JP2007039927A - Pile foundation structure - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent a structure from collapsing without improving a liquefied ground over a wide range. <P>SOLUTION: This pile foundation structure 1 comprises a footing 3 forming a part of a viaduct as an upper structure and a pile 5 buried in the liquefied ground 4 as a ground and in which a head part is joined to the footing 3. Only the covered area 6 of the liquefied ground 4 which surrounds the pile 5 so as to abut on the peripheral surface thereof is improved. When only the covered area 6 for the pile 5 is improved by filling a chemical around the pile 5 supporting the viaduct 2, when a completely liquefied state is expected, a covering rate t/D is set to 4 and larger. When it is sufficient to estimate a medium liquefied state, the covering rate t/D is set to 3 or larger or 1 or or larger. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a pile foundation structure mainly applied to a viaduct pile foundation.

  The liquefaction that occurs during an earthquake is that when horizontal vibrations act on the ground due to the earthquake, the pore water pressure between the sand particles rises due to the shear deformation of the ground, and the effective stress becomes zero with the increase in the pore water pressure, and between the sand particles. This is a phenomenon in which stress cannot be transmitted and fluidity increases, eventually leading to the collapse of the building by losing the vertical support force. Needless to say, it tends to occur in loose saturated sandy ground (hereinafter, liquefaction is likely to occur) The ground is called liquefied ground).

  In Japan, the devastating damage caused by such liquefaction has long been recognized by the Niigata Earthquake, and various liquefaction countermeasures have been researched and developed.

  As a typical countermeasure against liquefaction, when an existing structure is erected, the ground strength is improved by injecting chemicals into a wide area of the liquefied ground that extends downward, and shear deformation during an earthquake is suppressed. It is.

JP-A-3-5528 JP 2002-30649 A

  Here, in the case of a pile foundation, when the ground around the pile is liquefied during an earthquake, the horizontal ground reaction force does not act on the pile. In other words, because ground resistance decreases due to liquefaction, the pile bears most of the horizontal force during earthquakes from existing structures, causing a large bending moment at the pile head and causing bending failure. As a result, the entire pile cannot support the load of the existing structure.

  However, conventional liquefaction measures that improve the ground strength by injecting chemicals etc. over a wide range of liquefied ground not only have poor construction efficiency, but also, for example, pile foundations that support viaducts, along the bridge axis of the viaduct It was necessary to improve the ground over a long section, and there was a problem of lack of reality in terms of construction period and construction cost.

  The present invention has been made in consideration of the above-described circumstances, and an object thereof is to provide a pile foundation structure capable of reliably preventing the collapse of a structure without improving the ground of the liquefied ground extensively. And

  In order to achieve the above object, a pile foundation structure according to the present invention is a pile foundation structure configured to support an upper structure with a pile embedded in the ground as described in claim 1. Of the ground, only the covering region surrounding the pile so as to abut on its peripheral surface is improved.

  Moreover, the pile foundation structure which concerns on this invention is a pile foundation structure comprised so that an upper structure may be supported by the pile which consists of two or more embedded in the ground, as described in Claim 2, The ground is divided into a plurality of covering regions surrounding the piles so as to abut on their peripheral surfaces and an intermediate region extending outside the covering region, and only the plurality of covering regions are improved. It will be.

  Moreover, the pile foundation structure which concerns on this invention sets the coverage t / D to 4 or more, when the diameter of the said pile is D and the thickness of the said coating | coated area | region is set to t.

  Moreover, the pile foundation structure which concerns on this invention makes the coverage t / D 2 or more, or 1 or more, when the diameter of the said pile is D and the thickness of the said coating | covering area | region is set to t.

  As a result of research and development from various viewpoints in order to review the inefficient conventional method of improving the ground over a wide area as a measure to prevent liquefaction, the present applicant did not improve the ground of the liquefied ground over a wide area. We have found a new finding that the ground can be improved only by improving the ground around the pile.

  That is, in the pile foundation structure according to the present invention, only the covering region surrounding the pile so as to abut on its peripheral surface is improved in the ground, and even with such partial ground improvement, liquefaction prevention is prevented. It was confirmed by a verification test that sufficient effects can be obtained.

  It is arbitrary how much the thickness of the covering region is set, but in the case of the invention according to claim 2, that is, in the case of a group pile, the thickness where the covering regions of a pair of adjacent piles can maintain a non-contact state. Limit.

  On the other hand, the lower limit value of the thickness of the covering region may be set as appropriate depending on how much liquefaction is expected. When the diameter of the pile is D and the thickness of the covering region is t, the covering rate t / If D is set to 4 or more, the soundness of the pile can be ensured even if it is in a completely liquefied state, and the collapse of the upper structure can be prevented.

