JP7582129B2 - Reinforcement structure for existing tower foundations - Google Patents

Description

This invention relates to a reinforcement structure for an existing steel tower foundation that has been found to lack compressive strength.

Generally, in the design of the foundations of existing power transmission towers, when it is found that the existing tower foundation is not strong enough to withstand compressive loads, the compressive bearing capacity of the existing tower foundation is increased to ensure the safety factor of the existing tower foundation against compressive loads.

Figure 4 shows a conventional reinforcement structure for an existing tower foundation 101, and is a diagram showing a mat-type pile foundation 102 that is used to reinforce the existing tower foundation 101. This mat-type pile foundation 102 is formed by integrating the existing tower foundation 101 (four inverted T-shaped concrete foundations 103) with the mat-type pile foundation 102, and multiple piles 104 integrated with the mat-type pile foundation 102 increase the compressive bearing capacity. The construction method for constructing the mat-type pile foundation 102 on the existing tower foundation 101 is disclosed in Patent Document 1.

JP 2002-256573 A

When driving piles 104 into the soil 106 on a site where existing facilities exist, the mat-type pile foundation 102 shown in Figure 4 has the problem that it takes time to carry out preparatory work such as confirming the position of the foundation floor plate 105 that has already been buried in the ground, and the piles 104 cannot be positioned efficiently due to headroom restrictions (restrictions on aboveground space to prevent piles to be driven into the ground from coming into contact with overhead wires or cross arms that support the overhead wires).

The present invention aims to efficiently ensure the safety factor against compressive loads of existing steel tower foundations without driving new piles into the ground.

The present invention relates to a reinforcement structure for an existing tower foundation 2, in which an inverted T-shaped concrete foundation 6 corresponding one-to-one to each of the four legs 5 of a tower 1 is buried in soil 8, and one of the four legs 5 of the tower 1 is supported by the inverted T-shaped concrete foundation 6. The present invention replaces at least a part of the soil 8 directly above the foundation floor plate 7 of the existing tower foundation 2, on which a compressive load acts, with a lightweight embankment 10 that is lighter than the soil 8, lays a buoyancy-suppressing concrete 11 on the lightweight embankment 10, suppresses the lifting of the lightweight embankment 10 due to the buoyancy acting on the lightweight embankment 10 with the weight of the buoyancy-suppressing concrete 11, and determines the weight of the lightweight embankment 10 so that the safety factor of the existing tower foundation 2 is at least 2.00, thereby reducing the compressive load acting on the foundation floor plate 7. The safety factor (Fc) is determined as follows, in accordance with the concept of Article 60 of the Technical Standard for Electrical Equipment and the Design Standard for Power Transmission Supports.
Fc=(QC+KC)/(C+WC+WS+WS3)≧2.00
QC: compressive bearing capacity of the soil KC: bearing capacity of the pile 17 C: compressive force from the steel tower 1 WC: weight of concrete WS: weight of the soil 8 directly above the foundation floor plate 7 WS3: weight of the lightweight embankment 10

The present invention can efficiently ensure the safety factor against compressive loads of an existing tower foundation by making at least a portion of the soil directly above the foundation slab of the existing tower foundation, on which compressive loads act, into lightweight embankment that is lighter than the soil, without driving new piles into the ground.

1(a) is a front view of the tower, and FIG. 1(b) is a plan view of the tower as viewed from the direction of arrow C in FIG. 1(a) (plan view showing the state in which a load is applied). 2(a) and 2(b) are diagrams showing the reinforcing structure of an existing tower foundation, in which FIG. 2(a) is a plan view showing the reinforcing structure of an existing tower foundation, and FIG. 2(b) is a cross-sectional view taken along line A1-A1 in FIG. 2(a). FIG. 3(a) is an enlarged cross-sectional view of part B in FIG. 2(b), and FIG. 3(b) is a diagram showing a modified example of the reinforcing structure of the existing tower foundation shown in FIG. 3(a). 4(a) and 4(b) are diagrams showing a conventional reinforcement structure for an existing tower foundation, in which FIG. 4(a) is a plan view showing a conventional reinforcement structure for an existing tower foundation, and FIG. 4(b) is a cross-sectional view taken along line A2-A2 in FIG. 4(a).

The following describes an embodiment of the present invention in detail with reference to the drawings.

Figure 1 shows a simplified view of a steel tower 1, with Figure 1(a) being a front view of the steel tower 1 and Figure 1(b) being a plan view of the steel tower 1 as viewed from the direction of arrow C in Figure 1(a) (plan view showing the state in which a load is applied). Figure 2 shows the reinforcing structure of an existing steel tower foundation 2. In Figure 2, Figure 2(a) is a plan view showing the reinforcing structure of an existing steel tower foundation 2, and Figure 2(b) is a cross-sectional view taken along line A1-A1 in Figure 2(a). Figure 3(a) is an enlarged cross-sectional view of part B in Figure 2(b), and Figure 3(b) is a diagram showing a modified example of the reinforcing structure of an existing steel tower foundation 2 shown in Figure 3(a).

