JP6855665B2 - Pile foundation structure and how to reinforce existing piles - Google Patents

Description

The present invention relates to a technique for reinforcing a pile foundation, and more specifically, to a technique for reinforcing a pile foundation by improving the ground around the pile.

Civil engineering structures such as piers, abutments, and retaining walls, or building structures such as apartment buildings and office buildings are extremely heavy and, in principle, are not allowed to sink after operation. Therefore, such a structure is constructed on a support layer such as a bedrock or a compact sand layer, which can be expected to have a considerable bearing capacity.

If the support layer is in a relatively shallow position, it can be excavated to a predetermined depth to expose the support layer and the foundation of the structure can be built directly on this support layer (so-called "direct foundation"). .. On the other hand, when the support layer is located at a relatively deep position, a considerable amount of excavation is required to expose the support layer, and a larger-scale foundation needs to be constructed, so that the direct foundation is not realistic. Therefore, in this case, a "pile foundation" is generally adopted instead of a direct foundation.

The pile foundation is based on a columnar pile constructed in the ground, and one large-diameter pile itself may be used as the pile foundation, but usually the pile foundation is composed of a plurality of piles. .. In addition, the piles that make up the pile foundation, except for special piles such as friction piles, are partially penetrated into the supporting ground or placed on the supporting ground.

As explained so far, pile foundations are used in cases where the supporting ground is deep, and the piles reach that deep supporting ground. In other words, there is an "intermediate layer" on the supporting ground, which has a relatively low ground strength, and the intermediate layer is considerably thick, and most of the piles are in this intermediate layer.

The pile foundation transfers the load of the above-ground structure to the supporting ground, so it is important to consider the axial force in the design of the pile. In principle, a large horizontal force is not applied to the pile in the usual design, but in the design at the time of an earthquake, a considerable horizontal force is naturally applied as a load at the time of an earthquake. When a horizontal force is applied to a long pile (usually the pile head), bending moment and shear force (section force) are generated in the pile, and especially the intermediate layer is soft ground (peat, organic clay, silt, or unconsolidated). In the case of the offshore stacking of Yui), a significantly large cross-sectional force tends to be generated.

By the way, as civil engineering is also called empirical engineering, the design concept has been improved so that it becomes a more appropriate structure every time a major natural disaster is experienced. In particular, in the 1995 Hyogoken Nanbu Earthquake, many important structures were damaged by the seismic load such as the collapse of the viaduct. Japan Road Association) ”(hereinafter,“ Road Bridge Specification ”) has been significantly revised.

The 1996 revision of the Road Bridge Specification changed the seismic design method, and as a result, structures that were evaluated to withstand earthquakes under the previous design methods became load during earthquakes under the new standard. In some cases, it was unbearable. In other words, based on new experience, there are cases where it is evaluated that the structures already in service cannot actually withstand an earthquake, that is, some kind of reinforcement is required. Of course, pile foundations are no exception, and there are many existing pile foundations that require reinforcement under the new standards.

Conventionally, the so-called "additional pile construction method" in which new piles are added has been the mainstream for reinforcing existing pile foundations. This additional pile construction method is not the idea of directly reinforcing the existing piles that make up the pile foundation, but the design idea of reinforcing the entire pile foundation by increasing the number of piles. Specifically, the upper foundation such as footing. The structure will be wider than it is now, and a new pile will be constructed under the widened part.

As mentioned above, it has been pointed out that the additional pile construction method requires widening of the footing and addition of piles, requires a considerable construction period, and requires a large construction cost. Especially when it is the foundation of urban expressways, the lengthening of reinforcement work will have a great impact on socioeconomics. Further, although the footing is widened for the additional piles, there is also a problem that the footing cannot be widened when there is a problem on the ground, and the additional pile method cannot be adopted in the first place.

Therefore, the applicant of the present application has developed a CPR method (Confining Pile Reinforcement Method) that solves the above problems, and presents this method in Patent Document 1.

