Undrained Stress Research Articles

Slope stability analysis for undrained loading conditions

AbstractThe current state of art for limit equilibrium analysis of slope stability problems lacks a satisfactory procedure for stability evaluation under general, rapid (undrained) loading conditions. Some procedures are available for the analysis of rapid drawdown, but these suffer from several shortcomings and, furthermore, are not applicable to other types of rapid loading. An approach is presented which overcomes these limitations. The approach integrates four components‐establishment of soil behaviour on the basis of laboratory testing, estimation of steady‐state conditions in the slope using a boundary value analysis, estimation of distribution of undrained strength in the slope using undrained stress paths, and identification of the critical slip surface followed by calculation of its factor of safety. The approach is illustrated through its application to the stability analysis of an earth dam under rapid drawdown and earthquake conditions.

Strain Space Plasticity Model for Cyclic Mobility

ABSTRACT The cyclic mobility model proposed in this paper is of a generalized plasticity—multiple mechanism type. The salient feature of the present approach is that the concept of the multiple mechanism, within the framework of plasticity theory defined in strain space, is used as a vehicle for decomposing the complex mechanism into a set of one dimensional mechanisms. The other feature is that the undrained stress path is idealized with the concept of liquefaction front, which is defined in the effective stress space as an envelope of stress points gradually approaching the failure line. Once coded into the finite element program, most of the existing models suffer from serious difficulty in the numerical solution process when effective stress path becomes very close to the failure line. A numerically robust approach is proposed in this paper by introducing a scheme for gradually enlarging scale of shear strain with progress of cyclic mobility.

Pore Pressure : Stress Ratio Relationship for Soft Bangkok Clay

ABSTRACT A series of triaxial tests was performed on undisturbed samples of soft Bangkok clay, which were consolidated under different constant stress ratios, to study the pore pressure development under monotonic loading conditions. The test results are interpreted through a pore pressure : stress ratio relationship. The behaviour of isotropically consolidated samples in the normally consolidated state shows a linear relationship between the pore pressure and the stress ratio under monotonic loading conditions and the same patterns were also established for other clays such as kaolin, Drammen clay and Boston Blue clay. Overconsolidated samples, and anisotropically consolidated samples showed bi-linear variation on the same plot. The linear/bilinear variation in the pore pressure-stress ratio relationship was used to develop simple expressions for the undrained stress paths for different initial conditions of the samples, using simple soil constants.

A unified elasto-viscoplasticity model for clays, part II: Verification

The elasto-viscoplasticity model presented in Part I is now examined in detail in this paper. The physical meaning of model parameters as well as appropriate procedures for their determinations are described herein. Based on this procedure, the model constants were determined for three groups of soils. Numerical simulations demonstrated the model's capability to capture the time-dependent behaviors, including creep (both drained and undrained), undrained stress relaxation, and drained tests with various effective stress paths. Quantitatively, the rate effect is shown to be captured by the model.

Normalized behavior of clay under irregular cyclic loading

A systematic analysis of the undrained stress–strain behavior of clay under irregular cyclic simple shear loading is presented. Seven specimens of an offshore clay consolidated to overconsolidation ratios of 1, 2, and 4 were subjected to different combinations of variable and nonsymmetric cyclic amplitudes using the Norwegian Geotechnical Institute (NGI) direct simple shear device. The test results show that (1) the behavior under such loads is influenced by several different factors, (2) among these factors the loading history and cyclic stiffness degradation are predominant, and (3) the irregular cyclic loading stress–strain curves can be described quite well by five rules that incorporate only these two factors. Four out of these five rules are the extensions of two original and two extended Masing rules to the behaviour of cyclically degrading clay. The fifth rule is new. The effects of the S-shaping of the stress–strain curves and the rate of loading on the applicability of the rules are also discussed. The stress–strain curves are presented in the normalized form with respect to the vertical effective consolidation stress. In this form they show quantitatively the same trends, indicating that such normalization is applicable to irregular cyclic loading. Key words: clay, earthquake loading, laboratory test, ocean soil, overconsolidation, simple shear test, soil dynamics, strain rate effect, repeated loading.

An empirical evaluation of pressuremeter test data

In situ pressuremeter tests have in recent years come into wide use in the determination of the parameters to be used for foundation design. Although theoretically correct analytical expressions have been developed for the radial expansion pressure versus radial strain relationship, these are generally cumbersome and difficult to use and in practice a graphical procedure is commonly used to derive the maximum undrained shear stress and the undrained modulus value from the test data.To overcome the disadvantages that this graphical procedure involves an empirically-derived analytical relationship based on Kondner's hyperbolic stress–strain relationship is presented here, by means of which all the derived parameters can be obtained simply and without recourse to graphical construction.

