Stress Path Tests Research Articles

Investigation of the Strain-Softening Behavior of Granular Soils with a New Multiaxial Cell

Abstract The special considerations leading to the design of a new multiaxial cell used for strain-softening studies are presented. This cell has the special features of being able to accommodate large strain without introducing boundary imperfections, allowing observation of shear band formation, and permitting strain path control. The observed strain-softening behavior of a quartz sand under multiaxial stress condition, i.e., σ'1 ≠ σ'2 ≠ σ'3, are presented. Both stress path tests and strain path tests were conducted. In a stress path test, the specimen was loaded in the major principal direction in a deformation-controlled mode but with the stress response constrained along a prescribed path using computer control. Hence strain-softening response can still be observed in a controlled way. Two types of softening, prefailure and postfailure softening, are identified. Prefailure softening will occur along certain strain paths, and the associated deformation is homogeneous. In postfailure softening under a multiaxial stress condition, only limited homogeneous deformation can be sustained, and eventually there will be shear band formation. However, this shear band is not triggered by boundary imperfections but is an inevitable response of the specimen to shearing in the postfailure domain under a multiaxial stress condition.

Stress‐Path Dependent Shear Strength of Sand

Failure envelopes of sands are often considered to be nonlinear depending both on relative density and stress level. Triaxial tests that were performed on clean alluvial‐quartzitic Zbraslav sand were used for analyzing the form of the failure envelope. In addition to relative density and stress level, the stress path (tests with constant‐cell pressure were compared with tests with constant‐mean‐stress level) has been found to affect the shear resistance as well. Only tests with constant‐mean stress produce nonlinear‐failure envelopes. Up to the peak‐stress difference, no shear bands were observed. Rough platens were used to simplify the procedure since the specimen's restraint was found not to affect the shear resistance for diameter to height ratio 1:2. The writer uses the percolation theory to provide the physical interpretation of the experimental results.

Behaviour of loose sand under simultaneous increase in stress ratio and principal stress rotation

The drained behaviour of loose sands under simultaneous increase in stress ratio and principal stress rotation is investigated. The hollow cylinder torsional device, which permits independent control of four stress parameters, namely effective mean normal stress [Formula: see text], stress ratio R, intermediate principal stress parameter b, and the inclination ασ of the major principal effective stress [Formula: see text] to the vertical, is adopted as the testing device. Drained tests carried out on saturated sand indicate that deformations under increasing R and ασ are path independent, if the final stress state is within the approximate bounds of R < 2 and ασ < 45°. With increasing stress ratio R and (or) principal stress rotation, deformations gradually become path dependent. Once loaded to a stress state within the domain R < 2 and ασ < 45°, the strain response under subsequent principal stress rotation is shown to be independent of the previous loading history. It is demonstrated that the strain response under any general increasing R − ασ path in the domain of R < 2 and ασ < 45° can be predicted using the results of a limited number of tests characterizing that domain. Strain increment direction αΔε is shown to be approximately coincident with and totally governed by the stress increment direction αΔσ, when the stress increment direction αΔσ is preferentially inclined towards the vertical deposition direction. Key words : sand behaviour, hollow cylinder torsional device, principal stress rotation, stress-path testing.

Stress path testing of boom clay from Mol, Belgium : Taylor, R N; Coop, M R Proc 26th Annual Conference of the Engineering Group of the Geological Society, The Engineering Geology of Weak Rock, Leeds, 9–13 September 1990P77–82. Publ Rotterdam: A A Balkema, 1993 (Engineering Geology Special Publication No. 8)

Stress path testing of boom clay from Mol, Belgium : Taylor, R N; Coop, M R Proc 26th Annual Conference of the Engineering Group of the Geological Society, The Engineering Geology of Weak Rock, Leeds, 9–13 September 1990P77–82. Publ Rotterdam: A A Balkema, 1993 (Engineering Geology Special Publication No. 8)

Strain states during stress path testing of the chalk : Loe, N M; Leddra, M J; Jones, M E Proc 33rd US Symposium on Rock Mechanics, Santa Fe, 3–5 June 1992P927–936. Publ Rotterdam: A A Balkema, 1992

Strain states during stress path testing of the chalk : Loe, N M; Leddra, M J; Jones, M E Proc 33rd US Symposium on Rock Mechanics, Santa Fe, 3–5 June 1992P927–936. Publ Rotterdam: A A Balkema, 1992

