JPH1161849A - Base isolation structure foundation on soft ground - Google Patents

Abstract

PROBLEM TO BE SOLVED: To surely exert the base isolation effect of a building by constructing an upper structure on a foundation base plate supported by foundation piles and a ground improving body via a base isolation layer, and arranging a base isolation device in the base isolation layer. SOLUTION: A foundation base plate 11 is supported by foundation piles 21 and a block-like ground improving body 30 generated to surround multiple foundation piles 21 among all foundation piles 21. An upper structure 10 is constructed on the foundation base plate 11 via a base isolation layer 12, and base isolation devices 13, 13 supporting the upper structure 10 are arranged in the base isolation layer 12. A deep layer mixed/stirred structural body using a cement hardening agent is preferably used for the block-like ground improving body 30. Laminated rubber isolators may be horizontally arranged at uniform intervals for the base isolation devices 13. The ground rigidity is increased, the relatively short-period component excels at the time of an earthquake, the resonance with a building is prevented, and the base isolation effect is surely exerted.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a seismic isolation structure foundation on soft ground, and in particular, to suppress the horizontal displacement of the foundation part on soft ground so that the seismic isolation device provided on the foundation part can function effectively. It relates to seismic isolation foundation on soft ground.

[0002]

2. Description of the Related Art In recent years, construction of middle to high-rise buildings has been widely carried out in a waterfront area called a waterfront. Many such coastal areas have been reclaimed in the past by reclaiming relatively shallow sea coastal areas, and the buried soil and the lower layers are composed of sandy layers, soft silt, and cohesive soils that may be liquefied. The ground is often thick.

By the way, it is known that a relatively long-period component of a transmitted ground motion is predominant in such soft ground during an earthquake. Therefore, when a building with a seismic isolation structure foundation is constructed on such soft ground,
In a soft ground, a prominent long-period component approaches the natural period of a building, causing a resonance phenomenon, and the vibration of the building may be significantly amplified. For this reason, buildings with seismic isolation foundations had to be built on relatively high quality ground.

On the other hand, the applicant has proposed a structure 50 based on a continuous underground wall 60 shown in FIG.
-27810). This structure 50 has a surface 51
A foundation pile 54 and a continuous underground wall 60 are provided so as to reach the support ground 53 through the surface ground 52 from above, and the superstructure (building) is supported based on the continuous underground wall 60. . Further, the continuous underground wall 60 and the upper structure 61
The rigidity of the seismic isolation device 62 interposed between the
1 was set to be different from the resonance frequency of the surface ground 52. As a result, in the illustrated structure having the continuous underground wall 60, the rigidity of the entire foundation is increased 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 is improved. Can be shortened, and the natural period of the upper structure 61 is increased by disposing a known seismic isolation device 62 (laminated rubber isolator) having a predetermined rigidity below the upper structure 61 to increase the resonance frequency. Can be prevented from matching.

[0005]

However, the foundation shown in FIG. 8 has a continuous underground wall 60 made of reinforced concrete surrounding a plurality of foundation piles 54, so that the weight of the upper structure 61 is large like a large-scale building. Although it is particularly effective for large buildings, there is a problem that construction costs for foundations are excessive for relatively small buildings. Further, of the composite foundation consisting of the continuous underground wall 60 and the foundation pile, the continuous underground wall 60 having a small wall thickness must be accurately constructed, and the construction is troublesome.

Accordingly, an object of the present invention is to solve the above-mentioned problems of the prior art, to make short-period components of the foundation ground during an earthquake prominent, to prevent resonance with the natural period of the building, and to improve the seismic isolation effect. An object of the present invention is to provide a seismic isolation structure foundation on soft ground that can be obtained reliably.

[0007]

In order to achieve the above object, the present invention is supported by a foundation pile and a block-like ground improvement formed to include a plurality of the foundation piles. An upper structure is constructed on a base slab via a seismic isolation layer, and a seismic isolation device for supporting the upper structure is provided in the seismic isolation layer.

