JP7735150B2 - Design method for caisson piles - Google Patents

Description

The present invention relates to a design method for deep foundation piles.

Caisson piles are used for applications such as bridge pier foundations. Caisson piles are designed to withstand a specified seismic force, and the design seismic force may be increased due to revisions to applicable design standards. In such cases, seismic reinforcement of existing structures may be necessary. Structures supported by caisson piles (such as bridge piers) are usually installed above ground, making seismic reinforcement relatively easy. However, because caisson piles are buried underground, seismic reinforcement of the caisson piles themselves is relatively difficult.

Patent Document 1 discloses a method for seismic reinforcement of caisson piles. Multiple anchors are fixed to the sides of the caisson piles and radially distributed in the surrounding ground, with their tips anchored in the bedrock. This method eliminates the need to thicken the caisson piles themselves, thereby reducing construction costs and time.

Japanese Patent Application Laid-Open No. 2021-105266

The method described in Patent Document 1 is promising as a method for seismic reinforcement of caisson piles, but no design method has been established for caisson piles reinforced with anchors. Traditionally, design models that model caisson piles and the surrounding ground have been used as design methods for caisson piles, so it would be desirable to be able to use conventional design models for caisson piles reinforced with anchors.

The present invention aims to provide a design method for caisson piles that allows conventional design models to be used for caisson piles reinforced with anchors.

The present invention relates to a design method for a caisson pile in which a plurality of anchors are provided around the caisson pile at a predetermined depth level in the on-site ground so as to maintain stability when the caisson pile is surrounded by on-site ground that cannot maintain stability of the caisson pile. The design method of the present invention includes the steps of: determining a ground constant of a virtual surrounding ground, which is assumed to be greater than a ground constant determined according to the geology of the on-site ground so as to maintain stability of the caisson pile, based on a design model that models the caisson pile and the surrounding ground that constitutes the predetermined periphery of the caisson pile; determining specifications for the multiple anchors so that the horizontal displacement of the caisson pile during an earthquake when the surrounding ground is composed of the on-site ground and the multiple anchors are provided at the predetermined depth level is smaller than the horizontal displacement when the surrounding ground is composed of the virtual surrounding ground without all the multiple anchors; and incorporating the virtual surrounding ground into the design model and designing the caisson pile using the design model.

The present invention provides a design method for caisson piles that allows conventional design models to be used for caisson piles reinforced with anchors.

1 is a conceptual diagram of a deep foundation pile to which the present invention is directed. FIG. 1 is a conceptual diagram showing an example of a design model of a conventional caisson pile. FIG. 3 is a conceptual diagram of a ground spring used in the design model shown in FIG. 2 . FIG. 1 is a schematic step diagram showing the flow of a design method for a deep foundation pile according to one embodiment of the present invention. FIG. 1 is a conceptual diagram of a first model. This is a conceptual diagram showing the horizontal displacement of a deep foundation pile. 1 is an example of a load-displacement curve obtained from the first model and the second model. FIG. 10 is a conceptual diagram of a second model. FIG. 10 is a conceptual diagram showing a method for determining anchor specifications.

One embodiment of the design method for caisson piles of the present invention will be described with reference to the drawings. This embodiment targets caisson piles supporting bridge piers, but the structure supported by the caisson piles is not limited in any way. Figure 1 shows the concept of the caisson pile targeted by the present invention. Figure 1(a) shows a side cross-sectional view of the caisson pile and its superstructure, and Figure 1(b) shows a cross-sectional view along line A-A in Figure 1(a). The caisson pile 1 is a roughly cylindrical reinforced concrete structure embedded in the ground 5, with a pier 2 supported on its top. The cross-sectional shape of the caisson pile 1 may be polygonal, such as a square or octagon, or oval. The pier 2 supports the bridge girder 3. Anchors 4 are provided around the side of the caisson pile 1. The anchors 4 are made of steel, such as PC steel wire or deformed steel bars, and one end is fixed to the caisson pile 1 and the other end is fixed to the ground. By placing anchors 4, deformation of the caisson pile 1 due to seismic loads is suppressed, improving the stability of the caisson pile 1. The anchors 4 may or may not be prestressed. Multiple anchors 4 are installed radially and horizontally from the side of the caisson pile 1. The anchors 4 are installed at different depth levels of the caisson pile 1, and the anchors 4 at each depth level are installed at the same angular position. However, the number, length, cross-sectional area, and angular position of the anchors 4 at each depth level may differ from each other, and the number of rows of anchors 4 is not particularly limited.

