Luca Masini

Fabrication and testing of a high resolution extensometer based on resonant MEMS strain sensors

A novel type of linear extensometer with exceptionally high resolution of 4 nm based on MEMS resonant strain sensors bonded on steel and operating in a vacuum package is presented. The tool is implemented by means of a steel thin bar that can be pre-stressed in tension within two fixing anchors. The extension of the bar is detected by using two vacuum-packaged resonant MEMS double- ended tuning fork (DETF) sensors bonded on the bar with epoxy glue, one of which is utilized for temperature compensation. Both sensors are driven by a closed loop self-oscillating transresistance amplifier feedback scheme implemented on a PCB (Printed Circuit Board). On the same board, a microcontroller-based frequency measurement circuit is also implemented, which is able to count the square wave fronts of the MEMS oscillator output with a resolution of 20 nsec. The system provides a frequency noise of 0.2 Hz corresponding to an extension resolution of 4 nm for the extensometer. Nearly perfect temperature compensation of the frequency output is achieved in the temperature range 20–35 °C using the reference sensor.

An Approach to Evaluate the Efficiency of Compensation Grouting

AbstractCompensation grouting is often employed as a mitigation technique to reduce the settlements induced by tunneling. The efficiency of compensation grouting, defined as the ratio of the volume of heave induced at the ground surface to the volume of injected grout, is strongly dependent on grout properties, injection characteristics, and soil properties. In clayey soils, site observations and laboratory tests show high values of compensation efficiency immediately after injection. However, the efficiency can decrease with time if positive excess pore-water pressures develop in the soil during the injection process. Conversely, in sandy soils, a loss of volume occurs because of pressure filtration of the grout during and soon after the injection process, which reduces the compensation efficiency. This paper describes an analytical model that can be used to evaluate the volume loss produced by pressure filtration of cement-bentonite grouts as a function of soil, grout, and injection parameters. The magn...

Seismic Performance of Geosynthetic-Reinforced Earth Retaining Walls Subjected to Strong Ground Motions

There is general evidence that the good performance of geosynthetic-reinforced earth retaining walls (GRWs) observed after strong seismic events can be attributed to their capacity to redistribute seismic-induced deformations within the reinforced zone, provided that the reinforcements are characterised by adequate extensional ductility. Therefore, it is desirable to promote the activation of internal (or local) plastic mechanisms, involving the reinforcement strength, since the design phase. In this study, the seismic performance of two earth retaining walls is compared. Specifically, a pseudo-static approach is adopted to conceive a GRW with a seismic resistance, expressed by the critical seismic coefficient kc, that involves activation of an internal plastic mechanisms, and a conventional retaining wall, in which the same critical seismic coefficient kc is attained for an external (or global) plastic mechanism. Finite difference dynamic analyses are carried out to evaluate the seismic performance of the two walls. In the analyses, a real acceleration time history is applied at the base of the models. The results of the dynamic analyses show that, compared to the wall in which external plastic mechanisms develop, the wall designed to activate internal plastic mechanisms exhibits a better seismic performance, with lower permanent displacements computed at the end of the seismic event.