Mapping the state of fracture around cavities

The state of stress around cavities is heterogeneous. Consequently, the state of fracture may also vary from point to point. Under compressive loading, cavity may be in one of four possible states: pre-fracture (pre-microfracture initiation), microfracture propagation (initiation to the onset of dilata dilatancy to failure) and post-failure. These four states are separated by the crack initiation stress, the crack damage stress and the failure stress. fracture events with confining pressure is examined with reference to three intact rocks: a brittle granite, a semi-brittle limestone and a ductile sal

The maximum principal stress (σ₁) at crack initiation, crack damage (onset of dilatancy), yielding and failure are established as a function stress (σ₃). For a single intact rock, all four fracture events can be represented using one function (the Rocker function) with a single fac various fracture states.

The proposed fracture criteria, based on the experimental data, are combined with the existing state of stress to prepare a fracture map around an elli intact Lac du Bonnet granite. The state of stability is expressed through a newly defined stability factor, the unconfined strength ratio (USR), wh to the traditional safety factor. In contrast to the conventional safety factor in rock mechanics (SFstrength/σ₁), which is σ₃ 3–σ₁, space of the stability diagram.     

A semi‐empirical method for the liquefaction analysis of offshore foundations

This paper presents a relatively simple method for three‐dimensional liquefaction analysis of granular soil under offshore foundations. In this method, the Mohr–Coulomb model, which defines the elasto–plastic stress–strain relationship under monotonic loading, is modified to accommodate the plastic strains generated by cyclic loading. The effects of cyclic loading, evaluated from the results of laboratory tests on saturated samples of soil, are incorporated into the model. The method is implemented in an efficient finite element program for analyses of three‐dimensional consolidating soil. The practicability of the model is demonstrated by analysis of a typical offshore foundation, and the predictions of the numerical analysis are compared with the observed behaviour of the foundation. Copyright © 2000 John Wiley & Sons, Ltd.     

Calculations of upper-mantle P-wave velocity anisotropies

Discovery of anisotropy for upper-mantle P-wave velocities has resulted recently in many laboratory determinations of velocities for natural mantle material, either by direct measurement on rock samples or through calculations based on single-crystal elastic constants, fabrics and modal compositions Comparison of field and laboratory data provides gross constraints for convection processes and flow mechanisms. As natural peridotites yeidl a wide range of maximum seismic anisotropies, their depth of origin and deformation history are inferred from pyroxene geobarometry and geothermometry, and from stress estimates derived from neoblast grain sizes. Large maximum anisotropies (ca. 1.0 km/s) are typical of samples with fabrics strengthened by low-temperature/highstress deformation related to emplacement. Other highly anisotropic mantle material includes peridotites beneath continental rifts and thinned continental crust. Maximum anisotropies for ordinary suboceanic and infracontinental peridotites are generally in the range of the field data (0.3–0.6 km/s) and support a model with [010]_ol nearly vertical (in accord with observations on ophiolites) and [100]_ol subparallel to the spreading direction. On the basis of fabric data, seismic anisotropy should decrease with depth.     

The vourinos ophiolite complex, Greece: The tectonite suite

Classical structural analysis, in combination with new techniques including pyroxene thermobarometry and recrystallized-grain- and subgrain-size paleopiezometry, has been used to deduce the tectonic history of the Vourinos Ophiolite Complex, Greece. Results can be used to infer the variation with depth of differential stress in the upper mantle and indicate that these rocks were subjected to an initial stress level of 10–100 bar over a depth interval from 100 to 40 km, respectively. Subsequently, they underwent mylonitization, associated with stresses of about 2500 bar at 30 km depth. Rocks equilibrated at the deepest levels commonly have the highest structural positions within the ultramafic tectonites and also show appreciable Al depletion. Lack of annealing recrystallization in the mylonitic rocks, together with low stress levels and an abnormally high pyroxene geotherm for the Complex lead to the interpretation of the Complex being the remnant of a mantle diapir, with an overlying magma chamber, located at a spreading ridge.     

