Steel tower foundation structure and method for constructing the foundation structure

Abstract

The object of the present invention is to solve the problems of the prior art, namely, to provide a tower foundation structure that can resist the lifting load associated with power transmission towers (ST), and a method for constructing such a foundation structure. [Solution] The steel tower foundation structure of the present invention is a foundation structure that supports a steel tower having two or more main legs, and comprises a concrete foundation and screw piles. The concrete foundation is provided on each main leg, and the screw piles are inserted into the ground and a portion of them is anchored to the concrete foundation. The self-weight and circumferential frictional resistance of the screw piles resist the uplift load associated with the steel tower.

Description

Technical Field

[0001] The present invention relates to the foundation of a steel tower, and more specifically, to a steel tower foundation structure using screw piles and a method for constructing the foundation structure.

Background Art

[0002] A transmission steel tower is one of the extremely important facilities for stably supplying power to consumers. Naturally, it has a robust structure and, in principle, settlement (especially uneven settlement) is not allowed. Therefore, important and heavy structures such as transmission steel towers are constructed on a supporting layer where a considerable bearing capacity can be expected, such as bedrock or compacted sand layers. For example, in the case of a transmission steel tower, it is often structured such that a foundation is constructed on the supporting layer and the main leg members are fixed to the foundation, that is, a configuration that obtains support from the supporting layer through the foundation is common.

[0003] When the supporting layer is at a relatively shallow position, it is possible to excavate to a predetermined depth to expose the supporting layer and directly construct a foundation (so-called "direct foundation") on this supporting layer. On the other hand, when the supporting layer is at a relatively deep position, a considerable amount of excavation is required to expose the supporting layer, and furthermore, the construction of a large-scale foundation is necessary, making a direct foundation unrealistic. Therefore, in such cases, a "pile foundation" is adopted instead of a direct foundation.

[0004] Since the pile foundation involves relatively large costs for each of the material cost of the piles, transportation cost, cost associated with drilling, and depreciation of construction machinery including drilling machines, the construction cost tends to increase compared to the direct foundation. Especially when constructing a transmission steel tower in mountainous areas where there is no traffic of people or vehicles, it is inevitable to adopt deep foundation piles because drilling machines cannot be transported to the site, and as a result, one has to accept a further increase in construction costs. Therefore, as long as there are no design problems, it is the mainstream to adopt a direct foundation.

[0005] Figure 15 is a front view (with a cross-sectional view of the underground section) of a typical power transmission tower ST. Because the supporting layer is relatively shallow, a direct foundation is used for this power transmission tower ST. Specifically, an inverted T-shaped concrete foundation (hereinafter simply referred to as "inverted T-shaped foundation") is installed in the ground, and the main leg members ML of the power transmission tower ST are fixed to this inverted T-shaped foundation. Such inverted T-shaped foundations are now widely used because they reduce construction costs. Although Figure 15 shows two main leg members ML and an inverted T-shaped foundation for convenience, this power transmission tower ST has four main leg members ML, each with four inverted T-shaped foundations.

[0006] Incidentally, in an inverted T-shaped foundation, power transmission towers (STs) are subjected not only to compressive loads due to their own weight, but also to uplift loads (also called pull-out forces). For example, when wind loads act on a power transmission tower (ST), or when an earthquake causes a load due to the tower's own weight, a horizontal force acts on the tower (ST) as shown in Figure 15. At this time, the vertically long power transmission tower (ST) tends to rotate as a whole due to this horizontal force (arrow A in the figure). Then, in accordance with this rotation of the power transmission tower (ST), an uplift load in the pull-out direction (arrow B in the figure) acts on one inverted T-shaped foundation (left side in the figure), and a force in the compressive direction (arrow C in the figure) acts on the other inverted T-shaped foundation (right side in the figure).

[0007] Figure 16 is a schematic partial cross-sectional view showing the elements by which an inverted T-shaped foundation resists the uplift load associated with a transmission tower. As shown in this figure, an inverted T-shaped foundation can resist the uplift load of a transmission tower through the following elements: the self-weight of the inverted T-shaped foundation (hereinafter referred to as "foundation self-weight WC"), the self-weight of the soil on the footing of the inverted T-shaped foundation (hereinafter referred to as "soil self-weight WS"), the shear resistance between the soil and the ground (hereinafter referred to as "ground shear resistance FS"), and the frictional resistance between the side of the footing and the ground (hereinafter referred to as "footing frictional resistance FF").

