Aves, Peter Charles; (1996) Mechanics of compaction and dilatancy in triaxially stressed rocks, under simulated crustal conditions studied by pore volumometry. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Triaxial and hydrostatic deformation experiments have been conducted on Darley Dale sandstone, Penmaenmawr microgranodiorite and Gypsum, to investigate, (i) compaction and dilatant mechanisms occurring during deformation under a variety of stress conditions, (triaxial and hydrostatic), (ii) the effect of confined fluid flow conditions (undrained), on triaxial deformation characteristics of sandstone, (iii) the effects of varying the Upstream Reservoir Volume - URV, (necessary to measure pore fluid pressure), under undrained conditions on the deformation characteristics of sandstone. The different volumes are analogous to different permeabilities of surrounding (capping) rocks in the crust, since a degree of fluid flow is permitted into the URV depending the URV size. (iv) the effects (mechanical and chemical) of fluid (water) on deformation by varying pore fluid pressure and temperature during suites of drained and undrained triaxial deformation experiments, and,(v) the mechanisms occurring during drained dehydration of gypsum, and during the triaxial deformation of undrained dehydrating gypsum. Results from drained and undrained hydrostatic compaction of Darley Dale sandstone show a smooth concave hydrostat indicating no pore collapse up to a confining pressure of =450MPa. The unloading curves indicate restricted fluid outflow - a consequence of compaction induced low permeability. The elastic and inelastic components of pore volume loss under this pressure range is determined. Mean stress induced compaction is shown to be chiefly elastic below 500MPa confining pressure. Analysis of the data using established poro-elasticity theory shows that from experimental and published rock bulk moduli data, and corrected Skempton B co-efficients (from undrained compaction data), drained pore fluid volume loss under a uniform mean stress can be predicted. Drained compaction results on Penmaenmawr microgranodiorite are also presented. Drained pore volume compaction data is also used in conjunction with pore volume data from triaxial deformation experiments to examine the components of pore volume change attributable to compaction and to dilatancy. The results indicate that under cataclastic flow the majority of compaction is deviatoric stress induced and is largely inelastic. The form of the dilatant pore volume increase curve during brittle faulting deformation at the onset of acoustic emission is found to be exponential, corroborating its connection with acoustic emissions which also initially increase exponentially. Results on drained and undrained triaxial deformation experiments on Darley Dale sandstone indicate that a confined fluid mass environment (low permeability in surrounding/capping rock) causes a variation in effective confining pressure during deformation which can change the deformation mode from brittle faulting failure to cataclastic flow. A large fluid system volume necessary to monitor fluid pressure (the Upstream Reservoir Volume - URV) causes transitional deformation behaviour between that expected under completely undrained conditions and drained conditions. Extrapolation of the URV variation data has allowed an attempt at determination of a DRV correction factor to predict pore fluid pressure change under zero URV. The pore fluid pressure change due to the initial application of differential stress (divided into deviatoric stress and effective mean stress) is presented in three dimensional space. The plot allows compaction as a function of either of these stresses to be viewed. The result is valid only for the suite of tests used, although the method is valid for all conditions, and indeed is used in this study for a large suite of experiments. A suite of drained and undrained triaxial deformation results are analysed in 3-D space of pore fluid volume, deviatoric stress, and effective mean stress. The results show how the effect of the URV on effective and deviatoric stress causes the rock to behave with different apparent poro-elastic properties. The drained surface (plotted independently of undrained data), confirms that deviatoric stress causes compaction (under constant effective mean stress) during cataclastic flow, and dilatancy during brittle faulting failure. Quantification of the surface has implications for rock poro-elastic behaviour prediction under any stress field. The effect of elevated temperature of the deformation on sandstone and microgranodiorite is presented, and a decrease in the brittle/cataclastic transition pressure with temperature of 1MPa/l1C is found. The effect is attributed to thermal cracking of the rock matrix. Finally, results of a study on the factors affecting the dehydration of gypsum is presented. The results indicate a complicated interaction of chemical dehydration, compaction assisted fluid flow, and pressure assisted permeability increase restricting fluid flow, as found by other workers. Disentanglement of this interaction is not attempted since it requires further experimentation. The experimental programme necessitated the modification of a high pressure triaxial deformation cell to allow pore fluid volume and pressure measurements to be made simultaneously with acoustic emissions and axial stress and strain on deforming rock specimens. The modifications, and associated equipment and technique developments are described.

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