Numerical modelling of fault reactivation in carbonate rocks under fluid depletion conditions – 2D generic models with a small isolated fault

This generic 2D elastic-plastic modelling investigated the reactivation of a small isolated and critically-stressed fault in carbonate rocks at a reservoir depth level for fluid depletion and normal-faulting stress conditions. The model properties and boundary conditions are based on field and laboratory experimental data from a carbonate reservoir. The results show that a pore pressure perturbation of −25 MPa by depletion can lead to the reactivation of the fault and parts of the surrounding damage zones, producing normal-faulting downthrows and strain localization. The mechanism triggering fault reactivation in a carbonate field is the increase of shear stresses with pore-pressure reduction, due to the decrease of the absolute horizontal stress, which leads to an expanded Mohr's circle and mechanical failure, consistent with the predictions of previous poroelastic models. Two scenarios for fault and damage-zone permeability development are explored: (1) large permeability enhancement of a sealing fault upon reactivation, and (2) fault and damage zone permeability development governed by effective mean stress. In the first scenario, the fault becomes highly permeable to across- and along-fault fluid transport, removing local pore pressure highs/lows arising from the presence of the initially sealing fault. In the second scenario, reactivation induces small permeability enhancement in the fault and parts of damage zones, followed by small post-reactivation permeability reduction. Such permeability changes do not appear to change the original flow capacity of the fault or modify the fluid flow velocity fields dramatically.     

Adsorption behaviors of Cd^2＋ on Fe2O3/MnO2 and the effects of coexisting ions under alkaline conditions

This study describes the adsorption features of cadmium on Fe2O3 and MnO2 in alkaline saline conditions. The adsorption reached equilibrium in 6 hours under alkaline conditions. The absorption of cadmium on Fe2O3 and MnO2 was consistent with Freundlich absorption isotherms, and the corresponding adsorption capacities were 16.3 and 16.7 mg·g-1, respectively. Moreover, the adsorption quantity of cadmium on Fe2O3 and MnO2 rose with increasing pH from acidic to neutral, and reached the maximum at pH= 9. The coexisting chlorides reduced the adsorption capacity of Fe2O3 and MnO2. The influence intensities of different cations follow the order of CaCl2>>KCl>NaCl. However, the influence of sodium salts on the capacities of Fe2O3 and MnO2 to adsorb cadmium appeared more complicated: the relatively low concentrations of sodium salts could reduce the adsorption capacity; with increasing concentrations of sodium salts, e.g. NaCl and NaNO3. The adsorption capacity decreased continually. Moreover, due to the competition adsorption and precipitation effects, the adsorption capabilities of Na2CO3, NaH2PO4 and Na2HSO4 could also be reduced and cadmium concentrations in the solution were reduced as well.     

Elastoplastic constitutive modelling of soil–structure interfaces under monotonic and cyclic loading

This paper is dedicated to the non-linear numerical modelling of the soil–structure interface. Thus, in a first part, after the presentation of the constitutive model, the soil–structure interface interaction is treated in terms of direct shear test simulations. A strategy for the interface model parameters’ identification is also presented. This strategy is linked to the similitude of soil–structure interface behavior and the soil behavior, regarding the interface surface roughness. In a second part, the performance of the numerical simulations are verified numerically against published results for soil–structure experimental shear tests. Finally, as an application, interface stress paths are studied in axially loaded pile–soil systems and load transfer mechanisms are identified.     

Gravitational instabilities triggered by fluid overpressure and downslope incision – Insights from analytical and analogue modelling

Fluid overpressure at the base of low-permeability strata reduces effective stress, allowing for gravitational sliding of the overlying cover. The force driving sliding is the slope-parallel component of the weight of the cover, whereas the resisting forces are the friction at the base of the cover and the buttressing resistance to shortening, which can be critically reduced by incision at the base of the slope. We developed an analytical model and undertook a series of analogue experiments to better understand the evolution of a sedimentary cover sliding above a low-permeability layer subjected to fluid overpressure. Where a downslope buttress was present, the sliding sheet length decreased with increasing pore-fluid pressure. In the absence of such buttress, the slide's length increased exponentially with increasing pore-fluid pressure. Another important difference dealt with geometry and kinematics. Buttressed slides consisted of one large slope-parallel mass rigidly translated and bounded by downslope thrusts and upslope normal faults. With increasing pore-fluid pressure, the contractional structures propagated upslope. By contrast, non-buttressed slides showed intense strain: deformation started with normal faults forming near the incision, then propagating upslope throughout the slide's evolution.     

