JP2022144694A - Tensile dispersion type ground anchor reinforcing method - Google Patents

Abstract

To provide a reinforcing method to rationally obtain location requiring reinforcement material for a tensile dispersion type anchor.SOLUTION: In a tensile dispersion type ground anchor reinforcing method, a plurality of tendons 1A, 1B, 1C formed of PC steel twisted wire is inserted in a borehole; unbonded portion of the tendons coated with PE on an anchor head 10 side is free length 1a to 1c; portion of the tendons not coated with PE on depth side of the borehole is anchorage length 2a to 2c; length is different for each free length; and a stress dispersion portion is formed by disposing a plurality of boundary locations between the free length and the anchorage length of the tendons so as to be displaced from each other at a prescribed dispersion interval in longer direction. Reinforcement material is disposed at a portion at which stress degree exceeds a prescribed acceptable value (Fa) from a stress distribution generated in a grout at the anchor body length portion by tension anchoring of the tendons after hardening of the grout G injected in the borehole.SELECTED DRAWING: Figure 1

Description

The present invention relates to a reinforcement method for a tension distribution ground anchor.

Many methods of tension distribution ground anchors have been proposed as shown in prior art documents.
A ground anchor is abbreviated as an anchor below.
In conventional tension anchors, the unbonded portion is called the anchor free length portion, and the bonded portion is called the anchor body length portion. In this specification, the unbonded portion of each tendon is called the free length, and the bonded portion is called the fixed length. The boundary position between the free length and the fixed length that is closest to the anchor head side among the multiple tendons contained in the entire anchor is defined as the boundary position between the anchor free length part and the anchor body length part, and the anchor body length on the anchor head side A portion where the boundary positions between the free length and the fixed length of each tendon are sequentially shifted from the end of the portion is called a stress dispersion portion.
FIG. 2 of prior art document 1 (Japanese Patent No. 2655820) shows a basic conventional tension anchor. (See FIG. 7 (1).)
A tension anchor is composed of an anchor head, a tension part (anchor free length) and an anchor body (anchor body length). The anchor body is integrally formed by the adhesion force between the tendon (tension material) and the grout, and transmits the tensile force (tension force) from the tensile portion to the ground by frictional resistance with the installation ground. When a tensile force (tensile force) is applied to the anchor body, the stress generated in the grout is concentrated near the boundary position between the anchor free length portion (11) and the anchor body long portion (12) and reaches the maximum stress level. b, which often exceeds the design value a of the anchor body resistance (pull-out shear resistance). In order to solve this phenomenon, as shown in FIG. 1 of Prior Art Document 1, by using a plurality of tension steel materials with different lengths, the stress generated near the boundary between the free length and the fixed length of each tension steel material is reduced. A tension dispersion type anchor was proposed in which the stress is dispersed in the depth direction so that the maximum stress does not exceed the design value. (See FIG. 7 (2).)

In Patent Document 2 (Japanese Patent No. 2847054), in order to prevent a large stress from being locally concentrated in the anchor body and to equalize the stress distribution over the entire length of the anchor body, each strain steel is free-stretched. The boundary position between the length and the fixing length is shifted to form a so-called tension dispersion type, and a cylindrical body formed by spirally winding a wire without any gaps to prevent the occurrence of harmful cracks in the anchor body or A steel tube surrounds the tension steel to form an anchoring length. By doing so, the strength of the grout in the cylinder is increased by the restraining effect of the cylinder or the steel pipe, and the pull-out resistance of the anchor body is increased because the cylinder in the free length portion expands outward by the Poisson's ratio. can. In short, cracking of the grout is prevented by providing the restraining steel on the outer periphery of the tension steel over the entire length from the free length to the fixed length.

In Patent Document 3 (Japanese Patent No. 2847043), similar to the technique of Patent Document 2, a steel pipe is provided on the outer circumference of the tension steel material over the entire length of the anchoring length to constrain the anchor body, whereby the grout in the steel pipe is fixed. The anchoring force is increased by averaging the generated stress to prevent cracks from occurring in the grout portion, and by integrating the tension steel material and the grout in the steel pipe.
Prior Art Document 4 (Japanese Patent No. 3860941) proposes a differential tensioning method and a difference method as tensioning methods for tension dispersion anchors.
Various fixing methods have been proposed for ground anchors, but those not directly related to the reinforcing method of the present invention are excluded from the following description.

