JPH1161848A - Pile stress reducing structure on soft ground - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce horizontal stress increase of vertical support piles at the time of an earthquake by providing the vertical support piles supporting a structure constructed on the soft ground and diagonal piles driven at prescribed tilt angles to surround the bottom face region of the structure. SOLUTION: Multiple vertical support piles 11 supporting a structure 1 constructed on the soft ground 2 are provided, and diagonal piles 12 are driven at prescribed angles to surround the bottom face region of the structure 1. The diagonal piles 12 are driven from the foundation bottom face and peripheral edge portion of the structure 1 so that their chips reach an intermediate layer which is a nonliquefying layer 3 under the soft ground 2. The horizontal force generated when a horizontal load is applied is borne as the horizontal component of the axial force of the diagonal piles 12, and the stress increase of the vertical support piles 11 can be suppressed. The stationary load is borne by the vertical support piles 11, and the horizontal load at the time of an earthquake is shared by the vertical support piles 11 and the diagonal piles 12.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a structure for reducing pile stress in soft ground, and more particularly to a structure for reducing stress acting on piles during liquefaction and lateral flow during an earthquake.

[0002]

2. Description of the Related Art Structures built on soft ground are generally supported by pile foundations on deeper support layers. When a large earthquake occurs in such soft ground, for example, when the soft ground is a loose alluvial sand layer, liquefaction occurs. In a seaside area where soft ground where liquefaction occurs occurs, a shoreline structure such as a seawall may move from a rear side (land side) to a front side (sea side) due to an earthquake. When the structure supporting one of the grounds moves in this way, a lateral flow occurs in which the ground on the back moves in the horizontal direction. A large horizontal displacement may occur on an abutment foundation such as a road bridge provided at such a position during an earthquake. In the Hyogoken-Nanbu Earthquake, the piles supporting the abutment generated excessive stress on the pile supporting the abutment due to the lateral flow of the back soil along with the seawall and the movement of the abutment. Damage was reported.

Conventionally, in order to prevent damage to a pile foundation due to a horizontal force due to an earthquake, in soft ground, ground improvement or the like is performed over a predetermined range including a foundation portion to prevent liquefaction and the like. Large diameter piles have also been used to increase the breaking strength of the piles. However, with regard to liquefaction and lateral flow, phenomena were not fully understood, and an appropriate design method was not established. Also, it is necessary to perform some seismic retrofitting work on the existing shoreline structure if there is a possibility of lateral flow.

[0004] By the way, since the inclined pile can bear a predetermined horizontal force by the horizontal component of the axial force, it is known that it plays an effective role for stabilizing the foundation on which a large horizontal force acts. For this reason, it is often used, for example, for suppressing the forward displacement of an abutment or the like on which a large ground pressure acts due to an uneven load due to a backfill, such as a foundation on which a ground portion receives a large horizontal force.

FIG. 6 is a schematic configuration diagram showing an example of a self-supporting substructure constructed on a liquefied layer 50 taking advantage of the inclined pile. FIG. 1 shows an upper structure 51 of a space truss located underwater, and a substructure 60 connected to the upper structure 51 by a fixed pedestal 52 and supporting the upper structure 51. The substructure 60 includes a plurality of slant piles 61 made of a concrete pile. The slanted pile 61 is cast at a predetermined angle with respect to the vertical direction. Pile 6
A ground improvement body 62 for solidifying the cement milk to increase the supporting force is formed on the front end side of 1. Further, a not-shown tension member (tie rod) is inserted from the upper end of the slant pile 61 to the inside of the ground improvement body 62 to integrate the slant pile 61 and the ground improvement body 62.

[0006]

The inclined pile used as shown in FIG. 6 is designed to bear a vertical load component of the above-ground structure. For this reason, it is necessary to construct the inclined pile so that the tip reaches the support layer. However, if the slant pile is cast from the bottom of the structure outward from the bottom of the structure at a predetermined inclination to a deep support layer, the tip position may be out of the site in some cases. For this reason, it has been difficult to use the slant pile for the foundation structure in a building or the like that is often constructed to fill the land.

[0007] It is known that a relatively long-period component is dominant in soft ground during an earthquake. On the other hand, structures having seismic isolation devices are increasing recently, and the predominant period of such structures is long. Therefore, when a seismic isolation device is used for a structure in soft ground, the predominant long-period component in the soft ground acts on the structure incorporating the seismic isolation device, and it approaches the long-period natural period of the structure. A resonance phenomenon may occur. In order to prevent this resonance phenomenon, it is conceivable to change the natural period of the seismic isolation device itself to a longer period, or to shorten the natural period of the ground-foundation system. In the former method, it is difficult to design a seismic isolation device such as a laminated rubber isolator, and it is expected that the displacement of the structure is also increased. For this reason, in the load-displacement (P-Δ) relationship of the pile, it is expected that an excessive displacement occurs and the pile is frequently damaged.