Long-term spatio-temporal hydrochemical and 222Rn tracing to investigate groundwater flow and water–rock interaction in the Gran Sasso (central Italy) carbonate aquifer

In the Gran Sasso fissured carbonate aquifer (central Italy), a long-term (2001–2007) spatio-temporal hydrochemical and ²²²Rn tracing survey was performed with the goal to investigate groundwater flow and water–rock interaction. Analyses of the physico-chemical parameters, and comparisons of multichemical and characteristic ratios in space and time, and subsequent statistical analyses, permitted a characterisation of the hydrogeology. At the regional scale, groundwater flows from recharge areas to the springs located at the aquifer boundaries, with a gradual increase of mineralisation and temperature along its flowpaths. However, the parameters of each group of springs may significantly deviate from the regional trend owing to fast flows and to the geological setting of the discharge spring areas, as corroborated by statistical data. Along regional flowpaths, the effects of seasonal recharge and lowering of the water table clearly cause changes in ion concentrations over time. This conceptual model was validated by an analysis of the ²²²Rn content in groundwater. ²²²Rn content, for which temporal variability depends on seasonal fluctuations of the water table, local lithology and the fracture network at the spring discharge areas, was considered as a tracer of the final stages of groundwater flowpaths.     

Atlanta’s urban heat island under extreme heat conditions and potential mitigation strategies

The urban heat island (UHI), together with summertime heat waves, foster’s biophysical hazards such as heat stress, air pollution, and associated public health problems. Mitigation strategies such as increased vegetative cover and higher albedo surface materials have been proposed. Atlanta, Georgia, is often affected by extreme heat, and has recently been investigated to better understand its heat island and related weather modifications. The objectives of this research were to (1) characterize temporal variations in the magnitude of UHI around Metro Atlanta area, (2) identify climatological attributes of the UHI under extremely high temperature conditions during Atlanta’s summer (June, July, and August) period, and (3) conduct theoretical numerical simulations to quantify the first-order effects of proposed mitigation strategies. Over the period 1984–2007, the climatological mean UHI magnitude for Atlanta-Athens and Athens-Monticello was 1.31 and 1.71°C, respectively. There were statistically significant minimum temperature trends of 0.70°C per decade at Athens and −1.79°C per decade at Monticello while Atlanta’s minimum temperature remained unchanged. The largest (smallest) UHI magnitudes were in spring (summer) and may be coupled to cloud-radiative cycles. Heat waves in Atlanta occurred during 50% of the years spanning 1984–2007 and were exclusively summertime phenomena. The mean number of heat wave events in Atlanta during a given heat wave year was 1.83. On average, Atlanta heat waves lasted 14.18 days, although there was quite a bit of variability (standard deviation of 9.89). The mean maximum temperature during Atlanta’s heat waves was 35.85°C. The Atlanta-Athens UHI was not statistically larger during a heat wave although the Atlanta-Monticello UHI was. Model simulations captured daytime and nocturnal UHIs under heat wave conditions. Sensitivity results suggested that a 100% increase in Atlanta’s surface vegetation or a tripling of its albedo effectively reduced UHI surface temperature. However, from a mitigation and technological standpoint, there is low feasibility of tripling albedo in the foreseeable future. Increased vegetation seems to be a more likely choice for mitigating surface temperature.     

Deterministic and Probabilistic Stability Analysis of Soil Slope – A Case Study

This paper deals with the soil slope stability analysis along the road section (National Highway-05) near Luhri area, Himachal Pradesh, India. The area constantly experiences local as well as regional slides which enhances the siltation, reduce carrying capacity of the Sutlej river with time and further increases the water level on the banks of the slope. High traffic congestion puts the area at high risk zone and therefore, stability analysis was done using probabilistic as well as deterministic approach under static conditions through limit equilibrium method (LEM) and finite element method (FEM). FEM was used for stability analysis, mainly to obtain deformational characteristics, under static conditions and compared with LEM results. The results of both the methods help in identifying the zones of vulnerability in the case study. Further, the values of safety factor obtained from both the methods are in close approximation. The study also reveals that the slope is critically stable and a remedial measure has been suggested by modifying the geometry of the slope, which needs proper protection from any external factors like rainfall and earthquakes that may cause unforeseeable circumstances.     