  Moreover, if the coverage t / D is 2 or more, or 1 or more, the soundness of the pile can be sufficiently ensured in the case of a medium-scale liquefied state.

  Here, in the present specification, the state where the pore water pressure ratio (excess pore water pressure Δu / initial upper loading pressure σv ′) is about 0.5 to 0.7 is liquefied in the medium scale, and the state where the pore water pressure ratio is almost 1 This is called liquefaction.

  Specific examples of the construction method for forming the coating region include a chemical solution injection method and a deep mixing treatment method.

  The deep mixing treatment method is a method of increasing the strength of the ground by supplying a solidifying agent such as cement slurry and powdered cement into the ground and stirring and mixing in situ. As a mixing method, a machine using an earth auger etc. In any case, the deep-mixing method is wide-ranging and can be selected as appropriate in consideration of the soil properties of the ground where the covered region is to be formed.

  Hereinafter, an embodiment of a pile foundation structure according to the present invention will be described with reference to the accompanying drawings. Note that components that are substantially the same as those of the prior art are assigned the same reference numerals, and descriptions thereof are omitted.

(First embodiment)
FIG. 1 is a view showing a pile foundation structure according to the present embodiment. As can be seen from the figure, the pile foundation structure 1 according to the present embodiment is embedded in the footing 3 constituting a part of the viaduct 2 as the upper structure and the liquefied ground 4 as the ground, and is headed by the footing 3. It is comprised from the pile 5 to which the part was joined.

  Here, in the liquefied ground 4, only the covering region 6 surrounding the pile 5 so as to abut on the peripheral surface thereof is improved.

  In order to construct the pile foundation structure 1 according to the present embodiment, only the covering region 6 of the pile 5 is improved by injecting a chemical solution around the pile 5 that supports the viaduct 2 that is an existing structure.

  About a chemical | medical solution, what is necessary is just to select suitably from the well-known chemical | medical agent used when improving a liquefied ground.

  In injecting the chemical solution and improving the ground only in the covering region 6, when the diameter of the pile 5 is D and the thickness of the covering region 6 is t, the covering ratio t / D is 1 or more, desirably 2 or more, and more desirably. Should be set to 4 or more.

  In setting the coverage ratio t / D, depending on how easy the liquefied ground 4 is liquefied or how much the design earthquake motion is assumed, if a completely liquefied state is assumed, The coverage t / D is preferably 4 or more.

  On the other hand, if it is necessary and sufficient if a medium-scale liquefied state is assumed, the coverage t / D may be 2 or more, or 1 or more.

  Here, the completely liquefied state means a state in which the pore water pressure ratio is substantially 1, and the medium-scale liquefied state means that the pore water pressure ratio (excess pore water pressure Δu / initial initial applied pressure σv ′) is about 0.5. It means a state of ˜0.7.

  As described above, according to the pile foundation structure 1 according to the present embodiment, even if only the covering region 6 of the pile 5 is improved, the liquefied ground 4 is liquefied and cannot support the viaduct 2. It becomes possible to prevent the situation in advance.

  Moreover, according to the pile foundation structure 1 which concerns on this embodiment, even if a complete liquefaction state is assumed by making coverage ratio t / D 4 or more, the soundness of the pile 5 and also the collapse of the viaduct 2 are destroyed. It can be prevented in advance, and by setting the coverage ratio t / D to 2 or more, or 1 or more, the soundness of the pile 5 and the collapse of the viaduct 2 can be prevented in the case where a medium-scale liquefaction state is assumed. Can be prevented.

  In the present embodiment, the covered region according to the present invention is improved by ground injection, but instead of this, the ground may be improved by a deep mixing treatment method.

(Second Embodiment)
Next, a second embodiment will be described. Note that components that are substantially the same as those of the first embodiment are denoted by the same reference numerals, and description thereof is omitted.

  Drawing 2 is a figure showing the pile foundation structure concerning a 2nd embodiment. As can be seen from the figure, the pile foundation structure 21 according to the present embodiment includes a footing 23 constituting a part of the viaduct 22 as an upper structure and a liquefied ground 4 as a ground embedded in the footing 23. It is comprised from the multiple pile 25 to which the part was joined.

  Here, the liquefied ground 4 is divided into a plurality of covering regions 26 that surround each pile 25 so as to abut on their peripheral surfaces, and an intermediate region 27 that extends outside the covering region, and a plurality of coverings. Only the area 26 has been improved.

