NATM stress cells
NATM stress cells are used to control the pressure acting between a support (for example, a bending in a tunnel) and a prompting soil.
The cell consists of a rectangular steel cavity filled with a special oil and connected to an electric transducer, which transforms every change in pressure acting on the cavity in a variation of the electric signal with standard output 4÷20 mA.
The stress cells have been planned in different versions, which makes them versatile and suitable for every mounting need. They are supplied in different shapes:
• without hydraulic tube and with the transducer directly mounted horizontally or vertically on the cell;
• with hydraulic tube for recharge and with transducer directly mounted on the cells or on the tube.
The second type of cell described above has the main feature to be pressurizable through a charging valve installed on the end of the hydraulic Rilsan Tube, whose standard length is 1.5 meters. The repressurization of a stress cell is required, for example, when the cavity is embedded in the concrete of a tunnel covering. During the consolidation phase, the covering undergoes a contraction, which can determine a contact defect between the surface of the cavity and the concrete. By means of repressurization, it is possible to ensure the normal operation of the instrument, restoring the adhesion of the cavity to the concrete over the entire surface.
The transducer data can be read through the portable control unit or by using an automatic data acquisition system designed to provide continuous monitoring.