How to unload an existing ground anchor

Abstract

To provide a method for unloading an existing ground anchor capable of relatively easily and safely performing unloading work without requiring special skill.SOLUTION: The method for unloading the existing ground anchor includes the steps of: demolishing protective concrete 11 covering and protecting an anchor head protruding from a slope 7 to expose the anchor head; and releasing a tension applied to an anchor by moving a steel pedestal 2 in a slope ground direction by the tension of the anchor by demolishing the concrete that forms the slope 7 on the back of a base plate 2a of the steel pedestal 2 that is fixed to the slope 7 and supports the anchor head.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to a method for unloading an existing ground anchor, and more specifically to a method for unloading an existing ground anchor that easily and safely unloads the PC steel rod that serves as the tendon of an existing ground anchor that remains in a tensioned state, i.e., releases the remaining tensile force. [Background technology]

[0002] Ground anchor construction involves inserting prestressing steel rods (tensioning materials) into relatively small drilled holes and injecting grout to anchor them in the ground. The tensile strength of the prestressing steel rods is used to resist sliding forces such as landslides. As shown in Figure 1, the anchors used in ground anchor construction consist of a prestressing steel rod (tendon) 1 inserted into a drilled hole 8 drilled into the ground 9 through a precast slope 7, and a tensioning device that applies tension to the prestressing steel rod 1 by pulling the end of the prestressing steel rod 1 protruding from the drilled hole 8. For example, the tensioning device consists of an angled steel base 2 with a triangular side, a bearing plate 3 that is in surface contact with the slope of the steel base 2 and supports the head of the prestressing steel rod 1 by inserting it through, the threaded head of the prestressing steel rod 1 protruding from the bearing plate 3, and a nut 4 that is tightened onto the threaded head of the prestressing steel rod 1 after tensioning it.

[0003] The PC steel rod 1 is tensioned when tension is applied to it with the base plate 2a of the steel base 2 in close contact with the slope 7, and after the nut 4 is tightened, the PC steel rod 1 tries to contract and enters a tensioned state. The anchor head exposed from the slope 7 is covered with protective concrete 11 to prevent the anchor from popping out if the PC steel rod 1 corrodes and breaks due to deterioration caused by external corrosion, etc.

[0004] However, existing ground anchors are of an older type that was used before double corrosion protection of the anchor body became mandatory, and there is a risk that the anchor member (PC steel rod 1) will corrode and fracture. If the PC steel rod 1 corrodes and fractures, there is a risk that the protective concrete 11 covering the anchor head will break and fall from the slope. When carrying out repair work to prevent such a falling accident from occurring, it is necessary to perform an unloading process to release the tension on the PC steel rod 1 of the existing ground anchor. Conventional unloading methods include loosening the nut by lift-off and unloading by gas cutting the anchor head.

[0005] The lift-off method of loosening the nut involves using a jack to pull up the anchor head and loosen the nut, thereby removing the load. However, there is no guarantee of safety against the PC steel rod breaking when loosening the nut. In addition, a crane is required to move the equipment used to lift up the anchor head. Furthermore, because this is specialized work, it is difficult to secure workers, which limits the number of construction companies that can be commissioned to carry out the work.

[0006] On the other hand, the gas cutting method involves unloading the anchor head (the underside of the nut) by cutting it with a burner, but because the length of the PC steel rod that protrudes when cut is unknown (unpredictable), a device is required to prevent this protrusion, which takes time to design, and again, there is a problem in that the number of construction companies that can be commissioned with this method is limited.

[0007] In addition to the above methods, other methods have been proposed, such as using a specially shaped unloading coupler (Patent Publication No. 6311056) and a method in which a compression grip is crimped onto the tip of the PC steel rod to create a reduced diameter section in the PC steel rod, and then this reduced diameter section is cut off (Patent Publication No. 9-53232).However, these are all special construction methods and do not provide sufficient consideration to safety. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Patent No. 6311056 [Patent Document 2] Japanese Patent Application Publication No. 9-53232 Summary of the Invention [Problem to be solved by the invention]

[0009] As mentioned above, conventional methods for unloading existing ground anchors have had problems in that the safety of the work is not ensured, it is difficult to secure workers because it is special work, and the number of contractors available is limited. Therefore, an object of the present invention is to provide a method for unloading existing ground anchors that does not have such problems, that is, that does not require special skills and allows unloading work to be performed relatively easily and safely. [Means for solving the problem]

[0010] The invention described in claim 1 for solving the above problem includes the steps of: demolishing protective concrete that covers and protects an anchor head protruding from a slope made of concrete, thereby exposing the anchor head; and demolishing the concrete that constitutes the slope behind the base plate of a steel base that is fixed to the slope and supports the anchor head, thereby moving the steel base a predetermined amount in the direction of the slope ground by the tension of the anchor, thereby releasing the tension applied to the anchor. the predetermined amount corresponds to an amount of expansion and contraction of the anchor, and the amount of expansion and contraction of the anchor is calculated based on the residual tensile force of the anchor on the basis of elastic displacement theory; A line marking is made at a position corresponding to the predetermined distance from the lower end of the side surface of the base plate of the steel pedestal. This is a method for unloading an existing ground anchor.

