Temporary Flat Jacks
Oil Inflation – When flat jacks are to be used for a limited period and subsequently removed inflation is usually done with hydraulic fluid. For small numbers of jacks, requiring relatively small volumes of oil, simple hand pumps may be used for inflation. Larger jacks or large numbers of jacks may require electric or diesel powered pumps.
Hydraulic Circuits – Single or multiple flat jacks can be inflated individually or on inter-connected circuits. Each jack has a needle valve for locking off and may be fitted with a pressure gauge if required.
Permanent Flat Jacks
Where a force is required to be permanently applied to a structure, flat jacks are inflated under pressure with an epoxy resin by means of a special resin injection cylinder. The epoxy resin hardens over a period of time to leave the force permanently applied. This type of application is suitable for pre-loading new or additional steelwork in structures, underpinning works and structural bearing installation.
This method is often used where sufficient room is not available for wedging or grouting the jacking gap.
In certain circumstances, it may be necessary to carry out the initial inflation on hydraulic oil with resin inflation being carried out at a later date. An oil to resin transfusion system is then required.
Complex Flat Jack Systems
In applications using multiple flat jacks the hydraulic pumping circuitry can be arranged appropriately using pipe and compression fitting or flexible hoses and quick release couplings.
Individual jacks are usually fitted with manual needle valves. Pressure gauges may be fitted to individual jacks, downstream of their needle valves, instead of or in addition to a common ring main gauge. Differing loads in neighboring jacks may thus be locked in and monitored as necessary.
For especially complex operations solenoid valves, electronic pressure transducers and computer control, monitoring and data logging systems may be used.
Calibrated Flat Jacks for Weighing
A flat jack which has been pre-opened, fitted with a pressure gauge or transducer, sealed and then calibrated in a press can be used as a robust, accurate load cell. This load cell can be placed in tandem with a standard flat jack of the same size.
The twin jacks are then installed beneath a structure at weighing positions. The weight of the structure taken by inflation of the standard flat jack registers a pressure in the calibrated flat jack which, by reference to its calibration graph, provides an accurate weight.
Using this principle, offshore structures ranging from modules of several hundred tonnes to complete integrated decks of over 30,000 tonnes to an accuracy of +/- 1%. Where necessary for lighter structures, an arrangement is available with greater accuracy.
