Foundation pile structure in ground including soft ground and construction method of foundation pile structure

Abstract

Since the main pile can be constructed by a normal construction method and the countermeasure pile can be constructed in parallel with the construction work of the main pile, the main pile can be constructed while improving the soft ground with little influence of a construction period.SOLUTION: In the ground having a soft ground layer near the ground and a layer capable of supporting a load below the soft ground layer, the vertical load of a ground structure is supported by a main pile 10 reaching the layer capable of supporting the load. The countermeasure pile 20 is formed in a range including a soft ground layer in the vertical direction, and is formed by filling a hydraulic material in a countermeasure pile hole formed by being kneaded while pressing a pile hole wall. A plurality of the countermeasure piles 20 are arranged so as to surround the periphery of the main pile 10 in a plan view, the countermeasure piles 20 are not overlapped with the main pile 10, and the adjacent countermeasure piles 20 are also not overlapped with each other.SELECTED DRAWING: Figure 7

Description

[Technical Field]

[0001] This invention relates to a foundation pile structure on ground including soft ground that supports an above-ground structure constructed on soft ground with foundation piles and improves the soft ground, and a method for constructing a foundation pile structure on ground including soft ground to realize this foundation pile structure. [Background technology]

[0002] (1) The vertical load of the aboveground structure was transmitted through the foundation piles to the layer that could support the load, thereby supporting the aboveground structure. Also, near the ground level, various necessary reinforcement measures were implemented to prevent horizontal loads from acting on the foundation piles during a major earthquake. In this case, if the ground near the ground level is soft, it cannot support the horizontal load, which can lead to the destruction of the foundation piles, so in some cases it is necessary to improve the entire soft ground near the ground level using various construction methods. In particular, in the case of soft ground that is prone to liquefaction, special measures are required. In such cases, it takes many days to improve the entire soft ground, which makes construction inefficient.

[0003] (2) In this context, one proposal, intended for above-ground structures such as houses, proposes a structure in which columnar improvement bodies are continuously constructed between the support piles to be constructed, a wall-like improvement body is formed between the support piles, and the lower ends of the support piles are connected to a pre-formed plate-like improvement body (Patent Document 1). Note that the support piles in this case were not prefabricated piles, but rather one of the columnar improvement bodies was extended to the plate-like improvement body (supporting ground).

[0004] (3) In another proposal, a structure has been proposed in which a ground improvement body is formed around the outer periphery of a single support pile to a depth of 10 m or less from the ground surface (Patent Document 2). In this case, after the ground improvement body is formed, the support pile is driven through the ground improvement body.

[0005] (4) In another proposal, a structure has been proposed in which a ground improvement body is formed along the center line of adjacent support piles (Patent Document 3). Since this ground improvement body is formed along the center line of the support piles, it is formed in a grid pattern in plan view. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2020-16038 [Patent Document 2] Japanese Patent Application Laid-Open No. 2005-320692 [Patent Document 3] Japanese Patent Application Laid-Open No. 2007-262782 Summary of the Invention [Problem to be solved by the invention]

[0007] The proposal described in Patent Document 1 had the problem that it took several days to construct the plate-shaped improvement body and to form the plate-shaped improvement body by successively using the columnar improvement body, and it was also not possible to replace the columnar improvement body at the corners with prefabricated piles. Furthermore, in the proposal in Patent Document 2, the support piles are driven after the ground improvement body is installed, which poses the problem of requiring several days to construct the ground improvement body. Furthermore, when a large horizontal load is applied, such as in a major earthquake, the ground improvement body is generally damaged before the support piles, and since the deformed ground improvement body cannot return to its original shape, gaps form between the support piles and the ground improvement body. Furthermore, when the ground improvement body is damaged, the deformed ground improvement body cannot return to its original shape, so the unity between the support piles and the ground improvement body cannot be maintained, and there is a high risk that the effectiveness of the ground improvement work will be impaired in the future in this ground (around the support piles). In addition, in the counterproposal described in Patent Document 3, after the support piles are constructed, the surface layer is excavated using a shallow mixing treatment method to construct an improvement body, which requires a long construction period to construct an improvement body of a specified thickness and requires the improvement body to be fixed to the support piles, which creates the problem of complicated construction work. [Means for solving the problem]

[0008] This invention solves the above-mentioned problems by placing countermeasure piles for ground improvement around the main piles that support the vertical load of the above-ground structure so as not to overlap with the main pile holes of the main piles, and in the vicinity of the periphery of the main pile holes.

[0009] In other words, this structural invention is a structure in which, in a ground where there is a soft ground layer near the ground and a layer capable of supporting load below that, the vertical load of an above-ground structure is supported by main piles that reach the layer capable of supporting the load, and is a foundation pile structure in ground that includes soft ground, characterized in that the soft ground layer is reinforced as follows. (1) The main pile was constructed by excavating to a layer capable of supporting the load, constructing a main pile hole, and burying a structure in the main pile hole. (2) The countermeasure piles were formed vertically in an area including the soft ground layer, and the countermeasure pile holes were formed by pressing down on the pile hole walls and kneading the material into them, and then a specified hydraulic material was filled into the countermeasure pile holes. (3) The countermeasure piles are arranged in multiple numbers so as to surround the main pile in the plan view, and are arranged so that the countermeasure piles do not overlap with the main pile and adjacent countermeasure piles do not overlap with each other.