The record of Himalayan erosion preserved in the sedimentary rocks of the Hatia Trough of the Bengal Basin and the Chittagong Hill Tracts,Bangladesh

The Cenozoic sedimentary succession of Bangladesh provides an archive of Himalayan erosion. However, its potential as an archive is currently hampered by a poor lithostratigaphic framework with limited age control. We focus on the Hatia Trough of the Bengal Basin and the adjacent fold belt of the Chittagong Hill Tracts which forms the outermost part of the west‐propagating Indo‐Burmese wedge. We present a basin‐wide seismic stratigraphic framework for the Neogene rocks, calibrated by biostratigraphy, which divides the succession into three seismically distinct and regionally correlatable Megasequences (MS). MS1 extends to NN15‐NN16 (ca. 2.5–3.9 Ma), MS2 to NN19‐NN20 (ca. 0.4–1.9 Ma) and MS3 to present day. Our seismic mapping, thermochronological analyses of detrital mineral grains, isotopic analyses of bulk rock, heavy mineral and petrographic data, show that the Neogene rocks of the Hatia Trough and Chittagong Hill Tracts are predominantly Himalayan‐derived, with a subordinate arc‐derived input possibly from the Paleogene IndoBurman Ranges as well as the Trans‐Himalaya. Our seismic data allow us to concur with previous work that suggests folding of the outer part of the west‐propagating wedge only commenced recently, within the last few million years. We suggest that it could have been the westward encroachment and final abutment of the Chittagong Hill Tracts fold belt onto the already‐uplifted Shillong Plateau that caused diversion of the palaeo‐Brahmaputra to the west of the plateau as the north‐east drainage route closed.     

Late Cretaceous India–Madagascar fit and timing of break-up related magmatism

A U–Pb zircon age of 91.2 ± 0.2 Myr from western India (St. Mary islands) confidently links India with the Late Cretaceous magmatic province in Madagascar (≈ 84–92 Ma), and the U–Pb age is within analytical error of the U–Pb age of the Analalava gabbro pluton (91.6 ± 0.3 Myr) in northeastern Madagascar. Palaeomagnetic data from India and Madagascar allow us to postulate a new India–Madagascar fit (Euler latitude = 14.24°, longitude = 38.8° and rotation angle = –69.2°). This fit is applicable to the Late Cretaceous, directly prior to and during the early phase of Madagascar–India separation. In our Late Cretaceous reconstruction, south-west India runs roughly subparallel with the first known break-up related magnetic anomaly (A34); it maintains a close connection between Mada-gascar and India, but places India slightly rotated compared to the eastern margin of Madagascar and more northerly compared with some reconstructions. St. Mary magmatism is linked to the initial break-up between India and Madagascar, and magmatism probably resulted from rift-related extensional processes initially induced by the Marion hotspot underlying southern Madagascar during the Late Cretaceous.     

Three‐dimensional finite elements for the analysis of soil contamination using a multiple‐porosity approach

A three‐dimensional finite‐element model of contaminant migration in fissured clays or contaminated sand which includes multiple sources of non‐equilibrium processes is proposed. The conceptual framework can accommodate a regular network of fissures in 1D, 2D or 3D and immobile solutions in the macro‐pores of aggregated topsoils, as well as non‐equilibrium sorption. A Galerkin weighted‐residual statement for the three‐dimensional form of the equations in the Laplace domain is formulated. Equations are discretized using linear and quadratic prism elements. The system of algebraic equations is solved in the Laplace domain and solution is inverted to the time domain numerically. The model is validated and its scope is illustrated through the analysis of three problems: a waste repository deeply buried in fissured clay, a storage tank leaking into sand and a sanitary landfill leaching into fissured clay over a sand aquifer. Copyright © 2005 John Wiley & Sons, Ltd.