JP3918461B2 - Construction method for underwater foundation - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a construction method for an underwater foundation, and more particularly to a construction method for constructing an underwater foundation by effectively applying a suction load.
[0002]
[Prior art]
When constructing foundations in the water, in the case of pile foundations, mechanical forces such as hammering and vibration can be used as pushing force, but for example, in deep foundations such as large revetments and offshore structures, Such mechanical force makes it difficult to set the foundation structure.
[0003]
Therefore, as one of the methods for constructing such a deep water foundation, a construction method called a suction foundation construction method in which a caisson housing with a skirt is submerged in the bottom of the ground using a pressure difference (suction). There is.
[0004]
The skirt foundation that is laid down by using such suction is expected to expand its application as a new foundation form that replaces the caisson foundation and the steel pipe sheet pile caisson foundation.
[0005]
FIGS. 4 and 5 show an example of a construction method for such a suction foundation. In the suction foundation construction method shown in FIG. 4, the caisson housing 1 has a cylindrical side wall 2 and a side wall 2. The bottom plate portion 3 that closes the lower end side of the bottom plate portion 3 and the skirt portion 4 that extends below the outer peripheral edge of the bottom plate portion 3 are provided.
[0006]
The skirt portion 4 is formed in a cylindrical shape. A pumping passage 5 is formed through the wall thickness of the skirt portion 4, and a pumping pipe 6 is provided so as to pass through the bottom plate portion 3. When constructing the underwater foundation, the caisson housing 1 is sunk in the underwater ground 7.
[0007]
At this time, the lower end side of the skirt part 4 is penetrated into the bottom ground 7 to prevent the inflow of water, and in this state, the water in the skirt part 4 passes through the drainage passage 5 and the bottom plate part 3. 6, thereby reducing the pressure in the skirt portion 4, generating a suction load inside and outside the skirt portion 4, and using this suction load, the caisson housing 1 is set in the bottom of the ground. .
[0008]
After that, in order to use the caisson housing 1 as a foundation, as shown in FIG. 5, a curable grout 8 such as mortar is provided between the bottom plate portion 3 and the water bottom ground 7 after being laid down to a predetermined depth. It is effective to remove the water from the bottom ground 7 surrounded by the skirt portion 4 through the pumping pipe 6 and to transfer the suction load to the bottom ground 7.
[0009]
Such an underwater foundation construction method is relatively easy to construct, and the caisson housing 1 sufficiently embedded in the submerged ground 7 has an advantage that resistance to sliding, falling, and pulling is remarkably increased.
[0010]
However, as shown in FIGS. 4 and 5, when the water bottom ground 7 includes the soft layer 7a and the support layer 7b below the soft layer 7a, the tip of the skirt portion 4 reaches the support layer 7b. Thus, the curable grout 8 is filled between the soft layer 7 a and the bottom plate portion 3. In such a caisson housing 1, since a load is applied to the soft layer 7a, continuous settlement occurs due to consolidation of the soft layer 7a even after the superstructure is completed.
Therefore, conventionally, in order to prevent such continuous settlement, {circle around (1)}. A method of removing the soft layer 7a. (2). A method in which the soft layer 7a is consolidated in advance or stabilized by ground improvement. (3). When the caisson housing 1 is inserted into the suction portion, any auxiliary method of applying a larger preload load to the tip of the skirt portion 4 by applying a larger load than that in service has been adopted.
[0011]
However, such conventional underwater foundation construction methods have the following technical problems.
[0012]
[Problems to be solved by the invention]
That is, in the auxiliary method (1), a great excavation and removal cost is required due to the removal of the soft layer 7a. Further, in the auxiliary method (2), it takes a long time for the soft layer 7a to settle down, and the construction period is very long.
[0013]
Furthermore, in the auxiliary method (3), if the specification can withstand a preload load larger than the load at the time of service, the structure of the caisson housing 1 becomes excessively large, resulting in a very uneconomic design.
[0014]
The present invention has been made in view of such conventional problems, and the object of the present invention is to construct an underwater foundation that can be economically constructed while avoiding a prolonged construction period. Is to provide.
[0015]
[Means for Solving the Problems]
To achieve the above object, the present invention provides a caisson housing including a cylindrical side wall, a bottom plate portion formed at the lower end of the side wall, and a skirt portion extending below the bottom plate portion. In the construction method of an underwater foundation to be submerged in the bottom ground by applying a suction load, after placing a pile foundation in which the lower end reaches the support layer in the bottom bottom ground of the caisson frame to be submerged, the pile foundation The caisson housing is laid down so that the tip of the skirt portion reaches the support layer from above, and the two are integrated to construct an underwater foundation for sharing the service load between the pile foundation and the caisson housing It is a construction method, and the pile foundation is a preload load that can be applied to the tip of the skirt portion when the caisson housing is submerged in the water bottom ground, and an upper work is constructed on the caisson housing. Based on the use load, the difference between the two load estimating operation to the difference so as to share the load bearing capacity of the pile foundation.
