tests29E.Summary and Conclusions311.Statement of Principles312.Equations for Undrained Shear Strength323.Summary of Important Points for Triaxial33Compression Tests on Normally Consolidated Simple ClayIII.STRENGTH BEHAVIOR OF OVERCONSOLIDATED SIMPLECLAY (Compression Tests Only)A.B.C.D.CID Tests35351.Stress-Strain2.Effective Stress Paths and FailureEnvelope363.Water Content versus Log Stress37Data35CIU Tests371.Stress-Strain2.Effective Stress Paths and Failure Envelope 383.Water Content versus Log StressData3740Unique Relationship Among Water Content, Shear40Stress and Effective Stress1.Relationship2.Effect of Ko ConsolidationBetween CIU and CID TestsSummary of Effect of Overconsolidation40on Strength 43Behavior for Isotropic Consolidation andReboundv40
Page No.IV.V.VI.1.Strength Data versus ConsolidationPressure and Water Content versusLog Stress432.Stress-Strain Characteristics443.Log OCR versus Strength Parameters45HVORSLEV PARAMETERS46A.Historical Development46B.Significance48C.Hvorslev Parameters for Simple Clay49TRIAXIAL EXTENSION TESTS52A.Types of Stress Systems52B.Triaxial Extension Tests on Simple Clay541.CIU Tests on Normally ConsolidatedSamples542.CID Tests on Normally ConsolidatedSamples573.Tests on Overconsolidated Samples5860SUMMARYA.Background60B.Variables Considered61C.Strength Principles62APPENDIX A -List of References64APPENDIX B -Notation67vi
LIST OF FIGURESI-.1-2II-1-2Sample in Triaxial CellIsotropic Consolidation and Rebound Curves forSimple ClayStress-Strain: CID (2"rc) Compression Testson Normally Consolidated Simple ClayMohr Circles at Failure for CID ( ac)Compression Tests on Normally onsolidatedSimple Clay-3Effective Stress Path and Strength Envelope forCID Test B-4Various Effective Stress Paths for CID Tests on-5Stress-Strain: Loading and Unloading CID ComPression Tests on Normally ConsolidatedSimple Clay-6Hyperbolic Stress-Strain RelationshipNormally Consolidated Simple Clayfor CIDCompression Tests on Normally ConsolidatedSimple Clay-7Water Content versus Log Stress for CID Compression Tests on Normally ConsolidatedSimple Clay-8Stress-Strain:-9Total and Effective Stress Paths for a CIU TestConsolidated Simple Claywith-10CIU Compression Tests on Normallyc 4 kg/cmEffective Stress Paths and Axial Strains for CIUCompression Tests on Normally ConsolidatedSimple Clay-11Water Content versus Log Stress for CIU Compression Tests on Normally Consolidated SimpleClay-12Water Content, Shear Stress and Effective StressRelationship for CIUand CID Compression Testson Normally Consolidated Simple Clayvii
II-13Effect of K Consolidation on Effective Stress Pathsfor C-fCompressiondated Simple Clay-14Tests on Normally Consoli-Effect of Ko Consolidation on Water Content versusLg Stress for C Compression Tests on NormallyConsolidated Simple Clay-15Total and Effective Stress Paths for UU CompressionTests on Normally Consolidated Simple Clay withac 4 kg/cm-16III-1Ratio of Undrained Strength to Consolidation PressureStress- Strain: CID ( 3 a 2) Compression Tests onNormally Consolidated and Highly OverconsolidatedSamples of Simple Clay-2Effect of OCR on Stress-Strain Behavior for Loadingand Unloading CID Compression Tests on SimpleClay-3Hyperbolic Stress-Strain Relationships for Loadingand Unloading CID Compression Tests on NormallyConsolidated and Highly Overconsolidated Samplesof Simple Clay-4Effective Stress Paths for Loading and UnloadingCID Compression Tests on Overconsolidated SimpleClay with a Maximum Past Pressure of 8 kg/cm z-5Water Content versus Log Stress for Loading andUnloading CID Compression Tests on Overconsolidated Simple Clay with a Maximum Past Pressureof 8 kg/cm-6Stress-Strain:2CIU Compression Tests on NormallyConsolidated and Highly Overconsolidated Samplesof Simple Clay-7Effect of OCR on Stress-Strain Behavior for CIUCompression Tests on Simple Clay-8Effect of OCR on Hyperbolic Stress-Strain Relationship for CIU Compression Tests on Simple Clay-9Effective Stress Paths for CIU Compression Testson Overconsolidated Simple Clay with a MaximumPast Pressure of 8 kg/cm 2viii
III-10Rendulic Plot Showing Ko Consolidation and CIUStress Paths for Overconsolidated Simple Clayof Isotropic and Ko Consolidationand-11Comparison-12Strength Data from CIU Compression Tests onNormally Consolidated and OverconsolidatedSimple Clay-13Strength Data from CID Compression Tests onNormally Consolidated and OverconsolidatedSimple Clay-14Water Content versus Log Stress for CIU and CIDRebound Curves for Simple ClayCompression Tests on Normally Consolidatedand Overconsolidated Simple Clay-15Hyperbolic Stress-Strain Relationships for CIUand CID Compression Tests on Normally Conconsolidated and Overconsolidated Simple Clay-16Stress Difference versus Pore Pressure andVolume Change for CIU and CID CompressionTests on Normally Consolidated and Overconsolidated Simple Clay-17Log OCR versus (su at a//sU at acm), versusu/ ac and d/ , an versus d/Su for TriaxialCompression Tests on Simple Clay-18Log OCRversus ef, versus ( al/IV-1-2V-13) at Failure,and versus Af and Awf for Triaxial CompressionTests on Simple ClayHvorslev Parameters (Drained Direct Shear Tests)Hvroslev Parameters from Triaxial CompressionTests on Simple ClayTypes of Stress Systems-2Stress Systems in the Field-3Stress-Strain: CIU Compression and ExtensionTests on Normally Consolidated Simple Clayix