Development of negative skin friction on driven piles in soft Bangkok clay: Discussion

pled finite-element analysis presented in the paper. Also, (7) The current finite-element analysis incorporated a the short-term pull-out tests contributed to the determinarigid-elastic model for the end-bearing stratum (Table 2b tion of normalized skin friction a in [ I ] (pp. 396-397 and of the authors' paper) and was not based on the Cam-clay Table 1). theory. (4) The topmost soil layer shows slight overconsolida(8) Detailed descriptions of the numerical analysis are ti011 due to crusting. As a result, it is expected that the given by Wong (1991). The boundary effects of the finiteM value for the topmost soil layer would be greater than element mesh did not contribute significantly to the negative the underlying soft clay layer. The Cam-clay parameters skin friction analysis. As mentioned earlier, the results were were determined by triaxial stress path tests conducted on mainly sensitive to the interface behaviour. The finite-element samples recovered from various depths. The values given mesh shown in Fig. 13 of the authors' paper is drawn to in Table 2 are consistent with previous studies carried out for scale, where the thin interface elements closest to the pile surBangkok subsoils. As explained earlier, as a result of disface were 0.1 m in thickness. The adjacent elements increase turbance (softening), the shear modulus at the soil-pile interin thickness radially outwards. As the height of the embankface is smaller than that of the surrounding undisturbed soil. ment was only 2.0 m, its construction was assumed to be This is not an inconsistency, but rather a feature in the rapid (instantaneous) for the purpose of analysis. numerical model. ( 5 ) The authors apologize for a misprint. The unit weight of concrete should be 2.5 t/m3 and not 1.5 t/m3. Wong, Y.K. 1991. Numerical simulation of negative skin fric(6) The negative skin friction analysis is mainly sensition on driven piles. M.Eng. thesis, Asian Institute of tive to the soil-pile interface behaviour rather than to the Technology, Thailand. effects of the embankment boundaries. Therefore, the error introduced by the axisymmetric finite-element idealization was considered to be small in the numerical model.

Collapse behavior of sand

Loose cohesionless materials can collapse during either static or dynamic loading, resulting in a rapid buildup of pore pressure and associated reduction in shear resistance. As the cohesionless material collapses, it rapidly looses resistance until the acting shear stress decreases to the available residual or steady-state strength. Specially designed stress-path testing has been performed on sand to investigate this collapse process. Results from this test program and previously published data show that a state boundary can be defined when a cohesionless material moves from peak to steady state along a constant void ratio stress path regardless of whether it is loaded drained or undrained. Further, it is demonstrated that the state boundary represents a surface in the effective mean normal stress–deviator stress–void ratio space. Hence, flow slides and liquefaction can be initiated when the stress path followed during either drained or undrained loading attempts to cross this state boundary surface. Key wordy : sand, collapse, liquefaction, stress path, state boundary, triaxial test.

Stress path tests to investigate yielding of Ko consolidated soil : Allman, M A; Atkinson, J H; Jordan, D O Proc 6th Australia-New Zealand Conference on Geomechanics, Christchurch, 3–7 February 1992P298–301. Publ New Zealand: New Zealand Geomechanics Society, 1992

Stress path tests to investigate yielding of Ko consolidated soil : Allman, M A; Atkinson, J H; Jordan, D O Proc 6th Australia-New Zealand Conference on Geomechanics, Christchurch, 3–7 February 1992P298–301. Publ New Zealand: New Zealand Geomechanics Society, 1992

Progressive destructuring of Bothkennar clay. implications for sampling and reconsolidation procedures

