JP3677705B2 - Base isolation structure on soft ground - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a base-isolated structure foundation on soft ground, and in particular, a base-isolated structure on soft ground in which the horizontal displacement of the base portion in soft ground is suppressed to effectively function the base-isolated device provided on the base section. Regarding the basics.
[0002]
[Prior art]
In recent years, construction of medium to high-rise buildings has been developed in a coastal area called the waterfront. Many of these coastal areas have been constructed by reclaiming relatively shallow coastal areas in the past, and the buried soil and its lower layer are soft sands, soft silts, and clay soils that can be liquefied. The ground is often thickly deposited.
[0003]
By the way, it is known that relatively long-period components of seismic motion transmitted in these soft grounds during earthquakes are dominant. Therefore, when a building with a seismic isolation foundation is constructed on such soft ground, the long-period component outstanding in the soft ground is close to the natural period of the building, causing a resonance phenomenon, and the vibration of the building is remarkable. There is a risk of amplification. For this reason, buildings with seismic isolation structures had to be constructed on relatively good quality ground.
[0004]
On the other hand, the applicant has proposed a structure 50 based on the continuous underground wall 60 shown in FIG. 8 (see Japanese Patent Laid-Open No. 8-27810). This structure 50 is provided with a foundation pile 54 and a continuous underground wall 60 that extend from the ground surface 51 through the surface ground 52 to the support ground 53, and the upper structure (building) is formed on the basis of the continuous underground wall 60. It comes to support. Furthermore, the rigidity of the seismic isolation device 62 interposed between the continuous underground wall 60 and the upper structure 61 was set so that the resonance frequency of the upper structure 61 was different from the resonance frequency of the surface ground 52.
As a result, in the structure having the illustrated continuous underground wall 60, the rigidity of the entire foundation is enhanced by the continuous underground wall 60 provided so as to surround the foundation pile 54, and the ground portion surrounded by the continuous underground wall 60. While the known seismic isolation device 62 (laminated rubber isolator) having a predetermined rigidity is disposed below the upper structure 61, the natural period of the upper structure 61 is lengthened, and the resonance frequency is reduced. Can be prevented from matching.
[0005]
[Problems to be solved by the invention]
However, since the foundation shown in FIG. 8 is provided with a continuous underground wall 60 of reinforced concrete so as to surround a large number of foundation piles 54, it is particularly effective when the weight of the upper structure 61 is large as in a large-scale building. There is a problem that the construction cost of the foundation becomes excessive in a relatively small building. Moreover, the continuous underground wall 60 with thin wall thickness must be constructed | assembled accurately among the composite foundations of the continuous underground wall 60 and a foundation pile, and construction is troublesome.
[0006]
Therefore, the object of the present invention is to eliminate the above-mentioned problems of the prior art, make the short-period component of the foundation ground during an earthquake predominate, and not to resonate with the natural period of the building, ensuring a seismic isolation effect. It is to provide a seismic isolation structure foundation on soft ground.
[0007]
[Means for Solving the Problems]
In order to achieve the above object, the present invention comprises a composite foundation structure that supports an upper structure with a foundation pile and a block-like ground improvement body constructed so as to include a plurality of the foundation piles. In addition, a shear resistor is provided between the lower bottom plate of the double foundation bottom plate constructed on the upper surface of the block-shaped ground improvement body, and is exempted between the lower bottom plate and the upper bottom plate of the double foundation bottom plate. A seismic device is provided to support an upper structure constructed on the upper bottom plate .
[0008]
At this time, it is preferable that the block-like ground improvement body is a deep mixed stirring structure using a cement hardener.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an embodiment of a base isolation structure on soft ground according to the present invention will be described with reference to the accompanying drawings.
FIG. 1 is a part of a schematic cross-sectional view showing an example in which the base isolation structure according to the present invention is applied as the base structure 20 of the upper structure 10. In the geological layer illustrated in the figure, soft layers are distributed under the surface buried layer. As this soft layer, there is an alluvial sand layer, a soft silt layer, and a cohesive soil layer that may cause liquefaction. Below that is a relatively hard cohesive soil layer that functions as a support layer, a dune sand layer and a gravel layer. For example, a gravel layer as a support layer has an N value of 50 or more.
