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.     

A sediment record of trophic state change in an Arkansas (USA) reservoir

Reservoir sediments are used cautiously in paleolimnological studies because of dating uncertainties, possible sediment disturbances and even concerns that indicators of trophic status may behave differently in reservoirs as opposed to natural lakes. We measured loss on ignition (LOI), carbon to nitrogen ratio (C:N), diatom abundance, total nitrogen (TN), total phosphorus (TP), TN:TP ratio, and carbon and nitrogen isotopes (δ¹³C and δ¹⁵N) in an 83-cm sediment core to track recent trophic status changes in Beaver Reservoir, Northwest Arkansas, USA. Measurements showed that LOI, TN, TP and diatom abundance increased significantly from the bottom to the top of the core (p < 0.001). The C:N ratio and δ¹³C indicated a predominantly algal source for organic matter in the sediments. Increases in TN and TP were positively correlated with human population growth (p < 0.01) and the TN:TP ratio recorded a shift from phosphorus to nitrogen limitation around 1990. This shift may have encouraged cyanobacterial growth that caused episodes of taste and odor problems in the reservoir. This study suggests that despite concerns about sediment dating and disturbance, reservoir sediments can provide valuable information on past water quality changes.     

Late Quaternary (24–10 ka BP) environmental history of the Neotropical lowlands inferred from ostracodes in sediments of Lago Petén Itzá, Guatemala

We inferred late Pleistocene and early Holocene (24–10 ka BP) environmental conditions in and around Lago Petén Itzá, Guatemala from ostracode remains in the lake sediments. Multivariate statistics were run on autecological information for 29 extant ostracode species collected in 63 aquatic ecosystems on the Yucatán Peninsula along a steep, increasing NW–S precipitation gradient and across a large altitudinal range. Conductivity and water depth are the most important factors that shape ostracode communities. Transfer functions were developed and applied to fossil ostracode assemblages in a ~76-m sediment core (PI-6, ~85 ka) taken in 71 m of water from Lago Petén Itzá, to infer past shifts in conductivity and water level. Results suggest climate was cold and wet during the Last Glacial Maximum (LGM). Alternating dry and wet conditions characterized the deglacial. Early Holocene climate was warmer and wetter. The LGM was characterized by low ostracode species richness (4 spp.) and abundance (<940 valves g⁻¹), dominance of benthic over nektobenthic taxa, abundant Physocypria globula, conductivity as low as 190 μS cm⁻¹, and clay-rich sediments with relatively high total organic carbon and low C/N ratios (<14), suggesting relatively deeper water at the core site associated with abundant precipitation. Greatest water depth at the core site during the LGM occurred late in the period and was ~50 m. The deglacial was characterized by drier conditions, higher ostracode species richness (6 spp.) and abundances up to 18,115 valves g⁻¹, dominance of nektobenthic species, and presence of shallow-water and littoral-zone indicators such as Heterocypris punctata and Strandesia intrepida, conductivity up to 550 μS cm⁻¹, C/N ratios as high as 37, and gypsum deposition. Lowest inferred lake depth at the core site during the deglacial was ~20 m. The early Holocene was characterized by high numbers of ostracode remains, up to 25,500 valves g⁻¹, and the presence of L. opesta and P. globula. Cytheridella ilosvayi was absent from late Pleistocene sediments, suggesting it colonized northern Central America during the Holocene.     

The DRASTIC-Sg model: an extension to the DRASTIC approach for mapping groundwater vulnerability in aquifers subject to differential land subsidence,with application to Mexico City