[0010] The present invention also provides a foundation pile structure for ground including soft ground, characterized by being configured as follows. (1) Set a reference circle of radius L from the center of the main pile with diameter D0. (2) The centers of countermeasure piles with a diameter of D1 were placed along the circumference of the reference circle.

[0011] The present invention also provides a foundation pile structure for ground including soft ground, characterized by being configured as follows. (1) Set a reference circle of radius L from the center of the main pile with diameter D0. (2) The centers of countermeasure piles with a diameter of D1 were placed along the circumference of the reference circle. (3) The radius L and diameter D0 are 1,5×D0 <L<2.5×D0 It was decided.

[0012] The present invention also provides a foundation pile structure for ground including soft ground, characterized by being configured as follows. (1) If the diameter of the main pile is D0 and the diameter of the countermeasure pile is D1, D0=D1 It was decided. (2) The radius of the reference circle L and the diameter of the main pile D0 are L=2×D0 It was decided.

[0013] The above-mentioned foundation pile structure for ground including soft ground is characterized in that it is configured as follows. (1) The walls of the countermeasure pile holes were gradually compressed more firmly as they moved vertically toward the ground. (2) The wall of the main pile hole for the main pile was gradually compressed more strongly in the vertical direction toward the ground, at least in the soft ground.

[0014] The present invention also provides a foundation pile structure for ground including soft ground, characterized by being configured as follows. (1) The main pile was constructed by burying a prefabricated pile in a pile hole. (2) The layer capable of supporting the load was set as the supporting ground layer that can exert tip bearing capacity by the main piles.

[0015] The invention of the construction method is a method for constructing a foundation pile structure on ground that includes soft ground, characterized in that a structure that supports the vertical load of an above-ground structure using main piles that reach a supporting ground layer at a predetermined depth and a soft ground layer near the ground is constructed as follows. (1) The foundation pile structure is a structure in which countermeasure piles for ground improvement are constructed so as to surround the main pile in a plan view. (2) The countermeasure pile hole for the countermeasure pile is excavated with a first excavation rod while filling it with hydraulic material so that the hole wall is gradually compressed more firmly as it moves vertically toward the ground. (3) After the excavation of the countermeasure pile holes, or in parallel with the excavation of the countermeasure pile holes, The main pile hole for the main pile is formed by excavating the soft ground layer from near the ground surface with a first excavation rod so that the hole wall is gradually compressed more firmly as it approaches the ground surface in the vertical direction. (4) Next, in the main pile hole of the main pile, the first drilling rod is replaced with the second drilling rod, and the main pile hole is excavated using the second drilling rod down to the supporting ground layer. (5) Next, while injecting hydraulic material into the main pile hole from the second drilling rod or by other means, the second drilling rod is pulled up from the main pile hole to the ground, and a prefabricated pile is buried in the main pile hole. (6) Hydraulic material in the pile holes of the main piles. Once the hydraulic material in the pile holes of the countermeasure piles has solidified, the foundation pile structure is constructed.

[0016] The present invention also provides a method for constructing a foundation pile structure on ground including soft ground, characterized by the following configuration. (1) The first drilling rod is provided with a drilling head at the lower end of the rod body, which is attached to a ground drilling machine, corresponding to the diameter of the countermeasure pile hole or main pile hole to be excavated, and is provided with multiple hole wall kneading drums in the middle part of the rod body, and the hole wall kneading drums have an outer diameter larger than that of the kneading drums located at the top than that of the ones located at the bottom. (2) The second drilling rod is provided with a drilling head at the lower end of the rod body, which is attached to a ground drilling machine, corresponding to the diameter of the main pile hole to be drilled, and with multiple hole wall kneading drums in the middle part of the rod body.

[0017] The "hydraulic material" mentioned above refers to cement milk, etc., but in the case of main piles, it usually refers to the material filled into the main pile hole, and in the case of countermeasure piles, it refers to the material usually used when forming ground improvement columns.

[0018] Furthermore, the "structure" in the above refers to a reinforcing bar cage in the case of so-called cast-in-place piles, and to a prefabricated pile in the case of prefabricated pile construction.