[0008] However, when design calculations were performed during the planning stage, it was often determined that inverted T-shaped foundations could adequately support the compressive loads associated with power transmission towers (ST), but could not adequately resist the uplift loads. Inverted T-shaped foundations are used in cases where the bearing layer is shallow, but because the bearing layer is shallow, the inverted T-shaped foundation itself tends to be small. As a result, the foundation's self-weight (WC), soil's self-weight (WS), ground shear resistance (FS), and footing friction resistance (FF) are all calculated to be small values, meaning that the design result indicates insufficient resistance to uplift loads.

[0009] The lifting load on the inverted T-shaped foundation of power transmission towers (ST) has long been considered a problem that needs to be solved, and various technologies have been proposed to date. For example, Patent Document 1 proposes a reinforcement structure in which a new mat slab is added to reinforce an existing inverted T-shaped foundation, but without restricting the rotation of the inverted T-shaped foundation due to the lifting load, that is, a structure in which no bending moment is generated in the mat slab. [Prior art documents] [Patent Documents]

[0010] [Patent Document 1] Japanese Patent Publication No. 2022-062942 [Overview of the project] [Problems that the invention aims to solve]

[0011] Foundations using concrete mat slabs (hereinafter simply referred to as "mat foundations") require consideration of the overturning moment of the mat foundation itself, require excavation of a wider area of ​​ground, and also require a larger amount of concrete, resulting in higher construction costs compared to inverted T-type foundations. Considering the large number of power transmission towers (STs) that will be constructed in the future, this cost difference cannot be ignored, and therefore, a new inverted T-type foundation configuration capable of resisting the uplift load from power transmission towers (STs) was desired.

[0012] The object of the present invention is to solve the problems of the prior art, namely, to provide a tower foundation structure that can resist the lifting load associated with power transmission towers (ST), and a method for constructing such a foundation structure. [Means for solving the problem]

[0013] The present invention focuses on the idea of ​​combining a screw pile with an inverted T-shaped foundation and adding resistance from the screw pile to resist the uplift load associated with power transmission towers (ST), and is based on a completely new concept.

[0014] The steel tower foundation structure of the present invention is a foundation structure for supporting a steel tower having two or more main legs, and comprises a concrete foundation and screw piles. The concrete foundation is provided for each main leg, and the screw piles are inserted into the ground and a portion of them is anchored to the concrete foundation. The self-weight and circumferential frictional resistance of the screw piles resist the uplift load associated with the steel tower.

[0015] The tower foundation structure of the present invention can also be a structure in which a "pile block" is formed. This pile block is a structure formed by a plurality of screw piles arranged close enough to restrain the ground between the piles (the ground sandwiched between the piles), and the ground between the piles. In this case, the self-weight of the ground between the piles related to the pile block is taken into account, and it can resist the uplift load from the tower.

[0016] The tower foundation structure of the present invention may be provided with wall-shaped, cylindrical, or columnar pile block bodies, or it may be formed from a single pile block body. In this case, the shear resistance force of the ground on the wall surface of the pile block body can resist the uplift load from the tower.

[0017] The steel tower foundation structure of the present invention may also be provided with a pile block body comprising multiple screw piles arranged so that the pile spacing is less than three times the pile diameter. Note that this pile diameter includes the diameter of the screw pile blades.

[0018] The steel tower foundation structure of the present invention can also be a structure in which pile blocks are formed in the area of ​​the concrete foundation where the uplift load associated with the steel tower occurs.

[0019] The present invention relates to a method for constructing a foundation structure for a steel tower, comprising a pile insertion step and a concrete foundation formation step. In the pile insertion step, multiple screw piles are inserted into the ground for one concrete foundation, and in the concrete foundation formation step, a concrete foundation is formed so that a portion of the screw piles are anchored.

[0020] The foundation structure construction method of the present invention can also be a method for forming a "pile block" during the pile insertion process.

