Improved ground

Abstract

To provide an improved ground capable of suppressing consolidation settlement while minimizing the construction depth of columnar improvement by using the columnar improvement and a steel pipe pile together instead of integrating the columnar improvement and the steel pipe pile into a soil cement composite pile.SOLUTION: In the improved ground 1, two or more columnar improvements 2 are present in a length not reaching a bearing layer at a predetermined position below a building foundation 5, and a steel pipe pile 3 reaching the bearing layer is positioned between the two or more columnar improvements 2. For example, two columnar improvements 2 and 2 are present at the predetermined position, and the steel pipe pile 3 is located at a position corresponding to a gap between the two columnar improvements 2 and 2.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This invention relates to improved ground that is applied to soft ground to suppress settlement of buildings. [Background technology]

[0002] Conventionally, a soil cement composite pile has been known, which consists of a soil cement (columnar improvement) whose tip reaches the bearing layer and a steel pipe pile inserted into the soil cement.

[0003] Patent Document 1 discloses a method for constructing a soil cement composite pile, in which a soil cement column is constructed after confirming that the tip has reached the supporting layer based on changes in the excavation resistance value, and then a winged steel pipe pile having wings at least near the head, near the tip, and in the middle is inserted into the soil cement column to create a soil cement composite pile.

[0004] Patent Document 2 discloses a method for constructing a soil cement steel pipe composite pile, which involves using a drilling rod to excavate the ground where the composite pile is to be constructed, injecting a soil improvement material such as cement milk into the borehole to build a soil cement column, withdrawing the drilling rod from the borehole, and embedding a steel pipe pile into the soil cement column while rotating it.The steel pipe pile has a pressure-receiving section at least at its tip that increases the resistance from the ground and an adhesion-enhancing section at its shaft that increases frictional resistance with the surrounding area, and the pressure-receiving section at the tip of the pile is pushed through the bottom of the soil cement column, and at least the pressure-receiving section is embedded in a stratum that is deeper and harder than the stratum through which the borehole was excavated. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-201638 [Patent Document 2] Japanese Patent Application Laid-Open No. 2012-127082 Summary of the Invention [Problem to be solved by the invention]

[0006] However, while soil-cement composite piles, in which steel pipe piles are placed at the core of columnar improvement, have the advantage of high bearing capacity, they also have the disadvantage of being expensive and requiring a long construction period. Furthermore, because the columnar improvement must be cast and the steel pipe piles must be cast before the cement hardens, the columnar improvement and the steel pipe piles must be constructed simultaneously. Furthermore, in construction methods in which steel pipe piles penetrate columnar improvement, if the steel pipe piles have spiral blades around them, they disturb the ground around the steel pipe piles as they penetrate, resulting in very little friction between the outer surface of the steel pipe pile and the ground.

[0007] The objective of this invention is to provide improved ground that can suppress consolidation settlement while making the construction depth of columnar improvement as shallow as possible by using columnar improvement in combination with steel pipe piles, rather than integrating columnar improvement with steel pipe piles to form a soil cement composite pile. [Means for solving the problem]

[0008] In order to solve the above-mentioned problems, the improved ground of this invention is characterized in that two or more columnar improvements are present at a predetermined position below the foundation, with the length not reaching the supporting layer, and a steel pipe pile reaching the supporting layer is located between the two or more columnar improvements.

[0009] In the case of the improved ground described above, in ground improvement projects in areas with thick soft ground layers, rather than integrating the columnar improvement and the steel pipe piles and extending them to the supporting layer, the columnar improvement and the steel pipe piles are installed separately, and the columnar improvement is installed at a length that does not reach the supporting layer. This allows the columnar improvement to be designed to the shortest length possible to ensure bearing capacity through its peripheral friction and tip bearing capacity. This allows for the use of steel pipe piles (e.g., small-diameter steel pipes), which are easy to install, inexpensive, and durable, as a countermeasure against consolidation settlement, ensuring high economic efficiency. Furthermore, since the columnar improvement and the steel pipe piles are installed separately, the steel pipe piles can be installed even after the columnar improvement has solidified, eliminating the need to add expensive additives such as retarders to the columnar improvement cement. Furthermore, the columnar improvement and the steel pipe piles can be installed on different days, improving construction flexibility.

[0010] Three pillar-shaped improvements may be arranged in a triangular configuration or four pillar-shaped improvements may be arranged in a square configuration at the predetermined position, and the steel pipe pile may be positioned in the gap between the three or four pillar-shaped improvements. This allows the steel pipe pile to be easily positioned at the center of gravity of the multiple pillar-shaped improvements.

[0011] Six columnar improvements may be present at the predetermined positions, and the steel pipe piles may be positioned at the gaps between the six columnar improvements. This allows the steel pipe piles to be easily positioned at the center of gravity of each of the multiple columnar improvements.