[0019] In addition, the "layer capable of supporting loads" mentioned above refers to a tip support layer when the vertical load of a ground structure is supported mainly by tip bearing force, and a layer of ground that can exert peripheral friction force when the vertical load of a ground structure is supported mainly by peripheral friction force. Naturally, when both tip support and support by peripheral friction are used in combination, it includes both layers. [Effects of the Invention]

[0020] This invention has a structure in which countermeasure piles for ground improvement are placed around the main piles that support the vertical load of the above-ground structure so as not to overlap with the main pile holes of the main piles, and in the vicinity of the periphery of the main pile holes. This means that the main piles can be constructed using normal construction methods and can be constructed in parallel with the construction work of the main piles, which has the effect of minimizing the impact on construction time and allowing the main piles to be constructed while improving the soft ground. In particular, because the countermeasure piles do not overlap each other, the position of the countermeasure piles can be changed depending on the site conditions, such as the presence of underground obstacles. In addition, because countermeasure piles are placed around each main pile in plan view, the effect is that ground improvement is carried out at the heads of the main piles over a wide area. Also, because the countermeasure piles for each main pile are placed close to the countermeasure piles for the main piles adjacent to it, the effect is that the entire soft ground can be improved with a relatively small number of countermeasure piles. Furthermore, the main piles and countermeasure piles are constructed while kneading the hole walls of the main pile holes and countermeasure pile holes, so the ground is compacted by the countermeasure piles, and since the countermeasure piles are placed around the main piles, it is expected that the vertical bearing capacity of the main piles will also be increased. Furthermore, in the case of soft ground that is prone to liquefaction, even if excessive horizontal loads are applied to the ground due to earthquakes, etc., liquefaction near the ground surface can be significantly reduced, and above-ground structures can be effectively protected. Therefore, it is effective in efficiently increasing the horizontal bearing capacity of ground that has soft ground near the ground surface. Furthermore, since the improvement bodies are not surrounded by a wall, but are placed around the main piles at a distance from the main piles, and are also placed at a distance from each other, they can be effective as a liquefaction countermeasure. This is because, during an earthquake, excess pore water pressure in the ground rises and liquefaction occurs, but if an earthquake causes an increase in excess pore water pressure in the ground around the main piles, the water pressure will escape through the gaps between the improvement bodies to the outside of the ring shape of the improvement bodies, which is thought to suppress the increase in excess pore water pressure. [Brief explanation of the drawings]

[0021] [Figure 1] In Experimental Example A of the present invention, a model of the test specimen and the equipment arrangement are shown. [Figure 2] In experimental example A of this invention, the time history diagram (schematic) of excess pore water pressure for a seismic intensity of -440 mm depending on the placement of countermeasure piles is shown. [Figure 3] In Experimental Example B of the present invention, a model of the test specimen and the instrumentation arrangement are shown. [Figure 4] In Experimental Example B of the present invention, the number of steel pipes to be press-fitted as test specimens and the order of construction are shown. [Figure 5] In Experimental Example B of this invention, the time history of excess pore water pressure in the liquid-substratified layer is shown. [Figure 6] In Experimental Example B of this invention, the excess pore water pressure ratio is compared at the time when liquefaction occurred. [Figure 7] 1 is a conceptual diagram showing a plan view of the relationship between the main piles and countermeasure piles of the present invention. [Figure 8] 1 is a plan view showing the arrangement of main piles and countermeasure piles at a construction site of the present invention. [Figure 9] 10 is a plan view showing another arrangement of main piles and countermeasure piles at a construction site of the present invention. [Figure 10] 1 shows a schematic longitudinal cross-sectional view of the ground to which the present invention is applied, and of the main piles and countermeasure piles. [Figure 11] This shows a front view of the drilling rod used to excavate the main pile hole and countermeasure pile hole of this invention. DETAILED DESCRIPTION OF THE INVENTION

[0022] 1. Ground to which this invention is applied

[0023] (1) The ground to which this invention is applied (see the left side of Figure 10) has a supporting ground layer 34 (generally an N value of 30 or more) that can exert the tip bearing force of a precast pile at a predetermined depth. It is also assumed that above the supporting ground layer 34, near the ground level 30, there is a soft ground layer 32 (N value of about 0 to 20) that needs to have its horizontal bearing capacity strengthened in the event of a horizontal load caused by an earthquake or the like. Generally, in the soft ground layer 32, measures against liquefaction are required at a depth of about 10 m from the ground level, so the present invention is particularly suited to application to ground that is prone to liquefaction. Naturally, whether the ground is soft or prone to liquefaction cannot be determined simply by the N value, as this depends on the thickness of each layer and the quality of the ground.

[0024] 2. Placement of main piles and countermeasure piles

[0025] [1] In this invention, the main piles 10 are designed in a conventional manner according to the characteristics of the supporting soil layer 34 and the above-ground structure, and are determined in a conventional arrangement on the site with the required structure (see Figures 8 and 9). In addition to the main piles 10, countermeasure piles 20 are placed in the soft ground layer 32 near the ground 30 to strengthen the horizontal bearing capacity, etc.

[0026] [2] The main pile 10 used in the practice of this invention is constructed by burying a hollow concrete precast pile in a main pile hole (diameter D0) excavated from the ground 30. The lower end of the main pile (precast pile) 10 is anchored in the supporting ground layer 34 and bears the vertical load of the above-ground structure, so it has a structure appropriate for this. For example, the precast pile may have nodes protruding outward at the lower end or at other positions. In addition, to strengthen the vertical bearing capacity of the pile head, a steel pipe may be wrapped around the outer periphery or the pile may have a larger diameter, providing a structure similar to that of a specified precast pile. Furthermore, depending on the depth of the supporting ground layer 34, the main pile 10 can be constructed by connecting multiple prefabricated piles vertically (in the depth direction, vertical direction) in the direction of the ground depth, and the structure of each unit prefabricated pile is determined by the structure of the above-ground structure, the ground at the construction site, etc., just like the usual configuration. The main pile 10 is constructed by pre-digging a pile hole and filling it with a hydraulic material such as cement milk, then burying a pre-fabricated pile inside. In this case, the main pile hole is excavated while kneading the hole wall and pressing it radially (towards the surrounding ground). In this case, it is desirable to form the hole so that the pressing force gradually increases upward (towards the ground 30), especially near the ground 30 (the soft ground layer 32).

