JP6804728B2 - Liquefaction countermeasure structure design method for structures - Google Patents

Description

The present invention relates to a method for the design of liquefaction countermeasure structure structure creation for reducing damage due to liquefaction of a structure having a pile foundation.

When liquefaction occurs in the ground G having the non-liquefaction layer 2 on the liquefaction layer 1, for example, as shown in FIG. 13, the ground G above the liquefaction layer 1 (the upper non-liquefaction layer 2) ) Is greatly displaced. Then, for example, as shown in FIG. 14, when a structure (building) 4 having a pile foundation (pile) 3 is constructed on such a ground G, liquefaction occurs during an earthquake and the ground displacement suddenly changes. There is a risk that the pile 3 will be damaged at the boundary between the liquefied layer 1 and the upper non-liquefied layer 2, and the liquefied layer 1 and the non-liquefied layer 5 below the liquefied layer 1.

Conventionally, in order to prevent damage to the pile 3 (damage to the structure 4) due to the liquefaction of the ground G, the ground is improved so as not to liquefy the ground G, or the pile 3 is reinforced during liquefaction. Measures were often used to withstand the generated load F. However, as a countermeasure by ground improvement, especially when the liquefied layer 1 is continuously present in the deep part or a plurality of liquefied layers 1 are present with the non-liquefied layer in between, the liquefied layer 1 of the lowest layer is present. There is a problem that the cost of countermeasures becomes very high because it is necessary to improve the ground G having an enormous volume when it exists in a deep part. Further, there is a problem that it is difficult to adopt the measures for ground improvement and the measures for reinforcing the pile 3 for the existing pile foundation structure 4.

On the other hand, as shown in FIG. 15, for example, a liquefaction countermeasure has been proposed in which the rooting portion 4b of the structure 4 (superstructure 4a) is filled with a material (soft material 6) softer than the liquefaction layer 1. (See, for example, Patent Document 1). In this measure, the soft material 6 absorbs the force F acting on the structure 4 at the time of liquefaction to reduce the influence on the structure 4.

Further, for example, as shown in FIG. 16, a liquefaction countermeasure in which a lightweight material 7 is embedded around the structure 4 (superstructure 4a) has also been proposed. In this measure, the influence on the structure 4 is reduced by reducing the passive resistance due to the ground displacement during liquefaction.

Further, for example, as shown in FIG. 17 (FIG. 17 (a): cross-sectional view, FIG. 17 (b): plan view), the ground G around the structure 4 (superstructure 4a) is replaced with wall-shaped sand 8. Liquefaction countermeasures have also been proposed. In this measure, the replaced wall-shaped sand 8 is liquefied to suppress the ground displacement during liquefaction and reduce the influence on the structure 4.

Further, there is also a measure to provide a wall along the outer periphery of the structure 4, a measure called a so-called skirt wall method. In this skirt wall method, for example, as shown in FIG. 18, a foundation 4c is provided along the outer periphery of the structure 4. A connected rigid wall (skirt wall 10) is provided to reduce the horizontal load of the pile 3 (see, for example, Patent Document 2).

Japanese Unexamined Patent Publication No. 2000-178997 Japanese Unexamined Patent Publication No. 57-9925

However, in the measure of filling the rooting portion 4b of the structure 4 with the soft material 6, the ground G around the rooting portion 4b becomes soft, so that the shaking of the structure 4 (superstructure 4a) is caused by the rooting portion 4b. The effect of suppressing is reduced. Therefore, in the case of a small and medium-sized earthquake that does not cause liquefaction, on the contrary, the stress of the pile 3 may increase due to the shaking of the structure 4 and damage may occur.

Further, in the measure of burying the lightweight material 7 around the structure 4, there is a problem that it is necessary to bury the lightweight material 7 in a large area in order to surely reduce the passive resistance due to the ground displacement during liquefaction. was there.