Japanese Patent No. 2655820 Japanese Patent No. 2847054 Japanese Patent No. 2847043 Japanese Patent No. 3860941

In conventional tension (concentrated) anchors, as shown in FIG. (peak), and becomes a wavy shape that becomes smaller as it goes in the depth direction of drilling.
However, in the current anchor design method, the fixing length of the long anchor body is to be confirmed by calculating the design value a, which is the pull-out shear resistance averaged over the entire length of the anchor body length. As mentioned above, when the value calculated by this design method is compared with the actual stress distribution, the maximum value of the stress degree (peak) near the boundary between the free length and the anchorage length exceeds the grout design allowable value. Therefore, it cannot be said to be a rational design method.

In the tension distribution anchor shown in Patent Document 1 (Japanese Patent No. 2655820) (see FIG. 7 (2)), the free lengths (1a, 1b, 1c) of the first, second and third tendons and By displacing the boundary position between the anchorage lengths (2a, 2b, 2c) by the length of the anchorage length of the adjacent tendons, the stress generated in the grout is dispersed, and the free length of each tendon and the The maximum stress (wavy peaks) near the boundary with the anchoring length became smaller, and a rational stress distribution was formed.
However, displacing the boundary position between the free length and the anchorage length of each tendon by the anchorage length 2 (2a, 2b, 2c) in the longitudinal direction may increase the overall length of the anchor in some cases. Therefore, the drilling length is longer than that of the conventional tension (concentrated) type, and the cost is increased.

Patent Document 2 (Japanese Patent No. 2847054) and Patent Document 3 (Japanese Patent No. 2847043) disclose a method for solving this problem, in which the boundary position between the free length and the fixed length of each tendon is By shifting at a predetermined interval and overlapping the fixing length of each tendon to provide a so _- called stress dispersion part L3, the length of the entire anchor is shortened and stress is dispersed compared to the conventional tensile (concentrated) type. It becomes what was done.
Furthermore, as shown in FIG. 7(3), in Patent Document 3 (Japanese Patent No. 2847043), in addition to the tension dispersion type arrangement, the free length (1a, 1b, 1c) to the entire length of the anchor body (fixing lengths 2a, 2b, 2c) is proposed to be reinforced by arranging a reinforcing member 4 made of a steel pipe or a helical cylinder.

However, in the conventional reinforcement method, it is safe to prevent cracks occurring in the grout by providing the reinforcing material 4 over the entire length of the anchor body regardless of the stress distribution and the magnitude of the stress, but it tends to be overdesigned. is. This reinforcing method requires a long reinforcing member 4, which causes a problem of high cost.
In other words, it can be said that no design method has yet been established for tension dispersion anchors that rationally and economically determines the reinforcement range of reinforcing materials and the specifications and quantities of steel materials required, even though the stress is dispersed.

An object of the present invention is to provide a method for rationally and economically determining the range of reinforcing material and the amount of steel material according to the stress distribution situation dispersed in the grout, regarding the reinforcing method for the tension dispersion type anchor described above. and

In a tension distribution ground anchor formed by an anchor head, an anchor free length, and an anchor body length,
A plurality of tendons made of PC steel strands are inserted into the drilled hole, and the unbonded portion of each tendon is covered with PE on the anchor head side, and the back side of the drilled hole of the tendon is PE. The uncoated bond portion is defined as the fixed length, and the free lengths of the respective tendons are different, and the boundary position between the free length and the fixed length of the plurality of prestressing tendons is located in the depth direction of the drilled hole from the long portion of the anchor body. are staggered from each other at a predetermined distributed interval,
After the grout injected into the drilled hole has hardened, each of the tendons is fixed under tension to reinforce the portion where the stress degree exceeds a predetermined allowable value (Fa) from the stress distribution generated in the grout in the long portion of the anchor body. A reinforcement method for a tension distribution ground anchor characterized by placing reinforcements as areas.
Further, in the reinforcing method, the reinforcing material is made of steel, and the specifications and quantity of the reinforcing material are determined by the following formula (1).
fs×As≧σ ₀ ×A ₀ (1)
In formula (1),
fs: allowable stress level of reinforcing material As: cross-sectional area of reinforcing material σ ₀ : value exceeding a predetermined allowable value σ ₀ = σmax - Fa
σmax : Maximum value of stress in the above stress distribution (peak value)
Fa: Predetermined allowable value A ₀ : Effective cross-sectional area at the position where σ ₀ is generated A ₀ =Ah−nAp
Ah: drilling cross-sectional area at the position where σ ₀ occurs Ah = drilling diameter φ ² × π/4
Ap: Cross-sectional area of the tendon (PC steel wire) n: Quantity of the tendon (PC steel wire) in the cross section at the position where σ ₀ is generated Further, in the above reinforcement method,
The predetermined allowable value (Fa) is obtained by multiplying the design standard strength (fck) of the grout by an additional coefficient (γ), and the additional coefficient (γ) is determined according to the type of ground and is 2.0 or less. This is a reinforcement method for a tension distribution ground anchor characterized by:

