1. Field of the Invention
This invention relates to a process for lowering building structures, such as shutdown nuclear power plants or building structures which are to be erected as they are lowered, wherein a caisson is constructed below a bed plate of the building structure, the caisson comprising a reinforced top plate for supporting the building structure by means of supporting elements and a reinforced concrete ring, which depends from the rim of the top plate and constitutes a cutting edge. At least one transfer duct, adapted to be lowered with the top plate, is provided adjacent the rim of the top plate and superatmospheric pressure is maintained, by means of air locks, in a working chamber which is defined under the top plate and enclosed by the ring.
2. Summary of the Invention
Processes of the above kind are disclosed in German Pat. application P 38 02 910.3, which is not a prior publication. In such processes the working chamber formed in the caisson communicates with one or more laterally disposed transfer ducts of reinforced concrete, which are used for the transfer of persons and material. In the working chamber a superatmospheric pressure is to be maintained which is sufficient to displace ground water, and which may amount to about 2 bars if the ground water table is to be lowered. To maintain this pressure, compressed air locks are provided in a portion of the transfer duct which is disposed above the surface. To prevent entry into the ground of the portion of the transfer duct which contains the air locks, as the caisson is progressively lowered, the transfer ducts must be extended at suitable intervals and the air locks must be shifted to higher locations. For these operations, the working chamber must temporarily be relieved of superatmospheric air pressure. Such relief of superatmospheric air pressure from the working chamber may result in an inrush of water, which may have an adverse effect on equipment in the working chamber and accordingly should be avoided.
It is an object of the invention to provide a process of the kind described in which superatmospheric air pressure can be maintained in the working chamber of the caisson even when the transfer duct must be extended after corresponding lowering steps.
In accordance with the invention, in a process of the kind described, the top plate is connected at its rim to the bed plate by a sealing wall and is formed with an aperture, which connects the working chamber to the approximately disk-shaped space between the top plate and the bed plate, and air locks extending through the sealing wall of the transfer duct are provided between said disk-shaped space and the transfer duct. In this process, superatmospheric air pressure in the working chamber can be maintained even when the transfer ducts must be extended due to the progress of the lowering operation. Material to be handled and persons who enter and leave may pass through air locks, which are provided in the disk-shaped space between the top plate and the bed plate of the building structure, on the one hand, and the transfer duct or ducts, on the other hand. Through this arrangement of air locks, atmospheric pressure may be maintained in the transfer ducts.
The maintenance of superatmospheric air pressure in the working chamber affords a further advantage in that changeable and uncontrollable load conditions, which could otherwise occur in the working chamber when it is relieved of superatmospheric air pressure, will be substantially prevented. The superatmospheric air pressure in the working chamber will result in an exactly defined back pressure, which produces lifting forces opposing the weight of the building structure. The forces acting on the caisson will depend on the weight of the building structure less the frictional forces acting between the caisson and the building structure, on the one hand, and the surrounding soil, on the other hand, and less the lifting forces which are due to the superatmospheric air pressure in the working chamber. The lifting forces which are due to the superatmospheric air pressure in the working chamber may amount to about 50% of the lowering pressure if considerable frictional forces are to be overcome. If the working chamber were temporarily relieved of the back pressure, this might result in an undesired and uncontrollable subsidence or slipping of the caisson.
The opening in the top plate is suitably disposed near the transfer duct or ducts so that the transfer paths may be short and it may be sufficient to provide the transfer ducts in a larger height than over the remaining region of the disk-shaped space.