Even in the measures to replace the ground G around the structure 4 with sand 8, it is necessary that the groundwater level T is near the ground surface in order to ensure that the wall-shaped sand 8 is liquefied. Even the structure 4 below cannot be applied, and there is a problem that its adoption is greatly restricted.

Further, in the skirt wall construction method, a rigid wall (skirt wall 10) connected to the foundation 4c bears a horizontal force to reduce the horizontal load of the pile 3, and is a structure when the ground G is liquefied. It is not intended to reduce the horizontal force acting on the 4 or the pile 3. That is, the skirt wall 10 is provided to reduce the shearing force acting on the joint portion between the foundation 4c of the structure 4 and the pile 3 and reduce the damage to the joint portion, and is irrelevant to liquefaction. (It is not provided as a measure against liquefaction). Further, if such a skirt wall 10 is located in the non-liquefied layer 2 above the liquefied layer 1, the load (horizontal force) acting on the structure 4 at the time of liquefaction may be increased. Further, the pile stress generated at the boundary between the liquefied layer and the lower non-liquefied layer located deeper than the lower end of the skirt wall 10 may not be reduced.

In view of the above circumstances, a large area, without requiring the volume, to effectively provide a method of designing a liquefaction countermeasure structure to that structure creation allows damage liquefaction prevent (reduce) The purpose is.

In order to achieve the above object, the present invention provides the following means.

The method for designing a liquefaction countermeasure structure for a structure of the present invention is a structure for reducing damage caused by liquefaction of a structure constructed by providing a pile foundation on a ground having a non-liquefied layer on the liquefied layer. It is a design method of a structure for preventing liquefaction of an object, which is provided so as to surround the pile foundation, the upper end side is arranged in a non-liquefied layer above the liquefied layer, and the lower end side is the liquid. It is provided in a non-liquefied layer under the liquefied layer, and includes an edge cutting member that cuts the liquefied layer on the pile foundation side and the liquefied layer on the outside thereof, and the upper end portion of the edge cutting member is the structure. The structure specification setting step of setting the specifications of the structure and the ground springs of the pile foundation, the edge cutting member, and the rooting portion of the structure are arranged above the lower end of the structure. The ground spring setting step to be calculated, the general ground displacement calculation step to calculate the ground displacement at the time of an earthquake without the edge cutting member as the general ground displacement, and the ground displacement of the portion of the pile foundation located in the edge cutting member. Using the reference, the general ground displacement is given to the structure model composed of the beam spring model via the ground spring to calculate the displacement of the edge cutting member, and the displacement of the edge cutting member is the inside of the edge cutting member at the time of an earthquake. The step of calculating the ground displacement in the edge cutting member obtained as the ground displacement and the general ground displacement are given to the edge cutting member, the rooting portion of the structure, and the pile foundation located deeper than the lower end of the edge cutting member, and the edge cutting member. The difference between the displacement of the edge cutting member and the ground displacement updating step of the edge cutting member in which the ground displacement of the inside of the edge cutting member is given to the pile foundation to obtain the ground displacement of the inside of the new edge cutting member. The ground displacement determination step inside the edge cutting member and the ground inside the edge cutting member are determined by repeating the process until the edges converge, and the displacement of the edge cutting member at the stage where the difference in displacement of the edge cutting member converges is determined as the ground displacement inside the edge cutting member. It is characterized by including a pile stress calculation step of calculating the stress of the pile foundation at the time of an earthquake based on the ground displacement inside the edge cutting member determined in the displacement determination step.

According to the design method of liquefaction countermeasure structure structure creation of the present invention, a large seismic ground displacement during liquefaction, structures such as buildings pile stress becomes considerably large at the boundary between the liquid layer and the non-liquefied layer It is possible to significantly reduce the pile stress even for an object by providing an edge cutting member.