The base bottom slab 11 of the upper structure 10 has a double structure, and a seismic isolation device 13 is provided in the seismic isolation layer 12 sandwiched between the two bottom slabs 11A and 11B. In the present embodiment, in order to reduce the horizontal rigidity in the seismic isolation layer 12, a known laminated rubber isolator (hereinafter referred to as an isolator 13) as a seismic isolation device 13 is disposed with a uniform spacing in a plane. ing. The isolator 13 may be arranged so as to coincide with the columnar shape, or can be arranged regardless of the column position if the upper bottom plate 11A has sufficiently high rigidity.
On the other hand, the tip 21a of the pile foundation 21 is embedded in the support layer by a predetermined insertion length. In this embodiment, a cast-in-place pile is adopted as the pile foundation 21 because of the required support force, but various pile types such as steel pipe piles and ready-made PC piles are used depending on the field situation and application. be able to.
[0010]
In FIG. 1, a ground improvement body 30 is formed so as to surround a plurality (9 in the figure) of piles 21 at substantially the center position of the pile foundation. This ground improvement body 30 surrounds the circumference | surroundings of the pile 21 as a block in the state which closely contacted the small diameter columnar body which consists of soil cement, and is set as the planar shape shown in the figure (b). The construction depth reaches near the lower end of the soft layer.
In this way, by forming the ground improvement body 30 integrally with each pile at the substantially central position of the plurality of piles 21 reaching the support layer in the soft ground, the ground improvement body 30 has high rigidity at the time of an earthquake. Demonstrate and act to suppress the horizontal displacement of the entire foundation ground as much as possible.
Thus, the foundation ground including the pile foundation and the ground improvement body 30 is improved so that a relatively short period component is dominant. On the other hand, as shown in FIG. 1A, the upper structure 10 is supported by an isolator 13 disposed on the seismic isolation layer 12. For this reason, the rigidity of the horizontal direction of the base part which supports the upper structure 10 is suppressed low, and the natural period of the upper structure 10 becomes comparatively large. As a result, the response acceleration with respect to short-period vibration components such as earthquake motion is remarkably reduced, and the vibration of the upper structure 10 is greatly improved.
[0011]
Note that the basic structure, dimensions, horizontal rigidity, and other characteristics of the isolator 13 disposed in the seismic isolation layer 12 can be set as appropriate depending on the scale of the upper structure and the like, and various dampers (not shown) are provided as damping mechanisms. It is also possible.
[0012]
Hereinafter, the structure of the ground improvement body 30 constructed under the foundation bottom slab 11 on which the seismic isolation device of FIG. For simplification of the figure, the reference numerals of the piles 21 are given to only a part.
As shown in FIGS. 1 (a) and 1 (b), the ground improvement body 30 has a massive block shape at substantially the center of the bottom surface so as to include a plurality of piles 21 in plan view. For this reason, construction is easy and variation in the quality of the construction range can be suppressed. The strength reliability as the ground improvement body 30 is also high.
[0013]
FIG. 2A shows the ground improvement body 30 constructed with two blocks with respect to the foundation bottom plate 11. By dividing into two blocks as illustrated, the moment acting on the pile head of the pile 21 at the time of an earthquake can be dispersed, the stress of the foundation bottom slab 11 can be reduced, and the member thickness and the like can be reduced. Moreover, as shown in FIG.2 (b), it is also preferable to construct the ground improvement body 30 so that a planar shape may be circular and the whole may become a cylindrical shape as shown in FIG.2 (b). In this case, anisotropy in the strength and displacement resistance of the improved body can be eliminated, and the ground improvement effect of the ground improved body 30 can be exhibited in all directions.
[0014]
FIG. 3 shows an example in which an outer peripheral wall 31 made of a soil cement columnar body is constructed outside the foundation bottom slab 11 of the upper structure 10 in the ground where liquefaction may occur as shown in FIG. As shown in the figure, the outer peripheral wall 31 is constructed in a closed state so as to surround the pile foundation 21 that supports the upper structure 10.