[0012] The steel pipe pile may have an outer diameter that fits into the gap between the three or four columnar improvements placed close to each other. This allows the multiple columnar improvements and the steel pipe piles to be densely arranged in the narrow, predetermined position.

[0013] The steel pipe pile may have an outer diameter that fits into the gaps between the six columnar improvements that are placed close to each other. This allows the columnar improvements and the steel pipe piles to be densely arranged in the narrow, predetermined position.

[0014] A cover may be attached to the upper side of the steel pipe pile serving as the upper pile, and a filler material may be filled above the cover, thereby increasing the bending rigidity of the steel pipe pile and improving its horizontal bearing capacity during an earthquake.

[0015] The filler may be leveling concrete poured below the foundation, which eliminates the need for a separate process for filling the filler, thereby reducing construction costs.

[0016] The steel pipe pile serving as the lower pile may have a sealing cap at its lower end, whereby an air space forms inside the steel pipe pile above the sealing cap, generating buoyancy in the steel pipe pile, which can be used as a countermeasure against consolidation settlement. [Effects of the Invention]

[0017] According to the present invention, by using columnar improvement in combination with steel pipe piles, it is possible to prevent consolidation settlement while keeping the construction depth of columnar improvement as shallow as possible, even in areas with a thick layer of soft ground. [Brief explanation of the drawings]

[0018] [Figure 1] 1A and 1B are diagrams showing an outline of the improved ground of the embodiment, in which FIG. 1A is an explanatory diagram in plan view and FIG. 1B is an explanatory diagram in perspective view. [Figure 2] This is an explanatory diagram of the improved ground prepared for testing. [Figure 3] This is an outline of the load test on the improved ground constructed for the test (ground column diagram). [Figure 4] This is a graph showing the results of a load test on improved ground constructed for testing purposes. [Figure 5] 10A and 10B are diagrams showing an outline of improved ground in another embodiment, in which FIG. 10A is an explanatory diagram in plan view and FIG. 10B is an explanatory diagram in perspective view. [Figure 6] 10A and 10B are diagrams showing an outline of improved ground in another embodiment, in which FIG. 10A is an explanatory diagram in plan view and FIG. 10B is an explanatory diagram in perspective view. [Figure 7] 10A and 10B are diagrams showing an outline of improved ground in another embodiment, in which FIG. 10A is an explanatory diagram in plan view and FIG. 10B is an explanatory diagram in perspective view. [Figure 8] 1A to 1E are explanatory diagrams showing examples of steel pipe piles in improved ground. DETAILED DESCRIPTION OF THE INVENTION

[0019] An embodiment of the present invention will be described below with reference to the accompanying drawings. Figures 1(A) and 1(B) are explanatory diagrams showing an outline of an improved ground 1 according to the embodiment. In this improved ground 1, two adjacent columnar improvement columns 2, 2 are located at a predetermined position below a building foundation 5 (such as a footing), with a length that does not reach the bearing layer. A steel pipe pile 3 (small-diameter steel pipe) that reaches the bearing layer is located between the two columnar improvement columns 2, 2. The columnar improvement columns 2 are produced by discharging cement milk, a mixture of cement-based solidification material and water, into the soil and mixing and stirring it with the soil at that location. The diameter of the columnar improvement columns 2 can be, for example, 500 to 1200 mm. The steel pipe pile 3 has an outer diameter of, for example, approximately 50 to 165 mm. The outer diameter of the steel pipe pile 3 can be, for example, 10 to 20% of the diameter of the columnar improvement columns 2 employed.

[0020] Figure 2 shows an outline of the improved ground 1 prepared for the test. Each columnar improvement 2 has a diameter of 500 mm and a depth of 6 m. The steel pipe pile 3 has a diameter of 114.3 mm and a length of 24 m, consisting of four 6-m piles. The circumferential distance between the columnar improvements 2, 2 (for circular columnar improvements 2, 2, the distance between their centers minus the radius of each columnar improvement 2, 2) is approximately 130 mm, longer than the 114.3 mm diameter of the steel pipe pile 3. The tops of the columnar improvements 2 are located 300 mm below the ground surface, and a loading plate 41 measuring 0.5 m x 1.2 m is placed on top of the two columnar improvements 2, spanning them. Four strain gauges 42 for measuring the displacement of the steel pipe piles 3 and other structures are installed 200 mm below the loading plate 41.

[0021] Figure 3 shows an overview of the load test (soil column diagram) of the improved ground 1 constructed for the above-mentioned test. The vertical direction of this diagram represents the ground depth, and the corresponding relationship between the height of the columnar improvement 2 and the steel pipe pile 3 is shown superimposed on this diagram. This load test was carried out with the columnar improvement 2 supported on soft clayey soil and the steel pipe pile 3 (small-diameter steel pipe) supported on hard sandy soil. The load was applied in stages, forcing the improved ground 1 to a depth of approximately 80 mm.