At this time, it is preferable that the block-shaped ground improvement body is a deep mixing and stirring structure using a cement hardening material.

[0009]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view showing an embodiment of a base for a base isolation structure on soft ground according to the present invention. FIG. 1 is a part of a schematic structural sectional view showing an example in which a seismic isolation base structure according to the present invention is applied as a base structure 20 of an upper structure 10. In the stratum illustrated in the figure, a soft layer is distributed under a surface buried layer. Alluvial sand layers and soft silt layers, which may cause liquefaction,
Cohesive soil layer is located. Underneath are a relatively hard cohesive soil layer, a sedimentary sand layer and a gravel layer, which function as support layers. For example, a gravel layer as a support layer has an N value of 50 or more. The base slab 11 of the upper structure 10 has a double structure, and is provided with a seismic isolation device 13 in a seismic isolation layer 12 sandwiched between two bottom slabs 11A and 11B. In the present embodiment, in order to reduce the horizontal rigidity of the seismic isolation layer 12, a laminated rubber isolator (hereinafter, referred to as a seismic isolation device 13) is used.
It is described as an isolator 13. ) Are arranged at equal intervals in a plane. The isolator 13 may be arranged so as to match the column shape, or may be arranged regardless of the column position if the rigidity of the upper bottom slab 11A is sufficiently high. On the other hand, the tip 21a of the pile foundation 21 is inserted into the support layer by a predetermined insertion length. In this embodiment, cast-in-place piles are used for the pile foundation 21 in view of the required supporting force. Various types of piles, such as steel pipe piles and ready-made PC piles, are used according to the site conditions and applications. be able to.

In FIG. 1, a ground improvement body 30 is formed so as to surround a plurality (9 in the figure) of piles 21 located substantially at the center of the pile foundation. The ground improvement body 30 has a small-diameter columnar body made of soil cement in close contact with the periphery of the pile 21 as a block, and has a planar shape shown in FIG. 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 of the piles 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 during an earthquake. It behaves so as to minimize the horizontal displacement of the entire foundation ground as much as possible. As described above, the foundation ground including the pile foundation and the ground improvement body 30 is improved so that the relatively short-period component is dominant. On the other hand, FIG.
As shown in (a), the upper structure 10 is supported by an isolator 13 disposed on the seismic isolation layer 12.
For this reason, the rigidity of the foundation supporting the upper structure 10 in the horizontal direction is kept low, and the natural period of the upper structure 10 is relatively large. As a result, the response acceleration to a short-period vibration component such as a seismic motion becomes extremely small, and the vibration of the upper structure 10 is greatly improved.

The basic structure, dimensions, horizontal rigidity, and other characteristics of the isolator 13 provided on the seismic isolation layer 12 can be appropriately set according to the scale of the upper structure, and various dampers (not shown) as a damping mechanism. Can also be added.

Hereinafter, the configuration of the ground improvement body 30 installed under the base slab 11 on which the seismic isolation device of FIG. 1A is installed will be described. For the sake of simplicity of the drawing, the reference numeral of the pile 21 is given only to a part. The ground improvement body 30 is shown in FIG.
As shown in (a) and (b), the block has a massive block shape substantially at the center of the bottom surface so as to include a plurality of piles 21 in plan view. Therefore, the construction is easy, and the variation in the quality of the construction range can be suppressed. The strength reliability as the ground improvement body 30 is also high.

FIG. 2 (a) shows a ground improvement body 30 with a base slab 1
It is constructed with two blocks for one. By dividing into two blocks as shown in the figure, the moment acting on the pile head of the pile 21 at the time of the earthquake can be dispersed to reduce the stress of the foundation bottom slab 11, and the member thickness and the like can be reduced. In addition, it is also preferable to construct the ground improvement body 30 such that the improvement area has a circular planar shape and the entire area has a cylindrical shape as shown in FIG. 2B. In this case, the anisotropy in 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.