Triaxial stress and strain path tests have been carried out on high-quality Laval and Sherbrooke samples of lightly overconsolidated Bothkennar clay. The specimens were instrumented with local axial and radial strain and mid-plane pore pressure measurement, with precautions being taken to retain the existing pore water chemistry. The test programme imposed shear and volumetric strains of various magnitudes to assess the reduction in strength and stiffness caused by sampling and laboratory reconsolidation procedures. It is concluded that for this clay even very high quality tube samples will suffer a significant loss in mean effective stress, and some loss of structure during sampling, as a result of the imposed undrained shear strain cycle. Re-establishment of the initial effective stress level, by an appropriate stress path, will recover a proportion of the undisturbed undrained compressive strength that depends on the amount of destructuring: stiffness cannot be fully recovered. Reconsolidation procedures risk taking the specimen through the current yield surface, in which case large volumetric strains will occur and be accompanied by significant destructuring and, hence, irrecoverable loss of strength and stiffness. For natural clays, such as Bothkennar clay, the variability of structure within the deposit means that normalization solely with respect to effective stress cannot be used to allow for the disturbance caused by tube sampling, or to recover in situ soil properties. Des essais au triaxial avec chemin de contraintes et déformations ont été faits sur des échantillons Laval et Sherbrooke d'argile de Bothkennar un peu sur-consolidée de haute qualité. Les spécimens ont été instrumentés, les déformations locales axiales et radiales ainsi que la pression interstitielle à mi-plan étant mesurées, des précautions étant prises pour conserver la chimie de l'eau interstitielle existante. Le programme d'essaise impo-sava des déformations de cisaillement et de volume d'ampleurs variées pour établir la réduction de la résistance et de la raideur causée par les procédés d'échantillonnage et de re-consolidation en laboratoire. On a conclus que pour cette argile même des échantillons mince paroi de haute qualité verront une réduction importante de la contrainte effective moyenne et quelque déstructuration pendant l'échantillonnage, comme résultat du cycle de déformation de cisaillement non drainé imposé. Le rétablissement du niveau initial de la contrainte effective, par un chemin de contrainte approprié, permettra de récupérer une proportion de la résistance à la compression non drainée non remaniée dépendant de l'ampleur de déstructuration: la raideur ne peut jamais être pleinement retrouvée. Les procédures de re-consolidation risquent de mener l'échantillon au delà de la surface de fluage courante, au quel cas de grandes déformations de volume prendront place, et seront accompagnées par une déstructuration importante, et, donc, de pertes de résistance et de raideur irréparables. Pour des argiles naturelles telle que l'argile de Bothkennar, la variabilité de la structure au sein de la couche signifie que la normalisation par rapport à la contrainte effective seulement ne peut pas être utilisée pour prendre en compte le remaniement causé par l'échantillonnage au tube, en pour retrouver les propriétés du sol in-situ.

Mechanical properties of reconstituted Bothkennar soil

The basic behaviour of one-dimensionally normally consolidated and lightly overconsolidated reconstituted Bothkennar soil was investigated in laboratory triaxial stress path tests. The main programme of tests established a state boundary surface, containing critical state lines and one-dimensional and isotropic compression lines, and evaluated the basic material parameters. The compressibility of Bothkennar soil was found to be similar to that for high plasticity clay but the critical state friction angles were comparable with the values normally associated with coarse-grained soils. Additional tests demonstrated that the effect of ageing, over periods up to 200 hours, was to increase the apparent overconsolidation ratio. The results of a small number of tests on intact samples were examined within the basic framework developed for the reconstituted material. They illustrate the importance of accounting for the current states in interpreting the behaviour of intact and reconstituted samples. Le comportement fondamental du sol du Bothkennar reconstitué normalement et un peu surconsolidé a été étudié à l'aide d'essais triaxiaux avec chemin de contraintes. Le programme principal d'essais établit une surface limite d'état contenant les courbes d'état critique et les courbes de compression unidimensionnelles et isotropiques, il a également évalué les propriétés de base du matériau. On a trouvé que la compressibilité du sol de Bothkennar est semblable à celle d'argiles à haute plasticité mais que les angles de frottement à l'état critique avaient des valeurs comparables à celles normalement associées des sols à grains grossiers. Des essais supplémentaires démontrèrent que l'effet du vieillissement, pour des périodes jusqu'à 200 heures était d'accroître le degré de surconsolidation apparent. Les résultats d'un petit nombre d'essais sur des échantillons intacts furent examinés dans le cadre de base développé pour le matériau reconstitué. Ils illustrent l'importance de prendre en compte l'état présent pour interpréter le comportement des échantillons intacts et reconstitués.

Geotechnical properties of Quaternary deposits in the Belfast area

Abstract Soils deposited during the Quaternary era occur widely in Northern Ireland. Those in the Belfast area pose particular Engineering problems owing to their wide Variation, and the increasingly diverse range of construction activities in this major city. The soil types identified in the city area include two cohesive till sheets, separated by laminated clays and glacial sands. Overlying the glacial deposits is a soft, sensitive, post-glacial, estuarine clay known locally as Sleech. A field and laboratory-based programme of research has been conducted with the aim of improving understanding of the geotechnical behaviour of these deposits, and of identifying relevant Engineering properties. This paper presents results from this research, describing the soil types, common Engineering problems associated with them, and laboratory measured properties. The properties investigated relate to strength and deformation, including elastic yield parameters derived from stress path tests, and consolidation characteristics. The results show that the measured parameters depend on sampling techniques and preparation, as well as sample size and test procedure adopted. Thus any results require careful Interpretation and the practical applications of the findings are discussed in the paper.