V-4Stress Difference versus A Parameter for MUCompression and Extension Tests on NormallyConsolidated Simple Clay-5Effective Stress Paths for CIU Compression and-6Rendulic Plot ShowingEffective Stress Paths for-7Water Content versus Log Stress for CIU Compres-Extension Tests on Normally ConsolidatedSimple Clayand Extension Tests on NorCICompressionmally Consolidated Simple Claysion and Extension Tests on Normally Consoli-dated Simple Clayx
I.INTRODUCTIONA.Purpose of ReportIt has been stated many times that the strength of claysis probably the most confusing subject in the area of soil en-gineering. Not only is it difficult for students to grasp aunifying picture of strength behavior, but many practicingengineers and teacherssubject.also tend to be confused at times by theOne cause of this confusion is the lack of an organ-ized framework from which a person can start.It is thepurpose of this report to present such a framework whichwill be based on an admittedly simplified picture of thestrength behavior of saturated clays. However, once thisframework is firmly establishedyone can then study thebehavior of actual clays in terms of deviations from thissimplified picture, rather than as a set of isolated facts.By analogy, the behavior of gases, and at times fluids, isstudied in relationship to laws derived for ideal gases:under many conditions these laws yield very good approxi-mations to actual behavior but there are also cases whereinthe simplified laws are grossly inadequate.The same willbe true of the framework presented for the strength of clays,except that its general validity is of course less precise thanthat of the ideal gas laws.1
The principles to be presented were not generally formulated by the author but rather are primarily the result of aselective compilation from existing knowledge. The excellentwork on the remolded clays by Dr. Henkel and his associatesat Imperial College, as summarized by Henkel (1960)* andParry (1960), has served as background material for thereport.Although reference will be made to some of the otherpertinent work, a complete history of the research in this areais beyond the scope of the notes.B.Concept of the "Simple" ClayOn the basis of measured strength data on a number ofclays, an empirical set of strength principles was developedto describe the behavior of saturated clays during shear.Forexample, the principles of the "unique relationship betweenstrength and water content at failure and between strength andeffective stress at failure" for normally consolidated sampleswere first based on tests on undisturbed clays (Rutledge,1947; Taylor, 1948). These principles were extended tooverconsolidated samples from tests on the remolded Wealdand London clays (Henkel, 1960; Parry,1960). The "uniquerelationship between water content and effective stressduring shear" was derived from data on several remoldedclays (Rendulic, 1937; Henkel, 1960; Roscoe andPoorooshasb, 1963) and at least one undisturbed clay(Taylor,1955).:Appendix A contains a list of references2
Although the above principles were derived as workinghypotheses from actual strength data, there is no clay, or atleast no available data on a clay, wherein all of the principlesare followed exactly.Since a consistent set of data wasdesired to illustrate strength behavior and principles, theauthor was forced to create a clay which exactly conformedto the principles.This hypothetical soil is termed the "SimpleClay, ' in that its behavior is "not complicated or involved."The properties of the "Simple Clay" are patterned ingeneral after the data obtained on the remolded Weald clay,although practically all of the data were adjusted to varyingdegrees in order to be completely consistent.However, theresulting properties are believed to be fairly representativeof the strength behavior of many remolded and some undisturbedclays (very sensitive clays and clays with cementation areexcluded). Comments on the general applicability of theassumed properties to the behavior of actual clays will bePart II of this report treats somemade throughout the report.of the deviations in detail.C.Types of Shear TestsThe report is restricted to triaxial shear testing equipment since it representsa commonly used yet versatile typeof apparatus for which the stresses on the three principalplanes are always known, drainage can be controlled, and pore3
pressures can be conveniently measured.Suppose that a specimen of saturated clay is placed in atriaxial cell, a confining (cell) pressure a c* applied, and it isthen sheared by increasing the axial load P (see Fig. I-1).Depending upon the drainage conditions during the applicationof the confining pressure and during subsequent shearsthereare three basic types of shear tests:(1) Consolidated-Drained Test (called a CD test)(a) The drainage line is opened when the confiningpressure is applied. At equilibrium the confining pressure a c is equal to the consolidation pressure ac, and thepore pressure u is equal to zero.During consolidation,water would move out of the sample if the cell pressureac were larger than the effective stress ai initially actingon the sample.Conversely, the sample would imbibewater if a i had been greater than a c(b) During shear, in which the axial load P isincreased after consolidation has been completed, thedrainage line is also kept open. Furthermore, P isincreased slowly enough that insignificant excess porepressures develop during shear. In other words, wateris allowed to flow in or out of the sample in order thatthe pore pressure within the sample remains essentiallyequal to zero (relative to atmospheric pressure).;r Appendix B presentsthe list of symbols.4
