JP3320580B2 - Foundation construction method for offshore structures - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rubble foundation such as a breakwater constructed by constructing a rubble foundation at a predetermined height on a seabed and installing a caisson or the like on the rubble foundation. The present invention relates to a foundation construction method for a marine structure in which a friction member arranged between an upper surface of a foundation and a caisson is constructed by asphalt mastic.

[0002]

2. Description of the Related Art Offshore structures such as breakwaters and quays are constructed such that the seabed ground is leveled substantially uniformly and then stones and the like are dumped thereon to form a rubble foundation having a predetermined height. In general, a structure such as a caisson is placed and placed on a flat, flat surface. As the caisson that constitutes the offshore structure, a box-shaped member integrally formed of concrete, a member formed by coating concrete on a surface of a box-shaped structure made of steel plate with a predetermined thickness, and the like are used. . Since the above-mentioned caisson has a relatively small coefficient of friction with the upper surface of the rubble foundation, a sheet member for increasing friction is arranged on the lower surface of the caisson.

[0003] For example, in an example disclosed in Japanese Patent Publication No. Hei 5-19617, a rubber mat is integrally attached to the lower surface of a caisson, and the rubber mat increases the coefficient of friction between the caisson and the rubble foundation. It constitutes means for preventing slippage due to pressure. Also, the rubber mat is relatively expensive,
Due to the problem of workability and the like, a relatively thin material having a thickness of about 3 to 5 cm is often used. Therefore, when leveling the upper surface of the rubble foundation, it is necessary to make the unevenness as small as possible. On the other hand, as shown in Japanese Unexamined Patent Publication No. 4-176913, an asphalt mat is integrally attached to the bottom of a caisson. When the asphalt mat is used, the mat is relatively inexpensive, and the thickness of the mat can be arbitrarily formed. Can be favorably performed.

As described above, a marine structure constructed by arranging an asphalt mat on the lower surface of a caisson is constructed, for example, as shown in FIG. In the offshore structure 1 shown in FIG. 9, the rubble foundation 5 is constructed at a predetermined height after the surface of the seabed ground 2 is made substantially flat, and the rubble foundation 5 is constructed.
After flattening the upper surface of the asphalt mat 4, the asphalt mat 4 is laid, and the caisson 3 is placed on the asphalt mat 4. The asphalt mat 4 arranged between the rubble foundation 5 and the caisson 3 has a predetermined length and width, and
In many cases, a method of manufacturing a material having a thickness on land and laying it on a rubble foundation using a crane ship or the like is used. In contrast, when manufacturing a caisson, an asphalt mat may be integrally attached to the lower surface of the caisson, and a method of towing the caisson to the construction site and sinking it may be used.
This is performed as shown in the above-mentioned Japanese Patent Application Laid-Open No. 4-176913. And when using the construction method which sinks the caisson which attached the asphalt mat integrally on the site, it becomes possible to demonstrate the advantage that the on-site installation work of the offshore structure can be easily performed.

[0005]

As described above, when asphalt mats are manufactured at a harbor construction site, or when the asphalt mat is laid according to the upper surface, a method for manufacturing asphalt mats is used near the construction site. It is necessary to provide vacant lots, etc., asphalt plants and equipment for producing mats. Also, after manufacturing the asphalt mat,
After storing them temporarily and constructing the rubble foundation,
It is necessary to use a crane ship or the like in order to construct the asphalt mat immediately before constructing the caisson. However, at the construction sites of breakwaters and jetties on remote islands, there are many cases where there is no large open space near the construction site to produce asphalt mats. Means for producing each caisson are used. Then, the caisson is towed by a ship while floating on the sea, transported to the construction site, and can be submerged by injecting seawater into the caisson.

However, asphalt mats need to be transported to the site by barges or the like and laid on a rubble foundation using a crane ship. It is necessary to use it, and there is a problem that the construction unit price increases. In addition, harbor construction on remote islands, etc., requires construction in relatively rough seas, and when building rubble foundations in places where tidal currents are fast, construction using small stones is also possible. There is also the problem that it cannot be done. Furthermore, when laying asphalt mats on rubble foundations, port construction is being performed under a very large number of restrictions, such as taking into account the conditions of the sea area at the site. In addition to the above-mentioned problems, it is often difficult to secure large stones used for rubble foundations on remote islands, and for this reason, means such as transporting by boat from a distant mining site have been used. I have.

