GB2121854A - Marine structure constructing method - Google Patents

Abstract

A marine structure is constructed by placing on the seabed a topless, bottomless, hollow box-like caisson (1) having a pair of side walls (2) each consisting of arch shells, and a footing (3) extending along and outwardly from the bottom edge of each side wall, and introducing concrete into the cavity of the caisson on the seabed, and further introducing further filling, such as sand and earth, into the caisson cavity above the concrete layer. <IMAGE>

Description

SPECIFICATION Marine structure constructing method Background of the Invention This invention relates to marine structures, and more particularly, to a method of constructing a marine structure and an improved caisson.

Generally, marine structures for breakwater, revetment, quay and the like are constructed by fabricating a bottom-covered caisson on land, towing it by water to a destined site, and submerging it to rest on seabed. Since the caisson has a bottom cover in this case, the entire area of seabed ground on which the caisson is placed must be properly leveled. Such leveling operation is quite difficult and inefficient because of submarine operation, and the finishing accuracy is very low. Particularly at sites having a great depth of water, where the tide runs strong, and in muddy water, the ground leveling operation is very difficult and obstructive of overall construction process.

Another construction method uses a caisson free of a bottom cover. Bottomless caissons are economic since their area of contact with seabed ground is small so that the area of seabed ground required to be leveled is smaller than that required for bottom-covered caissons. Even in the case of bottomless caissons, an area of seabed ground must be leveled where it is irregular.

Under these circumstances, it was proposed to use provisional supports as shown in Fig. 6 and as disclosed in Japanese Patent Application Kokai No. 57-15727, for example. In this method, a limited area of seabed a on which provisional supports c are to rest is horizontally leveled with little treatment on the outside seabed area. A caisson d is placed on the provisional supports c on the leveled area and then filled with concrete at the bottom. This method can reduce the area of seabed to be leveled, but has several disadvantages. Where the depth of water is large, it is not necessarily easy to accurately rest the caisson don the provisional supports c. The provisional supports c are fabricated, transported, and submerged separately from the caisson d, adding to the cost of construction.Since only two provisional supports are placed longitudinal of the caisson, a great reaction results in the provisional supports when waves act on them immediately after placement. It is, therefore, required that the provisional supports be of great strength and the ground reaction be sufficient.

In addition to caissons having a rectangular cross section, caissons having a side wall consisting of a plurality of interconnected arcuate shells as shown in Fig. 5 are known in the art.

Such arcuate shell walls reduce the stress or bending moment exerted by external and internal forces due to their arch effect, considerably reducing the wall thickness or the material required with an economic benefit. In installing such an arcuate shell caisson on seabed using provisional supports as described above, calculation must be made on the assumption that the width of the caisson is an internal width B' rather than an external width B as shown in Fig. 5.

This is disadvantageous in view of stability calculation because of increased ground reaction, and thus uneconomic.

Summary of the Invention It is an object of the present invention to provide a method of constructing a marine structure in which a caisson may be steadily installed on a minimal leveled area without the need for accurate placement of provisional supports. on seabed.

Another object of the present invention is to provide a method of construction a marine structure without the need for fabricating and transporting provisional supports separately from a caisson body.

A further object of the present invention is to provide an improved arcuate shell type caisson whose external width is an effective width in structural calculation.

A still further object of the present invention is to provide an improved arched shell type caisson which requires minimized ground reaction when it is subjected to waves immediately after placement on seabed.

According to a first aspect of the present invention, there is provided a method of constructing a marine structure, comprising the steps of placing on seabed a hollow caisson which is open at the top and bottom and has a pair of side walls each consisting of at least one, preferably, a plurality of curved shells, and a footing extending outwardly from the bottom edge of each side wall; and introducing a filling into an interior cavity defined by the seabed and the hollow caisson. Preferably concrete is first introduced into the interior cavity of the caisson to form a lower concrete layer in staking relation to the seabed, and another filling material, for example, sand and earth, stone, silt and cement, industrial wastes, etc. is then introduced into the interior cavity above the concrete layer.

According to a second aspect of the present invention, there is provided a box-like caisson for use in constructing a marine structure, comprising a first pair of open top and bottom sides, a second pair of opposed longitudinal side walls each consisting of at least one, preferably a plurality of curved shells, a third pair of opposed transverse side walls, and a pair of footings extending longitudinally and outwardly from the bottom edge of the longitudinal side walls.

Brief Description of the Drawings The above and other objects, features, and advantages of the invention will be more fully understood by reading the following description in conjunction with the accompanying drawings, in which, Fig. 1 is a perspective view of one embodiment of the caisson according to the present invention; Fig. 2 is an elevational view of a marine structure constructed on seabed according to the present invention; Fig. 3 is a vertical cross section of the marine structure shown in Fig. 2; Fig. 4 is a plan view of the caisson shown in Fig. 1; Fig. 5 is a plan view of a prior art caisson; and Fig. 6 is an illustration of a caisson placed on a provisional support according to a prior art method.

Description of the Preferred Embodiments One preferred embodiment of the present invention will be described.