Japanese Patent No. 3643571

The CPR method shown in Patent Document 1 forms a reinforcing body by improving the ground around the existing piles, and is based on the design concept of directly reinforcing the existing piles, like the additional pile method. No need to build new piles. Since there is no need to install new piles, the construction is faster than the additional pile construction method, the overall construction cost is reduced, and in principle there are no land problems, so the CPR construction method is superior to the conventional construction method. I can say.

However, there was room for improvement in the conventional CPR method. FIG. 10 is a cross-sectional view showing a reinforcing structure of a pile foundation by a conventional CPR method. According to the analysis so far, if the reinforcing body is provided in the intermediate layer, an extremely large cross-sectional force (bending moment or shearing force) may be generated at the lower end of the reinforcing body, which is the boundary between the reinforcing body and the intermediate layer. Has been confirmed. Therefore, as shown in FIG. 10, the reinforcing body was constructed so as to reach the supporting ground in principle. That is, the dimension t (hereinafter, referred to as “thickness”) of the reinforcing body in the depth direction tends to increase, and there is a problem that the cost is pushed up by that amount. In other words, this point was thought to have room for improvement.

The object of the present invention is to solve the problems of the prior art, that is, to reduce the thickness of the reinforcing body while taking advantage of the CPR method that solves the problems of the conventional additional pile method. It is to provide a new CPR method that can be done.

The present invention has been made focusing on the point that the reinforcing body is composed of two layers and the rigidity of the lower layer is reduced as compared with that of the upper layer, and is an invention made based on an unprecedented idea. ..

The pile foundation structure of the present invention is a structure including a pile and a reinforcing body. This reinforcing body is provided in a part of the pile in the depth direction, and is composed of a "main reinforcing body" and a "buffer reinforcing body". However, the rigidity of the buffer reinforcing body is lower than the rigidity of the main reinforcing body.

The pile foundation structure of the present invention may have a structure in which the reinforcing body has two layers, the main reinforcing body in the upper layer and the cushioning reinforcing body in the lower layer, or the reinforcing body is sandwiched between the upper and lower two layers of the buffer reinforcing body3. It can also be a layered structure.

The pile foundation structure of the present invention may be a structure in which a pile and a reinforcing body are integrated. In this case, the reinforcing body can connect a plurality of piles.

The pile foundation structure of the present invention may be a structure provided with a hollow reinforcing body in a plan view (horizontal cross-sectional view). The reinforcing body in this case is arranged at a position on the outer circumference of the pile and away from the pile (a position not in contact with the pile).

The pile foundation structure of the present invention may also have a structure including a cushioning reinforcement body composed of a plurality of "divided layers" that are vertically overlapped. However, the rigidity of the lower partition layer is lower than the rigidity of the upper partition layer.

The pile foundation structure of the present invention can also have a structure in which the rigidity of the cushioning reinforcement is lower than that of the present reinforcing by making the planar shapes of the reinforcing body and the cushioning reinforcement different.

The pile foundation structure of the present invention can also have a structure in which the rigidity of the cushioning reinforcement is lower than that of the present reinforcing by forming the cushioning reinforcement into a multi-layer structure. In this case, the cushioning reinforcement body has a structure in which a plurality of "improved layers" with improved ground and "non-improved layers" without ground improvement are stacked one above the other.

The method for reinforcing the existing pile of the present invention is a method for reinforcing the existing pile, and is a method including a buffer reinforcing body forming step and the present reinforcing body forming step. Of these, in the buffer reinforcing body forming step, the buffer reinforcing body is formed by improving the ground around a part of the existing pile in the depth direction. On the other hand, in the present reinforcing body forming step, the present reinforcing body is formed by improving the ground around the pile directly above the cushioning reinforcing body. However, the cushioning reinforcing body is formed so as to have a rigidity lower than the rigidity of the main reinforcing body.

The pile foundation structure of the present invention and the method of reinforcing existing piles have the following effects.
(1) Compared to the additional pile construction method, the construction is faster and the overall construction cost is reduced.
(2) Compared with the conventional CPR construction method, the thickness of the reinforcing body can be reduced, so that the construction is faster and the overall construction cost is further reduced.
(3) Since it is not restricted by land problems, it can be used in many existing pile foundations.