[0027] [3] The countermeasure pile 20 used in the practice of this invention is constructed by excavating a countermeasure pile hole (outer diameter D1) from the ground 30 and injecting a hydraulic material such as cement milk into the countermeasure pile hole. In this case, the hydraulic material to be injected does not need to be as richly mixed as the hydraulic material injected into the main pile hole, and should be of a concentration similar to that used in ordinary ground improvement work. The outer diameter D1 of the countermeasure pile 20 is arbitrary, but as will be described later, from the viewpoint of construction efficiency, D1=D0 It is desirable to do so.

[0028] [4] The main piles 10 and countermeasure piles 20 are arranged as follows in plan view. (a) In FIG. 7( a ), a reference circle 25 with a radius and one side “L0” is formed around one main pile 10 , and the main pile 10 is positioned at the center of the reference circle 25 . (b) Countermeasure piles 20 are placed on the circumference of the reference circle 25. In this case, countermeasure piles 20 are placed two by two in the horizontal diameter direction and two in the vertical diameter direction in Figure 7(a), and countermeasure piles 20 are placed between these countermeasure piles 20. Therefore, a total of eight countermeasure piles 20 are placed approximately evenly. If there are underground obstacles (remains of existing foundations, rocks, etc.), the position of a particular countermeasure pile 20 can be changed to avoid them. (c) In this way, countermeasure piles 20, 20 that are adjacent to each other around the reference circle 25 are equally spaced at a distance L1 from the main pile 10 (Fig. 7(a)). The entire set of countermeasure piles 20, 20 that are spaced around one main pile 10 is called a countermeasure pile group 22. (d) Here, from the construction efficiency described later, D1=D0 In the following Experimental Examples A and B, D1=D0=D It is stated as such. In this case, from the experimental results described below, L0=2×D It is considered that the synergistic effect of the main piles 10 and the countermeasure pile group 22 can maximize the ground improvement effect with a small number of piles. This is also because construction efficiency is favorable. If "L0 > 2D", the distance from the main pile 10 will be large, and the distance between the adjacent countermeasure piles 20, 20 will be large, and it is thought that the effect of ground improvement will not be fully realized. Also, if "L < 2D", the distance between the "main pile 10 and countermeasure pile group 22" and the adjacent "main pile 10 and countermeasure pile group 22" will be large, and it is thought that the construction of additional countermeasure piles will be necessary in between (Figures 8 and 9). Also, if the countermeasure piles 20, 20 are too close, such as overlapping each other, the effect of compacting the countermeasure pile holes will not be realized. Also, similarly, L1=L0 In this case, six countermeasure piles 20 are arranged evenly around the reference circle 25 for one main pile 10 (not shown). (e) In addition, considering the above effects, "L0=2D" 1,5×D0 <L<2.5×D0 The degree is considered acceptable. (f) In the above, the countermeasure piles 20 are evenly arranged around the reference circle 25, but other arrangements are also possible. For example, a reference square 27 with a side length of "2 x L0" is set around the main pile 10, and first countermeasure piles 20A, 20A are arranged vertically and horizontally in Figure 7(b). Furthermore, second countermeasure piles 20B, 20B are arranged near the vertices of the reference square 27 (Figure 7(b)). In this case, the center distance between the main pile 10 and the four first countermeasure piles 20A is L0, but the distance between the main pile 10 and the four second countermeasure piles 20B is L01. L0 <L01 In this case, the distance between the countermeasure piles 20 is the same, L1. Note that an experimental example described later has shown that the effect of preventing liquefaction is greater when the countermeasure piles 20 are arranged using the reference circle 25 (FIG. 7(a)) than when the countermeasure piles 20 are arranged using the reference square 27 (FIG. 7(b)).

[0029] 3.Foundation pile structure 50

[0030] [1] With the main pile 10 and countermeasure pile 20 arranged as described above, the main pile 10 penetrates the soft ground layer 32 and is anchored in the supporting ground layer 34. It is preferable to position the lower end of the countermeasure pile 20 below the soft ground layer 32, but depending on the soft ground layer 32 and if ground improvement is sufficient, it is also possible for the lower end of the countermeasure pile 20 to remain within the soft ground layer 32.

