Flat Jacks are simple, practical devices for the civil or structural engineer wherever the application or control of large forces is required.
They are most often used in the solution of unforeseen problems such as in remedial measures or structural alteration. They are also used in new constructions in which they form part of the structural concept.
Flat Jacks consist of a mild steel capsule, circular in plan, with a cross section that is initially dumb-bell shaped.
The dished portions are fitted with machined steel thrust plates before the jack is placed between the surfaces to be jacked apart.
The jacks are fitted with two pipes – one inlet and one outlet. The inlet pipe is fitted with an isolator valve as standard and can also be fitted with a pressure gauge.
The outlet pipe is fitted with a plug and is normally only used to vent air from the system.
When the jack is inflated with a liquid the hydrostatic pressure is transmitted by the flat portions of the capsule through the thrust plates, while the toroidal rim of the jack deforms to allow the flat faces to move apart.
- Structural Pre-loading
- Bearing Replacement
- Control of Settlement
- Weighing of Large Structures
- Known Forces
- Precise Control
- Low Height for Limited Access
- Low Bearing Stresses
- Out of Parallel Tolerance Between Jacking Surfaces
When Flat jacks are to be used for a limited period and subsequently removed, inflation is normally done with hydraulic fluid, however certain circumstances may necessitate the use of water instead. For small numbers of jacks, requiring relatively small volumes of oil or water simple hand pumps may be used for inflation. Larger jacks or large numbers of jacks may require electric or diesel powered pumps.
Single or multiple Flat Jacks can be inflated individually or using inter-connected circuits. Each jack has a needle valve for locking off and may be fitted with a pressure gauge if required.
Hydraulic circuits of any complexity can be made with steel tubing or flexible hydraulic hoses.
When a force is required to be permanently applied to a structure, Flat Jacks are inflated under pressure with grout by means of a special purpose injection cylinder. The grout 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.
The mixing and pumping of grout requires considerable skill and experience. Incomplete mixing with hardener may result in a failure to set. Excessive accelerator may cause premature setting which will cause blockage, frustrate the Flat Jack operation and damage the pumping equipment.
For these reasons grout injection equipment is not hired to third parties and Fagioli contract services is strongly recommended.
Single special transfusion Flat Jack method:
At individual jack locations single special transfusion jacks, which are purpose designed and manufactured, can be transfused with grout under load. The pressure within the Flat Jack is balanced and maintained throughout the process of transfusing.
Additional Flat Jack method:
If adequate space is available two Flat Jacks may be installed one above the other. The first is inflated with hydraulic fluid and adjusted as necessary over a period of time until the required condition is achieved. The second Flat Jack is inflated directly with grout. The pressures in both Flat Jacks are balanced throughout until the oil or water Flat Jack has closed and the inflation is complete on the grout Flat Jack. This will then harden and both jacks will remain in place.