[0012] In one embodiment, the slope on the back surface of the base plate of the steel pedestal is removed by manual chipping.

[0013] In one embodiment, the chopping operation is performed using a jackhammer. [Effects of the Invention]

[0014] The method for unloading an existing ground anchor according to the present invention is as described above, and has the effect of enabling unloading work to be carried out relatively easily and safely without requiring special skills. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 2 is an exploded view showing the configuration of an existing ground anchor. [Figure 2] FIG. 1 is a perspective view showing the first step (cutting open the net cover) of the method for unloading an existing ground anchor according to the present invention. [Figure 3] FIG. 10 is a perspective view showing the second step (dismantling the protective concrete) of the method for unloading an existing ground anchor according to the present invention. [Figure 4] FIG. 10 is a perspective view showing the third step (exposing the anchor head) of the method for unloading an existing ground anchor according to the present invention. [Figure 5] FIG. 10 is a perspective view showing the fourth step (removing the slope on the back surface of the base plate of the steel pedestal) of the method for unloading an existing ground anchor according to the present invention. [Figure 6] FIG. 10 is a perspective view showing a fifth step (completion of unloading) of the method for unloading an existing ground anchor according to the present invention. [Figure 7] FIG. 10 is a perspective view showing the sixth step (removing the anchor head) of the method for unloading an existing ground anchor according to the present invention. [Figure 8] FIG. 10 is a perspective view showing the seventh step (filling mortar) of the method for unloading an existing ground anchor according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0016] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention provides a method for unloading an existing ground anchor, comprising the steps of: demolishing protective concrete covering and protecting an anchor head protruding from a slope to expose the anchor head; and demolishing the concrete constituting the slope behind the base plate of a steel base fixed to the slope and supporting the anchor head, thereby moving the steel base in the direction of the slope ground by the tension of the anchor, thereby releasing the tension applied to the anchor.

[0017] As described above, the existing ground anchor consists of a PC steel rod 1 that is inserted into a borehole 8 that has been drilled from the slope 7 to the ground 9 and is fixed to the ground 9 by injecting grout, an angled steel base 2 with a triangular side that supports the head of the PC steel rod 1 that protrudes from the borehole 8, a bearing plate 3 that makes surface contact with the sloped part of the steel base 2 and supports the head of the PC steel rod 1 that is inserted through it, and a nut 4 that is screwed onto the threaded part of the head of the PC steel rod 1 that protrudes from the bearing plate 3 (Figure 1).

[0018] The PC steel rod 1 is tensioned when tension is applied to it with the base plate 2a of the steel pedestal 2 in close contact with the slope 7, and the nut 4 is tightened via the bearing plate 3, causing the rod to contract and enter a tensile state. The anchor head exposed from the slope 7, i.e., the steel pedestal 2, bearing plate 3, nut 4, and the head thread of the PC steel rod 1 protruding from the nut 4, are then covered with protective concrete 11 to prevent it from popping out due to a break in the PC steel rod 1. Additionally, to prevent the anchor from popping out in the unlikely event that it does, a strip-shaped metal plate 10 is placed in a cross shape over the protective concrete 11 (see Figure 2). A net cover 12 may also be placed on top of this to prevent pieces of concrete from sliding off.

[0019] Next, the method for unloading an existing ground anchor according to the present invention will be described in detail in the order of steps with reference to the accompanying drawings.

[0020] Protective concrete demolition process In the method for unloading an existing ground anchor according to the present invention, first the net cover 12 is cut open to expose the installation location (Fig. 2), and the protective concrete 11 covering the anchor head protruding from the slope 7 is demolished to expose the head of the PC steel rod 1 that constitutes the anchor head, the nut 4 screwed into the head thread of the PC steel rod 1, the bearing plate 3 that receives the nut 4, and the steel base 2 that receives the bearing plate 3 (see Figs. 2 to 4).