As shown in Figure 1, for example, when the transmission line 4 is erected at an angle smaller than 180° (θ<180°) on the cross arm 3a (3b, 3c) of the tower 1, a horizontal angle load P (load P toward the left in Figure 1) due to the tension of the transmission line 4 acts on the tower 1. If the four legs 5 of the tower 1 are legs a, b, c, and d in the clockwise direction, this horizontal angle load P acting on the tower 1 generates a compressive load on legs b and c and a lifting load on legs a and d. As a result, a larger compressive load acts on the existing tower foundations 2 of legs b and c of the tower 1 compared to the existing tower foundations 2 of legs a and d. The compressive load acting on the existing tower foundations 2 of legs b and c is the sum of the tower load (the sum of 1/4 of the tower 1's own weight, the compressive load caused by the horizontal angle load P, 1/2 of the weight of the power lines 4 between the adjacent towers 1, 1, and the compressive load caused by the crosswind blowing on the tower 1), the weight of the inverted T-shaped concrete foundation 6, the weight of the soil, etc. (soil 8, lightweight embankment 10 described below, and buoyancy-suppressing concrete 11) located on top of the foundation floor plate 7 of the inverted T-shaped concrete foundation 6, and the underwater concrete 12 (see Figures 2 and 3(a)).

In the case of a steel tower 1 as shown in Figure 1, if it is found that the existing steel tower foundation 2 of legs b and c does not have sufficient strength to withstand the compressive load, it will be difficult to reduce the weight of the steel tower and the inverted T-shaped concrete foundation 6, so the weight of the soil 8 located on top of the foundation floor plate 7 will be reduced (see Figures 2 and 3(a)).

(Reinforcement structure for existing tower foundation)
As shown in Figures 2 and 3(a), the reinforcing structure of the existing tower foundation 2 according to this embodiment is such that a part of the soil 8 located directly above the foundation floor plate 7 of the inverted T-shaped concrete foundation 6 (within a range partitioned by an imaginary line 14 extending the outer edge of the foundation floor plate 7 upward) is replaced with a lightweight embankment 10, and buoyancy-suppressing concrete 11 is laid on the lightweight embankment 10. The buoyancy-suppressing concrete 11 is placed on the lightweight embankment 10 to suppress the lightweight embankment 10 from floating up due to buoyancy caused by moisture in the soil 8. The reinforcing structure of the existing tower foundation 2 according to this embodiment is not limited to that shown in Figures 2 and 3(a), and as shown in Figure 3(b), a roadbed 13 may be laid on the buoyancy-suppressing concrete 11 so that heavy machinery and the like can move on the roadbed 13.

As shown in FIG. 3(b), in the modified reinforcement structure of the existing tower foundation 2 according to this embodiment, soil 8 is placed on the foundation slab 7 of the inverted T-shaped concrete foundation 6, lightweight embankment 10 is placed on the soil 8, buoyancy-suppressing concrete 11 is placed on the lightweight embankment 10, and roadbed 13 is placed on the buoyancy-suppressing concrete 11. In the modified reinforcement structure of the existing tower foundation 2 according to this embodiment, the compressive load acting on the existing tower foundations 2 of legs b and c is the sum of the weight of the roadbed 13 and the weight of the root-wrapped concrete 15 in addition to the compressive load in the reinforcement structure of the existing tower foundation 2 according to this embodiment.

In Figures 3(a) and 3(b), the existing tower foundation 2 of legs b and c of the tower 1 is designed to support a compressive load with soil 8 located below the bottom surface 7a of the foundation floor plate 7 of the inverted T-shaped concrete foundation 6, with underwater concrete 12 and pebbles 16 between them, and with multiple piles 17 extending downward into the ground from the foundation floor plate 7 of the inverted T-shaped concrete foundation 6.

As shown in FIG. 3(a), the existing tower foundation 2 of the a-leg and the d-leg in this embodiment is designed to withstand the lifting load of the weight of the soil 8 directly above the foundation floor plate 7 of the inverted T-shaped concrete foundation 6, the weight of the lightweight embankment 10, and the weight of the buoyancy-suppressing concrete 11. As shown in FIG. 3(b), the existing tower foundation 2 of the a-leg and the d-leg in this embodiment is designed to withstand the lifting load of the weight of the soil 8 directly above the foundation floor plate 7 of the inverted T-shaped concrete foundation 6, the weight of the lightweight embankment 10, the weight of the buoyancy-suppressing concrete 11, the weight of the roadbed 13, and the weight of the root-wrapped concrete 15. In addition, in FIG. 3(a) and FIG. 3(b), the underwater concrete 12 is located in contact with the bottom surface 7a of the foundation floor plate 7 of the inverted T-shaped concrete foundation 6, and the pebbles 16 are located in contact with the underwater concrete 12, and the soil 8 supporting the compressive load is located adjacent to the pebbles 16.