The outer peripheral wall 31 initially functions as a temporary structure during the construction of the main body portion from the excavation stage of the underground portion of the upper structure 10. Furthermore, at the time of the main installation, the ground Gin in the outer peripheral wall 31 of the layer that may be liquefied and the ground Gout around the foundation function are divided. For this reason, it is possible to prevent the range of the excess pore water pressure that has risen locally on the ground Gout around the upper structure 10 from reaching the range within the ground Gin surrounded by the outer peripheral wall 31 within the time when the earthquake motion continues. Therefore, the increase in excess pore water pressure in the ground Gin range is not affected by the surrounding groundwater, so it can be minimized and the occurrence of liquefaction can be prevented or reduced. Decline can be prevented.
[0015]
In the present embodiment, a soil cement column wall equivalent to the ground improvement body 30 is used as the outer peripheral wall 31. For the soil cement column wall, use an auger or the like to mix a cement-based hardener into the soil in the ground where the hole is drilled to a specified diameter, and insert a stress bearing material such as H-section steel into the mixed part. Thus, a columnar body in which the soil cement part and the stress bearing material are integrated is formed, and the columnar bodies are arranged in the column direction to form one wall body.
[0016]
FIG. 4 is a schematic plan view showing an example in which the ground improvement body 30 is applied to the foundation of a structure having a planar shape elongated in one direction like an apartment house. As shown in the figure, by constructing the ground improvement bodies 30 at the end portions of the ends of the building foundation, it is possible to effectively act against the torsional behavior due to the eccentric horizontal force.
[0017]
In addition, in order to build the ground improvement body 30 mentioned above, if it is what is called a cement-type deep-layer mixing construction method, in addition to forming with a soil cement, in addition to an in-situ stirring method, an improved body formation method such as a jet mixing, a replacement type, etc. Can be applied. Moreover, it can carry out similarly to the example of construction of the conventional column wall or block also about the formation procedure of a block comprised by connecting a pile body, the amount of overlap, etc.
Portland cement and blast furnace cement are generally used as cement-based hardeners for soil cement, but depending on the soil properties of the target ground, mixed cement such as silica cement and fly ash cement can be used. .
It is also possible to use a delayed AE water reducing agent as an admixture to keep the cement slurry in a relatively long state, or to use bentonite as an admixture.
[0018]
By the way, since most of the load of the building as an upper structure is always borne by the pile foundation, the building vertical load borne by the ground improvement body is set small. At this time, since the shear force transmitted from the bottom of the building foundation to the ground improvement body during an earthquake is proportional to the vertical load acting on the top surface of the ground improvement body, it is expected that the shear force borne by the ground improvement body will be a small value. The Therefore, it is preferable to previously provide a shearing resistor for reliably transmitting a shearing force between the building bottom and the ground improvement body. Thereby, the shear force at the time of an earthquake can be reliably transmitted to a ground improvement body from a building bottom face.
[0019]
Hereinafter, an example of the shear resistor formed on the joint surface between the bottom surface of the building foundation and the upper surface of the ground improvement body will be described with reference to FIGS.
FIG. 5A shows a protrusion 41 made of a part of the foundation bottom slab 11 of the building protruding from the upper surface 30a of the ground improvement body. In this example, when placing the uppermost layer of the ground improvement body 30, a box-shaped frame having a predetermined shape is disposed to form a recess at a predetermined position on the improvement body upper surface 30a. A protrusion 41 is formed by driving the foundation bottom plate 11. FIG. 5B shows a modification in which protrusions 42 are formed on the ground improvement body upper surface 30a. In this modification, when the uppermost layer of the ground improvement body 30 is driven, the inside of the mold frame arranged in a predetermined shape may be driven integrally to form the protrusion 42 made of the improved body.
