Nonhydrostatic Loading Research Articles

Two-dimensional elastoplastic analysis of a long, cylindrical cavity under non-hydrostatic loading : Detourney, E; Fairhurst, C Int J Rock Mech Min SciV24, N4, Aug 1987, P197–211

Two-dimensional elastoplastic analysis of a long, cylindrical cavity under non-hydrostatic loading : Detourney, E; Fairhurst, C Int J Rock Mech Min SciV24, N4, Aug 1987, P197–211

Two-dimensional elastoplastic analysis of a long, cylindrical cavity under non-hydrostatic loading : Detourney, E; Fairhurst, C Int J Rock Mech Min SciV24, N4, Aug 1987, P197–211

Two-dimensional elastoplastic analysis of a long, cylindrical cavity under non-hydrostatic loading : Detourney, E; Fairhurst, C Int J Rock Mech Min SciV24, N4, Aug 1987, P197–211

Two-dimensional elastoplastic analysis of a long, cylindrical cavity under non-hydrostatic loading

This paper describes a two-dimensional elastoplastic model of a long, cylindrical cavity in an infinite rock mass subject to non-hydrostatic far-field stress loading. The rock is assumed to behave as an elastoplastic dilatant material that is characterized by a cohesive frictional yield strength. The analysis is based on a solution of the elastoplastic interface that is applicable when the underground cavity becomes completely surrounded by a zone of failed rock, and for stress conditions for which the problem is statically determined. This latter restriction limits the range of deviation from a hydrostatic stress condition that can be analyzed with the model. The stress and displacement field induced by excavation unloading of the cavity is determined on the basis of a proportional load trajectory that involves a monotonic decrease of internal pressure in the cavity. For this particular load history, the elastoplastic boundary grows in a self-similar manner. This allows both displacement and stress field to be calculated in a unit plane, defined by dividing all physical lengths by the characteristic size of the plastic zone. This feature of the solution eliminates the need to solve continuously for the stress and displacement during the evolution of the plastic region. Explicit formulation of the stress field in the plane and of the displacement field in the elastic region is provided; determination of the displacement field in the elastic region is obtained numerically using the method of characteristics. Some important specific results are discussed; a single parameter, the “obliquity” ( m), defined as the ratio between the stress deviator at infinity and the yield limit, is found to control the shape of the failed rock region and the uneven closure of the cavity. It is found also that the direction of maximum closure may become perpendicular to the maximum far-field compressive stress under some loading conditions.

Design charts for a deep circular tunnel under non-hydrostatic loading : St John C.M.; St John^C.M.; Fairhurst C. In: Rock Mechanics in Productivity and Protection (Proceedings of the Twenty-Fifth Symposium on Rock Mechanics, Evanston, Illinois, 25–27 June 1984)P849–856. Publ New York: AIME, 1984

Design charts for a deep circular tunnel under non-hydrostatic loading : St John C.M.; St John^C.M.; Fairhurst C. In: Rock Mechanics in Productivity and Protection (Proceedings of the Twenty-Fifth Symposium on Rock Mechanics, Evanston, Illinois, 25–27 June 1984)P849–856. Publ New York: AIME, 1984

Microstructural effects on wave propagation in solids

Two microstructural phenomena appear important in controlling the evolution of large amplitude stress waves in solids. First is the inherent microstructure of the material which leads to local stress concentration during passage of the wave and influences first order moduli through yielding, fracture, or phase transformation. Second is the evolution of microstructure during stress-wave deformation through heterogeneous yielding and flow. The first phenomenon appears to dominate behavior in stress waves on the order of the material strength while the second phenomenon becomes increasingly important at higher stress levels. Careful stress-wave experiments combined in some cases with pre- and post-shock microstructural analysis illustrate the effects. Studies on certain geological materials have revealed that microstructural effects coupled with nonhydrostatic stresswave loading leads to complicated wave propagation, including wave separation and rarefaction shocks. Further shock-wave work on metals and insulating solids indicates that a heterogeneous dynamic thermal and deformational microstructure controls shock transformation process and leads to unusual and useful changes in physical and mechanical properties.

