NUMERICAL EVALUATION OF THE TEMPORAL VARIATION OF VIBRATIONS INDUCED BY UNDERGROUND TRAINS

The change in the dynamic response of shallow soils as caught by two geophysical test campaigns is exploited to numerically predict the variation in the ground borne vibrations induced by the passage of underground trains. Multiple causes may lead to a variation in the perception of vibrations over time: from an increase in the train load to the roughness of the railway track, from the increase of the train speed to the modification of the dynamic response of the surrounding soil. In the present study, special attention is devoted to the effect of the hydrological regime on the latter. Two scenarios were calibrated on the results of repeated geodynamic tests on the same site in Milano but at two different times. The two investigations revealed differences in the dynamic response of above-ground soils which can be related to different saturation profiles. The passage of a typical convoy is simulated in the time domain. Differences between the two scenarios are highlighted together with the role played by the static axle load of the train. Furthermore, comparisons with recorded accelerations are provided to validate the model

A numerical assessment of variable saturation of the upper layers on the ground borne vibrations from underground trains: A case history

Ground borne vibrations generated by the passage of underground trains may change over time due to objective causes, such as increasing weight and speed of trains or ageing of the infrastructure components, as well as a variation in the dynamic response of the soil surrounding the tunnel. Among the possible causes of changes in the soil dynamic response, its hydrologic state has been seldom investigated. In this contribution, the role played by the conditions of the soil above the water table is addressed, starting from a case history in the city of Milano. Two-dimensional plane strain numerical models have been developed for the infrastructure. The models were calibrated on the results of two geophysical investigations performed at the same site in the city centre, but at two different times, which allowed distinguishing different dynamic responses. The system was excited by a synthetic load time history, matching a reference dynamic load spectrum included in Italian recommendations. Limitations of using this input on a 2D plane strain model were assessed by comparing the computed vibrations with experimental acceleration records collected on the tunnel. The results of the two numerical models are compared with those of a simulation performed assuming fully dry conditions above the water table. Overall, the set of analyses shows that even small changes in the dynamic response of the soil, interpretated as a consequence of variable saturation, may result in a change of a few decibels in the acceleration levels. Much larger accelerations are predicted on average with the simpler dry model, clearly showing the advantages of a more accurate modelling strategy

Poeti greci del Novecento

Introduzione di Filippomaria Pontani. Traduzioni di N. Crocetti, F. Pontani e F. M. Pontani. Collana «I Meridiani»

Ascent Trajectory Optimization and Neighboring Optimal Guidance of Multistage Launch Vehicles

Multistage launch vehicles are employed to place spacecraft and satellites in their operational orbits. If the rocket aerodynamics and propulsion are modeled appropriately, optimization of their ascent trajectory consists in determining the coast duration and the thrust time history that maximize the final mass at injection. This research derives all the necessary conditions for ascent path optimization of a multistage launch vehicle. With reference to an existing rocket, the indirect heuristic method is then applied, for the numerical determination of the overall ascent trajectory. An effective approach is used with the intent of satisfying the path constraint related to the maximum dynamical pressure in the atmospheric phase. Then, the recently introduced, implicit-type variable-time-domain neighboring optimal guidance is applied to the upper stage powered arc, for the purpose of obtaining the corrective control actions in the presence of nonnominal flight conditions. The guidance approach at hand, based on the second-order analytical conditions for optimality, proves to be rather effective (in terms of propellant budget), and guarantees very accurate orbit injection in spite of perturbations

Seepage and stability analyses of a zoned earth dam subjected to variable water heads: numerical simulations with Abaqus

The behaviour of a zoned earthen dam is analysed by means of a finite element model implemented in Abaqus. The analysed dam was built in Slovenia at the end of the 1980s for irrigation and flood protection purposes. In 2007, detection of a wet stain on the downstream slope forced the drawdown of the reservoir to carry out remedial works. Transient fully coupled hydro-mechanical analyses under partially saturated conditions have been conducted to investigate the behaviour of the dam from its construction to present. Available in-situ measurements have been exploited to calibrate the model and to assess the reliability of the predictions. To address the effects of the wet stain and of the remedial works on the dam body, stability analyses have been performed at significant times. Future scenarios have been analysed, using provided information on the time schedule of the reservoir impounding after completion of the remedial works. The results show that the high pore water pressures measured inside the dam body may be explained by damage experienced by the irrigation pipelines and that remedial works are necessary to restore the dam functionality

A simple device to immobilize protists for electrophysiology and microinjection

We present a simple device to mechanically immobilize motile cells such as ciliates. It can be used in particular for intracellular electrophysiology and microinjection. A transparent filter with holes smaller than the specimen is stretched over an outlet. A flow is induced by either a peristaltic pump or a depressurized tank, mechanically entraining cells to the bottom, where they are immobilized against the filter. The cells start swimming again as soon as the flow is stopped. We demonstrate the device by recording action potentials in Paramecium and injecting a fluorescent dye into the cytosol