The cross-sectional view which shows the pile foundation structure of this invention. Explanatory drawing which shows the construction situation of constructing a reinforcing body using a jet grout method. The plan view which shows the effective cross section of the reinforcing body formed by a plurality of improved bodies. (A) is a plan view showing a connected type reinforcing body in which the entire area is improved, and (b) is a plan view showing a connected type reinforcing body in which the ground is improved except for a part in the area. Top view showing a stand-alone reinforcement. FIG. 5 is a cross-sectional view showing a reinforcing body 300 having a three-layer structure in which the reinforcing body is sandwiched between upper and lower two-layer cushioning reinforcing bodies. (A) is a cross-sectional view showing the main reinforcing body and the impact reinforcing body having different cross-sectional shapes, (b) is a plan view of the arrow AA, and (c) is a plan view of the arrow BB. (A) is an overall sectional view including a buffer reinforcing body provided with a non-improved layer, and (b) is a partial sectional view showing a buffer reinforcing body provided with a non-improved layer. The result of the response analysis at the time of the earthquake for the pile foundation of the pier. The cross-sectional view which shows the reinforcement structure of the pile foundation by the conventional CPR method.

An example of the pile foundation structure of the present invention and the method of reinforcing the existing pile will be described with reference to the drawings. In addition, it is divided into "pile foundation structure" and "reinforcement method of existing piles", and each component will be described in detail. For the contents common to the pile foundation structure and the reinforcement method of existing piles, an example of the pile foundation structure It will be explained in. Therefore, in the method of reinforcing existing piles, only the contents peculiar to the method will be explained.

1. 1. Pile foundation structure FIG. 1 is a cross-sectional view showing the pile foundation structure 100 of the present invention. As shown in this figure, the pile foundation structure 100 is composed of a pile 200 and a reinforcing body 300 for reinforcing the pile 200, and supports the upper structure 400 on a support layer. A part of the tip (lower end) of the pile 200 is penetrated into the support layer, and a part of the pile head (upper end) is embedded in the footing 410 which is the foundation of the structure 400 (not shown). The reinforcing body 300 has a two-layer structure consisting of a main reinforcing body 310 in the upper layer and a cushioning reinforcing body 320 in the lower layer.

Here, the upper structure 300 is illustrated as a pier, but of course, the structure 300 may be another structure 300 such as a retaining wall or an apartment house. Further, although two piles 200 are shown in FIG. 1, the present invention can be implemented even if the structure is supported by piles 200 of an arbitrary number and arrangement (n rows × m rows, etc.). Of course, various piles 200 such as cast-in-place concrete piles, existing concrete piles, and steel pipe piles can be adopted, and further, not only the existing piles 200 but also the piles 200 are reinforced by the reinforcing body 300 even when newly installed. be able to.

(Reinforcement body)
The reinforcing body 300 can be constructed by improving the ground in the intermediate layer around the pile 200. Since this ground improvement is performed at a relatively deep position, it is preferable to adopt the high-pressure injection stirring method (jet grout method) as the method. Of course, other conventional construction methods may be adopted depending on the site conditions. FIG. 2 is an explanatory view showing a construction situation in which the reinforcing body 300 is constructed by using the jet grout method. The jet grout method will be briefly described below.

First, the ground improvement machine BM is installed at a predetermined position on the ground, and a rod is inserted into the ground to drill a hole. When the hole can be drilled to the desired depth, the cement-based hardener and compressed air are simultaneously injected from the side surface of the rod tip. Then, by gradually pulling up the rod while rotating it, a substantially columnar improved body is constructed. Therefore, the improved body constructed by the jet grout method has a substantially circular shape in a cross-sectional view as shown in the plan view of FIG. In order to make the reinforcing body 300 into a desired shape, it is preferable to construct a plurality of improved bodies while wrapping a part of the reinforcing body 300. For example, in FIG. 3, 24 improved bodies are constructed so as to wrap and orbit, and a hollow rectangular reinforcing body 300 is constructed in a horizontal cross-sectional view (plan view). As shown in this figure, it is advisable to design the effective cross section (hatched portion) of the reinforcing body 300 in consideration of the lap width of the improved body.