Elasticity and plasticity in soils

The note draws attention to the distinction between elasticity and plasticity in soils and investigtes the behavior of soils in the laboratory cylindrical compression test as a result of this distinction. Stress and strain paths obtained in drained and undrained cylindrical compression tests on a variety of undisturbed and remodeled soils are examined. Elastic behavior is identified with linearity of the drained strain path or the undrained stress path whereas plasticity is identified with nonlinear paths. The method suggested is unable to separate simultaneous elastic or plastic components of strain. /TRRL/

Secondary Consolidation and Strength of a Clay

The undrained strength and stress path of a soft marine clay subjected to different periods of secondary consolidation under isotropic stress was examined. The phenomenon of buildup of pore-water pressure has been shown to be due to the arresting of secondary compression. The effect of this phenomenon on the undrained strength and stress path was also studied. The results have been explained using the Cam-clay model. It is concluded that secondary compression would cause increase in undrained strength and stiffening of the stress-strain characteristics of bay mud, while the effect of the pore-water pressure buildup was to produce steeper undrained stress paths at small strains only.

Closure to “Undrained Stress Distribution by Numerical Methods”

Closure to “Undrained Stress Distribution by Numerical Methods”

Discussion of “Undrained Stress Distribution by Numerical Methods”

Discussion of “Undrained Stress Distribution by Numerical Methods”

Discussion of “Undrained Stress Distribution by Numerical Methods”

MR. CHEN CONGRATULATES THE AUTHOR FOR PRESENTING AN INTERESTING PAPER ON THE FINITE-ELEMENT METHOD. A CORRECTION IS GIVEN TO THE MATRIX FOR AN ELASTIC ISOTROPIC MATERIAL. AN EQUATION INTRODUCING TWO MATRICES FOR THE DISPLACEMENT INSIDE A TRIANGULAR ELEMENT AS RELATED TO THE DISPLACEMENT AT THE NODE COULD BE REPRESENTED BY A SINGLE MATRIX WHICH IS PRESENTED. MR. BARRETT COMMENTS THAT IT WOULD NOW APPEAR THAT THERE ARE TWO METHODS OF TREATING UNDRAINED STRESS DISTRIBUTION: (1) USE OF TOTAL STRESS PARAMETERS, AND (2) USE OF EFFECTIVE STRESS PARAMETERS. COMPARISONS OF STRESS DISTRIBUTIONS UNDER AND WITHIN LONG ELASTIC EMBANKMENTS FOR VARIOUS POISSON'S RATIOS HAVE BEEN CONDUCTED BY SEVERAL PEOPLE. THESE ARE DISCUSSED AND IT IS SHOWN THAT THEY ARE DIFFERENT FROM THOSE OF THE UNDRAINED CASE OF THE AUTHOR. THIS DISCREPANCY COULD EXIST FOR SEVERAL REASONS: (1) THE BOUNDARY CONDITIONS, (2) COARSENESS OF THE AUTHOR'S NETWORK, AND (3) THE TWO METHODS OF TREATING UNDRAINED STRESS DISTRIBUTIONS LEAD TO DIFFERENT RESULTS. IT WOULD BE HELPFUL IF THE AUTHOR WOULD REVIEW HIS PAPER IN THE LIGHT OF THESE POINTS AND GIVE SOME INDICATION AS TO WHETHER UNDRAINED STRESS DISTRIBUTIONS WITH VARIOUS EFFECTIVE STRESS PARAMETERS HAVE BEEN STUDIED AND AS TO HOW TWO EQUATIONS SHOULD BE ADJUSTED FOR UNDRAINED UNSATURATED SOILS. REFERENCES: UNDRAINED STRESS DISTRIBUTION BY NUMERICAL METHODS, JOHN T. CHRISTIAN, ASCE PROCEEDINGS PAPER 6243, NOVEMBER, 1968.

4. Undrained stress distribution by numerical methods: J.T. Christian.J. soil Mech. Fndns Div. ASCE, 94 (6), 1333–1345 (nov. 1968)

4. Undrained stress distribution by numerical methods: J.T. Christian.J. soil Mech. Fndns Div. ASCE, 94 (6), 1333–1345 (nov. 1968)