[0021] The foundation structure construction method of the present invention may further include a lifting test step and a pile spacing determination step. In the lifting test step, screw piles are inserted into the ground while varying the pile spacing, and then the screw piles are lifted. In the pile spacing determination step, the pile spacing at which it has been confirmed that the ground between the piles can be restrained by the lifting test step is determined as the planned pile spacing. In this case, in the pile insertion step, screw piles are inserted into the ground so that multiple screw piles are arranged at the planned pile spacing. [Effects of the Invention]

[0022] The steel tower foundation structure and foundation structure construction method of the present invention have the following effects. (1) Screw piles can be installed at low cost, and inverted T-shaped foundations can also be constructed at low cost, thus reducing the construction costs associated with building new transmission towers. (2) Although the installation of screw piles is added to the conventional inverted T-type foundation, the tower foundation structure can be constructed easily and quickly. As a result, the sharing of the tower can be accelerated, meaning that power supply can be started earlier. (3) The amount of concrete is reduced compared to a mat foundation. Moreover, the amount of excavated soil is suppressed compared to a mat foundation, that is, the amount of waste soil is reduced, so the environmental burden associated with waste soil treatment can be reduced.

Brief Description of Drawings

[0023] [Figure 1] Front view showing a transmission tower provided with the tower foundation structure of the present invention. [Figure 2] (a) is a partial cross-sectional view showing the tower foundation structure, and (b) is a plan view seen from above of the tower foundation structures arranged at four locations. [Figure 3] (a) is a front view showing a screw pile provided with a slightly larger-diameter blade member at a part of the tip side, and (b) is a front view showing a screw pile provided with a slightly smaller-diameter blade member in the lower half. [Figure 4] Partial cross-sectional view schematically showing each element by which the tower foundation structure of the present invention resists the lifting load. [Figure 5] Model diagram explaining the frictional force on the pile circumferential surface of the screw pile. [Figure 6] Plan view schematically showing a "pile block body" formed by arranging a plurality of screw piles in proximity. [Figure 7] Graph showing the relationship between the "load without considering the earth and sand weight" borne by each screw pile and the pile spacing. [Figure 8] (a) is a horizontal cross-sectional view schematically showing a columnar pile block body composed of four screw piles and the ground between the piles, (b) is a plan view schematically showing six columnar pile block bodies arranged in a concrete foundation, and (c) is a horizontal cross-sectional view schematically showing the columnar pile block body. [Figure 9] (a) is a horizontal cross-sectional view schematically showing a cylindrical pile block body, (b) is a horizontal cross-sectional view schematically showing a pile block body composed of a linear wall body, and (c) is a horizontal cross-sectional view schematically showing a pile block body composed of a curved wall body. [Figure 10] (a) is a front view schematically showing the lifting area when the transmission tower rotates towards the right, and (b) is a front view schematically showing the lifting area when the transmission tower rotates towards the left. [Figure 11] (a) is a schematic partial front view showing the pile block body to be placed in the lifting area, and (b) is a schematic horizontal cross-sectional view showing the pile block body to be placed in the lifting area. [Figure 12] A flowchart illustrating the main steps of the basic structure construction method of the present invention. [Figure 13] A step diagram showing the main steps of the basic structure construction method of the present invention. [Figure 14] (a) is a perspective view showing a hydraulic excavator equipped with a rotating device for inserting a screw pile into the ground, and (b) is a schematic partial side view showing the state in which the screw pile and the arm are connected by an intermediate rod. [Figure 15] A front view showing a typical power transmission tower. [Figure 16] A schematic partial cross-sectional view showing the various elements of the inverted T-shaped foundation that resist the uplift load associated with the transmission tower. [Modes for carrying out the invention]

[0024] An example of an embodiment of the transmission tower foundation structure and foundation structure construction method of the present invention will be described with reference to the figures. The present invention can be implemented for various transmission towers having two or more main leg members ML, but for convenience, the example described here will be for a power transmission tower ST.

[0025] 1. Tower foundation structure First, the tower foundation structure of the present invention will be explained in detail with reference to the diagrams. The foundation structure construction method of the present invention is a method for constructing the tower foundation structure of the present invention. Therefore, the tower foundation structure of the present invention will be explained first, followed by a detailed explanation of the foundation structure construction method of the present invention.

