Current Effective Stress Research Articles

Accurate Effective Stress Measures: Predicting Creep Life for 3D Stresses Using 2D and 1D Creep Rupture Simulations and Data

Operating structural components experience complex loading conditions resulting in 3D stress states. Current design practice estimates multiaxial creep rupture life by mapping a general state of stress to a uniaxial creep rupture correlation using effective stress measures. The data supporting the development of effective stress measures are nearly always only uniaxial and biaxial, as 3D creep rupture tests are not widely available. This limitation means current effective stress measures must extrapolate from 2D to 3D stress states, potentially introducing extrapolation error. In this work, we use a physics-based, crystal plasticity finite element model to simulate uniaxial, biaxial, and triaxial creep rupture. We use the virtual dataset to assess the accuracy of current and novel effective stress measures in extrapolating from 2D to 3D stresses and also explore how the predictive accuracy of the effective stress measures might change if experimental 3D rupture data was available. We confirm these conclusions, based on simulation data, against multiaxial creep rupture experimental data for several materials, drawn from the literature. The results of the virtual experiments show that calibrating effective stress measures using triaxial test data would significantly improve accuracy and that some effective stress measures are more accurate than others, particularly for highly triaxial stress states. Results obtained using experimental data confirm the numerical findings and suggest that a unified effective stress measure should include an explicit dependence on the first stress invariant, the maximum tensile principal stress, and the von Mises stress.

Suction Stress of Clay Over a Wide Range of Saturation

Quantifying the relationship between water retention and internal stress state of unsaturated soils has long been a challenge. Current effective stress formulations can effectively capture the relationship for coarse-grained soils, where capillary mechanisms dominate internal stress state over a wide range of saturation, but fail for fine-grained soils, particularly at low to intermediate saturation, where surface adsorption mechanisms dominate water retention and corresponding stress state. Internal stress state of compacted kaolinite specimens spanning saturation (S) from 0 to 0.73 is measured through Brazilian tensile strength tests. Changes observed in strength, strain, and stiffness are analyzed to calculate evolution of internal stress with saturation. A model is proposed to quantify the relationship between water retention and internal stress for fine-grained and coarse-grained soils at saturations over the full range (0 ≤ S ≤ 1). Performance of the model is evaluated by comparison with experimental results from the literature.

Secondary settlement estimation in surcharge preload subject to time effect of secondary consolidation coefficient

In order to eliminate the settlement underestimation in surcharge preload engineering, a study based on Bjerrum’s creep diagram and the tangent slope definition of secondary consolidation coefficient was carried out to analyze the time effect of secondary consolidation coefficient of over consolidated soil, and a time–growth model for it was formulated. As Bjerrum’s creep diagram is an idealized model, oedometer tests were performed to improve the above time–growth model of secondary consolidation coefficient for the purpose of achieving a better agreement with the actual ground situations. It is found that secondary consolidation coefficient of over consolidated soil not only decreases with the ratio of historical maximum to current effective stress of soil (OCR), but also increases with the development time of secondary consolidation. No matter how large OCR is, the long-term time effect of secondary consolidation coefficient of over consolidated soil is all significant. Based on the above results, a model for settlement estimation was formulated and a case study to estimate it indicates that the settlement estimated by our method is 2–5 times larger than that estimated by the previous method. Moreover, the larger the OCR is as well as the longer the service life is, the larger the difference between our method and the previous method is. Thus, the post-construction secondary settlement in surcharge preload engineering will be underestimated when neglecting the time effect of secondary consolidation coefficient in over consolidated state.

Overpressure within upper continental slope sediments from CPTU data, Gulf of Lion, NW Mediterranean Sea

Data from in situ piezocone tests (CPTU) and laboratory analyses are utilized for the interpretation of the stress history of Quaternary sedimentary sequences in the upper continental slope of the Gulf of Lion, northwestern Mediterranean Sea. A CPTU based preconsolidation pressure profile referenced to the current effective stress indicates that the deposit is underconsolidated from 12 meters below the seafloor (mbsf) down to at least 150 mbsf. Excess pore pressure below 12 mbsf is further supported by results from oedometer and dissipation tests. Subseafloor pockmarks and indications of free gas in seismic reflection profiles reveal four main overpressure sources (SI–SIV) with overpressure ratios >0.3 at subseafloor depths coinciding with levels where the dominantly silty-clayey sediment contains increased proportions of sand. We relate the excess pore pressure related to free gas due to gas exsolution processes and sea level variations driven by Pleistocene sea level changes.

Collapse of a Model Strip Footing on Dense Sand Under Vertical Eccentric Loads

The paper focuses on the behaviour of a model strip footing, resting on a saturated dense sandy soil, subjected to centred or eccentric vertical loading. Experimental tests, carried out on a small-scale physical model, are able to reproduce effective stress levels equivalent to those prevailing in prototype problems, thanks to the maintenance of a downward steady-state seepage in the soil. The test program consists of three series of tests, each corresponding to an imposed value of hydraulic gradient, and each involving five load eccentricities; one series, in particular, is carried out with still water. Relevant points of load–settlement curves are related to the evolution of soil-footing collapse mechanism, evidenced by the distortion of some vertical coloured sand strips. The collapse mechanism is formed either by one or two sliding surfaces, depending on both load eccentricities and hydraulic gradient values. Significant differences are shown to occur between centred and eccentric loading footing response. Shear strength parameters obtained from back-analyses carried out on load values recorded at the appearance of each sliding surface on the free soil surface, in both hypotheses of associated and non-associated flow rule validity, are adopted to draw, for each test, a theoretical collapse mechanism consisting, in plane strain, of a log-spiral line with adjacent-end tangents; the obtained theoretical sliding surfaces, in turn, are compared to the experimental ones, showing that these latter are either stress characteristic or zero-extension lines depending mainly on cumulative footing displacements and current effective stress level in the soil.