It is sectional drawing which shows the liquefaction countermeasure structure of the structure which concerns on one Embodiment of this invention. It is a flow chart which shows the design method of the liquefaction countermeasure structure of the structure which concerns on one Embodiment of this invention. It is a figure used for the explanation at the time of calculating the general ground displacement in the design method of the liquefaction countermeasure structure of the structure which concerns on one Embodiment of this invention. It is a figure used for explanation at the time of calculating the ground displacement inside the edge cutting member in the design method of the liquefaction countermeasure structure of the structure which concerns on one Embodiment of this invention. It is a figure used for explanation at the time of calculating the ground displacement inside the edge cutting member in the design method of the liquefaction countermeasure structure of the structure which concerns on one Embodiment of this invention. It is a figure used for explanation at the time of calculating the ground displacement inside the edge cutting member in the design method of the liquefaction countermeasure structure of the structure which concerns on one Embodiment of this invention. It is a figure used for explanation at the time of calculating the ground displacement inside the edge cutting member in the design method of the liquefaction countermeasure structure of the structure which concerns on one Embodiment of this invention. It is a figure used for explanation at the time of calculating the ground displacement inside the edge cutting member in the design method of the liquefaction countermeasure structure of the structure which concerns on one Embodiment of this invention. It is a figure used for explanation at the time of calculating the ground displacement inside the edge cutting member in the design method of the liquefaction countermeasure structure of the structure which concerns on one Embodiment of this invention. It is a figure which shows the model used in the demonstration experiment. It is a figure which shows the fitting state of the insulating member (wall material) of the model used in the demonstration experiment. It is a figure which shows the result of the demonstration experiment, (a) is the bending moment generated in the pile foundation, (b) is the figure which shows the shearing force generated in the pile foundation. It is a figure which shows the displacement at the time of liquefaction of the ground which has a non-liquefaction layer on the liquefaction layer. It is a figure used for the explanation about the damage of a structure by liquefaction. It is sectional drawing which shows the liquefaction measures of the conventional structure. It is sectional drawing which shows the liquefaction measures of the conventional structure. It is sectional drawing and plan view which show the liquefaction measures of the conventional structure. It is sectional drawing which shows the liquefaction measures of the conventional structure.

Hereinafter, with reference to FIGS. 1 to 13, a method for designing a liquefaction countermeasure structure for a structure and a liquefaction countermeasure structure for a structure according to an embodiment of the present invention will be described. The present embodiment relates to a structure for preventing or reducing damage to a structure (pile foundation structure) during liquefaction, and a method for designing the structure.

First, as shown in FIG. 1, the structure 4 of the present embodiment is constructed on the ground G having a non-liquefied layer (upper non-liquefied layer) 2 on the liquefied layer 1, and is built in the ground G. It is constructed by supporting the superstructure 4a with a plurality of piles (pile foundations 3) cast. Further, the superstructure 4a is constructed by rooting the lower end side (rooting portion 4b) into the upper non-liquefied layer 2.

The liquefaction countermeasure structure 15 of the structure of the present embodiment includes the liquefaction layer 1 on the side of the plurality of piles 3 (the liquefaction layer 1 immediately below the structure 4 in the present embodiment) and the liquefaction layer 1 outside the liquefaction layer 1. The edge cutting member 16 is provided in the ground G on the outer side in the horizontal direction of the structure 4 so as to cut off the edges. Further, the edge cutting member 16 of the present embodiment is continuously provided along the outer periphery of the structure 4, and is provided with the lower end portion 16a side embedded in the liquefaction layer 1.

Further, the edge cutting member 16 has an upper end portion 16b arranged in the upper non-liquefied layer 2 and a lower end portion 16a side in the non-liquefied layer (lower non-liquefied layer) 5 under the liquefied layer 1. It is provided with rooting.

