Statistical estimation of the relative efficiency of natural attenuation mechanisms in contaminated aquifers

Using monitored natural attenuation is an increasingly popular strategy for dealing with contaminated aquifers. This paper provides a statistical methodology for the estimation of the relative efficiency of natural attenuation mechanisms. The methodology provides estimates, with associated measures of uncertainty, of the relative efficiency of four types of bio-degradation (oxidation using oxygen as the electron-acceptor, denitrification, iron reduction and sulfate reduction). A data set from Trecate, Italy, is analysed using the methodology. The analysis shows that sulfate is the main cause of hydrocarbon removal on this site. It is also seen that oxidation using oxygen seems to be more preferential than the other reactions, in the sense that this reaction is relatively more efficient than other reactions at locations where the hydrocarbon concentration is low.     

Hybrid simulation of a partial-strength steel–concrete composite moment-resisting frame endowed with hysteretic replaceable beam splices

This paper deals with the seismic response assessment of a steel–concrete moment-resisting frame (MRF) equipped with special dissipative replaceable components (DRCs): the dissipative replaceable beam splices (DRBeS), which combine large energy dissipation with ease of replacement. The evaluation of the full potential of DRBeS requires a system-level investigation, that is, a six-story MRF, whereby the hysteretic effects of beam splices partial-strength joints are considered on the global response of the structural system. Therefore, an OpenSees finite element (FE) frame model, based on previous experimental campaigns with cyclic displacements on partial-strength joints, and a Matlab model validated on OpenSees, were used for a more complex experimental activity via hybrid simulation (HS). The aim of the simulations was twofold: (i) to increase knowledge of the non-linear behaviour of steel-concrete composite partial-strength MRFs; and (ii) to study the effectiveness of the DRBeS components for increasing the recovery of functionality after a major seismic event. Therefore, to appreciate the performance of the partial-strength MRF at damage limitation (DL), significant damage (SD) and near collapse (NC) within the performance-based earthquake engineering (PBEE) approach, HSs were carried out. In such instances, the ground floor was physically tested at full scale in the laboratory and the remainder of the structure was numerically simulated. Relevant results showed that the DRBeS were capable of dissipating a significant amount of hysteretic energy and of protecting the non-dissipative parts of partial-strength joints and the overall structure with an ease of replacement.     

Improving the GNSS positioning stochastic model in the presence of ionospheric scintillation

Ionospheric scintillations are caused by time- varying electron density irregularities in the ionosphere, occurring more often at equatorial and high latitudes. This paper focuses exclusively on experiments undertaken in Europe, at geographic latitudes between ~50°N and ~80°N, where a network of GPS receivers capable of monitoring Total Electron Content and ionospheric scintillation parameters was deployed. The widely used ionospheric scintillation indices S4 and s_j{\sigma_{\varphi}} represent a practical measure of the intensity of amplitude and phase scintillation affecting GNSS receivers. However, they do not provide sufficient information regarding the actual tracking errors that degrade GNSS receiver performance. Suitable receiver tracking models, sensitive to ionospheric scintillation, allow the computation of the variance of the output error of the receiver PLL (Phase Locked Loop) and DLL (Delay Locked Loop), which expresses the quality of the range measurements used by the receiver to calculate user position. The ability of such models of incorporating phase and amplitude scintillation effects into the variance of these tracking errors underpins our proposed method of applying relative weights to measurements from different satellites. That gives the least squares stochastic model used for position computation a more realistic representation, vis-a-vis the otherwise ‘equal weights’ model. For pseudorange processing, relative weights were com- puted, so that a ‘scintillation-mitigated’ solution could be performed and compared to the (non-mitigated) ‘equal weights’ solution. An improvement between 17 and 38% in height accuracy was achieved when an epoch by epoch differential solution was computed over baselines ranging from 1 to 750 km. The method was then compared with alternative approaches that can be used to improve the least squares stochastic model such as weighting according to satellite elevation angle and by the inverse of the square of the standard deviation of the code/carrier divergence (sigma CCDiv). The influence of multipath effects on the proposed mitigation approach is also discussed. With the use of high rate scintillation data in addition to the scintillation indices a carrier phase based mitigated solution was also implemented and compared with the conventional solution. During a period of occurrence of high phase scintillation it was observed that problems related to ambiguity resolution can be reduced by the use of the proposed mitigated solution.     