[0031] [2] As described in the construction method below, the main piles 10 and countermeasure piles 20 are excavated while kneading the hole walls of the main pile holes and countermeasure pile holes. Therefore, a relative density increase range 23 is set around the constructed main piles 10 and countermeasure piles 20, and the soft ground layer 32 is strengthened in conjunction with the effects of the main piles 10 and countermeasure piles 20 (Figure 7). In this case, the relative density increase range 23 is set as follows for the countermeasure pile 20 with a diameter D1 and the main pile 10 with a diameter D0, respectively: 1.2×D1~1.5×D1 1.2×D0~1.5×D0 is considered to be appropriate.

[0032] [3] The main piles 10 and countermeasure piles 20 (countermeasure pile group 22) formed in the above manner constitute a foundation pile structure 50 (FIGS. 7 to 10).

[0033] 4. Construction method of foundation pile structure 50

[0034] [1] In this construction method, the first drilling rod 40 used for drilling the upper part of the main pile hole and the countermeasure pile hole has a drilling head 41 equipped with drilling blades 42, 42 attached to the lower end of the rod body 40a, and a first kneading drum 43, a second kneading drum 44, and a third kneading drum 45 attached above it, respectively (Figure 10). The first drilling rod 40 may be, for example: Diameter D0 = Diameter D1 = 950 mm For excavation of Drilling diameter of drilling head 41 = 750 mm Outer diameter of first kneading drum 43 = 750 mm Outer diameter of the second kneading drum 44 = 850 mm Outer diameter of the third kneading drum 45 = 950 mm It is set as follows. The second drilling rod, which excavates the lower part of the main pile hole, is a normal drilling rod with a drilling head (shaft drilling diameter of about 950 mm, enlarged drilling diameter of about 1260 mm) attached to the rod body. A mixing drum (outer diameter: about 950 mm) is provided above it (not shown). In this case, the lower end of the main pile hole (i.e., the part inside the supporting ground layer 32) is made into a main pile hole with an expanded diameter, but it can also be made into a straight main pile hole without expanding the diameter.

[0035] [2] The countermeasure pile holes for the first countermeasure pile 20A and the second countermeasure pile 20B are excavated to a predetermined depth using the first excavation rod 40. The hole walls are formed by gradually compressing the material as they move vertically toward the ground 30. Cement milk (hydraulic material) is injected into the excavated countermeasure pile holes. The cement milk is mixed with the soil and sand in the countermeasure pile holes to make soil cement. Furthermore, the cement milk can be injected by discharging it from the first excavation rod 40 while it is excavating, or by another means after the first excavation rod 40 is raised to the ground 30. Furthermore, the timing of injection of the cement milk is also optional in this construction method.

[0036] [3] After or in parallel with the excavation of the countermeasure pile hole, the upper part (the ground surface 30 side) of the main pile hole for the main pile 10 is similarly excavated with the first excavation rod 40. The main pile hole is also formed so that the hole wall gradually becomes more strongly compacted as it approaches the ground surface in the vertical direction. In addition, excavation is carried out from near the ground surface 30 to a depth that passes through the soft ground layer 32.

[0037] [4] Next, in the main pile hole of the main pile 10, the first drilling rod 40 is pulled up to the ground 30, replaced with a second drilling rod, and the main pile hole is excavated using the second drilling rod down to the supporting ground layer 34.

[0038] [5] Next, after the main pile hole has been excavated to the bottom end, the second drilling rod is pulled above ground while the specified cement milk is injected into the main pile hole.

[0039] [6] Bury prefabricated piles into the main pile hole filled with cement milk (not shown).

[0040] [7] The cement milk in the countermeasure pile hole solidifies while the lower part of the main pile hole is excavated and the precast pile is buried, thereby shortening the construction time. In addition, since the diameter D0 of the main pile 10 and the diameter D1 of the countermeasure pile are the same, the upper part of the main pile hole and the countermeasure pile hole can be excavated with the same first excavation rod 40, eliminating the need to exchange the excavation rod for the countermeasure pile with another excavation rod for the main pile.

[0041] [8] Also, when multiple heavy machinery for excavating pile holes is used, while the main pile hole is being excavated with the second excavation rod, other countermeasure pile holes and the top of the main pile hole within the construction site can be excavated with the first excavation rod 40 (see Figure 8).

[0042] [9] In the same manner, all the main piles 10 and countermeasure piles 20 within the site are constructed (Figure 8). Once the injected cement milk has solidified, the main piles 10 will be able to support above-ground structures, and the main piles 10 and countermeasure piles 20 will be used to construct a foundation pile structure 50 that improves and strengthens the soft ground layer 32 (Figures 8 to 10).

[0043]

[10] Although the above-mentioned method is efficient for constructing the main piles 10, any other method can also be adopted (not shown), such as the inner excavation method, in which the main pile holes are excavated while prefabricated piles are buried at the same time.

[0044]

[11] Depending on the construction site, if the spacing between adjacent main piles 10, 10 is wide due to reasons such as the weight of the aboveground structure being small, and if there are not enough countermeasure piles 20 for ground improvement, an additional countermeasure pile group 22 can be constructed between the combination of countermeasure pile group 22 and main piles 10 in Figure 7 (Figure 9). In this case, in the additional countermeasure pile group 22, a countermeasure pile 20 is set at the center instead of the main pile 10.