Civil engineering structures such as bridges are designed in accordance with applicable design standards. However, revisions to these standards can increase the design seismic force, necessitating seismic reinforcement work. While bridge piers (2) are typically reinforced as a method for seismic reinforcement of bridges, seismic reinforcement of the caisson piles (1) themselves may also be necessary. Possible methods for seismic reinforcement of caisson piles (1) include thickening the caisson piles (1) themselves or improving and strengthening the ground surrounding the caisson piles (1). However, thickening the caisson piles (1) requires large-scale excavation, making construction difficult and posing significant challenges, particularly in mountainous regions. While ground improvement is effective for relatively soft ground, caisson piles (1) are generally installed on solid ground or bedrock in mountainous regions, posing numerous challenges, such as the difficulty of drilling holes and constructing improved structures due to the hardness of the ground and bedrock. For these reasons, seismic reinforcement work for caisson piles (1), especially in mountainous regions, is rarely carried out. In contrast, the construction method of reinforcing the caisson piles 1 with anchors 4 is considered to be a promising method because it can improve the earthquake resistance of the caisson piles 1 without increasing their thickness (fewer construction issues), and it is effective even on hard ground.

To date, caisson piles have been designed using a design model that models the caisson pile and the surrounding ground. Figure 2 shows an example of a design model. The ground springs consist of a horizontal compression spring k1 and a horizontal shear spring k2 on the front of the caisson pile 1, a vertical shear spring k3 on the front and back of the caisson pile 1, a vertical shear spring k4 on the side of the caisson pile 1, and a vertical compression spring k5 and a horizontal shear spring k6 in the ground below the caisson pile 1. Ground springs k1-k4, which simulate the lateral ground, are installed at multiple depth levels depending on the physical properties of the surrounding ground. Ground springs k5 and k6, which simulate the ground below, are installed horizontally. Seismic loads considered include horizontal seismic force H, vertical seismic force V, and bending moment M. Dedicated design software is used for the design, and the design model shown in Figure 2 is constructed within the software. By inputting data on the caisson piles and ground into the design software, the displacement and stress of the ground and caisson piles can be calculated.

Figure 3 shows the concept of soil springs used in the design model shown in Figure 2. Soil springs are defined by the elastic modulus E and soil strength. Soil strength refers to the internal friction angle φ and cohesion C; the elastic modulus E and soil strength together are sometimes referred to as soil constants. Soil springs are treated as linear springs when displacement is small, and when a certain displacement is reached, they undergo plastic deformation and bear a certain load (soil reaction). The elastic modulus E represents soil rigidity when displacement is small. Soil strength (internal friction angle φ and cohesion C) is an index that represents the load-bearing capacity of the soil in the plastic region, and the load that the soil can bear after plastic deformation is determined depending on the soil strength. The internal friction angle φ is used for sandy soil, and cohesion C is used for clayey soil.

However, conventional design methods (design software) cannot take into account the reinforcing effect of anchors. Developing a design method that can directly consider the reinforcing effect of anchors requires a great deal of time and effort, so it is desirable to apply a design method that is already in practical use and has a proven track record. Therefore, in this embodiment, the reinforcing effect of anchors is evaluated using the elastic modulus and strength of the ground, allowing conventional design methods (design software) to be used. When performing ground improvement, the elastic modulus and strength of the surrounding ground after improvement are entered into the design software. In contrast, in this embodiment, the reinforcing effect of anchors is evaluated by replacing it with ground improvement that has an equivalent effect. The design method for caisson piles in this embodiment will be explained step by step below. Figure 4 is a schematic step diagram showing the flow of the design method.

(Step S1)
A design model is created that models the caisson pile and the surrounding ground. The design model shown in Figure 2 is created by inputting the shape of the caisson pile, ground data, etc. into the design software.

(Step S2)
First, the depth level at which the anchors will be installed is determined. The anchors are preferably installed at a depth level where there is a hard layer of rock or other surrounding ground. Furthermore, to efficiently resist bending moments, it is preferable to install them as high as possible on the caisson pile. Figure 1(a) shows four layers of ground. Since layer L1 is a surface layer or backfill soil and is not suitable for anchor installation, the anchors are installed in layer L2. Then, based on the design model created in step S1, a virtual surrounding ground is calculated at the depth level at which the anchors are installed. The virtual surrounding ground is ground that has physical properties capable of withstanding a specified increased seismic force in the absence of anchors, and corresponds to reinforced ground obtained by ground improvement work on the existing ground. In the following explanation, the above-mentioned depth level is referred to as the anchor installation level. However, the anchor installation level does not need to coincide with the stratum classification and may span multiple layers.