Effect of recent stress history on the stiffness of overconsolidated soil

The stress-strain behaviour of overconsolidated soil is non-linear and the stiffiiess depends on the current state and stress history. In addition to the overall stress history which is best described by the overconsolidation ratio, the recent stress history- either a sudden change in the direction of the stress path or a period of time at a constant stress state-has a major effect on subsequent stiffiiess. The effects of a change in the direction of the loading path on the stiffness of overconsolidated soil were examined in laboratory stress path tests on reconstituted samples of London Clay. Further tests were carried out on reconstituted samples of other soils with a range of plasticities. The test results showed that the behaviour of over-consolidated soil is largely inelastic and highly nonlinear, and that the recent history has a major effect on the subsequent stiffiiess, particularly for relatively small changes of stress or strain. For reconstituted samples of London Clay the stiffiiess near the start of a loading path was found to vary by up to an order of magnitude following different rotations of the stress path. For other soils with lower plasticities the variations were smaller but still significant. Le comportement contrainte-déformation d'un sol surconsolidé est non-linéaire et al rigidité dépend de l'etat existant et de l'historique des contraintes. En addition à l'historique général des contraintes, qui est le mieux representé le rapport de surconsolidation, l'historique recent des contraintessait un changement abrupt dans la direction du chemin des contraintes sait une période de temps dans un état de contrainte constante-a un eftet primordial sur la rigidité ultérieure. Les effets d'un changement dans la direction du chemin de chargement sur la rigidité du sol surconsolidé ont été examinés au cours des essais concernant les chemins des contraintes effectués en le laboratoire sur des échantillons reconstitute d'argile de Londres. D'autres essais ont été effectués sur des échantillons d'autres sols avec diverses de plasticites. Les résultats des essais montrèrent que le comportement du sol surconsolidé est en grande partie non-élastique et très non-linéaire et que l'historique récent a un effet primordial sur la rigidité ultérieure, particulièrement pour des changements relativement petits de la tension ou de la contrainte. On trouvé que pour des èchantillons reconstitués d'argile de Londres la rigidité dans le voisinage d'un chemin de chargement variait jusqu'à un ordre de grandeur siuvant les differentes rotations du chemin des contraintes. Pour d'autres sols ayant des plasticités inférieures les variations étaient moins grandes mais encore significatives.

Continuous loading oedometer testing of soils

Abstract The conventional oedometer test specified in BS 1377: 1975, in which the loads are applied in increments, is simple and robust, but provides relatively little data on the compression and consolidation parameters of the soil or on its preconsolidation or yield pressure. Continuous loading oedometer tests are capable of measuring continuous variations of compression and consolidation parameters and can often identify the yield pressure more precisely. Data logging and control systems developed for stress path testing systems have been applied to oedometer tests and it is now possible to carry out continuous loading tests routinely. Analysis of the test data is relatively straightforward. Tests on reconstituted soil samples demonstrate that the measured parameters do not depend significantly on the type of test.

Some recent developments in triaxial testing systems for cohesionless soils

The stress states and their changes that can be attained in the conventional triaxial testing method are reviewed and compared to those attained in other more sophisticated testing methods. These advanced tests have been developed to extend the ranges of the stress states and the changes in these states that can be controlled. In conventional triaxial testing on solid cylinder specimens σ 2 ' is equal either to σ 1 ' or to σ 3 ' and only a jump rotation of 90 in the principal stress directions can be achieved. It is argued that in spite of this limitation the triaxial testing method is still a useful means to measure the strength and deformation characteristics of soils. The results, however, should be corrected to account for the actual states of stress in the field. Furthermore, some recent advances in the methods and equipment for triaxial testing and other related kinds of testing are reviewed with an emphasis on the importance of automation and simplification. Several examples are presented where automated controlled stress and/or strain path tests can be performed by means of a simple triaxial apparatus using various electronic transducers, microcomputers, and pneumatic pressurizing systems.