Further, as described above, when the seabed is relatively soft ground such as sand or mud, the rubble foundation is constructed after the ground is improved, but when the seabed is rock, For example, a construction method as shown in FIG. 10 is used. In the example shown in FIG. 10, the excavated portion 2b having a predetermined depth D is constructed on the seabed 2a of the bedrock, a rubble foundation 5 is constructed in the excavated portion, and the caisson 3 and the like are used as the asphalt mat 4. To build offshore structures. However, when the above-mentioned construction method is used, there is a problem that it takes a lot of time to excavate a solid rock, and the construction cost increases. In addition, it is not necessary to construct a rubble foundation especially for rock masses that can withstand the load of caisson, etc., but it is difficult to flatten the support foundation with the conventional method. Therefore, there is a problem in that once the bedrock is excavated, the construction of the rubble foundation and the construction of the caisson support base require a double effort.

[0008]

SUMMARY OF THE INVENTION The object of the present invention is to solve the above-mentioned problem of harbor construction on a remote island or the like, in which asphalt mastic is directly cast on the surface of a rubble foundation, and the fluidity of the asphalt mastic naturally increases. With the aim of providing a method of building an asphalt mastic layer in place of asphalt mat using flattening, placing a caisson on it and building a marine structure, To enable the construction of offshore structures by making it possible to build rubble foundations using small stones in fast-moving waters, and, when the seabed is rocky, to use asphalt mastic on the rocks. It is an object of the present invention to provide a method of constructing a foundation for a marine structure that can easily construct a support base by constructing a layer.

[0009]

SUMMARY OF THE INVENTION The present invention relates to a breakwater or the like constructed by constructing a rubble foundation at a predetermined height on the seabed ground and placing a caisson or the like on the rubble foundation via a friction member. Offshore structures. In the present invention, the asphalt mastic heated to have fluidity is poured into the upper surface of the rubble foundation, and the asphalt mastic is made to have substantially uniform thickness and substantially uniform flatness by using the fluidity of the asphalt mastic. An asphalt mastic layer is formed, and a structure such as a caisson can be positioned on the asphalt mastic layer. Further, in the present invention, it is also possible to form an asphalt mastic on the slope portion of the rubble base to form a slope portion protective layer, and to position a structure such as a caisson on the asphalt mastic layer. .

[0010] Further, in the present invention, a construction is performed in which the rubble foundation is divided into a plurality of layers, and the upper surface of one layer of the rubble foundation is coated with asphalt mastic having fluidity by heating. Every time when constructing and constructing the layer of the rubble foundation, repeatedly constructing the asphalt mastic and constructing the rubble foundation of a predetermined height, and after constructing the rubble foundation, Casting asphalt mastic on the surface to form an asphalt mastic layer with a substantially uniform thickness and a substantially uniform flatness,
A structure such as a caisson can be placed on the asphalt mastic layer. In addition to the above configuration,
When the seabed for constructing the offshore structure is rock, while performing work to correct the unevenness or slope of the rock,
Work is performed to remove the part of the rock mass with insufficient strength, form an asphalt mastic layer having a substantially uniform flat surface on the rock mass, and mount a structure such as a caisson on the asphalt mastic layer. It can also be removable.

[0011]

As described above, according to the present invention, after the top surface is flattened and a rubble foundation is constructed, asphalt mastic is flowed on the upper surface to form an asphalt mastic layer with less unevenness, and the asphalt By placing a caisson on the mastic layer, the coefficient of friction between the caisson and the rubble foundation can be set large. In addition, asphalt mastic flowing on the rubble foundation as described above is in a state where large unevenness does not occur due to fluidity in a state of being heated to a high temperature, and by a pressing action when a caisson is placed, Since it is naturally formed flat, the asphalt mastic layer can uniformly contact the lower surface of the caisson to exert its function as a friction increasing member. Furthermore, in the present invention, in the sea area where the tide is fast,
A rubble foundation is constructed by stacking small-diameter stones, and the surface is covered with asphalt mastic, and the stones are mutually fixed by asphalt mastic to prevent stones from being washed away by tide. Therefore, a rubble foundation on which relatively small stones are piled can be constructed, and construction work of a marine structure can be easily performed even at a construction site where large stones cannot be obtained.