  In order to construct the pile foundation structure 21 according to the present embodiment, the covering region 26 of each pile 25 is injected by injecting a chemical solution around the pile 25 that supports the viaduct 22 that is an existing structure, as in the first embodiment. Only improve the ground.

  About a chemical | medical solution, what is necessary is just to select suitably from the well-known chemical | medical agent used when improving a liquefied ground.

  In injecting the chemical solution to improve the ground only in the covering region 26, when the diameter of the pile 25 is D and the thickness of the covering region 26 is t, the covering ratio t / D is 1 or more, desirably 2 or more, and further desirably. Is set to 4 or more, and an upper limit value of the coverage ratio t / D is determined so that the respective covering regions 26 do not contact each other.

  In setting the coverage ratio t / D, depending on how easy the liquefied ground 4 is liquefied or how much the design earthquake motion is assumed, if a completely liquefied state is assumed, The coverage t / D is preferably 4 or more.

  On the other hand, if it is necessary and sufficient if a medium-scale liquefied state is assumed, the coverage t / D may be 2 or more, or 1 or more.

  As described above, according to the pile foundation structure 21 according to the present embodiment, the liquefied ground 4 can be liquefied to support the viaduct 22 only by improving the ground of only the covering region 26 of the plurality of piles 25. It is possible to prevent an unexpected situation such as being lost.

  Moreover, according to the pile foundation structure 21 which concerns on this embodiment, even when a complete liquefaction state is assumed by making coverage ratio t / D 4 or more, the soundness of the pile 25 and also collapse of the viaduct 22 are destroyed. It can be prevented in advance, and by setting the coverage rate t / D to 2 or more, or 1 or more, the soundness of the pile 25 and the collapse of the viaduct 22 can be prevented in the case where a medium-scale liquefaction state is assumed. Can be prevented.

  In the present embodiment, the covered region according to the present invention is improved by ground injection, but instead of this, the ground may be improved by a deep mixing treatment method.

  Next, in order to confirm the effect of the pile foundation structure according to the present invention, an indoor model test was performed, and the outline thereof will be described below.

In the test, assuming that the ground was improved by chemical injection only in the vicinity of the pile foundation peripheral surface of the railway structure, the degree of liquefaction and the size of the improved range were used as parameters.
In addition, both static loading experiments and dynamic experiments (vibration experiments) were performed for the test.
(1) Static loading experiment
(a) Outline of the experiment As shown in Fig. 3, the experiment was conducted using a rectangular shear soil tank (length 2600mm, width 1000mm, height 750mm) and four single pile models placed in the shear soil tank. . The relative density of the ground was created with Dr = 55%. Silica sand No. 6 was used for the model ground, and it was created by the underwater dropping method.

  As the input wave, a sine wave (maximum acceleration 100 gal) was used for the purpose of simulating a completely liquefied state.

  As shown in Fig. 3, the measurement equipment is equipped with a pore water pressure meter and an accelerometer in the model ground, a strain gauge for the single pile model, an accelerometer and a displacement meter for the earthen ring, and the ground surface and footing section. Each was equipped with a displacement meter.

The single pile model is made of acrylic rigid resin material (ABS) with pile diameter D = 20mm, pile length 750mm, bending stiffness 287000kN / m ^2, and the lower end is fixed to the bottom of the shear soil tank. Was made free.

The single pile model is solidified with polymer to imitate the ground improvement near the periphery of the pile by chemical injection, and the strength and relationship between the actual scale pile strength and the actual strength by chemical injection are set to the same strength (E50 = 485 kN / m ² ).

  A horizontal sectional view of the single pile model is shown in FIG. As can be seen in the figure, the single pile model is only ABS resin imitating a pile (hereinafter referred to as "no countermeasure"), and the periphery of the ABS resin is solidified with a polymer having a thickness corresponding to the pile diameter D (hereinafter referred to as , Referred to as “1D improvement”), the periphery of the ABS resin solidified with a polymer having a thickness corresponding to the pile diameter 2D (hereinafter referred to as “2D improvement”), the thickness around the ABS resin corresponding to the pile diameter 4D A total of four bodies (hereinafter referred to as “4D improvement”) solidified with the above polymer. The improved depth (polymer length) was the total length of the piles made of ABS resin for all cases.

  In the experimental method, such four single pile models were placed in a shear soil tank, and a horizontal load of 10.0 kN was applied statically in the short side direction of the shear soil tank. The excitation direction was set to be perpendicular to the loading direction (long side direction of the shear soil tank), and the inertial force was not generated in the single pile model.

(b) Experimental results Fig. 5 shows the bending moment distribution in the initial state (before liquefaction, pore water pressure ratio ≒ 0), medium-scale state (pore water pressure ratio ≒ 0.5) and complete liquefaction state (pore water pressure ratio ≒ 1.0). To FIG.