The rigidity, material, thickness, etc. of the edge cutting member 16 need not be particularly limited. Further, as long as the liquefaction layer 1 provided with the pile 3 is arranged so as to surround the liquefaction layer 1, it may be partially discontinuous, partially notched, or having holes. The edge cutting member 16 may be a member that does not allow soil to permeate as much as possible even if water passes through it when the ground is liquefied.
Further, in the present embodiment, the edge cutting member 16 is provided so that the upper end portion 16b side thereof is in contact with the rooting portion 4b of the structure 4. On the other hand, the edge cutting member 16 may be provided with its upper end arranged in the upper non-liquefied layer 2. At this time, it is more preferable that the upper end portion of the edge cutting member 16 is arranged above the lower end portion of the structure 4. Further, the edge cutting member 16 is provided so as to be separated from the rooting portion 4b, or the ground is improved between the edge cutting member 16 and the rooting portion 4b, and an intervening layer is provided between the edge cutting member 16 and the rooting portion 4b. May be good.

Further, the structure 4 of the present embodiment is provided with the edge cutting member 16 (liquefaction countermeasure structure 15 of the structure) as described above, and thus acts on the pile foundation 3 with the liquefaction of the liquefaction layer 1 at the time of an earthquake. The stress is obtained as shown in FIG. 2, and the pile foundation 3 is constructed based on the obtained stress (specifications such as the shape, strength, and rigidity of the pile foundation 3 are set).

First, as shown in FIG. 2 (and FIG. 1), specifications such as the shape of the pile 3 and the edge cutting member 16 and the proof stress and rigidity of each member (or as a whole) are set (Step 1: Structure specification setting). Process). Further, the pile spring, the edge cutting member spring (wall spring), and the rooting spring, that is, the ground spring of the rooting portion of the pile 3, the edge cutting member 16, and the structure 4 are calculated / set (Step 2: Ground spring setting step).

Further, as shown in FIGS. 2 and 3 (and FIG. 1), the ground displacement during an earthquake (hereinafter referred to as general ground displacement) in the absence of the edge cutting member 16 (liquefaction countermeasure structure 15) is calculated (Step 3: general). Ground displacement calculation process).

Then, as shown in FIGS. 2, 4, 5, 6 (and 1), the ground displacement of the portion of the pile 3 located in the edge cutting member 16 is set to 0 (based on), and the general ground displacement is set to the ground. The displacement of the edge cutting member 16 is calculated by giving it to the structure model via a spring, and the displacement of the obtained edge cutting member 16 is defined as the internal ground displacement surrounded by the edge cutting member 16 (Step 4, Step 5: Ground inside the edge cutting member). Displacement calculation process).

Further, in the present embodiment, as shown in FIGS. 2, 4, 5, 6, 7, 8, 9 (and 1), general ground displacement is applied to the edge cutting member 16 and the rooting 4b. In addition, the ground displacement in the edge cutting member is given to the pile 3 to calculate the ground displacement in the new edge cutting member 16 (Step 6: ground displacement updating step in the edge cutting member). This operation is repeated until the difference in displacement of the edge cutting member 16 converges. Then, the displacement of the edge cutting member at the stage where the difference in displacement of the edge cutting member 16 has converged is defined as the ground displacement in the edge cutting member (Step 7, Step 8: ground displacement determination step in the edge cutting member).

That is, since the deformation of the edge cutting member 16 is undecided at the initial stage, in the present embodiment, the deformation of the edge cutting member 16 is made equal to the ground deformation given to the pile by repeated calculation or the like, and the displacement at this time is set in the edge cutting member. Determined as ground displacement.

Further, in the present embodiment, in order to evaluate on the safe side, the rigidity of the edge cutting member 16 is set in consideration of only the rigidity of the surface (edge cutting member 16 that causes out-of-plane deformation) orthogonal to the direction of the seismic force.

Next, the pile stress in consideration of the inertial force and the ground displacement is calculated based on the determined ground displacement in the edge cutting member (Step 9: pile stress calculation step). At this time, for example, the pile stress is obtained by the simple sum or the square of the sum of squares of the pile stress due to the building inertial force and the ground displacement, or the pile stress is calculated by simultaneously loading the inertial force and the ground displacement.