In the method for constructing a foundation for a marine structure according to the present invention, the rubble foundation is divided into a plurality of layers without constructing the rubble foundation all at once due to waves and tidal currents in a tidal current area. When constructing the rubble foundation, asphalt mastic may be cast on the surface of each rubble foundation layer, and the rubble foundation layers may be stacked. In addition, it is possible to prevent the rubble foundation constructed of small stones from being washed away by the tide, and to cope with the situation of the construction site where only small stones can be used. In addition to the above configuration, when the seabed ground is a bedrock, it is not particularly necessary to excavate the bedrock to construct a rubble foundation, to level out unevenness of the construction ground, or to scrape off a part with insufficient strength. By performing only the construction, it is possible to configure a portion for placing asphalt mastic. Therefore, after the asphalt mastic is constructed on the bedrock so as to be flat, a structure such as a caisson can be placed, and the construction of the foundation can be simplified. Furthermore, in the present invention, it is only necessary to install an asphalt plant without using a crane ship such as a construction using asphalt mats or a yard for manufacturing asphalt mats. It can be used to easily perform asphalt mastic casting work.

[0013]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a marine structure according to the present invention. The example shown in FIG. 1 shows the structure of a marine structure 1 constructed by installing a caisson 3 on a rubble foundation 5 and, like the case of constructing a conventional marine structure, constructing a marine structure 1 A rubble foundation 5 is constructed at a predetermined height thereon, and a caisson 3 is installed via an asphalt mastic layer 10 provided on the rubble foundation 5.
In addition, the asphalt mastic constituting the asphalt mastic layer of the present invention is a mixture of asphalt and sand or other filler material using a plant, and heated to a high temperature to have fluidity. A material having a higher fluidity than the asphalt mastic used when producing the asphalt mat or the like can be used. And, when laying the asphalt mastic in water, even in a state where it is rapidly cooled by water, the asphalt fluidity can be exhibited in a good state, and the asphalt mastic is placed between the stones of the target rubble foundation. Is used as well as a property that enables a uniform thickness and uniformity of an asphalt mastic film to be formed on the surface of the rubble foundation.

In the present invention, as shown in FIGS. 2 to 4, for example, a temporary work scaffold 20 constructed from the land to the sea is used, and asphalt mastic having a high temperature and a high fluidity is used by using a bucket 15. The asphalt mastic layer 10 can be formed on the upper surface of the rubble foundation with a constant thickness and uniform flatness. The example shown in FIG. 2 shows a case where a small-scale offshore structure is constructed on an isolated island or the like. The temporary work scaffold 20 is constructed by extending the temporary work scaffold 20 from land, and is constructed on the temporary work scaffold 20. The crane 21 and the like are arranged. Then, the work of dumping the stones of the rubble foundation using the crane 21 and the construction of the asphalt mastic using the bucket 15 are performed, and the construction of the rubble foundation is performed without using the crane ship. It constitutes the equipment to obtain.

As described above, when the temporary work scaffold 20 is used, if the temporary work scaffold is arranged on the open sea side of the rubble foundation, the temporary work scaffold can exert the action of protecting the rubble foundation from the waves of the open sea. Therefore, the dumped stones can be protected from the tide even when building a rubble foundation by stacking relatively small stones. As shown in FIG. 2, after the rubble foundation is constructed to a predetermined length, the surface portion of the rubble foundation is leveled substantially flat, and the asphalt mastic 1 is formed using the bucket 15 suspended on the crane 21.
No. 1 is cast, and an asphalt mastic layer can be formed substantially flat using the fluidity of the asphalt mastic. As described above, when the asphalt mastic is cast using the bucket, as shown in FIG. 3, the lid member 16 of the bucket 15 is opened at a position close to the upper surface 6, and the flowability is large. The asphalt mastic 11 is allowed to flow.