By the way, in FIG. 1, the lower end of the footing 410 and the upper end of the reinforcing body 300 are in contact with each other, that is, the reinforcing body 300 is constructed directly under the footing 410 to reinforce the pile head, but depending on the structural conditions and construction conditions, the pile head is reinforced. As shown in FIG. 2, the reinforcing body 300 may be constructed so that a distance is provided from the lower end of the footing 410, that is, an intermediate layer is interposed between the footing 410 and the reinforcing body 300. In any case, it is not necessary for the lower end of the reinforcing body 300 to reach the support layer as shown in FIGS. 1 and 2, in other words, it is sufficient to construct the reinforcing body 300 in a part of the pile 200 in the depth direction. The thickness of the reinforcing body 300 (dimension in the depth direction of the reinforcing body) can be reduced.

The reinforcing body 300 is roughly classified into a type integrated with the pile 200 (hereinafter referred to as "connected type") and a type arranged at a position away from the pile 200 (hereinafter referred to as "independent type"). Can be done. FIG. 4 is a plan view (horizontal cross-sectional view) showing the reinforcing body 300 of the connected type, (a) shows the reinforcing body 300 in which the entire area is improved, and (b) shows a part of the area. Except for the ground-improved reinforcing body 300. As shown in this figure, the connecting type reinforcing body 300 is constructed so as to be integrated (closely adhered) to the pile 200, and moreover, the plurality of piles 200 are connected to each other. It should be noted that, as shown in FIG. 4A, the ground can be improved in the entire predetermined area, and all the piles 200 in the area can be connected by the reinforcing body 300, or horizontally as shown in FIG. 4B. It is also possible to construct the reinforcing body 300 as a hollow shape in a cross-sectional view, and to connect the outer peripheral piles 200 (12 in the figure).

On the other hand, when the reinforcing body 300 is an independent type, the reinforcing body 300 is constructed at a position away from the pile 200. FIG. 5 is a plan view (horizontal sectional view) showing the independent type reinforcing body 300. As shown in this figure, the independent type reinforcing body 300 is arranged on the outer periphery of the piles 200, and is constructed so as to surround the piles 200 at a position away from the piles 200. The independent type reinforcing body 300 does not directly reinforce the pile 200 like the connected type reinforcing body 300, but blocks the behavior of the external intermediate layer and the pile 200 through the intermediate layer inside the reinforcing body 300. Since it is possible to suppress the deformation of the body, it functions effectively as a reinforcing effect even if it is an independent type.

As described above, the reinforcing body 300 has a two-layer structure of the main reinforcing body 310 and the cushioning reinforcing body 320, and the reinforcing body is constructed by continuously constructing the main reinforcing body 310 in the upper layer and the cushioning reinforcing body 320 in the lower layer. 300 is formed (FIGS. 1 and 2). Alternatively, as shown in FIG. 6, the reinforcing body 300 can be formed as a three-layer structure in which the reinforcing body 310 is sandwiched between the upper and lower two-layer cushioning reinforcing bodies 320. In particular, when the reinforcing body 300 is constructed by providing a distance from the lower end of the footing 410 (that is, an intermediate layer is interposed between the footing 410 and the reinforcing body 300), this three-layer structure is effective. Then, the cushioning reinforcing body 320 is constructed with a rigidity lower than that of the main reinforcing body 310. Here, the rigidity is a concept including shear rigidity and flexural rigidity, and is a mechanical physical quantity determined by the cross-sectional area, the moment of inertia of area, the elastic modulus (Young's modulus), and the like. For example, in the case of the reinforcing body 300 constructed by the jet grout method, the rigidity of the reinforcing body 300 can be changed by changing the composition of the cement-based curing material used.