[0026] Figure 1 is a front view (with a cross-sectional view of the underground section) of a power transmission tower ST on which the tower foundation structure 100 of the present invention is installed, and Figure 2(a) is a partial front view (with a cross-sectional view of the underground section) of the tower foundation structure 100 of the present invention. As shown in these figures, the tower foundation structure 100 of the present invention supports a tower having two or more main leg members ML, such as a power transmission tower ST, and is an independent foundation structure provided for each main leg member ML. Although Figures 1 and 2(a) show two tower foundation structures 100, this power transmission tower ST has four main leg members ML, and therefore, in reality, the tower foundation structure 100 is provided in four locations as shown in Figure 2(b).

[0027] The transmission tower foundation structure 100 is composed of a structure such as an inverted T-shaped foundation (hereinafter referred to as "concrete foundation 120") and screw piles 110. The concrete foundation 120 supports the main leg members ML by burying a portion of the lower end of each leg member, and therefore is installed in the same number as the number of main leg members ML that the transmission tower ST is equipped with. The concrete foundation 120 is made of reinforced concrete with reinforcing bars RB, such as main reinforcement and distribution reinforcement, and the main leg members ML can also be fixed using these reinforcing bars RB. The screw piles 110 are inserted into the ground in a nearly vertical position (including vertical), and a portion of their upper end is anchored to the concrete foundation 120. However, multiple screw piles 110 are placed for each concrete foundation 120.

[0028] The screw pile 110 that constitutes the tower foundation structure 100 is composed of a pile body 111 and a blade 112, as shown in Figure 3. Various conventional types of screw piles 110 can be used, such as those with a slightly larger diameter blade 112 provided on a part of the tip, as shown in Figure 3(a), or those with a slightly smaller diameter blade 112 provided on the lower half, as shown in Figure 3(b).

[0029] Figure 4 is a schematic partial cross-sectional view showing the elements of the tower foundation structure 100 of the present invention that resist the uplift load PU. As previously described, when horizontal forces such as wind loads or earthquakes act on a power transmission tower ST, the vertically long power transmission tower ST exhibits a behavior in which it rotates as a whole in accordance with the horizontal force, and as a result, an uplift load PU acts on the tower foundation structure 100 as shown in Figure 4. Conventional inverted T-type foundations resist the uplift load PU by the weight of the foundation WC (the weight of the inverted T-type foundation), the weight of the soil WS (the weight of the soil on the footing of the inverted T-type foundation), the ground shear resistance FS (the shear resistance between the soil and the ground), and the footing friction resistance FF (the friction resistance between the side of the footing and the ground).

[0030] In contrast, the tower foundation structure 100 exhibits resistance not only from the foundation's own weight WC, the soil's own weight WS, the ground shear resistance FS, and the footing friction resistance FF, but also from the screw piles 110. That is, the self-weight of the screw piles 110 (hereinafter referred to as "pile self-weight WP") and the frictional resistance of the circumferential surface of the screw piles 110 located underground (hereinafter referred to as "pile friction resistance FP") resist the uplift load PU.

[0031] It is known that the screw pile 110, as a support pile, bears the compressive load due to the "pile surface friction force Rf" shown in Figure 5. This pile surface friction force Rf is determined by multiplying the columnar side surface, which consists of the "pile diameter D" and the "anchoring length L'", by the pile surface friction force τ according to the soil type. However, the pile diameter D is the outer diameter of the blade material 112, and the portion of the screw pile 110 to which the blade material 112 is provided is counted as the anchoring length L'.

[0032] As previously described, conventional inverted T-shaped foundations can adequately support the compressive load associated with power transmission towers ST, but are often found to be insufficient to resist the uplift load PU. Therefore, the tower foundation structure 100 of the present invention can be structured so that the screw pile 110 bears the resistance force that is insufficient against the uplift load PU. In other words, the concrete foundation 120 bears the compressive load associated with power transmission towers ST, and the screw pile 110 and the concrete foundation 120 resist the uplift load PU. Of course, it is also possible to configure the structure so that both the screw pile 110 and the concrete foundation 120 resist both the compressive load and the uplift load PU associated with power transmission towers ST.