[0016]
According to the construction method of the underwater foundation constructed in this way, after placing the pile foundation where the lower end reaches the support layer in the water bottom ground at the location where the caisson frame is to be set up, the tip of the skirt from above the pile foundation So that the caisson body is laid down so that both can reach the support layer , both are integrated, and the service load is shared between the pile foundation and the caisson body, so there is no need to share the load on the bottom ground, Consolidation subsidence does not occur when water service is applied.
[0017]
If consolidation settlement of the water bottom ground due to the service load does not occur, it is not necessary to remove or improve the ground, and it is not necessary to design the skirt portion to have a structure that can withstand a preload larger than the service load .
[0019]
The integration of the caisson housing and the pile foundation is accomplished by projecting the head of the front pile foundation into the space surrounded by the skirt under the bottom slab and injecting and filling a hardenable grout material into the space. Can be done.
[0020]
DETAILED DESCRIPTION OF THE INVENTION
DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments of the invention will be described in detail with reference to the accompanying drawings. 1 to 3 show an embodiment of a construction method for an underwater foundation according to the present invention.
[0021]
The underwater foundation shown in the figure includes a caisson housing 10 and a pile foundation 11. The caisson housing 10 includes a cylindrical or rectangular side wall portion 12, a bottom plate portion 14 that closes the lower end side of the side wall portion 12, and a skirt portion 16 that extends below the outer peripheral edge of the bottom plate portion 14. .
[0022]
The bottom plate portion 14 is formed in a disk shape having a predetermined thickness, and the peripheral edge is integrally fixed to the inner surface of the side wall portion 12. The skirt part 16 is formed in the same cross-sectional shape in which the lower side of the side wall part 12 is extended downward without changing the diameter as it is, and the pumping passage 18 has a predetermined interval in the circumferential direction within the wall thickness. A plurality of through holes are formed at intervals.
[0023]
A plurality of pumping pipes 20 are attached to the bottom plate portion 14 so as to penetrate the bottom plate portion 14 in the vertical direction. The caisson housing 10 configured as described above is constructed in a ground production yard or the like, towed to a foundation construction site, and is sunk in the submarine ground 26. In the construction method of the underwater foundation according to the present embodiment, before the caisson housing 10 is set, the pile foundation 11 is first placed in the bottom ground 26 as shown in FIG.
[0024]
The pile foundation 11 is composed of a plurality of pieces. The pile foundation 11 of the present embodiment is driven so as to pass through the soft layer 26a on the surface layer side of the water bottom ground 26, and the lower end thereof reaches the support layer 26b.
[0025]
In the case of a present Example, the load supporting force of the multiple pile foundation 11 is set as follows. In determining the load bearing capacity of the pile foundation 11, first, a preload load that can be applied to the tip of the skirt portion 16 when the caisson housing 10 is sunk on the water bottom ground 26 is obtained.
[0026]
This preload load includes the weight of the caisson housing 10 including the cylindrical side wall 12, the bottom plate portion 14 formed at the lower end of the side wall 12, and the skirt portion 16 extending below the bottom plate portion 14, It is obtained as an added value of the ballast load that can be applied to the caisson housing 10 and the suction load generated by the pressure reduction in the skirt portion 16 of the caisson housing 10.
[0027]
The ballast load in this case is determined from the weight of water injected into the side wall 12 and a weight that can be placed. The caisson housing 10 calculates a necessary supporting force in consideration of a predetermined safety factor in the service load when the superstructure is constructed, and an appropriate cross-sectional shape is designed according to a normal design method from the supporting force. Has been.
[0028]
Next, the difference between the service load when the superstructure is constructed in the caisson housing 10 and the above-described preload load is estimated, and the load amount of this difference is set as the load supporting force required for the pile foundation 11. The number, cross-sectional shape, arrangement interval, etc. of the pile foundation 11 are determined based on the supporting force.
[0029]
When such a determination of the pile foundation 11 is completed, the pile foundation 11 selected based on the determination is placed in the water bottom ground 26 as shown in FIG. The pile foundation 11 in this case is driven so that each is located in the skirt part 16 of the caisson housing 10 sunk after that.
[0030]
The pile foundation 11 is driven so that the pile heads of each pile protrude slightly upward from the surface of the water bottom ground 26, and when a predetermined number of placements are finished, then, as shown in FIG. Will be installed.