[0045]

[12] In addition, other construction methods can be adopted in the above. For example, some countermeasure piles 20 (for example, four) are formed for the first main pile 10, then some countermeasure piles 20 (for example, four) are formed for the second main pile, and then some countermeasure piles 20 (for example, four) are formed for the third main pile 10. Next, return to the first main pile 10 and form the remaining countermeasure piles 20 (four in this case), then form the remaining countermeasure piles 20 (four in this case) for the second main pile, and then form the remaining countermeasure piles 20 (four in this case) for the third main pile 10. Next, the first main pile 10, second main pile 10, and third main pile 10 can be constructed in any order (not shown). In any case, for one main pile 10, countermeasure piles 20 (countermeasure pile holes) will be constructed so as to surround the main pile 10, and then the main pile 10 (main pile holes) will be constructed.

[0046]

[13] Furthermore, in the above-mentioned construction, the main pile holes and countermeasure pile holes, and the countermeasure pile holes and the countermeasure pile holes are constructed so that they do not overlap, and as a result, the main piles 10 and the countermeasure piles 20, and the countermeasure piles 20 and the countermeasure piles 20 do not overlap, respectively.

[0047] 5. Other Embodiments

[0048] [1] In the above embodiment, the main pile 10 is configured as one pile, but the main pile 10 can also be configured as a so-called group pile from a plurality of unit prefabricated piles (not shown).

[0049] [2] In addition, in the above embodiment, the main pile 10 is constructed using a prefabricated pile, but the main pile 10 can also be constructed as a so-called cast-in-place pile by burying a reinforcing bar cage or the like in the main pile pile hole (not shown).

[0050] 6. Experimental Example A

[0051] [1] In this experiment, a 1G shaking table experiment was conducted in which the spacing and shape of countermeasure piles arranged in a pile-like fashion around the main pile were changed, and their effectiveness in suppressing liquefaction was examined.

[0052] [2] Experimental Overview A shear soil tank (1200mm wide x 1000mm high x 800mm deep) was used for the experiment. The outer diameter of the main pile was D, and the outer diameter of the countermeasure pile was also D. Figure 1 shows an example of the layout of the model and measuring instruments when the placement interval of the countermeasure piles is 2 x D and the layout shape is rectangular. The scale of the model was 1 / 25. Toyoura sand (specific gravity = 2.679 mg / m 3A two-layer ground structure was created, consisting of a liquefied layer and a non-liquefied layer, using a void ratio of 0.993 (maximum void ratio, 0.613). The liquefied layer was assumed to be loose sand equivalent to a 16m thick layer in actual ground, with a relative density of Dr = 36%. Below that, a non-liquefied layer with a relative density of Dr = 90% was provided. A suction prevention mat (10mm thick) was laid at the boundary between the liquefied and non-liquefied layers. This was done to efficiently recreate the liquefied layer when conducting cyclic shaking table experiments under different vibration conditions. The density of the liquefied layer was controlled by penetrating a high-pressure water jet nozzle into the liquefied layer from the ground surface to stir the ground, and a loose liquefied layer was recreated with controlled layer thickness. The groundwater level was set at the ground surface. The pile model consists of PVC pipes with an outer diameter of D = 50 mm for both the main piles and countermeasure piles, with the pile tips penetrating into the non-liquefaction layer. The pile heads are connected to the main piles and the surrounding countermeasure piles with bolts and nuts. As shown in Figure 1, the measuring instruments consist of accelerometers (A1 to A8) and pore water pressure meters (P1 to P5) placed at the same depth in the free ground (A1 to A3, P1 to P3) and between the piles (A4 to A7, P4 to P5). The excitation conditions were sinusoidal (2 Hz, 20 s), and the vibration table acceleration was set to 50, 60, 70, and 90 cm / s. 2 The placement intervals of the countermeasure piles were set to twice (2D) and three times (3D) the outer diameter of the main piles, and four placement shapes were used: rectangular and circular. A schematic diagram of the placement shapes of the countermeasure piles is shown in Figure 2.

[0053] [3] Experimental results Figure 2 compares the time history of excess pore water pressure between piles at a depth of 460 mm and in the free ground due to differences in pile arrangement, for each platform acceleration. 2 In the case of the free ground, the pore water pressure gauge (P2) reached a plateau immediately after the shaking, and at the same depth, excess pore water pressure equivalent to the effective overburden pressure was generated, resulting in a liquefaction state where the effective stress had disappeared. The pore water pressure gauge (P5) between the countermeasure piles also showed similar behavior, but the behavior was slightly smaller than that of the free ground. The 2D rectangular layout data is smaller than the other data because the measurement position was shallower. Platform acceleration: 60cm / s 2In the case of the 2D rectangular and 2D circular configurations, the behavior of excess pore water pressure is affected by the spacing and shape of the countermeasure piles. Without countermeasures, excess pore water pressure remains constant at a value equivalent to the effective overburden pressure from immediately after excitation until the end of excitation. On the other hand, the 3D circular configuration appears to behave similarly to the case without countermeasures, but reaches a peak immediately after excitation and then decreases during excitation. This trend becomes even clearer when the spacing between countermeasure piles is narrowed to 2D. While the maximum excess pore water pressure in the free ground reaches the effective overburden pressure in the 2D rectangular and 2D circular configurations, the maximum excess pore water pressure between the countermeasure piles does not reach the effective overburden pressure, and the excess pore water pressure dissipates during excitation. Additionally, the 2D circular configuration suppresses the rise in excess pore water pressure immediately after excitation, suggesting that the countermeasure piles suppress shear deformation of the ground around the main piles. Platform acceleration 50cm / s 2 In the figure, the differences in the placement interval and shape of the countermeasure piles become clearer. Without countermeasures, the ground reached a liquefaction state, but installing countermeasure piles can prevent liquefaction of the ground between the piles.