  As can be seen from these figures, regardless of the liquefaction state, it can be seen that the first fixed point moves in the direction of the pile head according to the improved diameter, and the generated maximum moment tends to decrease as the improved diameter increases. .

  Table 1 shows the degree of decrease in the generated sectional force for each improved liquefaction state for each liquefaction state on the basis of no measures taken.

  As can be seen in the table, in the initial state, the generated cross-sectional force is reduced according to the pile improvement diameter, and in the medium-scale liquefaction state, the degree of decrease compared to no countermeasure is 20% for "1D improvement" and "2D improvement". , “4D improvement” is 30%, and “1D improvement” and “2D improvement” are not much different from each other. Further, in the completely liquefied state, “4D improvement” is reduced by about 20%, but “1D improvement” and “2D improvement” are not much different from no countermeasures, and there is no effect of countermeasure work.

  That is, “4D improvement” is necessary to exert the countermeasure effect even in the completely liquefied state, while “1D improvement” or “2D” is necessary when taking measures for the medium-scale liquefied state. It was found that the “improvement” level is sufficiently effective.

(2) Vibration test Based on the results of the static experiment described above, the entire structure system is based on the target pile as a group pile in “1D improvement” that is considered to be effective within the scope of common sense as an implementation work. I tried to understand the behavior of the.
(a) Outline of experiment Figure 8 shows the circular large shear tank (φ = 1200mm, H = 750mm) used in the experiment. As can be seen from the figure, in this experiment, a pile foundation model of group piles (four piles) was placed in the center of the soil tank and vibration was applied.

  In the pile foundation model, 98N weight was placed on the pile head as superstructure on four piles, and both the pile tip and the pile head were rigidly connected. The model ground was prepared with a relative density of Dr = 60%. In addition, since the production specifications of other model grounds and pile foundation models are the same as in the static loading test, the description thereof is omitted here.

  As the input wave, a 0.5 to 5.0 Hz broadband wave (white noise) shown in FIG. 9 was used so that the resonance state of the input wave and the structure could be confirmed at each stage during excitation.

  Table 2 shows experimental cases of pile foundation models and model ground, and FIG.

  As can be seen from the table, C-1 to C-3 are filled with water up to the ground surface of the model ground so that the entire ground layer becomes a liquefied layer. On the other hand, in C-4 to C-5, the water level of the model ground is limited to a depth of 30 cm from the model ground surface so as to become a non-liquefied layer from the ground surface to a certain depth. Here, the relative density of the model ground is the same for both the liquefied layer and the non-liquefied layer, and it is created with Dr = 60% as described above.

  In addition, about the boundary part of a liquefied layer and a non-liquefied layer, it decided to arrange | position the improvement body layer (5 mm-10 mm) with water-stopping and small rigidity, and to prevent permeation of the water after ground preparation.

  In addition, as shown in FIG. 8, the measuring instrument installed a pore water pressure meter and an accelerometer in the model ground, attached a strain gauge to the pile foundation model, and further installed an accelerometer and a displacement meter on the earthen ring. A displacement meter was installed on the ground surface and footing part of the model ground.

(b) Experimental results
i) Pile acceleration response magnification In this experiment, a broadband wave (white noise) is used for the input acceleration waveform, and the maximum input acceleration at each time and time is different. Is difficult to do.

  Therefore, as shown in FIG. 11, in the medium-scale liquefaction state in which the excess pore water pressure is rising from the beginning and in the complete liquefaction state, the Fourier amplitude of the input acceleration and the response acceleration at the top of the pile is calculated, and the frequency within the section is further calculated. For each section, the ratio of the input energy to the response energy of the structure (response magnification = structure / input wave) was calculated, and this value was examined as the response magnification. Table 3 shows the calculation result of the response magnification.

  As can be seen from the table, in the medium-scale liquefaction state, the acceleration response magnification is almost constant in all cases and does not significantly affect the natural frequency change of the structure.

  On the other hand, in the completely liquefied state, the response magnification varies among the cases, and the acceleration response magnifications of C-2, C-3, and C-5 are none regardless of the presence or absence of the non-liquefied layer. It turns out that it has fallen compared with the countermeasure (C-1, C-4).

  This is presumably due to the fact that the natural frequency of the structure-ground coupled system changes and is affected by the ground resistance approaching zero.

ii) Pile behavior in the case of all liquefied layers (C-1 to C-3)
The pile moment was compared by picking up the time when the generated cross-sectional force was maximum. From the results of single pile behavior study (static loading test) to the mid-scale liquefaction state in the improved range 1D, there was a tendency to exert countermeasures. investigated.