[0031]
The caisson housing 10 is laid so as to cover the pile foundation 11 from above. In this installation, the lower end side of the skirt portion 16 is penetrated into the water bottom ground 26 to prevent the inflow of water. In this state, the water in the skirt portion 16 is passed through the pumping passage 18 and the pumping pipe 20. Thus, the pressure in the skirt portion 16 is lowered to generate a suction load on the inside and outside of the skirt portion 16, and the caisson housing 10 is sunk in the submarine ground 26.
[0032]
Then, the skirt portion 16 gradually penetrates into the water bottom ground 26, and the caisson housing 10 is laid down. As shown in FIG. 3, the bottom plate portion 14 comes close to the upper surface of the water bottom ground 26, and the skirt portion 16 When the lower end penetrates to the predetermined depth of the support layer 26b, the caisson housing 10 is completely set.
[0033]
In this set-up state, a space 28 defined by the surface of the water bottom ground 26 and the skirt portion 16 is formed on the lower surface side of the bottom slab portion 14, and the pile head of the pile foundation 11 protrudes into the space 28. ing.
[0034]
Next, in this state, filling and injection of the curable grout material 30 is performed in the space 28, and when the grout material 30 is cured, the caisson housing 10 and the pile foundation 11 are integrated to construct an underwater foundation. Is completed.
[0035]
Now, according to the construction method of the underwater foundation performed as described above, after placing the pile foundation 11 having the lower end reaching the support layer 26b in the water bottom ground 26 at the location where the caisson housing 10 is to be set, 11, the caisson housing 10 is laid down and integrated, and the service load is shared between the pile foundation 11 and the caisson housing 10, so that it is not necessary to share the load with the submarine ground 26. Consolidation settlement of the bottom floor does not occur when a load is applied.
[0036]
If consolidation settlement of the water bottom ground 26 due to the service load does not occur, it is not necessary to remove the soft layer 26a or improve the ground, and the skirt portion 16 needs to be designed to have a structure that can withstand a preload larger than the service load. Therefore, it is possible to construct economically while avoiding a prolonged construction period.
[0037]
That is, in the present embodiment, when the caisson housing 10 is designed under normal design conditions, the load that is insufficient due to the preload load that can be applied to the caisson housing 10 is shared by the pile foundation 11 that is previously placed. ing.
[0038]
According to such a construction method, regardless of the strength and thickness of the soft layer 26a, the caisson housing 10 can be made rational and economical, and the settlement characteristics can be improved.
[0039]
In the above embodiment, the case where the present invention is applied to the cylindrical caisson casing 10 is illustrated, but the implementation of the present invention is not limited to the caisson casing 10 of this shape. It is also possible to apply to the caisson housing 10 having other shapes such as a caisson housing having a shape partitioned by a partition wall.
[0040]
【The invention's effect】
As described above in detail in the embodiment, according to the construction method for an underwater foundation according to the present invention, it is possible to construct economically while avoiding a prolonged construction period.
[Brief description of the drawings]
BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a cross-sectional explanatory view of an initial process showing an embodiment of an underwater foundation construction method according to the present invention.
FIG. 2 is an explanatory cross-sectional view of a process performed subsequent to the process of FIG. 1;
3 is a cross-sectional explanatory diagram of a process performed subsequent to the process of FIG. 2. FIG.
FIG. 4 is an explanatory sectional view of an initial process showing an example of a conventional underwater foundation construction method.
FIG. 5 is an explanatory cross-sectional view of a process performed subsequent to the process of FIG. 4;
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 Caisson frame 11 Pile foundation 12 Side wall part 14 Bottom plate part 16 Skirt part 18 Pumping passage 20 Pumping pipe 26 Bottom ground 26a Soft layer 26b Support layer 30 Grout material

Claims (2)

A caisson housing including a cylindrical side wall, a bottom plate portion formed at the lower end of the side wall, and a skirt portion extending below the bottom plate portion is subjected to a suction load to be sunk in the water bottom ground. In construction method of underwater foundation,
In the submarine ground of the planned location of the caisson housing,
After placing the pile foundation where the lower end reaches the support layer, the caisson housing is sunk so that the tip of the skirt part reaches the support layer from above the pile foundation, and both are integrated,
An underwater foundation construction method for sharing in- service load between the pile foundation and the caisson housing ,
The pile foundation is based on the preload load that can be applied to the tip of the skirt when the caisson housing is submerged in the water bottom ground, and the service load when the superstructure is constructed on the caisson housing. Estimate the difference between both loads,
An underwater foundation construction method characterized in that this difference is shared as the load bearing capacity of the pile foundation . The caisson housing and the pile foundation are integrated by projecting the head of the front pile foundation into the space surrounded by the skirt portion under the bottom slab, and injecting and filling a hardenable grout material into the space. The construction method for an underwater foundation according to claim 1, wherein

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