Axial service limit state analysis of drilled shafts using probabilistic approach

Drilled shafts are, typically, designed by considering the axial ultimate limit state. In this design methodology, the axial displacement requirements are verified once the design is completed. As an alternative, drilled shafts may be designed by considering the axial service limit state. Service limit state foundation design is more efficient when done using the load and resistance factor design (LRFD) approach. Furthermore, reliability may be rationally incorporated into the design process that utilizes the LRFD method. In this paper, we develop probabilistic approaches for axial service limit state analysis of drilled shafts. The variability of shaft-soil interface properties is modeled by lognormal probability distribution functions. The probability distributions are combined with a closed-form analytical relationship of axial load-displacement curves for drilled shafts. The closed-form analytical relationship is derived based upon the “t–z” approach. This analytical relationship is used with the Monte Carlo simulation method to obtain probabilistic load-displacement curves, which are analyzed to develop methods for determining the probability of drilled shaft failure at the service limit state. The developed method may be utilized to obtain resistance factors that can be applied to LRFD based service limit state design.     

Modelling fundamental waiting time distributions for earthquake sequences

The distribution of waiting times between time-neighbouring events for a time series obeying the Omori law is examined theoretically and numerically with the aim of understanding the characteristics of these distributions, how these characteristics change (e.g. scale) with the parameters of the Omori series, and thus how empirical waiting time data may be correctly interpreted. It is found that the waiting time distribution, for a single Omori aftershock sequence, consists in general of two power law segments followed by a rapid decay at larger waiting times. The analyses are illustrated using real data from the SIL network on Iceland. This data often shows characteristics predominantly consistent with the Omori law, but there are significant exceptions. We conclude that waiting time distributions and related statistical analysis has meaningful potential for the analysis of earthquake data sets, as a step towards developing physical models of the earthquake process.     

Recent rapid salinity rise in three East Antarctic lakes

Research in East Antarctica has shown several recent environmental changes that may be linked to human impacts on climate. In order to detect the influence and context of these changes on coastal aquatic ecosystems we examined lake sediment cores from three lakes in the Windmill Islands, East Antarctica; Beall Lake, Holl Lake and ȁ8Lake Mȁ9. Cores were sectioned at␣2.5 mm intervals. Their diatom species composition was examined to detect changes in lake salinity using a diatom-salinity transfer function, and their algal pigment content was examined to detect photoautotrophic community responses to environmental change. Results showed that Holl Lake originated in a depression exposed by Holocene recession of the continental ice sheet and that Beall Lake and Lake M originated as isolated marine basins formed by changes in relative sea level. A general late Holocene trend of declining lake salinity was evident in all three lakes, interrupted by one short-term high salinity event in Beall Lake. This is consistent with a long-term positive moisture balance. This general decline in salinity has been followed by a remarkable recent rapid increase in salinity in all three lakes in the last few decades. We speculate that this rapid increase in salinity might be linked to changes taking place in the region including feedbacks resulting from decreasing sea ice extent as recorded in the nearby Law Dome ice core, and positive feedbacks in the catchments whereby reduced snow cover has led to decreased albedo, which in turn has caused increased evaporation and sublimation. Collectively these changes have shifted the lakes across a threshold from positive to negative moisture balance. A minor, but not rapid shift in the abundance of diatom pigments relative to pigments from green algae and cyanobacteria was also detected suggesting that some changes in photoautotrophic community composition have occurred. Measurements of modern nutrient levels are also higher than would be expected in Beall Lake and Holl Lake, given the extremely low sediment accumulation rates. This may be associated with a c. 300% increase in the population of Adélie penguins in the Windmill Islands recorded since the 1950s, or may a first signs of a rapid increase in catchment development and associated lake productivity as experienced in Antarctic and Arctic lakes subject to recent rapid regional warming. The most marked feature of the records is the rapid increase in salinity in all three lakes in␣the last few decades, which has occurred in lakes both with and without resident penguin populations.Dominic A. Hodgson and Donna Roberts contributed equally to this work     

Multiscale divergence between Landsat- and lidar-based biomass mapping is related to regional variation in canopy cover and composition

Background

Satellite-based aboveground forest biomass maps commonly form the basis of forest biomass and carbon stock mapping and monitoring, but biomass maps likely vary in performance by region and as a function of spatial scale of aggregation. Assessing such variability is not possible with spatially-sparse vegetation plot networks. In the current study, our objective was to determine whether high-resolution lidar-based and moderate-resolution Landsat-base aboveground live forest biomass maps converged on similar predictions at stand- to landscape-levels (10 s to 100 s ha) and whether such differences depended on biophysical setting. Specifically, we examined deviations between lidar- and Landsat-based biomass mapping methods across scales and ecoregions using a measure of error (normalized root mean square deviation), a measure of the unsystematic deviations, or noise (Pearson correlation coefficient), and two measures related to systematic deviations, or biases (intercept and slope of a regression between the two sets of predictions).

