Flat jacks are mainly used in the diagnosis of buildings and masonry with the purpose of establishing experimentally the actual static conditions of the structure under investigation. Other possible applications regard the measurement of stress in situ (e.g. in tunnels lining and in bridge piers).
The single-jack test methodology is based on the variation of the stress state in a point of the structure under examination due to a plane cut executed in the direction perpendicular to the surface. The stress release causes the cut to be closed to an extent which is assessed through convergency measurements between one or more points, situated in a symmetrical position with respect to the cut itself by using a high-precision strain gauge (see Mayes strain gauge). A special flat jack is then inserted within the cut and gradually put under pressure with a special hydraulic pump (see M2H16 Glötzl hand pump) until the previously measured convergency is eliminated. In these conditions the pressure within the jack is equal to the pre-existing force on the masonry, minus a constant that takes into account the relation between the area of the load jack and the area of the cut and the jacks stiffness coefficient (Km).
Two parallel jacks installed at about 50 cm from one another are used and the volume of material delimited by them is submitted to a mono-axial compression test to identify the deformability characteristics.
The employment of flat jacks can involve both a recovery installation and a disposable installation: in this case the jack is cemented to the cut and will serve for the medium and long-term measurement of the pressure in the point of installation.