FIG. 3 shows an example in which an outer peripheral wall 31 made of a soil cement column is constructed outside the base bottom slab 11 of the upper structure 10 on the ground where liquefaction may occur as shown in FIG. is there. As shown in the figure, the outer peripheral wall 31 is constructed in a closed state so as to surround the pile foundation 21 supporting the upper structure 10. Outer wall 31
Initially functions as a temporary structure from the excavation stage of the underground portion of the upper structure 10 to the construction of the main body portion. In addition, at the time of the permanent installation, it functions so as to separate the ground Gin in the outer peripheral wall 31 of the layer which may be liquefied and the ground Gout around the foundation. For this reason, the upper structure
It is possible to prevent the range of the excessive pore water pressure locally increased in the surrounding ground Gout from reaching the range in the ground Gin surrounded by the outer peripheral wall 31. Therefore, the rise of excess pore water pressure in the range of ground Gin is not affected by surrounding groundwater,
Liquefaction can be prevented or reduced, and a decrease in the horizontal resistance of the foundation can be prevented.

In the present embodiment, a soil cement column wall equivalent to the ground improvement body 30 is used as the outer peripheral wall 31. Soil cement pillar row wall using auger etc.
A cement-based hardening material is mixed into the in-situ soil of the ground portion drilled to a predetermined diameter, and a stress-bearing material such as an H-beam is inserted into the mixed portion to integrate the soil cement portion with the stress-bearing material. To form a columnar body, and further arranging the columnar body in the row direction to form one column.
It is a piece of wall.

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 flat shape elongated in one direction, such as an apartment house. By constructing the ground improvement body 30 at the wives at both ends of the building foundation as shown in the figure, it is possible to effectively act on the twisting behavior due to the eccentric horizontal force.

In order to construct the above-mentioned ground improvement body 30, if it is a so-called cement-based deep mixing method, besides forming with soil cement, in addition to the in-situ stirring method, improvement such as injection mixing, displacement method, etc. The body formation method can be applied.
The procedure for forming the blocks formed by connecting the piles, the amount of overlap, and the like can also be performed in the same manner as in the example of the conventional column wall or block construction. Portland cement and blast furnace cement are generally used as cement-based hardening materials used for soil cement, but it is possible to use mixed cements such as silica cement and fly ash cement depending on the properties of the soil on the target ground. . It is also possible to use a delayed AE water reducing agent as an admixture to maintain the state of the cement slurry for a relatively long time, or to use bentonite as an admixture.

By the way, since most of the load of the building as the upper structure is always borne by the pile foundation, the vertical load of the building borne by the ground improvement body is set to be small. At this time, 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 ground improvement body upper surface. You. Therefore, it is preferable to provide a shear resistor between the bottom of the building and the ground improvement body in advance for reliably transmitting the shear force. As a result, the shearing force during an earthquake can be reliably transmitted from the building bottom to the ground improvement body.

FIG. 5 shows an embodiment 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.
This will be described with reference to FIGS. FIG. 5A shows a projection 41 formed on the ground improvement body upper surface 30a as a part of the foundation bottom slab 11 of the building.
Is to be protruded. In this example,
When the uppermost layer of the ground improvement body 30 is cast, a box frame having a predetermined shape is arranged to form a recess at a predetermined position on the upper surface 30a of the improvement body, and the base bottom slab 11 is driven into this recess. Protrusion 4
1 is formed. FIG. 7B is a modified example in which a projection 42 is formed on the upper surface 30a of the ground improvement body. In the present modification, when the top layer of the ground improvement body 30 is driven, the inside of the formwork arranged in a predetermined shape may be integrally driven to form the protrusion 42 made of the improved body. In this way, a shear resistor in a state in which the irregular surface of a predetermined shape is meshed with the joint surface between the building base bottom surface 11a and the ground improvement body upper surface 30a. Thereby, integration of the building base bottom surface 11a and the ground improvement body upper surface 30a is achieved. Examples of the planar shape of the meshing surface of the concavo-convex shape may be various shapes other than the shapes shown in the respective drawings of FIG. Any of the shapes is preferably a shape in which the difference in the strength of the shear resistance with respect to the direction in which the shear force acts (the direction of the seismic force input) is reduced. For this reason, it is preferable to form a concentric or lattice-shaped uneven shear resistor over the entire surface of the ground improvement body upper surface 30a (see FIGS. 6A and 6B). In addition, it is preferable to set this uneven | corrugated shape part so that it may become finer than the construction interval of the pile of a building foundation.