[0054] [4] Summary A 1G shaking table experiment confirmed that liquefaction can be suppressed by placing countermeasure piles around the main piles. A 2D circular arrangement showed the greatest suppression effect. Therefore, a circular arrangement of countermeasure piles is preferable to a rectangular arrangement, and a greater suppression effect can be achieved by narrowing the spacing between the countermeasure piles.

[0055] 7. Experimental Example B

[0056] [1] In the above-mentioned Experimental Example A, the liquefaction suppression effect of countermeasure piles, in which pile-type improvements were placed around the support piles in liquefied ground, was verified in consideration of workability as a liquefaction countermeasure for support piles installed in liquefied ground, and it was revealed that a 2D circular arrangement provided the greatest suppression effect. In this experiment, a 1G shaking table experiment was conducted in which the construction methods of the countermeasure piles installed around the support piles were changed, and their liquefaction suppression effect was verified.

[0057] [2] Experimental Overview Figure 3 shows an example of the layout of the model and measuring instruments. The scale of the model was 1 / 25. The model ground was made of the same Toyoura sand as in Experiment A, and consisted of two layers: a liquefied layer and a non-liquefied layer. The relative density of the liquefied layer was set to 50%, and that of the non-liquefied layer to 90%. A suction prevention mat (10 mm thick) was laid at the boundary between the liquefied and non-liquefied layers. This was done so that the liquefied layer could be recreated when repeated experiments were conducted under different vibration conditions. The density of the liquefied layer was controlled by penetrating a high-pressure water jet nozzle into the liquefied layer from the ground surface to stir the ground, and a loose liquefied layer was recreated by controlling the layer thickness. The groundwater level was set at the ground surface. The pile models consisted of PVC pipes with an outer diameter of 38 mm for both the main piles and countermeasure piles. The countermeasure piles were spaced at twice the outer diameter (2D) and arranged in a circular shape. There were eight countermeasure piles. The difference in the pile construction method was to simulate the effect of stabilizing the hole wall caused by mixing during actual construction, by pressing a steel pipe with an outer diameter of 49 mm and a thickness of 5 mm into the PVC pipe using a hydraulic jack. The penetration length of the steel pipe was set to the bottom of the liquefied layer. Figure 4 shows the position and construction sequence of the steel pipes. The tip of the pile was embedded in the non-liquefied layer and connected to the bottom plate of the soil tank. The pile head was also restrained with a fixed plate to prevent the positional relationship between the main pile and countermeasure pile from changing during shaking. The excitation conditions were sine waves (2Hz, 20s) and the acceleration of the vibration table was 200cm / s 2 It was decided.

[0058] [3] Experimental results Figure 5 shows the time history of excess pore water pressure near the countermeasure piles at a depth of -470 mm and in the free ground for each case. The effective overburden pressure indicated by the dashed-dotted line in the figure indicates the effective overburden pressure at the same depth. In Case 1, where the steel pipe press-in process was omitted, the excess pore water pressure rose rapidly immediately after excitation and stabilized at a level equivalent to the effective overburden pressure after one second. In contrast, in Case 2, where five steel pipes were pressed in, it was found that the time it took for the excess pore water pressure to reach effective overburden pressure was slightly delayed, at two seconds. In Case 3, where steel pipes were pressed in on piles other than the main pile, the excess pore water pressure rose gradually and it took eight seconds for the excess pore water pressure to reach effective overburden pressure, prolonging the time to liquefaction. Finally, in Case 4, where steel pipes were pressed in on all piles, the maximum excess pore water pressure was 1.8 kN / m 2 , which shows that liquefaction does not occur. It can also be seen that excess pore water pressure dissipates slightly during shaking. Figure 6 compares the excess pore water pressure ratios of the other cases at the time when liquefaction occurred in Case 1. This shows that the excess pore water pressure ratio decreases as the number of steel pipes pressed in increases, regardless of the depth of the liquefied layer. Furthermore, when comparing the area near the piles with the free ground, it can be seen that the area near the main piles is slightly smaller in Cases 3 and 4. This is thought to be because the density of the surrounding area of ​​the piles and the lateral earth pressure increase by pressing in steel pipes that simulate the kneading effect, thereby suppressing shear deformation of the composite ground.

