Effective stress

About: Effective stress is a research topic. Over the lifetime, 3922 publications have been published within this topic receiving 97256 citations.

Frictional heating, fluid pressure, and the resistance to fault motion

Abstract: Expansion of pore fluid caused by frictional heating might have an important effect on the factional resistance and temperature during an earthquake and a controlling influence on the physics of the earthquake process When confined water is heated, the pressure increases rapidly (≳10 bars/°C) As Sibson (1973) has pointed out, this could cause a sharp reduction of effective normal stress and dynamic friction on the fault surface Whether or not this transient stress reduction occurs depends upon the tandem operation of several processes, any of which can break the chain that links frictional heat to frictional stress: the friction must cause an appreciable temperature rise (imposing conditions on the width of the shear zone and rate of conductive transport); the temperature rise must cause an appreciable fluid pressure rise (imposing conditions on the rate of pore dilatation or hydrofracturing, and the rate of Darcian transport); the fluid pressure rise must cause an appreciable reduction of friction (requiring the presence of a continuous fluid phase) Each process depends upon event duration, particle velocity, and the initial value of dynamic friction With the present uncertainty in the controlling parameters (principally permeability, width of the shear zone, initial stress, and factors controlling transient hydrofracture and pore dilatation) a wide variety of fault behavior is possible Limits to fault behavior for various ranges of the controlling parameters can be estimated from the governing equations, however, and results can be summarized graphically If the effective stress law applies and pore dilatation is unimportant, dynamic friction would drop from an initial value of 1 kbar to ∼100 bars when shear strain reached 10 for most earthquakes if the permeability were less than 01 μdarcy; the maximum temperature rise would be only ∼150°C irrespective of final strain If the permeability were ≳100 mdarcies, however, friction would be unaffected by faulting and temperatures could approach melting for shear strains ∼20 For permeabilities ∼1 mdarcy, friction could be reduced appreciably during large earthquakes, but during small ones it could not Combined with thermal effects, dilatational strain of a few percent of pore volume could lead to virtually frictionless faulting or increasing frictional resistance, dependeing upon its sign; unstable propagation of hydrofractures (after fluid pressure exceeded the least principal stress) could cause a sudden increase in fault friction Strengthening due to cooling and Darcian flow at the conclusion of an earthquake could occur in seconds or weeks depending upon event duration, transport parameters, and shear zone width; it might influence the redistribution of stress by aftershocks

Fundamentals of Liquefaction under Cyclic Loading

Abstract: The mechanism of progressive pore pressure increase during undrained cyclic simple shear tests on saturated sands has been examined in detail. The concepts developed provide a better understanding of the physical processes leading to liquefaction of horizontal sand deposits during earthquakes. A quantitative relationship between volume changes occurring during drained cyclic tests and the progressive increase of pore water pressure during undrained cyclic tests has been developed. The use of this relationship enables the build-up of pore water pressure during cyclic loading to be computed theoretically using basic effective stress parameters of the sand. The application of the theory to the case of undrained cyclic simple shear tests with uniform stress controlled loading is illustrated. Values of progressive pore water pressure increases predicted theoretically agree reasonably well with the nature of pore water pressure increases observed experimentally.

Craig's Soil Mechanics

Abstract: 1. Basic Characteristics of Soils 2. Seepage 3. Effective Stress 4. Shear Strength 5. Stresses and Displacements 6. Lateral Earth Pressure 7. Consolidation Theory 8. Bearing Capacity 9. Stability of Slopes

Basal mechanics of Ice Stream B, west Antarctica: 1. Till mechanics

Abstract: Data from laboratory geotechnical tests on till recovered from beneath Ice Stream B, West Antarctica, at the Upstream B camp (hereinafter the UpB till) show that failure strength of this till is strongly dependent on effective stress but is practically independent of strain and strain rate. These data support use of a Coulomb-plastic rheology in modeling of ice stream behavior and subglacial till deformation. Our testing program combined triaxial, ring shear, and confined uniaxial tests to investigate till strength and compressibility. Results show that the UpB till follows closely Coulomb's equation in which shear strength is a linear function of normal effective stress (apparent cohesion near zero and internal friction angle ϕ equal to 24°). Till compressibility is best described by a logarithmic function that relates void ratio to normal effective stress. In general, the behavior of the UpB till is consistent with other experimental evidence regarding mechanical behavior of granular materials. Based on our laboratory results we formulate the Compressible-Coulomb-Plastic till model in which there are three interrelated, primary state variables: shear strength, void ratio, and normal effective stress. This model is used in the second part of our study to simulate response of subglacial till to realistic effective stress forcings. These simulations demonstrate that the model is capable of reproducing fundamental aspects of subglacial till kinematics: (1) occurrence of tilt rate oscillations and negative tilt rates in tiltmeter records, and (2) distributed till deformation to depths of 0.1–1.0 m beneath the ice base. Our laboratory and modeling results substantiate application of the Compressible-Coulomb-Plastic model in simulations of the motion of Ice Stream B over its weak till bed.

Effective stress concept in unsaturated soils: Clarification and validation of a unified framework

Abstract: The effective stress principle, conventionally applied in saturated soils, is reviewed for constitutive modelling purposes. The assumptions for the applicability of Terzaghi’s single effective stress are recalled and its advantages are inventoried. The possible stress frameworks applicable to unsaturated soil modelling are reassessed in a comparative manner, specifically the Bishop’s single effective stress, the independent stress variables approach and the generalized stress framework. The latter considerations lead to the definition of a unified stress context, suitable for modelling soils under different saturation states. In order to qualify the implications brought by the proposed stress framework, several experimental data sets are re-examined in the light of the generalized effective stress. The critical state lines (CSLs) at different saturation states tend to converge remarkably towards a unique saturated line in the deviatoric stress versus mean effective stress plane. The effective stress interpretation is also applied to isotropic paths and compared with conventional net stress conception. The accent is finally laid on a second key feature for constitutive frameworks based on a unified stress, namely the sufficiency of a unique mechanical yield surface besides the unique CSL.