Results

Compared to forest inventory data (0.81-ha aggregate-level), lidar and Landsat-based mean biomass predictions exhibited similar performance, though lidar predictions exhibited less normalized root mean square deviation than Landsat when compared with the reference plot data. Across aggregate-levels, the intercepts and slopes of regression equations describing the relationships between lidar- and Landsat-based biomass predictions stabilized (i.e., little additional change with increasing area of aggregates) at aggregate-levels between 10 and 100 ha, suggesting a consistent relationship between the two maps at landscape-scales. Differences between lidar- and Landsat-based biomass maps varied as a function of forest canopy heterogeneity and composition, with systematic deviations (regression intercepts) increasing with mean canopy cover and hardwood proportion within forests and correlations decreasing with hardwood proportion.

Conclusions

Deviations between lidar- and Landsat-based maps indicated that satellite-based approaches may represent general gradients in forest biomass. Ecoregion impacted deviations between lidar and Landsat biomass maps, highlighting the importance of biophysical setting in determining biomass map performance across aggregate scales. Therefore, regardless of the source of remote sensing (e.g., Landsat vs. lidar), factors affecting the measurement and prediction of forest biomass, such as species composition, need to be taken into account whether one is estimating biomass at the plot, stand, or landscape scale.

     

Retreat dynamics of the eastern sector of the British–Irish Ice Sheet during the last glaciation

The findings of BRITICE-CHRONO Transect 2 through the North Sea Basin and eastern England are reported. We define ice-sheet marginal oscillation between ~31 and 16 ka, with seven distinctive former ice-sheet limits (L1–7) constrained by Bayesian statistical analysis. The southernmost limit of the North Sea Lobe is recorded by the Bolders Bank Formation (L1; 25.8–24.6 ka). L2 represents ice-sheet oscillation and early retreat to the northern edge of the Dogger Bank (23.5–22.2 ka), with the Garret Hill Moraine in north Norfolk recording a significant regional readvance to L3 at 21.5–20.8 ka. Ice-marginal oscillations at ~26–21 ka resulted in L1, L2 and L3 being partially to totally overprinted. Ice-dammed lakes related to L1–3, including Lake Humber, are dated at 24.1–22.3 ka. Ice-sheet oscillation and retreat from L4 to L5 occurred between 19.7 and 17.3 ka, with grounding zone wedges marking an important transition from terrestrial to marine tidewater conditions, triggered by the opening of the Dogger Lake spillway between 19.9 and 17.5 ka. L6 relates to ice retreat under glacimarine conditions and final ice retreat into the Firth of Forth by 15.8 ka. L7 (~15 ka) represents an ice retreat from Bosies Bank into the Moray Firth.     

Nanogeochemistry of hydrothermal magnetite

Magnetite from hydrothermal ore deposits can contain up to tens of thousands of parts per million (ppm) of elements such as Ti, Si, V, Al, Ca, Mg, Na, which tend to either structurally incorporate into growth and sector zones or form mineral micro- to nano-sized particles. Here, we report micro- to nano-structural and chemical data of hydrothermal magnetite from the Los Colorados iron oxide–apatite deposit in Chile, where magnetite displays both types of trace element incorporation. Three generations of magnetites (X–Z) were identified with concentrations of minor and trace elements that vary significantly: SiO₂, from below detection limit (bdl) to 3.1 wt%; Al₂O₃, 0.3–2.3 wt%; CaO, bdl–0.9 wt%; MgO, 0.02–2.5 wt%; TiO₂, 0.1–0.4 wt%; MnO, 0.04–0.2 wt%; Na₂O, bdl–0.4 wt%; and K₂O, bdl–0.4 wt%. An exception is V₂O₃, which is remarkably constant, ranging from 0.3 to 0.4 wt%. Six types of crystalline nanoparticles (NPs) were identified by means of transmission electron microscopy in the trace element-rich zones, which are each a few micrometres wide: (1) diopside, (2) clinoenstatite; (3) amphibole, (4) mica, (5) ulvöspinel, and (6) Ti-rich magnetite. In addition, Al-rich nanodomains, which contain 2–3 wt% of Al, occur within a single crystal of magnetite. The accumulation of NPs in the trace element-rich zones suggest that they form owing to supersaturation from a hydrothermal fluid, followed by entrapment during continuous growth of the magnetite surface. It is also concluded that mineral NPs promote exsolution of new phases from the mineral host, otherwise preserved as structurally bound trace elements. The presence of abundant mineral NPs in magnetite points to a complex incorporation of trace elements during growth, and provides a cautionary note on the interpretation of micron-scale chemical data of magnetite.     