In this specific example, trial calculations were performed using design software, assuming that the horizontal seismic force on the existing structure had increased to 1.2 times the design value. Specifically, the elastic modulus E and strength (internal friction angle φ and cohesion C) of the ground spring, shown by the thick line in Figure 2, were changed to determine the ground conditions necessary to maintain the stability of the caisson piles. As a result, it was found that the elastic modulus E and ground strength of layer L2 needed to be increased by 1.5 times to maintain the stability of the caisson piles. These ground conditions are the target values when evaluating the reinforcing effect of anchors. Note that because layer L2 was assumed to be clayey soil, the internal friction angle φ was not taken into account.

(Step S3)
A first model is created that models the caisson pile and the virtual surrounding ground. The first model is a caisson pile-soil interaction model at the anchor installation level, created by cutting out stratum L2 and the caisson pile at the same level as stratum L2. The ground data for stratum L2 is input as an elastic modulus E and ground strength (adhesion force C) 1.5 times larger than that of the existing ground. The first model is a three-dimensional model based on the finite element method. To reduce calculation time and costs, the caisson pile is cut into a semicircular half model. Figure 5 shows the FEM model. Figure 5(a) shows an oblique view, Figure 5(b) shows a side view, and Figure 5(c) shows a plan view. A rectangular mesh, which is a sector divided radially, is placed around the caisson pile 1, a square mesh is placed outside of that, and finer meshes such as triangles and squares are placed between them. The reason for placing square meshes by dividing the sector in the radial direction is to set anchor elements at the boundaries of these meshes in the second model (described later), and to match the mesh configurations of the first and second models. A surface joint is set at the boundary between the caisson pile and the ground.

(Step S4)
From the first model, the relationship between the horizontal displacement of the caisson pile and the horizontal reaction force is determined. As shown in Figure 6, the caisson pile 1 is horizontally displaced by a certain amount, and the horizontal reaction force that the caisson pile 1 receives from the ground 5 is calculated. This calculation is repeated to determine the relationship between the horizontal displacement and the horizontal reaction force, as shown in Figure 7.

(Step S5)
A second model is created that models the caisson pile, anchor, and the actual surrounding ground. The second model is a caisson pile-soil interaction model at the anchor installation level, created by cutting out stratum L2, the anchor, and the caisson pile at the same level as stratum L2. The ground data for stratum L2 includes the elastic modulus E and ground strength (adhesion C) of the existing ground. The second model is a three-dimensional model based on the finite element method, and as described above, is essentially the same as the first model, except that anchor elements are set at the mesh boundaries. Figure 8 shows the FEM model. Figure 8(a) shows an oblique view, Figure 8(b) shows a side view, and Figure 8(c) shows a plan view. The ground mesh is omitted in Figure 8, but it is the same as Figure 5. The anchor element is a rod element that bears only tensile loads and is modeled based on the anchor's position, length, etc. Force is transmitted between the anchor element and the ground only at the end of the anchor element (the anchoring part on the ground side), and frictional forces between the side of the anchor element and the ground are not taken into account. The anchor specifications (number of anchors, length, placement, diameter, etc.) are set with reference to the elastic modulus E and ground strength of the hypothetical surrounding ground, but these are provisional settings at this stage and are finally determined in step S7. In Figure 8, the anchors 4 are installed horizontally, but it is also possible to install the anchors 4 diagonally downward from the caisson pile 1. In that case, the ground mesh is inclined to match the direction in which the anchors extend.

(Step S6)
The relationship between the horizontal displacement of the caisson pile and the horizontal reaction force is determined from the second model. As in step S4, the caisson pile 1 is displaced horizontally by a certain amount, as shown in Figure 6, and the horizontal reaction force that the caisson pile 1 receives from the ground 5 is calculated. This calculation is repeated to determine the relationship between the horizontal displacement and the horizontal reaction force, as shown in Figure 7. Note that either steps S3-4 or steps S5-6 can be performed first.

(Step S7)
The anchor specifications are determined based on the relationship between the horizontal displacement and horizontal reaction force of the caisson piles calculated in steps S4 and S6. As described above, the anchor specifications include the number, length, placement, diameter, etc. of anchors. The anchor specifications are determined so that, at the anchor installation level, the horizontal displacement of the caisson piles (second model) with anchors during an earthquake is smaller than the horizontal displacement of the caisson piles (first model) without anchors and with virtual surrounding ground. Specifically, the anchor specifications are determined so that the horizontal reaction force (horizontal load) in the second model is greater than the horizontal reaction force (horizontal load) in the first model for any horizontal displacement. Figure 9 schematically shows the horizontal displacement and horizontal reaction force (horizontal load) of the caisson piles in the first and second models. Figure 9 also shows the relationship between horizontal displacement and horizontal reaction force in the case where the ground constants of the existing ground are used and no anchors are installed (current model (E + C)). The first model (1.5E + 1.5C), which has a 1.5 times higher elastic modulus E and ground strength (adhesion C), has a larger horizontal reaction force at any horizontal displacement compared to the current model. In other words, the horizontal displacement is smaller when the same load is applied. The initial gradient between the current model and the first model reflects the difference in elastic modulus E, and the maximum load reflects the difference in adhesion C. In the figure, the hatched area is the area where the horizontal load is larger than in the first model, and this area is the target area for anchor reinforcement.