Effect of Non-Hydrostatic Stress on Chemical Processes During Diagenesis: ABSTRACT

Non-hydrostatic stress facilitates porosity reduction in sandstones at elevated temperatures. Experimental results indicate that chemical equilibrium between fluids and solids in fluid-saturated, porous rocks will not be attained under conditions of directed stress. In rocks in which pore fluid pressures are less than lithostatic (Pf < PL), downward directed stress at grain contacts can produce an inhomogeneous distribution of mineral solubilities. High strain at grain contacts causes higher solubilities of the solids and leads to dissolution (pressure solution), whereas growth may occur at solid/water interfaces with low surface strain. The driving free energy for coupled dissolution and growth reactions under non-hydrostatic loading is proportio al to PL - Pf. Consequently, the magnitude of the End_Page 917------------------------------ departure from solid/fluid equilibrium during diagenesis will increase with depth of burial if the pore fluid pressure is hydrostatic. When mineral/fluid equilibrium is not reached, the mineral assemblage developed may be controlled by kinetic rather than thermodynamic factors. Solid phases not stable at the prevailing temperature and fluid pressure may form if high local solubilities create supersaturation in the bulk fluid with respect to many solids. The growth of minerals which decreases the total supersaturation rapidly is favored. Hence, fast-growing less-stable phases (for example, clays, zeolites, and aragonite) may form or persist at the expense of more stable but more slowly forming phases (i.e., illite, feldspars, and calcite). Consequently, the influence of both kinetic factors and bulk phase equilibrium should be considered in evaluating the genesis of mineral assemblages formed during diagenesis and bur al metamorphism. End_of_Article - Last_Page 918------------

Shear stress enhancement of void compaction

Motivated by the success of the spherical model in predicting the volumetric compaction behavior of both porous rocks and metals to a hydrostatic pressure, we consider the applicability of the spherical model to nonhydrostatic loading conditions. Specifically, the spherical model is used to examine the influence of the presence of a shear stress on the volumetric compression of a porous solid. We first obtain the linear, elastic solution for a hollow sphere subject to homogeneous tractions on the outer boundary. Then, assuming that the matrix material is governed by the Drucker‐Prager yield criterion, we use the elastic solution to derive an analytic expression for the onset of yield in the hollow sphere. The expression for the initial yield surface shows that the presence of a shear stress hastens the onset of yield in the sphere in comparison to a hydrostatic loading condition. This result agrees well with experimental data which shows that, for porous solids, permanent crush‐up begins at a lower mean stress under a nonhydrostatic loading than when the applied loading is a hydrostatic pressure. At this point, due to difficulty in obtaining an analytic solution, we turn to a numerical scheme (finite element method) to extend the analysis of the hollow sphere problem into the elasto‐plastic range. The spherical model results clearly exhibit the experimental finding that the presence of a shear stress tends to enhance the volumetric compaction of porous solids in comparison to a hydrostatic loading condition. For both a porous rock and metal sample, agreement between the spherical model and experimental results is excellent.

Megalencephaly in infants and children. The possible role of increased dural sinus pressure.

Seven children studied because of clinical macrocephaly and suspected hydrocephalus ultimately proved to have megalencephaly apparently due to an increase in sagittal sinus venous pressure as established from infusion studies. Unexplainably, these patients were all males. All were seen initially between 2 and 8 months of age. Head enlargement exceeded two standard deviations in all seven. Pneumoencephalography, ventriculography, or computerized tomography demonstrated normal or minimally enlarged ventricles that did not progress in size. Isotope cisternography was abnormal. Studies of CSF formation and absorption demonstrated normal absorption rates but high calculated sagittal sinus pressures. Though therapy was usually not required, in one unusual infant, severe progressive macrocephaly with minimal hydrocephalus required a shunt. Another had a transient episode of acute hydrocephalus associated with a low CSF absorption rate and ventricular enlargement. In this report, we review the intracranial hydrodynamics of benign intracranial hypertension (BIH), communicating hydrocephalus, and the pathogenesis of megalencephaly. Benign intracranial hypertension and the type of megalencephaly demonstrated by our patients appear to develop similarly except that the presence of open cranial sutures may allow a transient nonhydrostatic loading of brain parenchyma in infants, resulting in mild, nonprogressive macrocephaly.

Employing the theory of plastic consolidation to describe the limiting behavior of a clayey soil

1. A description of the limiting behavior of a clayey soil is permitted within the framework of the theory of plastic consolidation. 2. The existence of a loading surface, the smoothness of the surface, and its normality to the vector of the increase in plastic deformation is confirmed experimentally. 3. The significant effect of the third invariant stress parameter is observed, even under hydrostatic loading. 4. The appearance of significant plastic anisotropy is established under nonhydrostatic loading. 5. As we approach the limiting state, the loading surface shifts, breaking away from the coordinate origin of the space occupied by the principal stresses.

Principles of structural stability of underground salt cavities

The principles of structural stability of underground salt cavities and the significance of the principles as they relate to other cavities are discussed. The theory of plasticity is applied to the evaluation of stress and strain conditions of the salt cavities. The concept of a yielded zone which develops around the cavities is introduced, and a theoretical development of the extent and stress distribution of the zone is illustrated through the use of ideal spherical and cylindrical cavities under uniform triaxial compression. Applicability of the concept to actual conditions, such as cavity irregularities, brittleness of formation, and nonhydrostatic loading, is discussed.