[0022] Figure 4 shows a graph of the results of the load test. The vertical axis represents the applied load (kN) and the horizontal axis represents the settlement displacement (mm) of the improved ground 1 under repeated load application. The load is shared by two columnar improvements 2 and one steel pipe pile 3 that make up the improved ground 1 under test. For example, if the load bearing capacity of the steel pipe pile 3 under a 1000 kN load is 400 kN as determined by strain gauge 42, the remaining 600 kN is borne by the two columnar improvements 2. The results of the load test were better than those calculated using the general formula. Furthermore, as shown above, even when the improved ground 1 under test was forcibly compressed to a large extent, the bearing capacity of the columnar improvements 2 remained high, and no stress concentration was observed on the steel pipe pile 3.

[0023] Thus, in the case of the improved ground 1 described above, in ground improvement work in areas with thick layers of soft ground, the columnar improvement 2 and steel pipe pile 3 are not integrated and reach the bearing layer, but rather the columnar improvement 2 and steel pipe pile 3 are installed separately, and the columnar improvement 2 is installed at a length that does not reach the bearing layer. As a result, the columnar improvement 2 can be designed to the shortest length possible to ensure bearing capacity through its peripheral friction and tip bearing capacity, while the steel pipe pile 3, which is easy to install, inexpensive, and has toughness, is used as a countermeasure against consolidation settlement, ensuring high economic efficiency. Furthermore, by using the steel pipe pile 3 only as a countermeasure against consolidation settlement, the design can be carried out without having to consider structural strength calculations, buckling considerations, or restrictions on construction length. Furthermore, if the columnar improvement 2 and the steel pipe pile 3 are separate, the steel pipe pile 3 can be constructed even after the columnar improvement 2 has solidified, eliminating the need to add expensive additives such as retarders to the cement of the columnar improvement 2. Furthermore, the columnar improvement 2 and the steel pipe pile 3 can be constructed on different days, improving construction flexibility. Since the steel pipe pile 3 is used in its straight state, there is no restriction on the penetration speed, making construction extremely easy and reducing construction costs. Furthermore, since construction can be carried out using soil improvement machines commonly used in small buildings such as houses, the method is highly versatile and can be constructed even in small sites.

[0024] Figure 5 shows a modified example of improved ground 1. The improved ground 1 shown in this figure has three columnar improvements 2 arranged in a triangular configuration at a predetermined position below the building foundation 5, and a steel pipe pile 3 is located at the gap between the three columnar improvements 2 (center of gravity). The steel pipe pile 3 has an outer diameter that fits into the gap between the three adjacent columnar improvements 2. The distance that constitutes the proximity of the columnar improvements 2 (the circumferential distance between the columnar improvements 2) is, for example, 0 to 200 mm. When the circumferential distance between the columnar improvements 2 is approximately 0 mm, the outer diameter of the steel pipe pile 3 is equal to or smaller than the diameter d1 of a circle tangent to the three columnar improvements 2 of the same diameter arranged in a triangular configuration (e.g., 500 mm in diameter). When the circumferential distance between the columnar improvements 2 is 200 mm or less, the outer diameter of the steel pipe pile 3 can be equal to or smaller than the diameter d2 of a circle tangent to the three columnar improvements 2 of the same diameter spaced apart. Furthermore, even when the peripheral distance between the columnar improvements 2, 2 exceeds 200 mm, the outer diameter of the steel pipe pile 3 can be set to be equal to or smaller than the diameter d1 or the diameter d2.

[0025] Figure 6 shows another modified example of the improved ground 1. The improved ground 1 shown in this figure has four columnar improvements 2 arranged in a square shape at a predetermined position below the building foundation 5, and a steel pipe pile 3 is located at the gap between the four columnar improvements 2 (center of gravity position). The steel pipe pile 3 has an outer diameter that fits into the gap between the four adjacent columnar improvements 2. The distance that is considered to be close between the columnar improvements 2 (the circumferential distance between the columnar improvements 2) is, for example, 0 to 200 mm. When the circumferential distance between the columnar improvements 2 is approximately 0 mm, the outer diameter of the steel pipe pile 3 is equal to or less than the diameter d3 of a circle tangent to four columnar improvements 2 of the same diameter arranged in a square shape (e.g., 500 mm in diameter). When the circumferential distance between the columnar improvements 2 is 200 mm or less, the outer diameter of the steel pipe pile 3 can be equal to or less than the diameter d4 of a circle tangent to four columnar improvements 2 of the same diameter spaced apart in this manner. Furthermore, even when the peripheral distance between the columnar improvements 2, 2 exceeds 200 mm, the outer diameter of the steel pipe pile 3 can be set to be equal to or less than the diameter d3 or equal to or less than the diameter d4.