FIG. 5 (c) shows an example in which a dowel is provided as a shear resistor. Dowel 43
First, the lower half is buried at a predetermined interval in the uppermost layer of the ground improvement body 30 for positioning, and in this state, the building foundation slab 1
1 is cast on the ground improvement body 30.
In this manner, the entire dowel 43 as a shear resistor is buried at the joint surface position between the building base bottom surface 11a and the ground improvement body 30, so that the shear force from the building is transmitted to the ground improvement body 30 without fail. . As the dowel, an H-section steel, a steel pipe, a concrete-filled steel pipe, a precast concrete (PCa) pipe, a PCa pile, or the like cut into a short length can be used. Further, it is preferable that the dowels are arranged at substantially equal intervals vertically and horizontally as shown in FIG. 6C.

Further, as shown in FIGS. 7 (a) and 7 (b), it is also possible to construct the building foundation slab 11 so as to surround the outer periphery of the upper end of the ground improvement body 30. In this case, since a part 30b of the outer peripheral side surface of the ground improvement body 30, which is the already-constructed portion, becomes a mold, it is preferable to remove dirt and deposits on the surface and roughen the surface. In this way, the surrounding wall 45 is protruded from the building base bottom surface 11a, and the upper end portion of the ground improvement body 30 is restrained, so that the shearing force from the building 10 as the upper structure can be surely reduced.
Can be transmitted to In the front views of FIGS. 5 and 7, some of the seismic isolation devices 13 and the piles 21 are illustrated for simplification of the drawings.
Are not shown.

As is apparent from the above description, according to the present invention, by increasing the ground stiffness, a relatively short-period component becomes dominant in the foundation ground during an earthquake, and the natural period of the building and The effect is that the resonance of the building can be prevented and the seismic isolation effect of the building can be reliably exerted.

[Brief description of the drawings]

FIG. 1 is a front view and a plan view showing an embodiment of a base for a base-isolated structure on soft ground according to the present invention.

FIG. 2 is a plan view showing another embodiment of a base-isolated structural foundation on soft ground.

FIG. 3 is a plan view showing another embodiment of the base for a base-isolated structure on soft ground.

FIG. 4 is a plan view showing another embodiment of the base for a base-isolated structure on soft ground.

FIG. 5 is a front view showing a partial cross section of a shear resistor between the foundation bottom slab and the ground improvement body.

FIG. 6 is a partial plan view showing an example of a planar shape of the shear resistor shown in FIG. 5;

FIG. 7 is a partial front view and a plan view showing the configuration of the surrounding wall at the upper end portion of the ground improvement body.

FIG. 8 is a front view showing an example of a conventional base-isolated structure foundation on soft ground.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Upper structure 11 Base bottom plate 12 Seismic isolation layer 13 Isolator (isolation device) 20 Foundation structure 21 Pile 30 Ground improvement body 31 Outer peripheral wall

 ──────────────────────────────────────────────────続 き Continuation of the front page (72) Yoshimi Yoshimi, 1-3-2 Shibaura, Minato-ku, Tokyo Shimizu Corporation (72) Inventor Nobuo Mori 2-3-2 Shibaura, Minato-ku, Tokyo Shimizu Corporation Inside the corporation

Claims (2)

[Claims] An upper structure is provided via a base isolation layer on a foundation bottom slab supported by a foundation pile and a block-shaped ground improvement body formed to include a plurality of the foundation piles. A seismic isolation base on soft ground, wherein the seismic isolation device is constructed and installed in the seismic isolation layer to support the upper structure. 2. The seismic isolation base on soft ground according to claim 1, wherein said block-shaped ground improvement body is a deep mixing and stirring structure using a cement hardening material.