The notation "CD test" therefore means,C Consolidated under the confining pressure.D Drained, in that full drainage is allowed during shear.A test which is isotropically consolidated prior todrained shear is denoted by CID, whereas CAD meansthat the sample had been anisotropically consolidatedprior to drained shear.CD tests are sometimes called Slow tests and S tests.(2) Consolidated-Undrained Test (called a CU test)(a) As with the previous test, the specimen is allowedc.to consolidate under the confining pressure(b) However, just prior to shear the valve in thedrainage line is turned off so that the sample can notimbibe or expell water during subsequent shearing.Therefore no drainage occurs during shear, i.e. the shearis undrained.In this case excess pore pressures willdevelop within the sample.The notation "CU test" therefore means,C Consolidated under the confining pressure.U Undrained during subsequent shear.If pore pressuresare measured during undrained shear, one then knows theeffective stressesa acting on the sample; such a test isdenoted by a bar over CU, i.e. CU test.As before,isotropic and anisotropic consolidation are designated by5
CIU and CAU respectively. CU tests are sometimes calledQc tests and R tests.(3) Unconsolidated-Undrained Test (called a UU test)(a) In this test, the drainage valve is turned offbefore the sample is placed in the cell and is kept offduring application of the confining pressure.The watercontent of the sample remains equal to the initial watercontent wi and likewise the effective stress remains equalto the initial effective stress ai . Consequently the porepressure in the sample increases by an amount exactly*equal to the confining pressure (rc , i.e.u u (a whereC11. -Ui1(b) As in the CU test, no drainage is allowed duringshear.The notation "UU test" therefore means:U Unconsolidated with respect to the confining pressure(however the specimen does have an effective stress equalto T.i)1U Undrained during subsequent shear.If pore pressuresare measured during shear, the test is designated by UU.The most common type of UU test is the simpleunconfined compression test wherein the confining pressureis zero and metal top and bottom plattens are used.UUtests are sometimes called Q tests.:'Since the sample is saturated, Skempton's B parameter is equal tounity.6
D.Statement of Strength PrinciplesWhat will be called the three basic strength principles aresummarized below. Subsequent sections will present stressstrain, stress path, and water content-stress data on the SimpleClay. At appropriate points, the strength principles, and theircorollaries,Principlewill be explained and applied to the data.IFor normally consolidated samples, or for overconsolidatedsamples with the same maximum past pressurec(consideringshear in compression and extension separately), there is an uniquerelationship between strength [maximum stress difference(a1 -3)Principlemax.] and effective stress at failure.IIFor normally consolidated samples, or for overconsolidatedsamples with the same maximum past pressure (considering shear incompression and extension separately),there is an unique relationshipamong water content, shear stressandeffective stress.Principle IIIFor both normally consolidated and overconsolidated samples(considering shear in compression and extension separately), there isan unique relationship among strength, water content at failure, andeffective stress at failure as expressed by the Hvorslev parameters.E.Variables ConsideredThe variables which will be considered are listed below.7
Only triaxial shear tests are treated.(1) Type of Shear Test(a)UU test(b)CU test(c)CD test(2) Overconsolidation Ratio (O.C.R. cm/ ac)(a) Normally consolidated samples with varying -c(b) Overconsolidated samples with varying acm and O.C.R.(3) Total Stress Path*(a) Loading:(b)a1 increased;3a 1 constant;3Unloading:constant.decreased.(4) Value of K (Kc rc/ -ac ratioof radial to axial consoli-dation pressures.)(a) K(b)c 1 (isotropic consolidation)Kc / 1 (anisotropicconsolidation)(5) Value of Intermediate Principle Stress a2(a)Triaxial(b) Triaxial extension:F.2 a3 compression:a2 1 r; a1 aaar; a 3 aConsolidation Curves for Simple ClayConsolidation and rebound curves for isotropic stresses areplotted in Fig. I-2 for the Simple Clay.Two facts should be noted.(1) Water content versus log consolidation pressure is a straightline for virgin compression.w(%) 25.4The equation is:- 7.65 log ac (kg/cm2)*Although an infinite number of stress paths are possible,the two chosenare the most
1. Stress-Strain Data 10 2. Mohr Coulomb Strength Criteria and 11 Stress Paths 3. Effect of Different Stress Paths 13 4. Stress-Strain Data for Different Stress 1, Paths and the Hyperbolic Stress-Strain Relationship 5. Water Content versus Log Stress 16 6. Review 17 B. CIU Tests 18 1. Stress-Strain Data 18 2.
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