(Thickness of buoyancy-reducing concrete)
In the reinforcing structure for the existing steel tower foundation 2 of this embodiment shown in Figures 2(b) and 3(a), as described above, the buoyancy-suppressing concrete 11 is intended to suppress the buoyancy acting on the lightweight embankment 10. If the weight of this buoyancy-suppressing concrete 11 per cubic meter is 2.3 g (kN/ ^m3 ) and the buoyancy per square meter acting on the lightweight embankment 10 is 1.0 g (kN/ ^m2 ), the thickness (t1) is calculated by the following formula 1.
(Equation 1)
t1 = 1.0 · g (kN/m ² ) / 2.3 · g (kN/m ³ ) = 0.44 m ... Formula 1

That is, if the buoyancy suppressing concrete 11 has a thickness of 0.44 m or more, it can resist the buoyancy acting on the lightweight embankment 10 and suppress the floating of the lightweight embankment 10. In this embodiment, the lightweight embankment 10 uses a waste glass foam material with high weather resistance. Since the lightweight embankment 10 has some water absorption, it is not expected that the buoyancy of the lightweight embankment 10 will be 1.0·g (kN/m ² ). However, to be on the safe side, the buoyancy suppressing concrete 11 is set to 1.0·g (kN/m ² ) and used as the base value for calculating the thickness (t1) of the buoyancy suppressing concrete 11. The lightweight embankment 10 is not limited to the waste glass foam material, and high-density polystyrene foam, resin storage material, etc. may be used as appropriate depending on the conditions of the construction site of the steel tower 1.

A modified example of the reinforcement structure of the existing steel tower foundation 2 shown in FIG. 3(b) is a structure in which a roadbed 13 is laid on the buoyancy-suppressing concrete 11 shown in FIG. 3(a). In this modified example shown in FIG. 3(b), the roadbed 13 is laid with a thickness of 0.15 m and high-strength concrete with a weight of 2.4 g (kN/m ³ ) per cubic meter. In this modified example, the thickness (t1) of the buoyancy-suppressing concrete 11 is calculated as follows. In FIG. 3(b), the upper end side of the inverted T-shaped concrete foundation 6 is covered with root-wrapped concrete 15. This root-wrapped concrete 15 is arranged from the buoyancy-suppressing concrete 11 to a position protruding above the roadbed 13, and is designed to prevent water from puddling between the upper end 6a of the inverted T-shaped concrete foundation 6 and the upper surface 13a of the roadbed 13.

In such a modified example of the reinforcing structure of the existing tower foundation 2 shown in FIG. 3( b ), the weight per square meter (kN/m ² ) of the roadbed 13 is calculated by the following Equation 2.
(Number 2)
0.15(m)×2.4・g(kN/m ³ )=0.36・g(kN/m ² )...Formula 2

In the modified example of the reinforcing structure of the existing tower foundation 2 shown in FIG. 3(b), the thickness (t1) of the buoyancy-reducing concrete 11 is calculated by the following Equation 3.
(Equation 3)
t1 = (1.0 g (kN/m ² ) - 0.36 g (kN/m ² )) / 2.3 g (kN/m ³ ) = 0.28 m ... Formula 3

In other words, in the modified reinforcement structure of the existing tower foundation 2 shown in Figure 3 (b), if the buoyancy-suppressing concrete 11 has a thickness t1 of 0.28 m or more, it can resist the buoyancy acting on the lightweight embankment 10 and suppress the floating of the lightweight embankment 10.