[0021] Typically, metal strips 10 are placed in a cross shape on top of the protective concrete 11 in case the anchors pop out, but the work of demolishing the protective concrete 11 is carried out through the gaps between the metal strips 10, leaving them in place (see Figure 3). Furthermore, considering the possibility of the anchors popping out, the work is carried out from the side rather than from the front of the anchors (see Figure 3). The work of demolishing the protective concrete 11 is carried out by hand, for example, using a jackhammer 21. Therefore, the unloading method of the present invention can be carried out by anyone who can operate a jackhammer 21, even if they do not have special expertise, and can therefore be used in general slope construction work.

[0022] Removal of the slope behind the base plate of the steel pedestal After demolishing the protective concrete 11 to expose the anchor head (Fig. 4), the concrete that makes up the slope 7 on the back of the base plate 2a of the steel pedestal 2 is demolished. This work is also carried out using a jackhammer 21, cutting out the concrete to a uniform depth from the periphery of the base plate 2a of the steel pedestal 2 to the back (see Fig. 5).

[0023] This cutting work is carried out so as to reduce the tension calculated in the cutting calculation. This cutting calculation is to find the amount of expansion and contraction of the PC steel rod 1 from the remaining tensile force of the PC steel rod 1. As an example, if the PC steel rod 1 is a Gebinde Stab with a diameter of 26 mm and a total tendon length of 8.80 m and the following specifications are used, the calculation will be as follows. Pe: Residual tensile force 338.00kN To: Initial load 0.00kN lsa: Tendon anchorage length 4.00m lsf: Tendon free length 4.50m Es: Young's modulus 235.00kN / mm2 As: Tendon cross-sectional area 530.90mm2

[0024] <Theoretical elastic displacement in design> δet=(Pe-To)*lsf / (Es*As) δet=12.19mm <Upper limit of allowable elastic displacement> δet'=1.1*δet δet' =13.41mm <Lower limit of allowable elastic displacement> δet" =0.9*δet δet" =10.97mm

[0025] Of the amounts of expansion and contraction (displacement) calculated using the three methods above, the maximum allowable upper limit elastic displacement is considered to be the amount of expansion and contraction (displacement), and the amount of expansion and contraction (displacement) is set to approximately 15 mm. At the site, before the manual cutting work, a red line or other marking is made 15 mm from the bottom end of the side of the base plate 2a of the steel pedestal 2. 5 Put (See Figure 4) Then, the concrete that makes up the slope 7 is chipped away almost uniformly from the periphery of the base plate 2a by hand, and as the chipping progresses, the steel base 2 is pulled in the direction of the slope ground by the tension (force that tries to shrink) of the PC steel rod 1, and moves. After the chipping progresses, a scale is used to check whether the calculated movement distance has been reached (line markings on the side of the base plate 2a). 5 coincides with the plane of slope 7.) Once this is confirmed, it is determined that the tension has been released and unloading is complete, and the manual cutting work is finished.

[0026] Anchor head removal process Once the unloading is completed by the above work, the nuts 4 will loosen, so remove the nuts 4 and bearing plate 3, and then remove the steel base 2 (see Figure 6). After that, the heads of the PC steel bars 1 protruding from the slope 7 and the ends of the metal fitting fixing bolts 13 that fixed the strip metal plate 10 are cut off (see Figure 7), and the space is filled with dry mortar 14 (see Figure 8). [Industrial Applicability]

[0027] The method for unloading an existing ground anchor according to the present invention is as described above, and has the effect of allowing unloading work to be carried out relatively easily and safely without requiring special skills, and therefore has great industrial applicability. [Explanation of symbols]

[0028] 1 PC steel bar 2 Steel pedestal 2a Base Plate 3 Support plate 4 nuts 7 Slope 8 Drilling 9 Ground 10 Metal strip 11 Protective Concrete 12 Net cover 13 Bracket fixing bolt 21 Jackhammer

Claims

1. The method includes the steps of: demolishing protective concrete that covers and protects an anchor head protruding from a slope that is made of concrete, thereby exposing the anchor head; and demolishing the concrete that constitutes the slope on the back side of a base plate of a steel base that is fixed to the slope and supports the anchor head, thereby moving the steel base a predetermined amount in the direction of the slope ground by the tension of the anchor, thereby releasing the tension applied to the anchor. the predetermined amount corresponds to an amount of expansion and contraction of the anchor, and the amount of expansion and contraction of the anchor is calculated based on the residual tensile force of the anchor on the basis of elastic displacement theory; A method for unloading an existing ground anchor, characterized in that a line marking is made at a position corresponding to the predetermined amount from the lower end of the side surface of the base plate of the steel pedestal.

2. 2. The method for unloading an existing ground anchor according to claim 1, wherein the slope on the back surface of the base plate of the steel pedestal is removed by manual chipping.

3. 3. The method for unloading an existing ground anchor according to claim 2, wherein the manual chipping work is carried out using a rock drill.

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