In this way, a shear resistor is formed in a state in which the irregular surface of a predetermined shape is engaged with the joint surface between the building foundation bottom surface 11a and the ground improvement body upper surface 30a. Thereby, integration with the building foundation bottom face 11a and the ground improvement body upper surface 30a will be achieved. As examples of the planar shape of the concave and convex meshing surfaces, various shapes other than the shapes shown in the respective drawings of FIG. 6 can be used. Any of these shapes is preferably a shape in which the difference in strength of shear resistance with respect to the direction in which the shear force acts (the direction in which the seismic force is input) decreases. Therefore, it is preferable to use an uneven shear resistor having a concentric or lattice shape over the entire upper surface 30a of the ground improvement body (see FIGS. 6A and 6B). In addition, it is preferable to set this uneven | corrugated shaped part so that it may become finer than the construction interval of the pile of a building foundation.
[0020]
FIG. 5C shows an example in which dowels are provided as shear resistors. The dowels 43 are first embedded in the uppermost layer of the ground improvement body 30 at predetermined intervals for positioning, and the building foundation bottom slab 11 is placed on the ground improvement body 30 in this state. Thus, since the whole dowel 43 as a shear resistance body is embedded at the joint surface position between the building foundation bottom surface 11 a and the ground improvement body 30, the shearing force from the building is reliably transmitted to the ground improvement body 30. . As the dowels, H-shaped steel, steel pipe, concrete filled steel pipe, precast concrete (PCa) pipe, PCa pile, etc. cut into short lengths can be used. Further, it is preferable that the dowels are arranged at substantially equal intervals vertically and horizontally as shown in FIG.
[0021]
It is also possible to construct the building foundation bottom slab 11 so as to surround the outer periphery of the upper end of the ground improvement body 30 as shown in FIGS. In this case, a part 30b on the outer peripheral side surface of the ground improvement body 30, which is an already-constructed part, becomes a mold, so that it is preferable to remove the dirt and deposits on the surface and roughen the surface. Thus, the surrounding wall 45 protrudes from the building foundation bottom surface 11a, and the upper end portion of the ground improvement body 30 is restrained to reliably transmit the shearing force from the building 10 as the upper structure portion to the ground improvement body 30. Can do. In addition, in the front view of FIG. 5, FIG. 7, illustration of a part of seismic isolation apparatus 13 and the pile 21 is abbreviate | omitted for the simplification of a figure.
【The invention's effect】
As is apparent from the above description, according to the present invention, by increasing the ground rigidity, a relatively short period component becomes dominant in the foundation ground during an earthquake, and resonance with the natural period of the building is prevented. The effect is that the building's seismic isolation effect can be demonstrated with certainty.
[Brief description of the drawings]
FIG. 1 is a front view and plan view showing an embodiment of a seismic isolation structure foundation on soft ground according to the present invention.
FIG. 2 is a plan view showing another embodiment of the seismic isolation structure foundation on the soft ground.
FIG. 3 is a plan view showing another embodiment of the seismic isolation structure foundation on the soft ground.
FIG. 4 is a plan view showing another embodiment of the seismic isolation structure foundation on the soft ground.
FIG. 5 is a front view showing a partial cross section of a shear resistance body between a base bottom plate and a ground improvement body.
6 is a partial plan view showing an example of a planar shape of the shear resistor shown in FIG. 5. FIG.
FIG. 7 is a partial front view and a plan view showing the configuration of the surrounding wall of the upper end portion of the ground improvement body.
FIG. 8 is a front view showing an example of a seismic isolation structure foundation on a conventional soft ground.
[Explanation of symbols]
10 Superstructure 11 Base bottom 12 Seismic isolation layer 13 Isolator (Seismic isolation device)
20 foundation structure 21 pile 30 ground improvement body 31 outer peripheral wall

Claims (2)

It consists of a composite foundation structure that supports an upper structure with a foundation pile and a block-like ground improvement body constructed so as to include a plurality of the foundation piles, on the upper surface of the block-like ground improvement body A shear resistor is provided between the lower bottom plate of the double foundation bottom plate to be constructed, and a seismic isolation device is disposed between the lower bottom plate and the upper bottom plate of the double foundation bottom plate, Seismic isolation base on soft ground, characterized by supporting superstructure constructed in   2. The base isolation structure on soft ground according to claim 1, wherein the block-like ground improvement body is a deep mixed stirring structure using a cement hardener.

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