Here, a demonstration experiment in which a sine wave having a period of 1 second, a steady wave of 30 waves, and a maximum acceleration of 100 cm / s ² is used as an input wave of seismic motion will be described.

In this demonstration experiment, the experimental model shown in FIG. 10 was used.
A shear soil layer of 800 mm × 400 mm × 325 mm was used as the soil tank. The lower non-liquefied layer 5 of the ground G has a thickness of No. 3 silica sand layer having a relative density of about 100% of 100 mm (actual conversion 3 m), and the liquefied layer 1 and the upper non-liquefied layer 2 have a relative density of No. 7 having a relative density of 50%. A silica sand layer was created by the aerial drop method. The pore fluid was silicon oil having a specific gravity of 1 and a viscosity of 30 cs, and the groundwater level was 70 mm from the ground surface (actual conversion 2.1 m).

The pile 3 has four brass pipes (pile spacing 96 mm) having a diameter of 12 mm, a wall thickness of 0.5 mm, and a length of 252 mm. The tip of the pile was pinned using a bearing. The pile head can be regarded as a rigid joint.

The foundation was made of aluminum with a flat surface of 150 mm × 164 mm and a thickness of 40 mm. In order to prevent the structural inertial force from prevailing, the structure is made only of the foundation part, and the weight of the structure is reduced so that it is suppressed to almost the same level as the surrounding ground.

The edge cutting member (wall material) 16 is an aluminum plate having a thickness of 0.3 to 4.0, and the tip is inserted into urethane so that vibration from the bottom surface of the soil tank is not propagated. Further, the edge cutting member 16 is made of Teflon (registered trademark) tape having a thickness of 0.08 mm and five plate-shaped members having a width of 29 to 36 mm on each side so that the bending rigidity around the vertical axis does not contribute to the result. Hereinafter, it was created by pasting with Teflon tape). Further, in order to eliminate the influence of the in-plane rigidity of the side wall, as shown in FIG. 11, a gap of 1 mm is left between the side wall and the front and rear surface walls (adjacent edge cutting members 16), and a 0.08 mm thick Teflon tape is used. I just fastened it. Strain gauges were attached to 7 cross sections of the pile 3 and the edge cutting member (excluding the thickness of 0.3 mm) 16.

The main model specifications and similarity rules are as shown in Table 1.

The experimental cases are as shown in Table 2. C0 without the edge cutting member 16 and C0.3, C1.0, C2.0, and C4 with the thickness changed in the range of 0.3 to 4.0 mm. There were a total of 5 cases of 0. Table 2 also shows the ratio of the bending rigidity of the edge cutting member 16 to the bending rigidity of the retaining wall in which the H-shaped steels of H400-200-8-13 are arranged at a pitch of 900 mm. That is, C0.3 uses the edge cutting member 16 having a rigidity ratio of less than 1 / 10,000 of the retaining wall and having a very small bending rigidity.

FIG. 12 shows the results of the demonstration experiment, and the bending moment distribution (FIG. 12 (a)) and the shear force distribution (FIG. 12 (a)) at the time (about 12.6 s) when the maximum bending moment is exhibited at the liquefaction layer boundary immediately after liquefaction. 12 (b)) is shown.

From these results, it was confirmed that the presence of the edge cutting member 16 reduces both the bending moment and the shearing force near the upper and lower boundaries of the pile head and the liquefaction layer 1 to about 2/3 to 1/2. Further, it was confirmed that there was almost no difference in the effect depending on the thickness of the edge cutting member 16, and that even a very thin edge cutting member 16 had a large effect of reducing the stress of the pile 3 due to the ground displacement during an earthquake.