Mexico City relies on groundwater for most of its domestic supply. Over the years, intensive pumping has led to significant drawdowns and, subsequently, to severe land subsidence. Tensile cracks have also developed or reactivated as a result. All such processes cause damage to urban infrastructure, increasing the risk of spills and favoring contaminant propagation into the aquifer. The effects of ground deformation are frequently ignored in groundwater vulnerability studies, but can be relevant in subsiding cities. This report presents an extension to the DRASTIC methodology, named DRASTIC-Sg, which focuses on evaluating groundwater vulnerability in urban aquifers affected by differential subsidence. A subsidence parameter is developed to represent the ground deformation gradient (Sg), and then used to depict areas where damage risk to urban infrastructure is higher due to fracture propagation. Space-geodetic SqueeSAR data and global positioning system (GPS) validation were used to evaluate subsidence rates and gradients, integrating hydrogeological and geomechanical variables into a GIS environment. Results show that classic DRASTIC approaches may underestimate groundwater vulnerability in settings such as the one at hand. Hence, it is concluded that the Sg parameter is a welcome contribution to develop reliable vulnerability assessments in subsiding basins.     

Simultaneous Estimation of Geologic and Reservoir State Variables Within an Ensemble-Based Multiple-Point Statistic Framework

Assessment of uncertainty due to inadequate data and imperfect geological knowledge is an essential aspect of the subsurface model building process. In this work, a novel methodology for characterizing complex geological structures is presented that integrates dynamic data. The procedure results in the assessment of uncertainty associated with the predictions of flow and transport. The methodology is an extension of a previously developed pattern search-based inverse method that models the spatial variation in flow parameters by searching for patterns in an ensemble of reservoir models. More specifically, the pattern-searching algorithm is extended in two directions: (1) state values (such as piezometric head) and parameters (such as conductivities) are simultaneously and sequentially estimated, which implies that real-time assimilation of dynamic data is possible as in ensemble filtering approaches; and (2) both the estimated parameter and state variables are considered when pattern searching is implemented. The new scheme results in two main advantages—better characterization of parameters, especially for delineating small scale features, and an ensemble of head states that can be used to update the parameter field using the dynamic data at the next instant, without running expensive flow simulations. An efficient algorithm for pattern search is developed, which works with a flexible search radius and can be optimized for the estimation of either large- or small-scale structures. Synthetic examples are employed to demonstrate the effectiveness and robustness of the proposed approach.     

Impact lithologies and their emplacement in the Chicxulub impact crater: Initial results from the Chicxulub Scientific Drilling Project,Yaxcopoil, Mexico

Abstract— The Chicxulub Scientific Drilling Project (CSDP), Mexico, produced a continuous core of material from depths of 404 to 1511 m in the Yaxcopoil‐1 (Yax‐1) borehole, revealing (top to bottom) Tertiary marine sediments, polymict breccias, an impact melt unit, and one or more blocks of Cretaceous target sediments that are crosscut with impact‐generated dikes, in a region that lies between the peak ring and final crater rim. The impact melt and breccias in the Yax‐1 borehole are 100 m thick, which is approximately 1/5 the thickness of breccias and melts exposed in the Yucatán‐6 exploration hole, which is also thought to be located between the peak ring and final rim of the Chicxulub crater. The sequence and composition of impact melts and breccias are grossly similar to those in the Yucatán‐6 hole. Compared to breccias in other impact craters, the Chicxulub breccias are incredibly rich in silicate melt fragments (up to 84% versus 30 to 50%, for example, in the Ries). The melt in the Yax‐1 hole was produced largely from the silicate basement lithologies that lie beneath a 3 km‐ thick carbonate platform in the target area. Small amounts of immiscible molten carbonate were ejected with the silicate melt, and clastic carbonate often forms the matrix of the polymict breccias. The melt unit appears to have been deposited while molten but brecciated after solidification. The melt fragments in the polymict breccias appear to have solidified in flight, before deposition, and fractured during transport and deposition.     