[0033] Incidentally, it is known that when the distance between the centers of piles becomes small, the axial compressive bearing force of the piles becomes smaller than that of single piles, due to the "group pile" effect. For example, the "Specifications for Road Bridges and Commentary - Substructure Section -" and the "Pile Foundation Design Handbook" stipulate that when the distance between the centers of piles is less than 2.5 times the pile diameter D, the group pile effect must be considered. Furthermore, it is known that ground anchors, which are used for various purposes such as earth retention, slope stabilization, and the stabilization of various structures, experience a "group effect" when their spacing is small, which reduces the ultimate pull-out force of the anchor body. Therefore, the "Ground Anchor Design and Construction Standards and Commentary (2000)" stipulates that if the spacing between ground anchors does not meet the condition of "more than 4 times the diameter of the anchor body and more than 1.5m", the group effect must be considered.

[0034] Thus, support piles that bear compressive loads and ground anchors that are subjected to strong tensile loads are generally planned with a considerable spacing to avoid group pile effects. Therefore, it is conceivable that the screw piles 110 constituting the tower foundation structure 100 of the present invention would also be arranged at a certain interval to maximize their effectiveness as individual piles. However, the inventors of the present invention have found that installing multiple screw piles 110 in a considerably close proximity improves resistance to uplift loads PU in particular.

[0035] Figure 6 is a schematic plan view showing a structure (hereinafter referred to as "pile block body 140") formed by arranging multiple screw piles 110 in close proximity. In this pile block body 140, because the spacing between the screw piles 110 is small, the ground between the screw piles 110 (hereinafter referred to as "inter-pile ground 130") is restrained by the screw piles 110. As a result, when the screw piles 110 are pulled up, the inter-pile ground 130 is also pulled up. In other words, the pile block body 140 is a structure composed of multiple screw piles 110 and inter-pile ground 130, and can resist the lifting load PU of the power transmission tower ST by the weight of the screw piles 110 and inter-pile ground 130.

[0036] To form the pile block 140, it is necessary to position the screw piles 110 close enough to restrain the ground 130 between the piles, and this pile spacing (hereinafter referred to as the "planned pile spacing") can be determined by various methods. For example, the planned pile spacing can be determined by conducting a test construction. In this test, multiple screw piles 110 are inserted into the ground at the site, and then simultaneously pulled up. When the ground 130 between the piles is pulled up together with the screw piles 110, it can be determined that the pile block 140 has been formed. The above series of procedures is then repeated while changing the pile spacing to determine the maximum spacing at which the pile block 140 is formed, which is then set up as the planned pile spacing. Of course, instead of conducting a test construction at the site, a test construction can also be performed at a test site that replicates the conditions at the actual site.

[0037] Furthermore, the planned pile spacing can also be determined by estimating the load borne by the screw piles 110 when the lifting load PU of the power transmission tower ST is applied. Figure 7 shows the results of estimating the load borne by each screw pile 110 (hereinafter referred to as "unit pile load") while changing the pile spacing as a design condition. In this case, it is assumed that screw piles 110 with a pile diameter D of 96 mm are installed, and the unit pile load when the pile spacing is 1.5 m is set as the standard value (1.0). As shown in this figure, when the pile spacing becomes 0.3 m, the unit pile load decreases sharply. At this time, it is thought that the unit pile load is reduced because the self-weight of the ground between the piles 130 resists the lifting load PU in addition to the pile friction resistance FP and the pile's own weight WP, meaning that a pile block body 140 is formed. Therefore, in this calculation, if screw piles 110 are placed at a pile spacing of 0.3m, which is approximately three times the pile diameter D, a pile block body 140 is formed. In other words, if screw piles 110 are placed at a spacing of less than three times the pile diameter D, a pile block body 140 is formed, meaning that "less than three times the pile diameter D" can be used as the planned pile spacing.

[0038] As previously described, the transmission tower foundation structure 100 can be configured such that the concrete foundation 120 bears the compressive load associated with the transmission tower ST, and the screw piles 110 and the concrete foundation 120 resist the uplift load PU. In this case, the planned pile spacing should be set to "less than three times the pile diameter D" of the screw piles 110.