However, when constructing a columnar improved body by the jet grout method, if the cement-based curing material is changed in the middle, a large setup change actually occurs. In other words, after constructing the improved body (cylindrical body) of the lower cushioning reinforcement body 320 part, the setup is once changed and then the improved body of the upper layer main reinforcing body 310 part is constructed, which is a little time-consuming. It takes. Of course, such a method may be adopted, but the rigidity of the buffer reinforcing body 320 can be reduced by changing the size (cross-sectional area) of the main reinforcing body 310 and the buffer reinforcing body 320 as shown in FIG. it can. 7A and 7B are views showing the main reinforcing body 310 and the cushioning reinforcing body 320 having different cross-sectional shapes. FIG. 7A is a cross-sectional view, and FIG. 7B is an arrow view of AA (shown in FIG. 4A). A plan view, (c) is a plan view taken along the line BB (shown in FIG. 4 (a)).

Looking at FIG. 7A, it can be seen that the main reinforcing body 310 has a larger external dimension (width) than the cushioning reinforcing body 320. Further, as shown in FIGS. 7 (b) and 7 (c), the buffer reinforcing body 320 has a larger hollow portion than the main reinforcing body 310. That is, the cross-sectional area of the cushion reinforcing body 320 in the horizontal cross-sectional view is smaller than that of the main reinforcing body 310, and therefore, the cushion reinforcing body 320 has lower rigidity than the main reinforcing body 310 as a whole. .. In addition, in order to change the cross-sectional area of the main reinforcing body 310 and the buffer reinforcing body 320, it is necessary to construct some improved bodies (columnar bodies) of only the main reinforcing body 310 (that is, more improvements than the buffer reinforcing body 320). The main reinforcing body 310 may be formed by the body).

In order to make the rigidity of the buffer reinforcing body 320 smaller than the rigidity of the main reinforcing body 310, an unimproved layer may be provided as a part of the cushion reinforcing body 320. 8 (a) is a view showing a buffer reinforcing body 320 provided with a non-improved layer, FIG. 8 (a) is an overall sectional view, and FIG. 8 (b) is a partial sectional view. In the cushioning reinforcement body 320 shown in this figure, an "improved layer 321" with ground improvement and a "non-improved layer 322" without ground improvement are alternately stacked, and the cushioning reinforcement is provided by providing the non-improved layer 322. The rigidity of the entire body 320 is reduced. In order to provide the non-improved layer 322, for example, in the case of the jet grout method, it is preferable to provide a section for pulling up the rod without injecting a cement-based curing material.

The buffer reinforcing body 320 may have only one layer, or may have two or more layers. That is, two or more layers of the buffer reinforcing body 320 are formed by stacking a plurality of partial layers (hereinafter, referred to as "divided layers") constituting the buffer reinforcing body 320. In this case, it is preferable to reduce the rigidity of the divided layer from the upper side to the lower side. For example, if the buffer reinforcing body 320 is formed in the order of the first divided layer, the second divided layer, and the third divided layer from the top, the second divided layer has a higher rigidity than the first divided layer. It is made lower, and the rigidity of the third partition layer is lower than that of the second partition layer.

(Analysis result)
Hereinafter, the analysis results carried out by the inventors of the present application in order to confirm the effect of the invention of the present application will be described.

FIG. 9 shows the results of an earthquake response analysis of the pile foundation of the pier. From the left, a case where the pile foundation is not reinforced, a case where the pile foundation is reinforced by the conventional CPR method (a reinforcing body with only one layer), and the present application. The cross-sectional force generated in the pile is shown for each case reinforced by the invention. In this analysis, the rigidity of the present reinforcing body 310 in the present invention is set to be about the same as that of the reinforcing body of the conventional CPR method, and the rigidity of the buffer reinforcing body 320 is set to 1/10 of the present reinforcing body 310. As a result, it can be seen that the cross-sectional force of the pile head is significantly reduced in both the conventional CPR method and the present invention as compared with the case where the pile foundation is not reinforced. Further, in the conventional CPR method, the peak of the cross-sectional force is shown at the lower end surface (boundary surface with the intermediate layer) of the reinforcing body, but the cross-sectional force is reduced at the lower end surface of the buffer reinforcing body 320 of the present invention as compared with the pile head. You can see that it has been done. Note that FIG. 9 shows the effect when the rigidity of the buffer reinforcing body 320 is set to 1/10 of the main reinforcing body 310, but the rigidity of the reinforcing body 320 is smaller than that of the main reinforcing body 310, not limited to 1/10. The inventors of the present application have confirmed that the value has a considerable effect.