Mechanical properties and permeability evolution in gas-bearing coal–rock combination body under triaxial conditions

With the development of deep mining in recent years, coal and gas compound dynamic disasters become increasingly serious. In this study, uniaxial and triaxial compression tests were conducted on gas-bearing coals, coal–sandstone combined bodies and coal–mudstone combined bodies and the permeabilities in the triaxial tests were measured simultaneously. The mechanical behavior and seepage characteristics of coals and coal–rock combination bodies under triaxial conditions were compared in details. The results show that the peak strength among three samples is: coal–sandstone combined body > coal–mudstone combined body > coal. If other conditions were held constant, the strength and the elastic modulus of all specimens show that tendency increases with the increment of the confining pressure or with the decrease in the gas pressure. The strength characteristics of all three specimens met the Mohr–Coulomb criterion, and the residual strength has an increasing trend with the increase in confining pressure. The permeability evolutions of gas-bearing coals and coal–rock combination bodies which are determined by the crack propagation in the coals and rocks are not exactly the same. This preliminary study is intended to deepen our understandings of the mechanisms of coal–gas compound dynamic disasters and provide theoretical bases for their predictions.     

Grain-size-sensitive creep of plagioclase accompanied by solution–precipitation and mass transfer under mid-crustal conditions

We report here on a study of three deformed granitoids: two mylonites and an ultramylonite from the inner ductile shear zone of the Ryoke metamorphic belt, SW Japan. Monophase layers composed of quartz, plagioclase or K-feldspar are present in all samples. The plagioclase-rich layers consist of grains 6–10 μm in size, and sometimes include patchy K-feldspar and quartz, indicating solution-precipitation. In the mylonite, the fine-grained plagioclase is mainly An_23–25 and, the composition of plagioclase porphyroclast is An_21–39 without any significant maximum. The An compositions together with textural observations indicate that fine-grained plagioclase nucleated from solution with mass transfer during deformation. In the ultramylonite, fine-grained plagioclase is widely changed to be An_15–37, indicating that the grain-size-reduction process includes fracturing of original plagioclase porphyroclasts in addition to the solution–precipitation process, which results in the composition concentrated around An₃₀. In all samples, the crystallographic orientations of fine-grained plagioclases are almost random and do not correlate with neighbouring porphyroclasts. Grain-size-sensitive creep occurred during rock deformation subsequent to the process of solution–precipitation that involved mass transfer via fluids.     

Mechanical properties and stress–dilatancy relationships of unsaturated soil under various cyclic loading conditions

Acta Geotechnica - Most studies investigating the effect of cyclic loading on soil properties have been conducted for saturated soils. Embankments such as fill dams, roads and railways are usually...     

Simultaneous transport of water and solutes under transient unsaturated flow conditions — A case study

The imbalance between incoming and outgoing salt causes salinization of soils and sub-soils that result in increasing the salinity of stream-flows and agriculture land. This salinization is a serious environmental hazard particularly in semi-arid and arid lands. In order to estimate the magnitude of the hazard posed by salinity, it is important to understand and identify the processes that control salt movement from the soil surface through the root zone to the ground water and stream flows. In the present study, Malaprabha sub-basin (up to dam site) has been selected which has two distinct climatic zones, sub-humid (upstream of Khanapur) and semi-arid region (downstream of Khanapur). In the upstream, both surface and ground waters are used for irrigation, whereas in the downstream mostly groundwater is used. Both soils and ground waters are more saline in downstream parts of the study area. In this study we characterized the soil salinity and groundwater quality in both areas. An attempt is also made to model the distribution of potassium concentration in the soil profile in response to varying irrigation conditions using the SWIM (Soil-Water Infiltration and Movement) model. Fair agreement was obtained between predicted and measured results indicating the applicability of the model.     