[0039] The pile block 140 can be formed in various shapes, such as columnar, cylindrical, or wall-like. For example, Figure 8 shows a columnar pile block 140 consisting of four screw piles 110 and ground between piles 130. Such columnar pile block 140 can be placed in multiple locations (six locations in the figure) of the concrete foundation 120, as shown in Figure 8(b). The columnar pile block 140 also has side walls, as shown by the dashed lines in Figure 8(a), and these side walls are in contact with the ground. Therefore, as shown in Figure 8(c), frictional resistance (hereinafter referred to as "block friction resistance FB") is generated between the side walls of the pile block 140 and the ground, and this block friction resistance FB can also resist the lifting load PU of the power transmission tower ST.

[0040] Figure 9(a) shows a cylindrical pile block 140 with an annular (donut-shaped) cross-section. In such a cylindrical pile block 140, an outer and inner circumferential wall surface is formed, and therefore, block friction resistance FB is generated on both the outer and inner circumferential wall surfaces. Figure 9(b) shows a wall-shaped pile block 140 with a straight cross-section, and Figure 9(c) shows a wall-shaped pile block 140 with a curved (semicircular) cross-section. In such wall-shaped pile block 140, a series of side walls are formed around the perimeter, and block friction resistance FB is generated on these side walls.

[0041] The pile block 140 requires the screw piles 110 to be placed close together, meaning that multiple screw piles 110 must be placed densely. Therefore, the pile block 140 requires more screw piles 110 than single screw piles 110 that are placed with sufficient spacing between piles, and consequently, the construction cost is higher. For this reason, it is preferable to place the pile block 140 only in the necessary areas of the concrete foundation 120, or more specifically, only in the areas where uplift loads PU are expected to occur (hereinafter referred to as the "uplift areas").

[0042] In some cases, it is anticipated that horizontal forces acting from a particular direction will have a particularly significant impact on certain power transmission towers (STs). For example, in power transmission towers (STs) where particularly large horizontal forces act when wind loads act on the transmission lines, it is anticipated that horizontal forces perpendicular to the direction of the overhead line will be dominant. In such cases, the effects of horizontal forces from two directions will be considered, as shown in Figure 10. Specifically, Figure 10(a) considers the effect of a horizontal force acting from one direction (the left in the figure), while Figure 10(b) considers the effect of a horizontal force acting from the other direction (the right in the figure).

[0043] In the case shown in Figure 10(a), a horizontal force is acting to the right on the transmission tower ST. As a result, an uplift load PU acts on the left tower foundation structure 100 (hereinafter specifically referred to as "left tower foundation structure 100L"). It is assumed that a particularly large uplift load PU acts on the outer part (left side in the figure) of the left tower foundation structure 100L. Therefore, in this case, the outer part (left side in the figure) of the left tower foundation structure 100L can be set as the uplift area. On the other hand, in the case shown in Figure 10(b), a horizontal force is acting to the left on the left side of the transmission tower ST. As a result, an uplift load PU acts on the right tower foundation structure 100 (hereinafter specifically referred to as "right tower foundation structure 100R"). It is assumed that a particularly large uplift load PU acts on the outer part (right side in the figure) of the right tower foundation structure 100R. Therefore, in this case, the outer part (right side in the figure) of the right tower foundation structure 100R can be set as the uplift area.

[0044] When a lifting area is defined, it is preferable to place pile block bodies 140 in the lifting area and screw piles 110 as single piles in the other areas. For example, in Figure 11, pile block bodies 140 are placed in the lifting area set in the left half of the left tower foundation structure 100L and the lifting area set in the right half of the right tower foundation structure 100R, while screw piles 110 as single piles are placed on the right side of the left tower foundation structure 100L and on the left side of the right tower foundation structure 100R. Note that if the tower foundation structure 100 is supported by the concrete foundation 120 against the compressive load associated with the power transmission tower ST, and resists the lifting load PU with the screw piles 110 and the concrete foundation 120, the screw piles 110 as single piles shown in Figure 11 can be omitted.