In other words, it was confirmed that it is important to prevent the liquefied ground from slipping through as a function / action of the edge cutting member 16, and the difference in effect due to the difference in rigidity is small. That is, it has been confirmed that the edge cutting member 16 may be a very thin member or a member having low rigidity as long as it is possible to prevent the liquefied ground from slipping through, and can exhibit an excellent pile stress reducing effect regardless of the rigidity. Was done.

Therefore, in the design method of the liquefaction countermeasure structure 15 of the structure and the liquefaction countermeasure structure 15 of the structure of the present embodiment, the ground displacement during an earthquake during liquefaction is large, and the liquefaction layer 1 and the non-liquefaction layer 2 are present. Even for a structure 4 such as a building in which the liquefaction stress becomes remarkably large at the boundary with 5 and 5, the liquefaction stress can be significantly reduced by providing the edge cutting member 16.

Although one embodiment of the liquefaction countermeasure structure of the structure and the design method of the liquefaction countermeasure structure of the structure according to the present invention has been described above, the present invention is not limited to the above one embodiment. It can be changed as appropriate without departing from the purpose.

For example, the edge cutting member 16 may be installed at the time of constructing the structure 4 (for example, the earth retaining wall may be used as the edge cutting member 16), or the sheet pile may be driven into the ground G as the edge cutting member 16 to drive the existing structure 4 into the ground G. The edge cutting member 16 (liquefaction prevention structure 15) may be added to the surface.

1 Liquefied layer 2 Upper non-liquefied layer 3 Pile (pile foundation)
4 Structure 4a Superstructure 4b Rooting part 4c Foundation 5 Lower non-liquefied layer 6 Soft material 7 Lightweight material 8 Sand 10 Skirt wall (wall, wall body)
15 Structure liquefaction countermeasure structure 16 Edge cutting member F Horizontal force (load)
G Ground T Groundwater level

Claims (1)

It is a design method for liquefaction countermeasure structures of structures to reduce damage caused by liquefaction of structures constructed with pile foundations on the ground where there is a non-liquefaction layer on top of the liquefaction layer.
It is provided so as to surround the pile foundation, and the upper end side is arranged in the non-liquefaction layer above the liquefaction layer, and the lower end side is arranged in the non-liquefaction layer under the liquefaction layer. Provided, the liquefaction layer on the pile foundation side and the liquefaction layer on the outside thereof are provided with an edge cutting member for edge cutting.
The upper end portion of the edge cutting member is arranged above the lower end portion of the structure.
The structure specification setting process for setting the specifications of the structure and
A ground spring setting step for calculating each ground spring of the pile foundation, the edge cutting member, and the rooting portion of the structure, and
A general ground displacement calculation process that calculates the ground displacement at the time of an earthquake when there is no edge cutting member as a general ground displacement, and
The displacement of the edge cutting member is calculated by giving the general ground displacement to the structure model composed of the beam spring model via the ground spring while referring to the ground displacement of the portion of the pile foundation located in the edge cutting member. The step of calculating the ground displacement inside the edge cutting member, which obtains the displacement of the edge cutting member as the ground displacement inside the edge cutting member at the time of an earthquake.
The general ground displacement is given to the edge cutting member, the rooting portion of the structure, and the pile foundation located deeper than the lower end of the edge cutting member, and the ground displacement inside the edge cutting member is given to the pile foundation. A step of updating the ground displacement inside the edge cutting member for obtaining the ground displacement inside the new edge cutting member, and
The ground displacement updating step inside the edge cutting member is repeated until the difference in displacement of the edge cutting member converges, and the displacement of the edge cutting member at the stage where the difference in displacement of the edge cutting member converges is used as the ground displacement inside the edge cutting member. The process of determining the ground displacement inside the edge cutting member to be determined, and
The structure is characterized by including a pile stress calculation step of calculating the stress of the pile foundation at the time of an earthquake based on the ground displacement inside the edge cutting member determined in the ground displacement determination step inside the edge cutting member. How to design a structure to prevent liquefaction of objects.

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