As shown in FIG. 3, the operation of placing the asphalt mastic 1 using the bucket is as follows.
By performing the process repeatedly at predetermined intervals, an asphalt mastic layer 10 having a predetermined thickness can be formed on the upper surface 6 as shown in FIG. At the same time, asphalt mastic with high fluidity enters between the stones, causing the stones on the upper surface 6 to adhere to each other. And the condition of the rubble foundation can be favorably maintained until the caisson is installed. As described above, the rubble foundation constructed by casting asphalt mastic on the upper surface is in a state where no large unevenness is formed due to the fluidity of the asphalt mastic. In addition, even if there is some unevenness on the surface of the asphalt mastic layer, when a heavy caisson or the like is installed, the pressure of the caisson causes the asphalt mastic layer to move between the flat part on the lower surface of the caisson and the unevenness on the upper surface. Thus, no gap is formed and the lower surface of the caisson is completely filled. Therefore, the asphalt mastic layer uniformly contacts the lower surface of the caisson, and the friction state between the caisson and the rubble foundation can be favorably maintained.

As shown in FIG. 2, in addition to the case of constructing a marine structure using a temporary work scaffold, as in the case of a conventional general marine structure construction method, a rubble foundation is constructed using a crane ship. Construction and casting of asphalt mastic layers can also be performed. Furthermore, a short temporary work scaffold was constructed from the land and construction was performed for a short section on the land side of the offshore structure.After that, a crane was installed using the constructed offshore structure, and construction of the rubble foundation and asphalt It is also possible to perform the work of casting a mastic layer. In other words, when using the construction method described above, the construction of the rubble foundation is divided into short sections and constructed according to the size of the caisson,
It is possible to use a construction method of forming a coating with an asphalt mastic layer and placing a caisson one by one on the rubble foundation to extend the offshore structure. And if the extended offshore structure is used as a scaffold for construction, all other work can be done from the land simply by transporting and installing the caisson at sea, so without using many large ships etc. This makes it possible to construct offshore structures.

Apart from the construction method as shown in FIG. 2, in the present invention, it is also possible to construct a rubble foundation in a sea area with a very fast tide using a relatively small stone. For example, in the example shown in FIG. 5, after assuming the lower part of the rubble foundation as one layer, and constructing the lower rubble layer 8 at a predetermined height,
A protective asphalt layer 12 is constructed by flowing asphalt mastic on the surface of the lower rubble layer 8, and small stones are fixed to each other via asphalt mastic in a state where the asphalt mastic enters between the stones, and are opposed to rough waves and tides. Let it do. Then, the quiet time of the next wave is selected, the upper rubble layer 9 is constructed, and asphalt mastic is cast on the surface of the upper rubble layer 9 as shown in FIG. Constituting a coating. Also, in the sea area where the waves are rough or the sea area where the tide is fast, the slope protection layer 13 is also applied to the side of the rubble foundation with a predetermined thickness,
To enable rubble foundations made of small stones to be protected from the tide. Further, before or at an arbitrary time when the caisson is constructed, asphalt mastic is caused to flow on the upper surface of the rubble foundation 5 to construct an asphalt mastic layer 10 having a predetermined thickness, and the caisson 3 is installed as shown in FIG. In this way, offshore structures can be constructed.

The offshore structure constructed as shown in FIG. 7 does not require the construction of a rubble foundation at one time.
It is possible to use a construction method such as dividing and constructing a layer of the rubble foundation within a quiet time or period of the wave. And, in the rubble foundation that is divided into multiple layers and constructed, the multiple layers are stacked in a state where the surface of the thin layer is covered with asphalt mastic, so that the rubble foundation is stable against tides and waves Can be maintained. In addition,
When constructing the offshore structure shown in FIG.
3 only needs to install the asphalt mastic between the stones to protect the stones on the slopes of the rubble foundation, and only apply the asphalt mastic layer of a certain thickness only on the upper surface of the rubble foundation. Since the support portion has a substantially uniform flatness, it is not necessary to use a large amount of asphalt mastic. Also, as described above, when constructing the offshore structure of FIG. 7, the construction method of sequentially extending the offshore structure from the land is used to shorten the section for constructing the temporary work scaffolding and the like. Also, construction can be performed without using a ship.

Each of the embodiments of the present invention described above is an example in which a rubble foundation is constructed when the seabed ground on which the structure is constructed is sandy or muddy. ,
Even when the seabed is rock, it can be dealt with simply by building an asphalt mastic layer. For example, FIG.
As shown in the figure, when the seabed ground is the bedrock 2a, the excavation portion 2c is formed by shaving the projection 2d or excavating a portion having a small supporting strength with respect to the ground for constructing a structure such as a caisson.
To construct. Then, rubble or the like is put into the excavated portion 2c of the weak portion to construct a substantially flat ground, and the leveling work is performed on the protruding portion 2d.
Asphalt mastic layer 10 corresponding to the construction part of
To construct.