  FIG. 12 shows the moment distribution and pore water pressure ratio distribution in the medium-scale liquefied state.

  As can be seen in the figure, in the case of no countermeasure (C-1), the maximum moment is generated at the pile head, whereas for C-2 with improved pile length, the inflection point and occurrence near the center of the pile Although the moment is almost the same as that of C-1 without countermeasures, it is understood that the generated cross-sectional force is suppressed at the pile head, and the effect of the countermeasure work is sufficiently exhibited.

  In addition, about C-3 of the pile head -30cm improvement, in the vicinity of the improved pile head, generated cross-sectional force is suppressed like C-2, but the generated moment in the center part of the pile is large. Recognize.

  By doing this, applying the improvement only to the shallow range (GL-30cm) causes a difference in the stiffness of the pile, and the generated moment increases at the boundary of the improved body, similar to the case of having a non-liquefied layer to be described later. It is considered that the mode has been changed.

iii) Pile behavior with non-liquefied layer (C-4 to C-5)
FIG. 13 shows a moment distribution diagram and a pore water pressure ratio distribution diagram in a medium-scale liquefaction state.

  The moment distribution in the case of having a non-liquefied layer is characterized in that an inflection point occurs at the layer boundary, but the generated cross-sectional force is suppressed in C-5 where countermeasures are taken in the medium-scale liquefied state. And it turns out that the clear inflection point is not produced in the layer boundary part.

  The inflection point at the layer boundary is thought to be affected by the presence or absence of liquefaction (ground stiffness ratio), but by making improvements, the difference in stiffness between the liquefied layer and the non-liquefied layer is reduced. This is considered to have led to the suppression of the generated cross-sectional force.

  Based on the above, it is considered that the influence of the inflection point can be reduced when the relationship between the ground strength of the non-liquefied layer and the rigidity of the structure and the relationship between the improved body and the structure of the liquefied layer are matched. It can be said that it is necessary to take countermeasures in consideration of the relationship between ground strength and improved body strength.

It is the figure which showed the pile foundation structure which concerns on 1st Embodiment, (a) is a vertical sectional view, (b) is a horizontal sectional view which follows an AA line. It is the figure which showed the pile foundation structure which concerns on 2nd Embodiment, (a) is a vertical sectional view, (b) is a horizontal sectional view which follows a BB line. It is a figure regarding the verification test of this invention, and the vertical sectional view of the shear soil tank used for the static loading test, and the arrangement | positioning figure of an apparatus. The vertical sectional view which showed the relationship between a single pile and the covering area | region surrounding it. The graph which showed the bending moment distribution and pore water pressure ratio of an initial state. A graph showing the bending moment distribution and pore water pressure ratio in a medium-scale liquefied state. The graph which showed the bending moment distribution and pore water pressure ratio of a complete liquefaction state. It is a figure regarding the verification test of this invention, and the vertical cross-sectional view of the shear soil tank used for the vibration test, and the arrangement | positioning drawing of an apparatus. The graph which showed the input acceleration waveform used for the vibration test. The schematic diagram which showed the experiment case. The graph which showed the excess pore water pressure rise process. The graph which showed the experimental result in the liquefied ground comprised only by the liquefied layer, and showed the bending moment and the pore water pressure ratio. The graph which showed the experimental result in the liquefied ground comprised by the liquefied layer and the non-liquefied layer, and showed the bending moment and the pore water pressure ratio.

Explanation of symbols

1,21 Pile foundation structure 2,22 Viaduct (superstructure)
3,23 Footing (pile foundation structure)
4 Liquefaction ground (ground)
5,25 Pile (Pile foundation structure)
6,26 Covered area

Claims (4)

A pile foundation structure configured to support an upper structure with a pile embedded in the ground, wherein only the covering region surrounding the pile so as to abut on the peripheral surface of the ground is ground. Pile foundation structure characterized by improvement. A pile foundation structure configured to support an upper structure with a plurality of piles embedded in the ground, and surrounds the ground by abutting each of the piles on their peripheral surfaces. A pile foundation structure that is divided into a plurality of covering regions and an intermediate region that extends outside the covering regions, and is formed by improving only the plurality of covering regions. The pile foundation structure according to any one of claims 1 and 2, wherein a coverage ratio t / D is 4 or more, where D is a diameter of the pile and t is a thickness of the covering region. The pile foundation structure according to any one of claims 1 and 2, wherein a coverage ratio t / D is 2 or more, or 1 or more, where D is a diameter of the pile and t is a thickness of the covering region.

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