[0045] 2.Foundation structure construction method Next, the method for constructing the foundation structure of the present invention will be explained in detail with reference to Figures 12 to 14. Note that the method for constructing the foundation structure of the present invention is the method for constructing the tower foundation structure 100 described so far. Therefore, explanations that overlap with those described for the tower foundation structure 100 will be avoided, and the explanation will mainly focus on aspects specific to the method for constructing the foundation structure of the present invention. In other words, anything not described here is the same as what was explained in "1. Tower Foundation Structure".

[0046] Figure 12 is a flowchart showing the main steps of the foundation structure construction method of the present invention, and Figure 13 is a step diagram showing the main steps of the foundation structure construction method of the present invention. The foundation structure construction method of the present invention can be composed of "test construction" and "main construction" as shown in Figure 12. In this test construction, the planned pile spacing (the pile spacing of screw piles 110 for forming the pile block body 140) is determined, and in the main construction, the steel tower foundation structure 100 is actually constructed. Therefore, in cases where the planned pile spacing has already been determined, the test construction can be omitted. On the other hand, even in cases where the planned pile spacing has been determined, test construction can be performed if the planned pile spacing needs to be reviewed.

[0047] In the test construction, as shown in Figure 12, the pile spacing of the screw piles 110 is first provisionally set (Step 211 in Figure 12), and then multiple screw piles 110 are inserted into the ground at that provisional spacing (Step 212 in Figure 12). At this time, the screw piles 110 can be inserted into the ground at the actual construction site, or they can be inserted into the ground at a test site where conditions similar to those at the actual site have been prepared.

[0048] When multiple screw piles 110 are inserted into the ground, the multiple screw piles 110 are simultaneously pulled up (Step 213 in Figure 12). If it is not confirmed that the ground between the piles 130 is pulled up together with the screw piles 110 (No in Step 214 in Figure 12), the series of steps (Steps 211 to 214) are repeated after setting an even smaller pile spacing. On the other hand, if it is confirmed that the ground between the piles 130 is pulled up together with the screw piles 110 (Yes in Step 214 in Figure 12), the pile spacing at that time is determined as the planned pile spacing (Step 215 in Figure 12). Alternatively, the pile spacing can be set from small to large in Step 211, and the series of steps (Steps 211 to 214) can be repeated while changing the pile spacing to determine the maximum spacing at which the pile block body 140 is formed as the planned pile spacing.

[0049] In this construction, as shown in Figure 13(a), the ground is excavated to construct a concrete foundation 120 (Step 221 in Figure 12), and then multiple screw piles 110 are inserted into the excavated area (Step 222 in Figure 12). However, the screw piles 110 are inserted so that their heads protrude partially from the bottom of the excavation, so that their heads are fixed to the concrete foundation 120. Of course, the multiple screw piles 110 are arranged to match the planned pile spacing determined in the test construction. When inserting the screw piles 110 into the ground, for example, a hydraulic excavator equipped with a rotating device (attachment) as shown in Figure 14(a) (hereinafter referred to as "hydraulic excavator BM with rotating device") can be used. This hydraulic excavator BM with rotating device has an attachment attached to the tip of the arm of a conventional hydraulic excavator, and the screw piles 110 are connected to this attachment and then pressed into the ground while rotating the screw piles 110 using an electric or hydraulic motor. Furthermore, the rotary-equipped hydraulic excavator BM is positioned on the ground, and the screw pile 110 is inserted into the ground of the excavation area by extending its boom and arm. Therefore, if the screw pile 110 is inserted too deeply and the reach of the arm is insufficient, it is advisable to connect the screw pile 110 and the arm using an intermediate rod, as shown in Figure 14(b).

[0050] Once the screw pile 110 is inserted into the ground, the reinforcing bars RB, such as main reinforcement and distribution reinforcement, are assembled as shown in Figure 13(b), and the formwork FM for the concrete foundation 120 is assembled (Step 223 in Figure 12). At this time, the main leg members ML can also be fixed with the reinforcing bars RB. Alternatively, the formwork FM can be assembled for the upper part of the concrete foundation 120 above the footing, and the natural ground can be used for the sides of the footing, thus omitting the assembly of the formwork FM. Once the reinforcing bars RB and formwork FM are assembled, concrete is poured using the formwork FM and the natural ground as shown in Figure 13(c) (Step 224 in Figure 12), constructing the concrete foundation 120 with the heads of the screw piles 110 anchored to it. After the concrete foundation 120 is cured until it has achieved sufficient concrete strength, the formwork FM is removed, and the footing portion of the concrete foundation 120 is backfilled as shown in Figure 13(d) (Step 225 in Figure 12). [Industrial applicability]

