In case of: radial and tangential stresses in the lining of tunnels and underground excavations; stress in rock masses, pressures under foundations; contact pressures on the surface of diaphragm and retaining walls, piers and abutments of viaducts; OTR suggest use of tunnel pressure cell.

NATM pressure cell are made up by a couple of steel membranes shaped and welded. They are linked to a pressure trasductor by an hydraulic tube and equipped with a pressurising valve for the attachment of a manual pump, in case of loss of load or due to adhesion to the structure. Cell  and hydraulic tube are vacuum saturated with deareated oil.

How is it installed?

For the radial model the sensible surface is perpendicular to the charge, so directed to the tunnel ray. Cells measure rock’s pressure on the tunnel arch. To prepare the installation surface: prepare the concrete and placed it on the future contact surface smoothing it out. Don’t leave exposed edges or rocks to ensure a smooth floor to the cell.

At the end fix the cell inside the concrete using loops on the sides of it self and repressurize the system only if it’s necessary.

For the tangential model, the cell has to be fixed parallel to the tunnel’s ray. The instrument will measure the inner pressure of the concrete of the tunnel arch. In this case too should be used the eyelets to fix the technology  in the right position with respect to the reinforcement of the structure. The high pressure hydraulic tube has to be choosen enough long to repressurize the instrument and the terminal. It have therefore to remain outside the concrete casting of the work.