[0026] Figure 7 shows another modified example of the improved ground 1. The improved ground 1 shown in this figure has six columnar improvements 2 arranged in a 2x3 pattern at a predetermined position below the building foundation 5, and steel pipe piles 3 are located at two gaps between the six columnar improvements 2 (the center of gravity is the midpoint of the line connecting these two gaps). The steel pipe piles 3 have an outer diameter that fits into the gaps between the four adjacent columnar improvements 2. The distance that constitutes the proximity between the columnar improvements 2, 2 (the circumferential distance between the columnar improvements 2, 2) is, for example, 0 to 200 mm. When the circumferential distance between the columnar improvements 2, 2 is approximately 0 mm, the outer diameter of the steel pipe pile 3 is equal to or less than the diameter d3 of a circle tangent to four columnar improvements 2 of the same diameter (for example, 500 mm diameter) arranged in a square shape. When the circumferential distance between the columnar improvements 2, 2 is 200 mm or less, the outer diameter of the steel pipe pile 3 can be set to the diameter d4 or less of the circle tangent to four columnar improvements 2 of the same diameter spaced apart. Furthermore, even when the circumferential distance between the columnar improvements 2, 2 exceeds 200 mm, the outer diameter of the steel pipe pile 3 can be set to the diameter d3 or less or the diameter d4 or less. As another example of an arrangement of six columnar improvements 2, a configuration using two sets of improved ground 1 as shown in Figure 5 may be used. In this case, three steel pipe piles 3 can be arranged.

[0027] Figure 8(A) shows an example of the allocation of steel pipe piles 3 in improved ground 1. In this example, the steel pipe piles 3 are allocated to one lower pile of 6 m, two middle piles totaling 12 m, and one upper pile of 3 m. When the steel pipe piles 3 are sunk, as shown in Figure 8(B), soil fills in from the tip of the lower pile to partway up the height, and this soil acts as a lid, creating an air layer above the soil lid.

[0028] In such an arrangement example of steel pipe piles 3, as shown in FIG. 8(C), a receiving cover 31 may be attached to the upper side of the steel pipe pile 3 serving as the upper pile, and a filler material 32 may be filled above the receiving cover 31. This increases the bending rigidity of the steel pipe pile 3 (upper pile) during an earthquake. The leveling concrete (basic concrete) poured below the building foundation 5 may be used as the filler material 32. This eliminates the need for an additional process for filling the filler material 32, thereby reducing construction costs. Note that even in the structure shown in FIG. 8(C), the soil that has filled in from the tip of the lower pile to partway up the height serves as a lid, and an air layer is created above the lid.

[0029] To increase the air space, the steel pipe pile 3 serving as the lower pile may have a sealing lid 33 at its lower end, as shown in Figure 8(D). In this case, compared to a configuration without the sealing lid 33, almost the entire interior of the steel pipe pile 3 above the sealing lid 33 becomes an air space, and the large buoyancy generated in the steel pipe pile 3 can be used to prevent consolidation settlement. In order to obtain an even larger air space, a structure in which the filler material 32 is not filled in the upper side of the steel pipe pile 3 can also be used, as shown in Figure 8(E).

[0030] Although the embodiments of the present invention have been described above with reference to the drawings, the present invention is not limited to the illustrated embodiments. Various modifications and variations can be made to the illustrated embodiments within the same scope as the present invention or within an equivalent scope. [Explanation of symbols]

[0031] 1: Improved ground 2: Column improvement 3: Steel pipe pile 5: Building foundations 31: Receiving lid 32: Filling material 33: Airtight lid 41: Loading plate 42: Strain gauge

Claims

1. Improved ground characterized by the presence of two or more columnar improvements at a predetermined position below the foundation, with the length not reaching the supporting layer, and a steel pipe pile reaching the supporting layer being located between the two or more columnar improvements.

2. The improved ground described in claim 1 is characterized in that there are three columnar improvements in a triangular arrangement or four columnar improvements in a square arrangement at the specified position, and the steel pipe pile is located in the gap between the three or four columnar improvements.

3. The improved ground described in claim 1 is characterized in that there are six columnar improvements at the specified positions, and the steel pipe piles are located in the gaps between the six columnar improvements.

4. Improved ground according to any one of claims 1 to 3, characterized in that a receiving cover is attached to the upper side of the steel pipe pile which serves as the upper pile, and filler material is filled above the receiving cover.

5. 5. The improved ground according to claim 4, wherein the filler is leveling concrete poured below the foundation.

6. 4. The improved ground according to claim 1, wherein the steel pipe pile serving as the lower pile has a sealing lid on the lower end side.

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