[0051] The steel tower foundation structure and foundation construction method of the present invention can be used for various types of steel towers having multiple main legs, including power transmission towers. According to the present invention, steel towers can be constructed at low cost, which means that the construction of new steel towers will be promoted, and thus a stable supply of electricity can be provided throughout the country. Therefore, this invention is not only industrially applicable but also has the potential to make a significant contribution to society. [Explanation of symbols]

[0052] 100 Steel tower foundation structure of the present invention 100L (Left tower foundation structure of the tower foundation structure) 100R (Right tower foundation structure of the tower foundation structure) 110 Screw piles (for the foundation structure of transmission towers) 111 (The body of a screw pile) 112 (Screw pile) blade 120 Concrete foundation (for the foundation structure of a transmission tower) 130 Ground between piles (of pile block bodies) 140 (Pile block body for the foundation structure of a transmission tower) BM hydraulic excavator with rotary device FB block friction resistance FF footing friction resistance FM formwork FP pile friction resistance FS Ground Shear Resistance ML (Main leg material of a power transmission tower) PU lifting load RB rebar ST transmission tower WC basic dead weight WP pile weight WS (Sediment Self-Weight)

Claims

1. A foundation structure for supporting a steel tower having two or more main legs, A concrete foundation is provided on each of the aforementioned main leg members, The system comprises a screw pile that is inserted into the ground and then anchored to the concrete foundation, Multiple screw piles are placed in each of the aforementioned concrete foundations. The self-weight and circumferential frictional resistance of the screw pile resist the uplift load associated with the tower. A steel tower foundation structure characterized by the following features.

2. A pile block body is formed by a plurality of screw piles arranged close enough to restrain the ground between the piles, and the ground between the piles. The self-weight of the ground between the piles related to the aforementioned pile block resists the uplift load from the aforementioned steel tower. The tower foundation structure according to feature 1.

3. The pile block body is wall-shaped, cylindrical, or columnar, The shear resistance force of the ground on the wall surface of the pile block resists the uplift load by the steel tower. The tower foundation structure according to feature 2.

4. Multiple screw piles are arranged such that the spacing between them is less than three times the pile diameter. The pile diameter is the diameter including the blades of the screw pile. The tower foundation structure according to claim 2 or 3, characterized by the features described herein.

5. The pile block is formed in the area of ​​the concrete foundation where the uplift load associated with the steel tower is expected to occur. The tower foundation structure according to claim 2 or 3, characterized by the features described herein.

6. A method for constructing a tower foundation structure that supports a tower having two or more main legs, The aforementioned tower foundation structure comprises a concrete foundation provided on each of the main leg members, and a screw pile anchored to the concrete foundation. For one of the aforementioned concrete foundations, a pile insertion process is performed in which multiple screw piles are inserted into the ground, The process includes a concrete foundation forming step of forming the concrete foundation so that a portion of the screw pile is anchored, The self-weight and circumferential frictional resistance of the screw piles constituting the tower foundation structure resist the uplift load associated with the tower. A method for constructing a foundation structure characterized by the following features.

7. In the pile insertion process, a pile block body is formed by inserting the screw piles into the ground such that the multiple screw piles are positioned close enough to restrain the ground between the piles, thereby forming a pile block body consisting of the multiple screw piles and the ground between the piles. The self-weight of the ground between the piles related to the pile block body constituting the foundation structure of the steel tower resists the uplift load by the steel tower. The method for constructing a foundation structure according to claim 6, characterized by its features.

8. The process involves inserting the screw piles into the ground while varying the pile spacing, followed by a lifting test process in which the screw piles are pulled up. The pile spacing determination step further comprises determining the pile spacing as the planned pile spacing, which is confirmed by the lifting test step to be able to restrain the ground between the piles, In the pile insertion step, the screw piles are inserted into the ground so that the multiple screw piles are arranged at the planned pile intervals. The method for constructing a foundation structure according to claim 7, characterized by its features.

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