Predation has no competition: factors influencing space and resource use by echinoids in deep‐sea coral habitats,as evidenced by continuous video transects

Predation and competition are highly influential factors determining space use in foraging animals, and ultimately contribute to the spatial heterogeneity observed within habitats. Here we investigated the influence of competition and predation on space and resource use via continuous video transect observations – a tool that has not previously been employed for this purpose. This study therefore also evaluates video data as a pragmatic tool to study community interactions in the deep sea. Observations were compiled from 15 video transects spanning five submarine canyons in the Bay of Biscay, France. Substrate choice, positioning on the coral, echinoid aggregate size, and the presence/absence of predators (e.g. fish and decapods) as well as competitors (both inter‐ and intra‐specific) were recorded. Two dominant co‐existing echinoid taxa, echinothurids and Cidaris cidaris (3188 total observations), were observed in the study. For the echinothurids, no significant trends were detected in the inter‐ and intra‐specific competition data. For Cidaris cidaris, significant shifts in substrate use were correlated to the presence of inter‐specific competitors (echinothurids), whereby an increase in dead coral substrate usage was observed. Highly significant patterns were detected amongst echinoids near fish and decapod predators. A shift to the base of the coral infra‐structure was correlated to the presence of fish, and fewer individuals were observed in the open areas of the reef and a greater number were found in the mid and top sections of the coral when in the presence of decapods. Aggregates formed irrespective of the presence of predators. Aggregations are likely to form for feeding and reproduction rather than for defensive purposes; and migration along the coral infra‐structure may be a predator‐driven behaviour as echinoids seek refuge from predators. Predation risk might play a stronger – or more detectable – role in structuring echinoid space and resource use in deep‐sea coral habitats. In addition, the study successfully detected patterns in the video data thereby demonstrating its potential usefulness for similar ecological studies on other deep‐dwelling megabenthos.     

A random solid‐porous model to simulate the retention curves of soils

This paper describes a random solid‐porous model capable of simulating the structure of porous materials. To this purpose, the grain and pore size distributions as well as the void ratio of the material are required. Solids and pores are distributed at random in the model's space according to a size strategy. Herein, the model is used to simulate the retention curves of soils. The Laplace equation is used to determine the size of the pores able to saturate or dry during a wetting or drying process, respectively. The continuous path principle is used to define those elements that effectively saturate or dry during these processes. With this procedure, it is possible to simulate the main retention curves as well as the scanning curves during wetting–drying cycles. Some experimental results reported in the international literature have been used to test the model. This model can be enhanced to study the mechanical behavior of unsaturated soils. Copyright © 2012 John Wiley & Sons, Ltd.     

Holocene laminated sediments from the southern Gulf of California: geochemical,mineral magnetic and microfossil study

Geochemical, magnetic mineral analyses and microfossils (radiolarians) are used to characterise a Holocene laminated sequence from the Alfonso Basin, northern Bay of La Paz, southern Gulf of California. In most cases, dark and light laminae show similar bulk chemical compositions and magnetic mineral properties. However, dark laminae contain relatively more terrigenous elements (Al, Si, Fe, K and Mg) supporting a pluvial origin related to the volcanic siliceous tuffs around the Bay of La Paz. In light laminae, calcium content is higher, representing biogenic input. The magnetic signal is dominated by low coercivity, fine‐grained low‐titanium titanomagnetites and magnetite. Hysteresis ratio parameters in domain state plots show that samples fall in the pseudo‐single domain field, indicating mixtures of single‐ and multi‐domain particles. Variations in hysteresis saturation magnetisation parameters for dark and light laminae reflect changes in relative mineral concentration. Results indicate a dominant volcanic source for magnetic minerals into the basin, associated with variable pluvial and/or aeolian transport. Low radiolarian abundance in dark laminae seems to be associated with larger penetration and longer residence time of oligotrophic subtropical waters into the bay that occurred during El Niño–Southern Oscillation (ENSO) events. Light laminae are the result of episodic pulses of high productivity. We suggest that dark/light laminae result from oceanographic and climatic cyclic processes, with main terrigenous input variation possibly associated with ENSO forcing of pluvial influx, and/or latitudinal migration of the Intertropical Convergence Zone. The laminated sequence of the Alfonso Basin formed a non‐annual depositional system, in contrast to the varved sediments characteristic of the central Gulf of California basins. Copyright © 2010 John Wiley & Sons, Ltd.