As shown in FIG. 10, the asphalt mastic layer 10 should be formed to have a predetermined thickness after reducing asperities as much as possible when the seabed can be constructed relatively flat. Can be constructed by Further, as shown in FIG. 8, in a state where the seabed ground is inclined to one side, in correspondence with the inclination,
One side is thick and the other side is thin, caisson 3
The asphalt mastic is cast so that the supporting surface of the asphalt is substantially flat. Further, as shown in FIG. 8, in the case of the construction method in which the bedrock constituting the seabed ground is used as it is as a support base of the structure, stones and the like are discarded in a portion having a relatively large inclination angle of the ground. By doing so, the surface portion can be made substantially flat, and then the asphalt mastic can be cast into a predetermined thickness. Therefore, as described above, by constructing the asphalt mastic layer, it is easy to construct the supporting foundation for the structure by using the ground rock as it is, without particularly constructing the rubble foundation on the seabed ground. can do.

In addition to the above-described configuration, the present invention combines the construction method using the seabed rock as it is and the construction method for constructing the rubble foundation in accordance with the condition of the seabed ground, and performs the construction. The asphalt mastic can be cast after the upper surface of the support base of the above structure is formed substantially flat. Then, by constructing the surface of the asphalt mastic layer cast on the support base as having a substantially uniform flatness, it becomes possible to easily carry out construction of a caisson or the like. Also, when using the method of the present invention, after forming the support base substantially flat,
By casting the asphalt mastic, the asphalt mastic enters the gaps in the stones and acts to fill the recesses, thus exhibiting the effect of integrating the support base such as a caisson, and supporting the structure. It can be performed in good condition. Furthermore, when the support base is rock, the construction method that uses the support strength of the rock as it is can reduce the construction cost of offshore structures, and asphalt mastics can be directly placed on the support base. By casting, it is possible to increase the coefficient of friction for supporting a caisson or the like and stably support the structure.

[0023]

The marine structure foundation construction method of the present invention is as follows.
Since it is constructed as described above, after building the rubble foundation by flattening the upper surface, asphalt mastic is flowed on the upper surface to form an asphalt mastic layer with less unevenness, and the asphalt mastic is formed. A caisson can be placed on the layer to increase the coefficient of friction between the caisson and the rubble foundation. In addition, asphalt mastic flowing on the rubble foundation as described above is in a state where large unevenness does not occur due to fluidity in a state of being heated to a high temperature, and by a pressing action when a caisson is placed, Since it is naturally formed flat, the asphalt mastic layer uniformly applied to the lower surface of the caisson comes into contact with the caisson, and it is possible to exhibit the function as a friction increasing member. Further, in the present invention, in a sea area with a fast tide, etc., a relatively small-diameter stone is piled up to form a rubble foundation, the surface of which is covered with asphalt mastic, and the stones are mutually fixed with asphalt mastic, and the stones are rubbed by the tide. Can be prevented from being washed away. And
A rubble foundation piled with relatively small stones can be constructed, and construction work for offshore structures can be easily performed even at construction sites where large stones cannot be obtained.

Further, according to the present invention, in a sea area where the tide is fast, the rubble foundation may not be constructed at one time due to the condition of the waves and the tide.
When constructing the rubble foundation in a state where the rubble foundation is divided into a plurality of layers, the rubble foundation can be constructed by placing asphalt mastic on the surface of each rubble foundation and stacking the rubble foundation layers. In addition, it is possible to prevent the rubble foundation constructed of small stones from being washed away by the tide, and to cope with the situation of the construction site where only small stones can be used. Furthermore, in the present invention, it is only necessary to install an asphalt plant without using a crane ship such as a construction using asphalt mats or a yard for manufacturing asphalt mats. It can be used to easily perform asphalt mastic casting work.

In addition to the effects of the present invention as described above, in the present invention, when the seabed ground is rock, the rock is used as it is as a support base for the structure, without constructing a rubble foundation. The asphalt mastic layer can be applied to a predetermined thickness. When the above-described construction method is used, the strength of the rock can be used as it is, and the construction cost of the support base of the structure can be reduced, and the construction can be easily performed. .