An object of the present invention is to solve the above-mentioned problems of the conventional technology and to provide an economical structure for reducing pile stress in soft ground having high horizontal resistance during an earthquake.

[0009]

In order to achieve the above object, the present invention provides a vertical support pile for supporting a structure constructed on soft ground, and a bottom surface of the structure on which the vertical support pile is disposed. A slanted pile which is cast at a predetermined inclination angle so as to reach an intermediate layer which is a non-liquefied layer under the soft ground from a base bottom surface and a peripheral portion of the structure so as to surround an area. A horizontal force generated when a horizontal load is applied is borne as a horizontal component of the axial force of the slant pile, thereby suppressing an increase in stress of the vertical support pile. The constant load is borne by the support pile, and the horizontal load during the earthquake is shared by the support pile and the slant pile, and the slant pile does not need to be constantly loaded.

At this time, the slant pile may be cast at a predetermined inclination angle from a peripheral portion of the structure to a bottom region (inside of the foundation).

Alternatively, the slant pile is cast at a predetermined inclination angle in a direction that resists lateral flow of the ground with respect to a direction in which lateral flow of the soft ground on which the structure is constructed may occur. Is also preferred.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a pile stress reduction structure for soft ground according to the present invention will be described below with reference to the accompanying drawings. FIG. 1A is a schematic sectional view showing a support state of a structure employing a pile stress reduction structure in soft ground according to the present invention. In the present embodiment, the alluvial sand layer 2 and the non-liquefied layer 3 that may be liquefied exist between the support layer 4 and the ground surface. The non-liquefied layer mentioned here refers to a cohesive soil or a relatively hard sandy soil (intermediate support layer). As shown in the figure, the structure 1 is supported by a plurality of vertical support piles 11. Further, a short slanted pile 12 having a predetermined inclination from the bottom surface of the structure 1 toward the alluvial sand layer 2 which is likely to be liquefied is cast obliquely outwardly as viewed from the structure 1. The length of this short slant pile 12 is the tip 12
The length a is set so as to reach the upper portion of the non-liquefied layer 3 that can exert a relatively dynamic supporting force. In addition, the number of castings is set so that, of the increased horizontal stress generated in the vertical support pile 11 at the time of the earthquake, a horizontal resistance force necessary for keeping the pile stress within an allowable value can be borne. Since the inclined pile 12 can bear the horizontal load due to the earthquake as the horizontal component of the axial force, the horizontal stress of the vertical support pile 11 can be reduced.

FIG. 1 (b) shows an example in which the above-described inclined pile 12 is installed by being inclined inward in the bottom area of the structure 1 on which the vertical support pile 11 is installed, that is, in a plane. Things. Since the slanted pile 12 is inclined inward and does not require a marginal site around the structure 1, the angle of the slanted pile 12 is set within the design-possible range to reduce the horizontal component resistance. Can be enhanced. Further, the tip of the short slanted pile 12 shown in each figure in FIG. 1 does not need to reach the support layer 4, and is located in the non-liquefied layer 3. Further, there is an advantage that the seismic retrofitting work of the foundation of the existing structure 1 can be performed from around the structure 1.

In the above description, the ground composed of the alluvial sand layer 2, the non-liquefied layer 3, and the support layer 4 has been described as an example. However, for example, the target ground composed of the ultra-soft clay layer 2 near the ground surface is also assumed. can do.

FIGS. 2 (a) and 2 (b) are schematic views showing examples of the angle of the slant pile 12 determined by the thickness of the liquefied layer on a site having the same area having a width B, for example. .
As shown in FIG. 2A, when the liquefied layer (alluvial sand layer 2) has a relatively large layer thickness (D1) from the ground surface,
When the tip 12a of the inclined pile 12 reaches the non-liquefied layer 3, the angle (θ1) of the inclined pile 12 becomes relatively small. On the other hand, as shown in FIG. 2B, the layer thickness (D
When 2) is thin, the angle (θ2) at which the slant pile 12 is driven can be increased, and the horizontal resistance component also increases. In the present embodiment, the tip 12a of the slant pile 12 is supported by the non-liquefied layer 3, but even if the non-liquefied layer 3 is a relatively soft viscous soil layer, the dynamic strength of the viscous soil layer 3 is increased. Therefore, the pile itself can be expected to have horizontal resistance. In addition, the ground at the pile tip 12a is
In the case of a slightly hard sandy soil (intermediate support layer), the support effect can be obtained more reliably. When the dynamic support force of the slant pile 12 is insufficient, it is preferable to increase the tip support force by using the slant pile 12 as a node pile or expanding the diameter of the pile tip. As the diameter expansion technology, a mechanical stirring type diameter expansion device in the middle digging method and the cast-in-place pile method can be used.