Lid tectonics-Preface

正The idea that plate tectonics may not have operated deep in Earth's Precambrian past has a long legacy.What predated plate tectonics is unknown,and advances in data-from geochemical,geological and tectonic,to paleomagnetic,as well as modelling ap-     

The role of horizontal resolution in simulating drivers of the global hydrological cycle

The role of atmospheric general circulation model (AGCM) horizontal resolution in representing the global energy budget and hydrological cycle is assessed, with the aim of improving the understanding of model uncertainties in simulating the hydrological cycle. We use two AGCMs from the UK Met Office Hadley Centre: HadGEM1-A at resolutions ranging from 270 to 60 km, and HadGEM3-A ranging from 135 to 25 km. The models exhibit a stable hydrological cycle, although too intense compared to reanalyses and observations. This over-intensity is explained by excess surface shortwave radiation, a common error in general circulation models (GCMs). This result is insensitive to resolution. However, as resolution is increased, precipitation decreases over the ocean and increases over the land. This is associated with an increase in atmospheric moisture transport from ocean to land, which changes the partitioning of moisture fluxes that contribute to precipitation over land from less local to more non-local moisture sources. The results start to converge at 60-km resolution, which underlines the excessive reliance of the mean hydrological cycle on physical parametrization (local unresolved processes) versus model dynamics (large-scale resolved processes) in coarser HadGEM1 and HadGEM3 GCMs. This finding may be valid for other GCMs, showing the necessity to analyze other chains of GCMs that may become available in the future with such a range of horizontal resolutions. Our finding supports the hypothesis that heterogeneity in model parametrization is one of the underlying causes of model disagreement in the Coupled Model Intercomparison Project (CMIP) exercises.     

Generalized Radial Flow in Synthetic Flow Systems

Traditional analysis methods used to determine hydraulic properties from pumping tests work well in many porous media aquifers, but they often do not work in heterogeneous and fractured‐rock aquifers, producing non‐plausible and erroneous results. The generalized radial flow model developed by Barker (1988) can reveal information about heterogeneity characteristics and aquifer geometry from pumping test data by way of a flow dimension parameter. The physical meaning of non‐integer flow dimensions has long been a subject of debate and research. We focus on understanding and interpreting non‐radial flow through high permeability conduits within fractured aquifers. We develop and simulate flow within idealized non‐radial flow conduits and expand on this concept by simulating pumping in non‐fractal random fields with specific properties that mimic persistent sub‐radial flow responses. Our results demonstrate that non‐integer flow dimensions can arise from non‐fractal geometries within aquifers. We expand on these geometric concepts and successfully simulate pumping in random fields that mimic well‐test responses seen in the Culebra Dolomite above the Waste Isolation Pilot Plant.     

The onset of magnetoconvection at large Prandtl number in a rotating layer I. Finite magnetic diffusion

Abstract

This paper develops further a convection model that has been studied several times previously as a very crude idealization of planetary core dynamics. A plane layer of electrically-conducting fluid rotates about the vertical in the presence of a magnetic field. Such a field can be created spontaneously, as in the Childress—Soward dynamo, but here it is uniform, horizontal and externally-applied. The Prandtl number of the fluid is large, but the Ekman, Elsasser and Rayleigh numbers are of order unity, as is the ratio of thermal to magnetic diffusivity. Attention is focused on the onset of convection as the temperature difference applied across the layer is increased, and on the preferred mode, i.e., the planform and time-dependence of small amplitude convection. The case of main interest is the layer confined between electrically-insulating no-slip walls, but the analysis is guided by a parallel study based on illustrative boundary conditions that are mathematically simpler.