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a marine structure constructed by the method of the present invention.

FIG. 2 is an explanatory view of construction work for asphalt mastic on a rubble foundation.

FIG. 3 is an explanatory diagram showing a state in which asphalt mastic flows down from a bucket.

FIG. 4 is an explanatory diagram of an operation of constructing an asphalt mastic layer on the upper surface of a rubble foundation.

FIG. 5 is an explanatory diagram of a construction in which a rubble foundation is divided into a plurality of layers and constructed.

6 is a cross-sectional view of a rubble foundation constructed by the method of FIG.

FIG. 7 is an explanatory view showing a state where a caisson is placed on the rubble foundation of FIG. 6;

FIG. 8 is a construction method of a marine structure in which an asphalt mastic layer is constructed on a bedrock and a caisson or the like is placed.

FIG. 9 is an explanatory view of a marine structure constructed using a conventional asphalt mat.

FIG. 10 is an explanatory view of a conventional example in which a marine structure is constructed on rock. It is explanatory drawing of a marine structure using the conventional asphalt mat.

[Explanation of symbols]

1 offshore structure, 3 caisson, 4 asphalt mat, 5 rubble foundation, 10 asphalt mastic layer, 11 asphalt mastic,
15 buckets, 20 temporary work platforms.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Tatsu Waki 1-2-29-4 Tsurugamine, Asahi-ku, Yokohama, Kanagawa Prefecture Inside World Engineering Co., Ltd. (72) Inventor Mitsuru Nonoda 2- 11-20 Tamagawa, Ota-ku, Tokyo No. Japan Road Co., Ltd. (72) Inventor Mitsuhiro Sato 3-13-1, Kyobashi, Chuo-ku, Tokyo Yuraku Building Inside Taisei Rotech Co., Ltd. (72) Inventor Katsuo Matsuzaki 2-10-9 Akasaka, Minato-ku, Tokyo (56) References JP-A-49-56414 (JP, A) JP-A-62-244921 (JP, A) JP-A-2-136142 (JP, A) JP-A 7-244 305322 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) E02D 23/02 E02B 3/06 301 E02D 23/00 E02D 23/16

Claims (4)

(57) [Claims] A rubble foundation is constructed at a predetermined height on the seabed ground and a caisson or the like is mounted on the rubble foundation via a friction member via a friction member. Pour the asphalt mastic heated to give fluidity to the upper surface, to form an asphalt mastic layer having a substantially uniform flatness with a substantially uniform thickness utilizing the fluidity of the asphalt mastic. A method of constructing a foundation for a marine structure, wherein a structure such as a caisson can be placed on the asphalt mastic layer. 2. A rubble foundation, such as a breakwater, constructed by constructing a rubble foundation at a predetermined height on the seabed ground and placing a caisson or the like on the rubble foundation via a friction member. Pour the asphalt mastic having the fluidity by heating the upper surface of the asphalt mastic and forming the asphalt mastic layer having the substantially uniform flatness with the substantially uniform thickness using the fluidity of the asphalt mastic. A marine structure characterized by forming an asphalt mastic on the slope portion of the rubble foundation to form a slope protection layer, and enabling a structure such as a caisson to be placed on the asphalt mastic layer. Basic construction method for objects. 3. A rubble foundation such as a breakwater constructed by constructing a rubble foundation at a predetermined height on the seabed ground and mounting a caisson or the like on the rubble foundation via a friction member. Is divided into a plurality of layers, and construction is performed to cover the upper surface of one layer of the rubble foundation with the asphalt mastic having fluidity by heating, and each time the rubble foundation layer is constructed By repeatedly constructing the asphalt mastic and constructing a rubble foundation having a predetermined height, and after constructing the rubble foundation, the asphalt mastic is substantially uniformly cast on the upper surface of the rubble foundation. Forming an asphalt mastic layer having a thickness and a substantially uniform flatness, and enabling a structure such as a caisson to be placed on the asphalt mastic layer; Law. 4. When the seabed on which the offshore structure is constructed is a bedrock, a work for correcting unevenness or an inclined portion of the bedrock is performed, and a work for removing a portion of the bedrock having insufficient strength is performed. Forming an asphalt mastic layer having substantially uniform flatness on the upper surface thereon, and enabling a structure such as a caisson to be placed on the asphalt mastic layer; .