Root‐related rhodochrosite and concretionary siderite formation in oxygen‐deficient conditions induced by a ground‐water table rise

Sedimentological, mineralogical, stable carbon and oxygen isotope determinations and biomarker analyses were performed on siderite concretions occurring in terrestrial silts to understand their formation and to characterize the sedimentary and diagenetic conditions favouring their growth. High δ¹³C values (6·4‰ on average) indicate that siderite precipitated in an anoxic environment where bacterial methanogenesis operated. The development of anoxic conditions during shallow burial was induced by a change in sedimentary environment from flood plain to swamp, related to a rise of the ground‐water table. Large amounts of decaying plant debris led to efficient oxygen consumption within the pore‐water in the peat. Oxygen depletion, in combination with a decrease in sedimentation rate, promoted anoxic diagenetic conditions under the swamp and favoured abundant siderite precipitation. This shows how a change in sedimentary conditions can have a profound impact on the early‐diagenetic environment and carbonate authigenesis. The concretions contain numerous rhizoliths; they are cemented with calcium‐rhodochrosite, a feature which has not been reported before. The rhodochrosite cement has negative δ¹³C values (?16·5‰ on average) and precipitated in suboxic conditions due to microbial degradation of roots coupled to manganese reduction. The exceptional preservation of the epidermis/exodermis and xylem vessels of former root tissues indicates that the rhodochrosite formed shortly after the death of a root in water‐logged sediments. Rhodochrosite precipitated during the initial stages of concretionary growth in suboxic microenvironments within roots, while siderite cementation occurred simultaneously around them in anoxic conditions. These suboxic microenvironments developed because oxygen was transported from the overlying oxygenated soil into sediments saturated with anoxic water via roots acting as permeable conduits. This model explains how separate generations of carbonate cements having different mineralogy and isotopic compositions, which would conventionally be regarded as cements precipitated sequentially in different diagenetic zones during gradual burial, can form simultaneously in shallow burial settings where strong redox gradients exist around vertically oriented permeable root structures.     

Evaluation of Land Suitability for Urban Land‐Use Planning: Case Study Dhaka City

The aim of this research is evaluation of land suitability for urban land‐use planning. Four factors and fourteen criteria were selected for suitability analysis and land‐use planning. Factors and criteria were defined based on literature survey, experts’ opinions, local contexts and availability of data. GIS‐based Analytical Hierarchy Process (AHP) was used as a Multi‐criteria Decision Making model. The study was conducted on a selected area of Dhaka city, which is one of the fastest growing mega‐cities of the world. The research result shows that highly suitable area (13%) should be used for urban residential zone; moderately suitable area (35%) should be designated as mixed use zone; low suitable area (42%) should be reserved for agricultural use and open spaces; and not suitable area (10%) should be protected from any types of activities except agriculture. The research approached an urban land‐use planning at a regional scale.     

An Agent‐Based Simulation of Residential Location Choice of Tenants in Tehran,Iran

Residential location choice modeling is one of the substantial components of land use and transportation models. While numerous aggregated mathematical and statistical approaches have been developed to model the residence choice behavior of households, disaggregated approaches such as the agent‐based modeling have shown interesting capabilities. In this article, a novel agent‐based approach is developed to simulate the residential location choice of tenants in Tehran, the capital of Iran. Tenants are considered as agents who select their desired residential alternatives according to their characteristics and preferences for various criteria such as the rent, accessibility to different services and facilities, environmental pollution, and distance from their workplace and former residence. The choice set of agents is limited to their desired residential alternatives by applying a constrained NSGA‐II algorithm. Then, agents compete with each other to select their final residence among their alternatives. Results of the proposed approach are validated by comparing simulated and actual residences of a sample of tenants. Results show that the proposed approach is able to accurately simulate the residence of 59.3% of tenants at the traffic analysis zone level.     

Analysis of anomalies in ADS-B and its GPS data

Traditionally, the surveillance component of the air traffic management system has been based on radar, which consists of two separate systems: primary radar and secondary radar, which both enable the measurement of the aircraft range and bearing to the radar station. Primary radar is based on signals emitted by a ground station simply being reflected off an object and detected by a ground-based receiver. Secondary radar also emits signals, but relies upon a transponder onboard the aircraft to emit a signal itself, modulated among others by a four-digit aircraft identity (Mode A), aircraft altitude (Mode C) and/or 24-bit unique address (Mode S). Typical accuracies of secondary radar are of the order of 0.03 NM in range and 0.07° in azimuth. However, no position integrity report is provided. Air traffic density is expected to significantly increase in the future. In order to maintain or enhance air travel efficiency, while maintaining safety, more accurate surveillance systems, with the required integrity, will be required. Automatic dependent surveillance–broadcast (ADS-B) is a new aviation surveillance system, envisioned to overcome the limitations of radar and to enhance surveillance performance and thereby increase airspace capacity. However, its high dependence on external systems such as onboard navigation and communication systems also increases the number of potential points of failure. It is important to understand and mitigate these failure modes before the system can reliably be implemented. The present study emerged as an exploratory research as part of a safety assessment framework development for the ADS-B system. It reviews the ADS-B failure modes, data collection and analysis of ADS-B and its corresponding onboard GPS data. The study identifies a set of failures common to certain aircraft models, with consistent error patterns. A key failure mode was found to be associated with the navigation data from the onboard GPS. We discuss the identified failure modes and investigate the nature and causes of these failures. The findings highlight some of the deficiencies of the current ADS-B system, which will need to be addressed before the ADS-B system can reliably be implemented.     

Behavior of Shallow Strip Footing on Twin Voids

This paper numerically examines the bearing capacity and failure mechanism of a shallow strip foundation constructed above twin voids. The voids may refer to caves, caverns, underground aqueduct or tunnels due to water seepage, chemical reaction or deliberately excavated in soil deposit. The ability of numerical model to accurately predict the system behavior is evaluated by performing verification analyses on existing researches. Subsequently, a parametric study carried out to reveal the influence of size of footing/voids and their location (i.e. depth, spacing, eccentricity) on the bearing capacity of footing. To clarify the failure mechanism, the distribution of shear strain in the soil for different scenarios is assessed. The parametric study provided a new framework to determine the bearing capacity and the mode of failure for footings on voids. Based on the results, a criterion can be issued to avoid collapse of footing/voids regarding the shape, location and size of voids. The results can also be used to design construction of a footing on existing voids while the acquired failure mechanisms can be appointed to develop analytical solutions for this problem. Results demonstrated that a critical depth for voids and a critical distance between them exist where the influence on the ultimate bearing capacity of footing disappears.