FIG. 3 shows an example in which a slant pile 12 is applied to a structure 1 constructed on the ground where lateral flow may occur. As shown in FIG.
In the structure 1 built near the back of the building, the seawall 5 may move in the sea direction during an earthquake, so that the ground may be permanently deformed in the direction of the arrow. Therefore, it is preferable to assume the direction in which permanent deformation occurs, and to incline the slant pile 12 so as to resist the direction in which the ground may deform. In the case of FIG. 3A, the resistance to horizontal deformation (lateral flow) can be increased by placing the slant pile 12 at a predetermined angle toward the seawall 5.

FIG. 3 (b) shows an example of a pile foundation structure 1 constructed on an inclined ground. Also in this case, there is a possibility that permanent deformation may occur in the direction of the arrow so as to slide on a slope during an earthquake. In this case, the stake 12
The resistance to deformation (lateral flow) that occurs as if sliding on a slope can be increased by casting.

FIG. 4 shows an embodiment in which the effect of the seismic isolation device 15 is exerted by using the short slant pile 12 for the structure 1 having the seismic isolation device 15 typified by a laminated rubber isolator. It shows the form. As shown in the figure, the horizontal displacement of the base plate 1B of the structure 1 during the earthquake is suppressed by placing the slant pile 12. further,
The predominant period between the ground and the foundation system is relatively short, and the possibility of resonance with the long-period seismic isolation device can be reduced. That is, the short-period component of the seismic motion input to the basic slab 1B is dominant in combination with the oblique resistance. This ground motion is the basic version 1B
It is transmitted to the upper structure 1A via the seismic isolation device 15 provided above. At this time, since the upper structure 1A has a relatively long natural period due to the presence of the seismic isolation device 15, the response acceleration of the upper structure 1A is significantly reduced, and the effect of the seismic isolation device 15 is exhibited without resonance.

In each of the above-described embodiments, an example has been described in which the direction in which the short slant pile 12 is driven is inclined inward or outward in the plane of the structure 1. On the other hand, in the structure 1 shown in FIGS. 5A to 5C, the inclined piles 12 </ b> A and 12 </ b> B whose inclinations are opposite to each other are provided.
Was placed in a plane along the outer edge of the structure 1, and a grid-wall-shaped pile driving region consisting of four sides was provided around the structure 1 having a square plane. That is, with such an arrangement of the inclined piles, the construction can be performed without the inclined shaft protruding outside from the foundation site. FIG. 5A is a partial perspective view schematically showing a basic portion of the structure 1. FIG. As shown in the figure, a plurality of slant piles 12A, 1A,
2B is cast so as to constitute an elevation. As a result,
A grid-shaped horizontal resistance member is obtained with a predetermined inclination facing in the opposite direction. (See FIGS. 5B and 5C). The oblique piles 12A and 12B as the horizontal resistance members are formed in a square shape in plan view so as to surround the area where the vertical support pile 11 is driven. At the time of an earthquake, the slant piles 12A and 12B arranged in a lattice wall shape along the periphery of the structure 1 bear the horizontal resistance component of the vertical support pile 11 disposed therein, so that the horizontal stress of the vertical support pile 11 is increased. Is reliably reduced.

[0020]

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing a basic structure of an embodiment of a pile stress reduction structure in soft ground according to the present invention.

FIG. 2 is a schematic structural cross-sectional view showing a state in which short slant piles are driven in a pile stress reduction structure in soft ground according to the present invention.

FIG. 3 is a schematic structural cross-sectional view for realizing a pile stress reduction structure in a ground where lateral flow (permanent deformation) is expected.

FIG. 4 is a schematic structural cross-sectional view for realizing one embodiment of a pile stress reduction structure in a structure including a seismic isolation device.

FIG. 5 shows a pile stress reduction structure in which a wall-like body composed of a group of slant piles is formed around a structure to bear a horizontal resistance component.

FIG. 6 is a schematic structural cross-sectional view showing a conventional example using a slanted pile as a liquefaction countermeasure.

[Explanation of symbols]

 Reference Signs List 1 structure 2 alluvial sand layer 3 clay layer 4 support layer 11 vertical support pile 12, 12A, 12B inclined pile 15 seismic isolation device

Claims (3)

[Claims] 1. A vertical support pile for supporting a structure constructed on soft ground, and a slant pile installed at a predetermined inclination angle so that the tip thereof reaches an intermediate layer below the soft ground. ,
A pile stress reduction structure in soft ground, wherein a horizontal force generated when a horizontal load is applied is borne as a horizontal component of an axial force of the slant pile to suppress an increase in stress of the vertical support pile. 2. The pile stress reduction in soft ground according to claim 1, wherein said slant pile is driven at a predetermined inclination angle from a peripheral portion of said structure toward a bottom surface region of said structure. Construction. 3. The slanted pile has a predetermined inclination angle in a direction in which lateral flow (permanent deformation) of soft ground on which the structure is constructed may occur, in a direction that resists lateral flow of the ground. The pile stress reduction structure according to claim 1, wherein the pile stress is reduced.