Modelling small strains for analysis in geotechnical engineering : Stallebrass, S E Ground EngngV22, N9, Jan/Feb 1990, P26–29

This paper specifies factors which should be considered, when investigating the small strain stiffness of soils in order to obtain a realistic prediction of ground movement. For many structures in overconsolidated soils, most of the soil experiences shear strains less than 0.1%. At these small strains, the stiffness of overconsolidated soil is nonlinear, and depends on the current state, overconsolidation ratio and recent stress history of the soil. To predict both the magnitudes and the distributions of ground movements adequately, soil models should incorporate all these characteristics. This behaviour can be modelled, using a nonlinear elastic relationship derived empirically from appropriate data, but these data can be obtained only by using special stress path testing, which usually requires local axial strain measurement. A new kinematic yield model has been developed, which uses easily obtained basic soil parameters to model the stress-strain behaviour of soil. This approach provides a potentially simpler method of predicting ground movements.

The Mexico Earthquake of September 19, 1985—Deformability of Mexico City Hard Deposits under Cyclic Loading

Buildings founded on point bearing piles performed better than those founded on friction piles in Mexico City during the earthquakes of September 19 and 20, 1985. Nevertheless a few buildings supported on end bearing piles tilted considerably after the earthquakes. Friction piles have been preferred over end bearing piles in Mexico City because the latter type of foundation has drawbacks resulting from the apparent emersion of structures brought about by regional subsidence. Experimental research into the behavior of the soils that form the bearing strata for pile tips shows that stress path testing can be used to obtain a deformability parameter which allows for the calculation of permanent deformations induced by earthquake loading below end bearing piles. The tests indicate that the deformability of these soils under cyclic loading is rather low. The use of the deformability parameter is illustrated by means of an example which shows that the behavior predicted on the basis of stress path testing and a simplified deformation analysis is consistent with observed field performance.

ELASTO-PLASTIC CONSTITUTIVE MODEL FOR SAND IN LOW AND HIGH PRESSURE

An elasto-plastic constitutive model was established in order to describe the stress-strain behavior of normally and over-consolidated sand for static loading in a wide stress region. The fundamental concept herein is based on the assumption that sand can be regarded as an isotropic material. The proposed model which involves eight parameters consists of two set of the yield, plastic potential and hardening functions. These functions are expressed based on a few basic assumptions and experimental evidences obtained from the multi-step stress path tests under 0.1MPa-10MPa confining pressure. This model can give good qualitative prediction of shear and volumetric strains for triaxial compression tests with various stress paths on normally and over-consolidated sand in a wide stress region.

Microcomputer stress path testing and data analysis system for research and practice : Woods, R I; Clinton, D B Proc Symposium on Computer Applications in Geotechnical Engineering, Birmingham, April 1986P137–145. Publ Birmingham: Midlands Geotechnical Society, 1986

Microcomputer stress path testing and data analysis system for research and practice : Woods, R I; Clinton, D B Proc Symposium on Computer Applications in Geotechnical Engineering, Birmingham, April 1986P137–145. Publ Birmingham: Midlands Geotechnical Society, 1986

Microcomputer stress path testing and data analysis system for research and practice : Woods, R I; Clinton, D B Proc Symposium on Computer Applications in Geotechnical Engineering, Birmingham, April 1986P137–145. Publ Birmingham: Midlands Geotechnical Society, 1986

Microcomputer stress path testing and data analysis system for research and practice : Woods, R I; Clinton, D B Proc Symposium on Computer Applications in Geotechnical Engineering, Birmingham, April 1986P137–145. Publ Birmingham: Midlands Geotechnical Society, 1986

A constitutive model and associated testing for soft rock

A general yet simplified constitutive model is proposed to characterize stress-deformation behaviour of rocks. It is based on the theory of elasto-plasticity and allows for factors such as hardening, volume changes, stress paths, cohesive and tensile strengths and variation of yield behaviour with mean pressure. It is applied to characterize behaviour of a soft rock, soapstone. The constants for the model are determined from a comprehensive series of laboratory tests under different initial confinements and stress paths by using a multiaxial testing device. The model is verified with respect to laboratory tests used for finding the constants and complex stress path tests not used for finding the constants. Overall, the proposed model involves less or equal number of constants compared to available models of similar capabilities and is easier to implement in (numerical) solution procedures.