[0059] [4] Summary A 1G shaking table experiment confirmed the possibility of improving the suppression effect of shear deformation as a composite ground by simulating the construction method of pile foundations. [Explanation of symbols]

[0060] 10 main pile 20 Countermeasure piles 22 Countermeasure pile group 23 Relative density increase range 25 base yen 27 Reference rectangle 30 Ground 32 Soft ground layer 34 Supporting ground layer 40 First Drilling Rod 41 Drilling head (first drilling rod) 42 Excavation blade 43 First kneading drum (first drilling rod) 44 Second kneading drum (first drilling rod) 45 Second kneading drum (first drilling rod)

Claims

1. A foundation pile structure on ground that includes soft ground, in which there is a soft ground layer near the ground and a layer below that that can support load, and which supports the vertical load of an above-ground structure with main piles that reach the layer that can support the load, characterized in that the soft ground layer is reinforced as follows. (1) The main pile was constructed by excavating to a layer capable of supporting the load, constructing a main pile hole, and burying a structure in the main pile hole. (2) The countermeasure piles were formed vertically in an area including the soft ground layer, and the countermeasure pile holes were formed by pressing down on the pile hole walls and kneading the material into them, and then a specified hydraulic material was filled into the countermeasure pile holes. (3) The countermeasure piles are arranged in multiple numbers so as to surround the main pile in the plan view, and are arranged so that the countermeasure piles do not overlap with the main pile and adjacent countermeasure piles do not overlap with each other.

2. 2. A foundation pile structure for ground including soft ground according to claim 1, characterized in that it is configured as follows: (1) Set a reference circle of radius L from the center of the main pile with a diameter of D0. (2) The centers of countermeasure piles with a diameter of D1 were placed along the circumference of the reference circle.

3. 2. A foundation pile structure for ground including soft ground according to claim 1, characterized in that it is configured as follows. (1) Set a reference circle of radius L from the center of the main pile with a diameter of D0. (2) The centers of countermeasure piles with a diameter of D1 were placed along the circumference of the reference circle. (3) The radius L and diameter D0 are 1.5×D0<L<2.5×D0 It was decided.

4. 4. A foundation pile structure for ground including soft ground according to claim 3, configured as follows: (1) If the diameter of the main pile is D0 and the diameter of the countermeasure pile is D1, D0=D1 It was decided. (2) The radius of the reference circle L and the diameter of the main pile D0 are L = 2 x D0 It was decided.

5. 2. A foundation pile structure for ground including soft ground according to claim 1, characterized in that it is configured as follows. (1) The walls of the countermeasure pile holes were gradually compressed more firmly as they moved vertically toward the ground. (2) The wall of the main pile hole for the main pile was gradually compressed more strongly in the vertical direction toward the ground, at least in the soft ground.

6. The foundation pile structure for ground including soft ground according to claim 1, characterized in that it is configured as follows: (1) The foundation pile structure is configured by burying prefabricated piles in pile holes for main piles. (2) The layer capable of supporting the load was set as the supporting ground layer that can exert tip bearing capacity by the main piles.

7. A method for constructing a foundation pile structure on ground that includes soft ground, characterized in that in ground that has a supporting ground layer at a predetermined depth and a soft ground layer near the ground, a structure that supports the vertical load of an above-ground structure using main piles that reach the supporting ground layer is constructed as follows. (1) The foundation pile structure is a structure in which countermeasure piles for ground improvement are constructed so as to surround the main pile in a plan view. (2) The countermeasure pile hole for the countermeasure pile is excavated with a first excavation rod while filling it with hydraulic material so that the hole wall is gradually compressed more firmly as it moves vertically toward the ground. (3) After the excavation of the countermeasure pile holes, or in parallel with the excavation of the countermeasure pile holes, The main pile hole for the main pile is formed by excavating the soft ground layer from near the ground surface with a first excavation rod so that the hole wall is gradually compressed more firmly as it approaches the ground surface in the vertical direction. (4) Next, in the main pile hole of the main pile, the first drilling rod is replaced with the second drilling rod, and the main pile hole is excavated using the second drilling rod down to the supporting ground layer. (5) Next, while injecting hydraulic material into the main pile hole from the second drilling rod or by other means, the second drilling rod is pulled up from the main pile hole to the ground, and a prefabricated pile is buried in the main pile hole. (6) Hydraulic material within the main pile hole. Once the hydraulic material in the countermeasure pile hole has solidified, the foundation pile structure is constructed.

8. 8. A method for constructing a foundation pile structure on ground including soft ground according to claim 7, characterized by comprising the following steps: (1) The first drilling rod is provided with a drilling head at the lower end of the rod body, which is attached to a ground drilling machine, corresponding to the diameter of the countermeasure pile hole or main pile hole to be excavated, and is provided with multiple hole wall kneading drums in the middle part of the rod body, and the hole wall kneading drums have an outer diameter larger than that of the kneading drums located at the top than that of the ones located at the bottom. (2) The second drilling rod is provided with a drilling head at the lower end of the rod body, which is attached to a ground drilling machine, corresponding to the diameter of the main pile hole to be drilled, and with multiple hole wall kneading drums in the middle part of the rod body.

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