Flat jacks are designed to carry out in-situ testing of masonry structures and rock.
 
The flat jack is constructed from two stainless steel plates welded around their periphery, with the narrow gap between the plates filled with hydraulic fluid. It is inserted into a slot cut into the structure to be monitored and gradually brought up to pressure with a special hydraulic pump.
 
As stress increases within the structure or rock,the fluid pressure within the cell rises as the plates are squeezed together and it is possible to derive the stresses acting in the structure test area.
 
A length of stainless steel tube connects the plates to a pressure transducer (VWDT 5000 or SGT 3000) that converts the pressure to an electrical signal which can be read directly with a MP12 readout or data logged.