Field Evidence for the Upwind Velocity Shift at the Crest of Low Dunes

Flow that is topographically forced by hills and sand dunes accelerates on the upwind (stoss) slopes and reduces on the downwind (lee) slopes. This secondary wind regime, however, possesses a subtle effect, reported here for the first time from field measurements of near-surface wind velocity over a low dune: the wind velocity close to the surface reaches its maximum upwind of the crest. Our field measurements show that this upwind phase shift of velocity with respect to topography is found to be in quantitative agreement with the prediction of hydrodynamical linear analysis for turbulent flows with first-order closures. This effect, together with sand transport spatial relaxation, is at the origin of the mechanisms of dune initiation, instability and growth.     

Tuning a Kalman filter carrier tracking algorithm in the presence of ionospheric scintillation

GPS Solutions - Strong ionospheric electron content gradients may lead to fast and unpredictable fluctuations in the phase and amplitude of the signals from Global Navigation Satellite Systems...     

Hazard-dependent soil factors for site-specific elastic acceleration response spectra of Italian and European seismic building codes

To define the seismic input in non-liquefiable soils, current seismic standards give the possibility to treat local site effects using a simplified approach. This method is generally based on the introduction of an appropriate number of soil categories with associated soil factors that allow modifying the shape of the elastic acceleration response spectrum computed at rocky (i.e. stiff) sites. Although this approach is highly debated among researchers, it is extensively used in practice due to its easiness. As a matter of fact, for standard projects, this method represents the driving approach for the definition of the seismic input. Nevertheless, recent empirical and numerical studies have risen doubts about the reliability and safety of the simplified approach in view of the tendency of the current soil factors of Italian and European building codes to underestimate the acceleration at the free surface of the soil deposit. On the other hand, for certain soil classes, the current soil factors seem to overestimate ground amplification. Furthermore, the occurrence of soil nonlinearity, whose magnitude is linked to both soil type and level of seismic intensity, highlights the fallacy of using constant soil factors for sites with a different seismic hazard. The objective of this article is to propose a methodology for the definition of hazard-dependent soil factors and simultaneously quantify the reliability of the coefficients specified in the current versions of Eurocode 8 (CEN 2005) and Italian Building Code (NTC8 2008 and revision NTC18 2018). One of the most important outcome of this study is the quantification of the relevance of soil nonlinearity through the definition of empirical relationships between soil factors and peak ground acceleration at outcropping rock sites with flat topological surface (reference condition).     

Bowing of marble slabs: can the phenomenon be arrested and prevented by inorganic treatments?

Bowing of thin marble slabs is a phenomenon affecting both historic monuments and modern buildings. In spite of the ubiquity and destructiveness of this phenomenon, no fully satisfactory treatment is currently available to arrest and/or prevent bowing. In this study, a treatment based on formation of hydroxyapatite (HAP) was investigated as a possible route to arrest and possibly prevent bowing of Carrara marble slabs. Four different formulations of the HAP treatment were tested and compared to ammonium oxalate and ethyl silicate (widely used in the practice of marble conservation). The treatments were applied onto pre-weathered and unweathered specimens to investigate their ability to arrest and prevent bowing, respectively. Marble behavior was studied in terms of residual strain and bowing after thermal cycles up to 90 °C in dry and wet conditions. Marble cohesion was assessed before and after the thermal cycles by ultrasound. The HAP treatments exhibited promising results, as the residual strain and the bowing after the cycles were always lower or equal to the untreated references, while marble cohesion was always higher. Surprisingly, ammonium oxalate caused marked worsening of marble thermal behavior. In the case of ethyl silicate, most of the initial benefit after consolidation was lost after the thermal cycles. In general, the results of the study point out the importance of evaluating marble thermal behavior to assess the suitability of any conservation treatment and suggest that treatments able to strengthen marble without causing excessive pore occlusion and stiffening are preferable to enhance durability to thermal cycles.