Zircon–quartz–calcite segregations in carbonate–alkaline metasomatic rocks of the western Baikal region and their petrogenetic implications

Fine-grained segregations up to 5 mm in size composed of graphic intergrowths of zircon, quartz, calcite and containing up to 0.8 wt % SrO have been found in albite–riebeckite and dolomite–biotite metasomatic rocks formed after alaskite granite. They contain magnetite, titanomagnetite (25.4 wt % TiO₂), cerite-(Ce,Nd), rutile (up to 1.2 wt % Nb₂O₅), as well as rare micrograins of monazite-(Ce), bastnaesite-(Ce), and barite (up to 5.7 wt % SrO). The fine-grained structure of mineral aggregates suggests a metacolloidal nature. It is assumed that the zircon–quartz–calcite assemblage was formed due to exchange decomposition reaction between the salt phase of hydrothermal solution with predominant Na₂CO₃, elevated Zr and, to a lesser extent, Fe, Ti, LREE, Nb contents and dissolved calcium and silica compounds of a Na₂SiO₃ type.     

Mesozoic paleogeography and paleoclimates – A discussion of the diverse greenhouse and hothouse conditions of an alien world

The Mesozoic was the time of the break-up of Pangaea, with profound consequences not only for the paleocontinental configuration, but also for paleoclimates and for the evolution of life. Cool greenhouse conditions alternated with warm greenhouse and even hothouse conditions, with global average temperatures around 6–9 °C warmer than the present ones. There are only sparse and controversial evidence for polar ice; meanwhile, extensive evaporitic and desertic deposits are well described. Global sea levels were mainly high, and the content of atmospheric O₂ was varying between 15 and 25%. These conditions make the Mesozoic Earth an alien world compared to present-day conditions. Degassing from volcanism linked to the rifting process of Pangaea and methane emissions from reptilian biotas were climate-controlling factors because they enhanced atmospheric CO₂ concentrations up to 16 times compared to present-day levels. The continental break-up modified paleopositions and shoreline configurations of the landmasses, generating huge epicontinental seas and altering profoundly the oceanic circulation. The Mesozoic was also a time of important impact events as probable triggers for “impact winters”; and for the Era at least nine huge (diameter > 20 km) impact structures are known. This paper presents an abridged but updated overview of the Mesozoic paleogeographic and paleoclimatic variations, characterizing each period and sub-period in terms of paleoclimatic state and main tectonic and climatic events, and provides a brief geologic, stratigraphic, paleoclimatic and taphonomic characterization of dinosaur occurrences as recorded in the Brazilian continental basins.     

Impact assessment of a saline waste lagoon on groundwater pollution near a coastal aquifer in India—synthesis of geoelectrical and hydrochemical studies

The present study is an attempt to assess the impact of a saline waste lagoon on the near subsurface through electrical resistivity tomography (ERT). Resistivity and IP imaging data have been collected on profiles close to the lagoon and at a far off location (control location). Water samples have been collected from the lagoon and a municipality drinking well close to it and analysed for the water chemistry. The geoelectrical sections indicate very low resistivity formations in the near subsurface in the vicinity of the lagoon as compared to the control profile. The water chemistry data from the monitoring well close to the profile also indicates very high total dissolved solids (8658 mg/L) and qualitatively supports the contamination of the near subsurface. The conductive formations in the vicinity of the lagoon can be attributed to the overflow from the lagoon or the seepage.     

Learning soil parameters and updating geotechnical reliability estimates under spatial variability – theory and application to shallow foundations

ABSTRACT

Field data is commonly used to determine soil parameters for geotechnical analysis. Bayesian analysis allows combining field data with other information on soil parameters in a consistent manner. We show that the spatial variability of the soil properties and the associated measurements can be captured through two different modelling approaches. In the first approach, a single random variable (RV) represents the soil property within the area of interest, while the second approach models the spatial variability explicitly with a random field (RF). We apply the Bayesian concept exemplarily to the reliability assessment of a shallow foundation in a silty soil with spatially variable data. We show that the simpler RV approach is applicable in cases where the measurements do not influence the correlation structure of the soil property at the vicinity of the foundation. In other cases, it is expected to underestimate the reliability, and a RF model is required to obtain accurate results.     

Coal Bed Methane potentiality — Case studies from Umaria,Korba and Ib-valley coals,Son-Mahanadi Basin

The Coal Bed Methane (CBM), a dreadful mining hazard, became eco-friendly clean gas for the third world country when humanity is facing challenges against pollution and energy crisis. It is observed from the studies that the Korba and Ib valley coals have potential for CBM exploration whereas Umaria coals have negligible potential. Though, the Sohagpur coalfields (east of Umaria coalfield) have ample scope of CBM exploration.