The second model, which includes anchors, has a larger horizontal reaction force at any given horizontal displacement than the current model and the first model. In other words, it experiences smaller horizontal displacement when subjected to the same load. As a result, the second model is evaluated as providing even safer results than the first model, which confirmed the stability of the caisson piles against increased seismic forces. Figure 7 also shows a case (2E + 2C) in which the soil elastic modulus E and soil strength (adhesion C) are doubled compared to the existing ground. By calculating the load-displacement curve for such a case, it is possible to prevent the anchor specifications from being overly cautious and optimize the design margin of the second model.

In other words, it can also be explained as follows: In the load-displacement relationship, the initial gradient is thought to reflect the elastic modulus E of the ground. Furthermore, because the load-displacement relationship becomes curved as the ground becomes plastic, differences in load at the same displacement are thought to reflect differences in adhesion C, which is the strength of the ground. Therefore, if the load-displacement curve obtained with the second model is located above the load-displacement curve obtained with the first model, the second model can be considered to have an elastic modulus E and adhesion C that are equal to or greater than those of the first model.

In the above specific example, step S7 ends because the second model provided a safe result for the first model, but the second model does not always provide a safe result for the first model. If the second model provides an unsafe result for the first model, the anchor specifications are revised and steps S5 to S7 are executed again. Alternatively, multiple anchor specifications may be set in step S5, and step S6 may be executed for multiple second models. In this case, multiple second models may be compared with the first model in step S7, and a second model that provides a safe result may be selected.

(Step S8)
The virtual surrounding ground is incorporated into the design model, and the caisson pile is designed using the design model. In this specific example, the ground data for layer L2 is input as 1.5 times the elastic modulus E and ground strength (adhesion C) of the existing ground, while the elastic modulus E and ground strength (adhesion C) of the existing ground are input for layers L1, L3, and L4. In reality, this design model was created in step S1, so there is no need to create a new design model in this step. As described above, since the second model has been confirmed to provide safer results than the first model, inputting the elastic modulus E and strength (internal friction angle φ and adhesion C) used in the first model into existing design software enables the design of anchor-reinforced caisson piles using the same procedure as before. As described above, in this embodiment, after confirming that anchor reinforcement has an effect equal to or greater than ground reinforcement, the elastic modulus E and strength of the reinforced ground are input into the design software, so the effects of anchor reinforcement can be incorporated into existing design models of caisson piles.

1. Deep foundation pile 4. Anchor 5. Ground

Claims (3)

A design method for a deep foundation pile in which a plurality of anchors are provided around the deep foundation pile at a predetermined depth level of the ground so as to maintain the stability of the deep foundation pile, and there is a ground that cannot maintain the stability of the deep foundation pile around the deep foundation pile .
Based on a design model that models the caisson pile and the surrounding ground that constitutes a predetermined periphery of the caisson pile, the ground constant of the virtual surrounding ground is assumed to be greater than the ground constant determined according to the geology of the local ground so as to maintain the stability of the caisson pile .
Determine the specifications of the plurality of anchors so that the horizontal displacement of the caisson pile during an earthquake when the plurality of anchors are installed and the surrounding ground is composed of the existing ground at the predetermined depth level is smaller than the horizontal displacement of the caisson pile during an earthquake when all of the plurality of anchors are absent and the surrounding ground is composed of the virtual surrounding ground ;
A design method for deep foundation piles, comprising: incorporating the virtual surrounding ground into the design model; and designing the deep foundation piles using the design model. Creating a first model that is a calculation model that models the caisson pile and the virtual surrounding ground at the predetermined depth level;
Obtaining a relationship between the horizontal displacement and horizontal reaction force of the caisson pile from the first model;
Creating a second model that is a calculation model that models the caisson pile, the plurality of anchors, and the on - site ground at the predetermined depth level;
Obtaining the relationship between the horizontal displacement and horizontal reaction force of the caisson pile from the second model;
and
2. A design method for deep foundation piles as described in claim 1, wherein the specifications of the anchor are determined so that the horizontal reaction force in the second model is greater than the horizontal reaction force in the first model for any horizontal displacement. The design method for deep foundation piles described in claim 2, wherein the first model and the second model are three-dimensional models based on the finite element method.