The effects of the present invention are as follows.
(1) In the reinforcement method of the present invention, the portion where the magnitude of the stress generated in the grout after being dispersed exceeds a predetermined allowable value is set as the reinforcement range, and the maximum stress degree (σmax ) exceeds a predetermined allowable value (Fa) (σ _{0 =} σ max - Fa) and the product (σ ₀ × A ₀ ), it is possible to rationally and economically determine the required quantity of reinforcing materials within the required range.
(2) In the reinforcement method of the present invention, only the ridges (peaks) in the wavy stress distribution that results in the grout are reinforced, so the reinforcement material can be short, and the tendon does not enter the drilled hole. Since the insertion is simple, the workability is greatly improved, and the processing of the reinforcing material becomes simple, resulting in cost reduction.
(3) The reinforcing method of the present invention eliminates the need for reinforcing materials in the troughs of the stress distribution. It is economical because the tendon and the grout can be securely fixed and integrated even if the diameter of the anchor hole is the same as the conventional drilling diameter without enlarging the diameter of the anchor hole for inserting the reinforcing material.
(4) With respect to the stress generated in the grout, the maximum value of the generated stress is suppressed to a predetermined allowable value, and by preventing the occurrence of cracks due to excessive stress, the pull-out shear between the tendon (strand) and the grout is reduced. It secures resistance (adhesion) and eliminates the risk of the tendon being pulled out of the grout due to bond failure, making it a highly reliable and safe tension dispersion anchor.

1 is a side view, a cross-sectional view, and a stress distribution diagram of a tension distribution anchor of the present invention; FIG. Sectional drawing of the PC steel strand wire made into a tendon. The tension control chart of the present invention. FIG. 2 is a stress distribution diagram of the tension distribution anchor of the present invention. A design embodiment of the reinforcement method of the present invention. Sectional drawing of the Example applied to slope stabilization. FIG. 2 is a cross-sectional view and stress distribution diagram of a tension distribution anchor of the prior art;

In this specification, unless otherwise specified, "longitudinal direction" means the longitudinal direction of the tension distribution anchor, and "cross section" means a cross section perpendicular to the longitudinal direction.
Further, the boundary position between the free length 1 (unbonded portion) and the fixed length 2 (bonded portion) of each tendon is hereinafter simply referred to as the boundary position.
Fig. 1 (1) shows the side surface of the tension dispersion anchor, Fig. 1 (2) shows the stress distribution in the grout along the longitudinal direction when the tension is fixed, and Fig. 1 (3) shows the longitudinal direction of the tension dispersion anchor. 1A, BB, CC, DD, and EE shown in FIG. 1(1) along the direction.
The prestressed tendons 1A, 1B, and 1C each use a PC steel strand wire, and the PC steel strand wire is bent into a U-turn to form a U-turn portion 2u at each tip.
In the stress distribution diagram of the long portion 12 of the fixing member in FIG. 1(2), the maximum value (peak) σmax (σ1, σ2, σ3) of the stress degree occurs at each boundary position of each tendon 1A, 1B, 1C. It shows that there is
In the cross-sectional view of FIG. 1(3), tendons 1A, 1B, and 1C are shown as a pair, and cross-sections of a total of six PC steel strand wires (1A to 1C) are shown.