[Al Structure Referring to Figs. 1-4, a hollow caisson 1 according to the present invention is illustrated in the form of a box-like caisson of concrete which is topless and bottomless or has a first pair of open top and bottom sides. The caisson 1 has a second pair of opposed longitudinal side walls 2 each consisting of a plurality of interconnected curved shells such as arcuate or arched shells, and a third pair of transverse straight side walls joining the longitudinal side walls at their ends. The caisson 1 is usually fabricated by integrating a plurality of arched shells into a shell side wall, and joining two shell side walls by straight side walls into a unit.

The above-described caisson is of a well-known configuration. According to the present invention, the caisson 1 is provided with an integral footing 3 at the bottom edge of each longitudinal shell side wall 2. This footing 3 is a shelf-like member formed integral with the caisson 1 and extends throughout the length of the side wall 2 and outwardly from the bottom edge of the side wall 2 to form a straight edge. In the illustrated embodiment, the caisson 1 has in its interior longitudinal and transverse intermediate partitions 4 which are extended between the opposed side walls to enhance the strength of the caisson. The transverse partitions connect the connections between adjacent shells in the side walls 2 and extend parallel to the straight side walls. The longitudinal partition connects the straight side walls and crosses with the transverse partitions.

The partitions 4 need not be provided with a footing at their bottom edge.

[B] Construction After the caisson 1 is fabricated on land, it is transported to a destined site on the sea.

Transportation may be carried out by various methods, for example, by attaching a ceiling plate to the top of the caisson 1 to render it buoyant, using a ship equipped with a crane by which the caisson is suspended, or by attaching floats to the caisson to allow it to float.

The seabed on which a marine structure is to be constructed is leveled. More specifically, the limited linear areas of seabed on which the footings 3 are to rest are roughly leveled.

Then, the caisson is gently submerged and rested on the leveled seabed area. Since the footings 3 are formed integral with the caisson 1, the caisson 1 may be placed in a steady manner irrespective of some irregularities in the seabed.

After placement, concrete is introduced into the interior cavity of the caisson 1 to form a lower concrete layer 4. Since the concrete layer 4 fills recesses in the seabed ground, it serves to stake the caisson to the ground, providing a substantial resistance to horizontal forces by waves together with the caisson itself.

Construction is completed by introducing a filling 5 of sand and earth on the concrete layer 4 in the cavity of the caisson 1. In addition to sand, examples of the filling material 5 include stones, mixtures of silt and cement, muddy deposits and cement, or the like, and industrial wastes.

The present invention has the following advantages.

(a) The caisson may be rested on seabed in a sufficiently steady manner only by leveling the seabed over a narrow area on which the footings land, eliminating the need for leveling the seabed over a wider area in contact with an entire bottom surface as required in the case of bottom covered caissons. The labor and time required for leveling operation and concomitant danger may be substantially reduced.

(b) The footings formed integral with the caisson lead to a substantial reduction in work and construction period because the two step construction of first submerging provisional supports and accurately submerging and resting a caisson body on them as in the prior art is unnecessary.

(c) The presence of footings allows the external width B (shown in Fig. 4) between the footing outer edges to be used as the effective width in stability calculation. The present invention thus allows the ground reaction to be reduced lower than possible for the effective width in the prior art.

(d) When the caisson undergoes horizontal forces by waves in an unsteady condition immediately after placement on seabed, the footing integral with the caisson allow the ground reaction to be reduced.

Claims (8)

1. A method of constructing a marine structure, comprising the steps of placing on seabed a hollow caisson which is open at the top and bottom and has a pair of side walls each consisting of at least one curved shell, and a footing extending outwardly from the bottom edge of each said side wall, and introducing a filling into an interior cavity defined by the seabed and the hollow caisson.

2. The marine structure constructing method according to claim 1 wherein the filling introducing step includes introducing concrete into the interior cavity of the hollow caisson to form a lower concrete layer in staking relation to the seabed, and introducing another filling material into the interior cavity above the concrete layer.

3. The marine structure constructing method according to claim 2 wherein the other filling material is selected from the group consisting of sand and earth, stone, silt and cement, muddy deposits and cement, industrial wastes, and mixtures thereof.

4. The marine structure constructing method according to claim 1 or 2 wherein each of said side walls of the caisson consists of a plurality of interconnected curved shells.

5. A box-like caisson for use in constructing a marine structure, comprising a first pair of open top and bottom sides, a second pair of opposed longitudinal side walls each consisting of at least one curved shell, a third pair of opposed transverse side walls each connecting said longitudinal side walls, and a pair of footings extending longitudinally and outwardly from the bottom edge of said longitudinal side walls.

6. The caisson according to claim 5 wherein said longitudinal side walls each consist of a plurality of interconnected curved shells.

7. The caisson according to claim 6 which further comprises at least.one transverse partition connecting the connections between adjacent shells in the longitudinal side walls and extending parallel to the transverse side walls.

8. The caisson according to claim 7 which further comprises at least one longitudinal partition connecting said transverse side walls.