(Volume of lightweight embankment)
In the reinforcing structure of the existing tower foundation 2 according to this embodiment shown in Fig. 3(a) and the modified example of the reinforcing structure of the existing tower foundation 2 shown in Fig. 3(b), the range of placement of the lightweight embankment 10 is directly above the foundation floor plate 7 of the inverted T-shaped concrete foundation 6. The weight of the lightweight embankment 10 is determined so that the safety factor (Fc) is at least 2.00 in Equation 4 created in accordance with "Article 60 of the Interpretation of Technical Standards for Electrical Equipment" and "Design Standard for Power Transmission Support Structures (JEC-127)."
(Equation 4)
Fc = (QC + KC) / (C + WC + WS + WS3) ≧ 2.00 ... Formula 4
Assuming Fc=2.00, the formula 4 is transformed to find the weight of the lightweight embankment (WS3) 10.
(Equation 5)
WS3 = (QC + KC) / 2.00 - (C + WC + WS) ... Formula 5
WS3: Weight of lightweight embankment 10 (kN)
QC: Compressive bearing capacity (kN)
KC: bearing capacity of pile 17 (kN)
C: Compressive force from tower 1 (kN)
WC: concrete weight (kN)
WS: Weight of the soil 8 directly above the foundation floor plate 7 (kN)
Here, if the weight per unit volume of the lightweight embankment 10 is γ·g (kN/m ³ ), the volume VS3 (m ³ ) of the lightweight embankment 10 can be calculated by Equation 6.
(Equation 6)
VS3=WS3/γ·g Formula 6
The lightweight embankment 10 with the volume (VS3) calculated by the above formula 6 is used to replace a part (or all) of the soil 8 directly above the foundation floor plate 7 of the inverted T-shaped concrete foundation 6.

(Effects of the embodiment)
As described above, the reinforcement structure for the existing tower foundation 2 according to this embodiment can efficiently ensure the safety factor against the compressive load of the existing tower foundation 2 by making at least a part (part or all) of the soil 8 directly above the foundation floor plate 7 of the existing tower foundation 2, on which the compressive load acts, into lightweight embankment 10 that is lighter than the soil 8, without driving new piles into the ground. Therefore, compared to the reinforcement structure for the existing tower foundation 2 according to this embodiment of the present invention, which uses the conventional mat-type pile foundation 102 to reinforce the existing tower foundation 101, the reinforcement structure for the existing tower foundation 2 according to this embodiment does not require time for preparatory work such as checking the foundation floor plate 7 of the existing tower foundation 2, and there is no restriction on work due to headroom restrictions, so that it is possible to improve work efficiency and significantly reduce construction costs.

In addition, the reinforcing structure of the existing tower foundation 2 according to this embodiment is applied only to the existing tower foundation 2 on which a compressive load acts, and is not applied to the existing tower foundation 2 on which a lifting load acts, so that the areas to be worked on can be narrowed down. As a result, the reinforcing structure of the existing tower foundation 2 according to this embodiment can narrow the scope of work compared to the reinforcing structure of the existing tower foundation 101 using the conventional mat-type pile foundation 102, and can shorten the construction period and power outage time.

In addition, the reinforcing structure for the existing tower foundation 2 in this embodiment does not use special construction methods such as those used for the conventional mat-type pile foundation 102, so it can be constructed using conventional construction methods such as excavation, backfilling, compaction, and leveling, making construction easier than with the reinforcing structure for the existing tower foundation 101 using the conventional mat-type pile foundation 102.

1... Tower, 2... Existing tower foundation, 5... Leg, 6... Inverted T-shaped concrete foundation, 7... Foundation slab, 8... Soil, 10... Lightweight embankment, 11... Buoyancy-reducing concrete, 17... Pile

Claims (3)

In a reinforcement structure for an existing steel tower foundation, an inverted T-shaped concrete foundation is buried in the soil in one-to-one correspondence with each of the four legs of a steel tower, and one of the four legs of the steel tower is supported by the inverted T-shaped concrete foundation.
at least a portion of the soil directly above the foundation floor plate of the existing tower foundation on which a compressive load acts is replaced with lightweight embankment that is lighter than the soil, buoyancy-suppressing concrete is laid on the lightweight embankment, the lifting of the lightweight embankment due to buoyancy acting on the lightweight embankment is suppressed by the weight of the buoyancy-suppressing concrete, and the weight of the lightweight embankment is determined so that the safety factor of the existing tower foundation is at least 2.00, thereby reducing the compressive load acting on the foundation floor plate,
The safety factor (Fc) is determined according to the following formula in accordance with Article 60 of the Technical Standard for Electrical Equipment and the Standard for Design of Support Structures for Power Transmission:
Fc=(QC+KC)/(C+WC+WS+WS3)≧2.00
A reinforcement structure for an existing tower foundation characterized by the following: QC: compressive bearing capacity of the ground KC: bearing capacity of the pile C: compressive force from the tower WC: weight of concrete WS: weight of the soil directly above the foundation floor slab WS3: weight of the lightweight embankment The lightweight embankment is a waste glass foam material.
The reinforcing structure for an existing steel tower foundation according to claim 1. The soil is positioned on the foundation floor plate, the lightweight embankment is positioned on the soil, the buoyancy-reducing concrete is positioned on the lightweight embankment, and the roadbed is positioned on the buoyancy-reducing concrete.
The reinforcing structure for an existing steel tower foundation according to claim 1 or 2.