On the applicability of cross-anisotropic elasticity to granular materials at very small strains

The paper examines the validity of assuming that granular material behaviour can be considered as cross-anisotropic, linear elastic, within a kinematic ‘kernel’ yield surface that is dragged through stress space with the current effective stress point. Data from more than 300 high-resolution triaxial probing and bender element tests performed on 15 samples are reported in which both small stress cycles and shear wave measurements in both the vertical and horizontal directions were conducted on sand and glass ballotini specimens under various effective stress states. It was found that behaviour is not fully elastic. However, provided the effective stress ratio, R, was kept below around 2·2, most features of the very small-strain behaviour could be described in terms of strongly anisotropic compliance terms that varied with void ratio and current effective stress state, apparently independently of the specimen's stress history. The anisotropy depended on the effective stress states. Specimens loaded to R values exceeding 2·2 that experienced shear-induced dilation followed different patterns of behaviour. The concept of a kinematic closed elastic yield surface, or ‘kernel’, proved to be an appropriate way of describing the pronounced effects of recent stress history on the apparent limits to the linear range observed in a wide range of stress probing tests.

On the applicability of cross-anisotropic elasticity to granular materials at very small strains

The paper examines the validity of assuming that granular material behaviour can be considered as cross-anisotropic, linear elastic, within a kinematic ‘kernel’ yield surface that is dragged through stress space with the current effective stress point. Data from more than 300 high-resolution triaxial probing and bender element tests performed on 15 samples are reported in which both small stress cycles and shear wave measurements in both the vertical and horizontal directions were conducted on sand and glass ballotini specimens under various effective stress states. It was found that behaviour is not fully elastic. However, provided the effective stress ratio, R, was kept below around 2·2, most features of the very small-strain behaviour could be described in terms of strongly anisotropic compliance terms that varied with void ratio and current effective stress state, apparently independently of the specimen's stress history. The anisotropy depended on the effective stress states. Specimens loaded to R values exceeding 2·2 that experienced shear-induced dilation followed different patterns of behaviour. The concept of a kinematic closed elastic yield surface, or ‘kernel’, proved to be an appropriate way of describing the pronounced effects of recent stress history on the apparent limits to the linear range observed in a wide range of stress probing tests.

Investigation of aquifer-system compaction in the Hueco Basin, El Paso, Texas, USA : C. Heywood, in: Land subsidence. Proc. international symposium, The Hague, 1995, ed F.B.J. Barends & others, (IAHS; Publication, 234), 1995, pp 35–45

The Pleistocene geologic history of the Rio Grande valley in the Hueco basin included a cycle of sediment erosion and re-aggradation, resulting in unconformable stratification of sediment of contrasting compressibility and stress history. Since the 1950s large groundwater withdrawals have resulted in significant water-level declines and associated land subsidence. Knowledge of the magnitude and variation of specific storage is needed for developing predictive models of subsidence and groundwater flow simulations. Analyses of piezometric and extensometric data in the form of stress-strain diagrams from a 16 month period yield in situ measurements of aquifer-system compressibility across two discrete aquifer intervals. The linear elastic behaviour of the deeper interval indicates over-consolidation of basin deposits, probably resulting from deeper burial depth before the middle Pleistocene. By contrast, the shallow aquifer system displays an inelastic component, suggesting pre-consolidation stress not significantly greater than current effective stress levels for a sequence of late Pleistocene clay. Harmonic analyses of the piezometric response to earth tides in two water-level piezometers provide an independent estimate of specific storage of aquifer sands.

Ultimate bored pile load capacity in granular soils: a case history against the current effective stress design theory in comparison to a penetration based design method : Mordhorst, C Proc 1st International Geotechnical Seminar on Deep Foundations on Bored and Auger Piles, Ghent, 7–10 June 1988P451–456. Publ Rotterdam: A A Balkema, 1988

Ultimate bored pile load capacity in granular soils: a case history against the current effective stress design theory in comparison to a penetration based design method : Mordhorst, C Proc 1st International Geotechnical Seminar on Deep Foundations on Bored and Auger Piles, Ghent, 7–10 June 1988P451–456. Publ Rotterdam: A A Balkema, 1988

Work as a criterion for determining in situ and yield stresses in clays

A method of interpreting conventional oedometer test data using work per unit volume as a criterion for determining both in situ effective and yield stresses in clay is presented. This technique was applied to the results of oedometer tests carried out on samples of natural clay deposits and on specimens consolidated anisotropically from a slurry to a known effective stress state. The work per unit volume – effective stress relationship, using arithmetic scales, can be approximated or fitted using linear relationships. The intersections of these fitted lines are demonstrated to provide accurate values for in situ current and yield (preconsolidation) stresses. The yield stress is defined as the intersection of the initial fitted line and the linear relationship observed at higher stresses. The current effective stress is indicated by the first significant divergence of the data from the initial fitted line. These relationships apply to both conventionally (horizontally) trimmed specimens and to vertically trimmed oedometer samples. It is hypothesized that the in situ effective and yield stresses (in both the vertical and horizontal directions) in a natural clay can be determined by the work per unit volume interpretation of oedometer tests carried out on horizontally and vertically trimmed specimens. Key words: in situ, stress, yield, oedometer, interpretation, clays, work, state, K0, preconsolidation pressure.