Generally, having to carry out checks in a long time (several months or even years), it often used an onsite data acquisition systems, able to pre-process the information and send them via modem to a remote operations center. Every settled tool will be connected to an acquisition control unit, processing and transmission the data. This unit is set as much as possible in a barycentric position closer to the sensors; thus, the cable lengths (a disturbing factor) are minimized and tools are in the same homologous transmission conditions to the control unit. A remote user can collect data by reaching by telephone this control unit, connected to the telephone network by a modem. If necessary, another user (personal computer) can locally connect to the data collection, to process and report alert threshold in situ. The end user, however, is able to process all the data put on regular files, by appropriate software used to generate numeric and graphics visualization of the information detected by the onsite sensors. A diagnostic module should also let you highlight any outliers or abnormal values against the measuring range; so, it is possible a daily monitoring of the tool proper functioning. After setting the suitable alarm thresholds for the measuring parameters, it is finally possible to activate the acoustic and visual mechanisms that reports to the operator the pre-established thresholds overcoming. All the collected informations are processed during a standard time period (for example, monthly), and every extraordinary phenomenon, that could generate the alteration of one of the monitored parameters, has to be collected.
CRACKS AND CREVICES MONITORING
The tool usually used consists of transducers of potentiometric displacement. These tools allow measuring the displacement, even a micrometric one, of cracks and microcracks with accuracy.
Measurement of the vertical tilt parameters variation
We use wall inclinometers to onsite tilt measurement (and tilt variation) of the wall to which they are fixed. Inclinometers are biaxial, with sensors set on rods in order to measure the variation of inclination between two points of the tilt.
In this kind of measurement, the tool used is a mercury assestimeter with a barometer cell sensor compensated in temperature (connected to one or more distinct mercury reservoirs, all placed at a maximum distance of 40 m from assestimeter). Assestimeters measure the pressure of a column of mercury comprised between the reference reservoir and a pressure transducer incorporated in the soil of which it follows the settlements. Through appropriate correlations, pressure variations allow to achieving the yielding of the monitored structural elements. If properly designed and installed, the system allows you to gauge the vertical dimension changes (settlements) in the order of 0.2 – 0.5 mm.
Control of the vibration behaviour
If required, it is possible to check the vibrational dynamic behaviour, over time too and even for long periods. In this case, one has to install accelerometers connected to common or single acquisition data units. Dividing the monitoring network into different parts is a necessary proceeding to avoid excessive lengths of connection cables for any type of instrumentation. In fact, excessive lengths do not provide sufficient guarantees about the data and monitoring reliability. The acquisition will take place via a portable system, connected to the network of electric energy, equipped with an internal battery and battery charger in order to ensure the operation even in case of voltage drop. Each voltage unit will be calibrated with a minimum threshold of intervention according to the monitored pulse; a sampling time will set compatibly with its frequency scheduled for the structural element under review.