2. Reinforcement method of existing piles The reinforcement method of existing piles of the present invention will be described with an example of adopting the jet grout method. First, as shown in FIG. 2, the ground improvement machine BM is installed at a predetermined position on the ground, the rod is inserted into the ground to make a hole, and the improved body of the buffer reinforcing body 320 part in the predetermined section while pulling up the rod ( Cylindrical body) is constructed. Then, while pulling up the rod as it is, this time, an improved body of the main reinforcing body 310 part is constructed in a predetermined section. When one improved body can be constructed, the position of the improved body is changed in a plane to construct adjacent improved bodies, and by repeating this, the buffer reinforcing body 320 and the main reinforcing body 310 are completed, that is, the reinforcing body 300 is completed. Let me. In addition, in order to make the rigidity of the buffer reinforcing body 320 smaller than that of the main reinforcing body 310, an improved body of only the main reinforcing body 310 portion (without the buffer reinforcing body 320) is constructed, or a non-injection rod pulling section is constructed. As described above, the improved layer 321 and the non-improved layer 322 are alternately stacked by providing the above, or the composition of the cement-based hardening material used in the present reinforcing body 310 and the cushioning reinforcing body 320 is changed. ..

The pile foundation structure of the present invention and the method for reinforcing existing piles are widely used such as pile foundations for bridges such as road bridges, railway bridges, and water pipe bridges, pile foundations for retaining walls and general houses, and reuse of existing piles. be able to. Considering that the invention of the present application can effectively and economically reinforce a structure in service, and can also reinforce a structure that was not subject to reinforcement due to land problems, it is not only industrially usable but also social. It can be said that this is an invention that can be expected to make a great contribution.

100 Pile foundation structure of the present invention 200 (pile foundation structure) pile 300 (pile foundation structure) reinforcement 310 (reinforcement) main reinforcement 320 (reinforcement) cushion reinforcement 321 (buffer reinforcement) improvement Layer 322 Non-improved layer (of buffer reinforcement) 400 Structure 410 (Structure) Footing BM Ground improvement machine

Claims (4)

A pile and a reinforcing body for reinforcing the pile are provided.
The reinforcing body formed by the ground improvement is provided in a part of the pile in the depth direction, and is composed of a main reinforcing body in the upper layer and a cushioning reinforcing body in the lower layer.
The rigidity of the buffer reinforcing body is made lower than the rigidity of the main reinforcing body by forming a configuration in which a plurality of improved layers with improved ground and non-improved layers without ground improvement are stacked one above the other.
The pile foundation structure is characterized by this. The reinforcing body is integrated with the pile and connects a plurality of piles.
The pile foundation structure according to claim 1. The reinforcing body has a hollow shape in a plan view, is located at a position away from the pile, and is arranged on the outer circumference of the pile.
The pile foundation structure according to claim 1. In the method of reinforcing existing piles
A step of forming a buffer reinforcing body by improving a part of the ground around the pile in the depth direction of the existing pile, and a step of forming the buffer reinforcing body.
By improving the ground around the pile directly above the buffer reinforcing body, the main reinforcing body forming step of forming the main reinforcing body is provided.
In the buffer reinforcing body forming step, the cushion reinforcing body is formed by vertically stacking a plurality of improved layers with improved ground and non-improved layers without ground improvement, thereby increasing the rigidity of the main reinforcing body. To reduce the rigidity of
A method of reinforcing existing piles, which is characterized by this.

Images