A flat-jack consists on a plane welded metallic reservoir, with two openings for the fluid input and (or) output, which are inflated with oil through a pressurization system. This technique was initially created for testing rocky terrain, having been exported to the evaluation of the stress state and mechanical behaviour of masonry structures. This technique is considered moderately destructive and has two setup outlines: using one (simple) or two (double) flat-jacks.
The simple flat-jack test allows determining the level of stress installed at a given point of a structure. The test is divided in two steps: (i) measurement of the reduction of the distance between two horizontal lines located below and above a cut done in the wall to introduce the flat-jack; (ii) determination of the pressure to introduce in the flat-jack to restore the position of the two lines before the cut. This pressure corresponds to the vertical stress installed in the wall at the level of the cut.
The double flat-jack test was created to determine on-site the behavior curves (stress vs deformation) of materials and, therefore, to estimate its elastic modulus and, through other relations established in the literature, the load capacity. This test is usually preceded by a simple flat-jack test. Then, a second cut, parallel to the first one, is made above the previous one where a second flat-jack is inserted. The two flat-jacks are connected to the same hydraulic pump that ensures the same pressure in both jacks. A set of displacement transducers is placed between the two flat-jacks to monitor the displacements for the different levels of pressure installed in the flat-jacks.