FIG. 1(3) shows cross sections AA, BB, CC, DD and EE of the tension distribution anchor. In the AA cross section, each tendon 1A, 1B, 1C has a coating portion 3, specifically a PE (polyethylene) coating 6 described later, and each tendon 1A, 1B, 1C is in an unbonded state. be. In the BB cross section, the prestressing tendons 1A, 1A are in a so-called bonded state in which the PE coating 6 of the rust-proof treated PC steel strand 500 shown in FIG. 2 is peeled off and the PC steel wire 50 is exposed. . In the CC cross section, the tendons 1C, 1C are in a bonded state with the sheath 3 removed and the PC steel strand exposed. In the DD cross section, the tendons 1B and 1B are also in a bonded state with the covering portion 3 removed, and all the tendons 1A to 1C are in a bonded state.
In this way, the prestressing tendons (1A, 1B, 1C) are arranged with predetermined dispersion intervals (L ₁ , L ₂ ) and the boundary positions are shifted in the longitudinal direction.
The U-turn portion 2u of each tendon 1A, 1B, and 1C is inserted into the drilled hole H so as to be positioned on the far side of the drilled hole H. The other end protrudes a required length from the drilled hole so that the anchor head 10 can be tensioned and fixed.

A tension distribution anchor as an embodiment is composed of an anchor head (tension anchoring end) 10, an anchor free length portion 11, an anchor body length portion 12, and a drilling extra length Hx. The drilled holes are filled with grout G, and after the grout G hardens, the tendons 1A, 1B, and 1C are tensioned and fixed at the anchor heads by the procedure described later.

The tendons 1A, 1B, and 1C are preferably made of PC steel stranded wire subjected to antirust treatment. FIG. 2 shows cross-sections of various antirust-treated PC steel strands. The upper part is a PC steel strand wire that has been treated for rust prevention, and the lower part is a polyethylene coating (PE coating) 6 that covers the outer periphery of the PC steel strand shown in the upper part, and fills the gap with grease or wax as a filler 7. It is a so-called unbonded type.
The following description is for PC steel stranded wire 500 that has been treated for rust protection. In the unbonded type, a polyethylene coating (PE coating) 6 is provided on the outer periphery of the PC steel stranded wire.
The A type is a seven-strand PC steel strand wire 50 in which six side wires 5S are wound around a central core wire 5C. is formed.
The B type has a double antirust layer in which a zinc-plated antirust layer 5Z is formed on the wire and a synthetic resin antirust layer 5P is further formed thereon.
In the C type, a comprehensive synthetic resin antirust layer 5Pc is formed on the outer periphery of seven PC steel strands, and synthetic resin is filled between the wires 5S.
In addition, in this C type, the outer grooves between the wires after the synthetic resin antirust layer is formed are shallower, and the bond effect (adhesion with grout) is lower than the A type and B type, It is preferable that the surface of the synthetic resin anticorrosive layer 5Pc be adhered with sand grains Sa to increase the adhesion force.

In the tension dispersion type anchor shown in FIG. 1, the anchor head uses a fixing tool (anchor head, wedge, anchor plate, etc.) to fix the tendon under tension, which is the same as in the prior art, and the problem to be solved by the present invention. Since it is not directly related to , detailed description is omitted.

As shown in FIG. 1, the prestressing tendons (1A to 1C) of the tension dispersion type anchor are provided with a PE coating 6 at the free lengths 1a, 1b, and 1c in an unbonded state. The stranded wire and the grout G do not adhere to each other, and in the fixing lengths 2a, 2b, and 2c, the PE coating 6 is peeled off to expose the internal rust-proof PC steel stranded wire 500, which is adhered to the grout G, It is in a so-called bond state. This is the same as the conventional tension type anchor, and is the basic configuration of the tension type anchor.
Also, the area from the anchor head 10 to the boundary position of the tendon with the shortest free length is defined as an anchor free length part 11, and the area from there to the terminal end of the anchorage length of the tendon with the longest anchorage length is defined as an anchor body length part 12. do. A stress dispersion portion L3 is formed from the starting point of the anchor body length portion ₁₂ .

In the stress distribution part L3, the boundary positions of each tendon 1A, _1B , 1C (consisting of a pair of strands of PC steel in the cross section) are shifted in the longitudinal direction by predetermined dispersion _intervals (L1 _, L2). This is the part to be placed. That is _, the stress dispersion portion L3 is the sum of the dispersion intervals L1 _{and L2} . In the illustrated example, L ₃ =L ₁ +L ₂ .
In the embodiment of FIG. 1, the tendon 1A (having the shortest free length) and the tendon _1B are provided with a dispersion interval L1 to shift the boundary position. Similarly, the tendon 1B and the tendon 1C (the one with the longest free length) _are provided with a dispersion interval L2 to shift the boundary position. Thus, a tension distribution anchor is formed.

Each dispersion interval (L ₁ , L ₂ ) is desirably determined according to the state of stress occurring at the boundary position of each tendon (1A, 1B, 1C). The distribution intervals may all be the same interval, but they can also be different intervals.
In the embodiment of FIG. 1, the fixation length 2 of the tendon is the same length (2a=2b=2c), but it is needless to say that the fixation length 2 is not limited to this and may be different. For example, 2a=2c+ _{L1 +} L2, 2b=2c ₊ L2. In other words, the tendons may have the same length at the bottom of the drilled hole.
In short, the fixing length L is determined so that the tension force of each tendon 1A, 1B, 1C can sufficiently secure the adhesive force between the PC steel strand wire and the grout.

In addition, at actual construction sites, due to insufficient grout filling, the influence of the permeable layer, construction errors, unevenness of the ground, etc., the adhesion between the tendon and grout becomes insufficient, and pull-out accidents occur. In order to prevent this, it is preferable to form the U-turn portion 2u by making the tip of the fixing long portion of each tendon U-turn. The present invention is not limited to this form, and if the adhesion between the grout and the tendon can be sufficiently secured, the U-turn portion 2u is not provided and the ends of the tendons (1A, 1B, 1C) are straight. may be

In the tension dispersion type ground anchor shown in FIG. 1, the maximum stress degree σmax (mountain) in the grout formed at each boundary position when each tendon (1A, 1B, 1C) has completed tension fixing is shown in FIG. 1 (2 ), they are σ1, σ2, and σ3, respectively, and although the values are different, they fluctuate depending on the tension and dispersion intervals of the tendons 1A, 1B, and 1C. This point will be described in detail together with the tension control chart of FIG. 3 and the stress distribution chart of FIG.

As shown in the stress distribution diagram in the grout of FIG. When only the maximum stress value σ1 at the boundary position of the tendon 1C where 1c is the longest exceeds the predetermined allowable value (Fa = γfck) of the grout G, the fine hatched area above the dotted line indicating this allowable value is set as the reinforcement range a, and the necessary reinforcement member 4 is installed.
Also, although illustration is omitted, in the tendon 1A or 1B, if the maximum stress degree σ2 or σ3 at the boundary position exceeds a predetermined allowable value (Fa), that region is similarly set as a reinforcement range. Install stiffeners.
The specifications and quantity of reinforcing materials are determined by the following formula (1).
fs×As≧σ ₀ ×A ₀ (1)
In formula (1),
fs: allowable stress of reinforcing material (preferably long-term allowable stress of steel used as reinforcing material)
As: cross-sectional area of reinforcing material (for example, effective cross-sectional area of steel pipe used as reinforcing material)
σ ₀ : value exceeding a predetermined allowable value σ ₀ =σmax−Fa
In FIG. 1(2), σ ₀ =σ1−γfck
σmax: Maximum value of stress in the above stress distribution (peak value)
In FIG. 1(2), σ1, σ2, and σ3 are all σmax.
Fa: Predetermined allowable value (Fa=γfck)
A ₀ : Effective cross-sectional area at the position where σ ₀ is generated A ₀ = Ah-nAp
FIG. 1 shows the effective cross-sectional area at the position where σ3 occurs.
Ah: drilling cross-sectional area at the position where σ ₀ occurs Ah = drilling diameter φ ² × π/4
Ap : Cross-sectional area of tendons (PC steel strands) n : Quantity of tendons (PC steel strands) in the cross section at the position where σ ₀ occurs

The reinforcing member 4 is preferably made of a steel member. For example, it can be appropriately selected and used from steel pipes with ribs having an uneven surface, spiral bars, corrugated steel sheaths, and the like. It is desirable that the length of the reinforcing member 4 is appropriately longer than both ends of the reinforcing range a.

Next, the method of tensioning the tendon and the stress distribution will be described based on the tension control chart of FIG.
FIG. 3 is a tension management diagram of the tension dispersion anchor of the present application, which was created following the tension management diagram (FIG. 1) of Patent Document 4 (Patent No. 3860941). It was created based on the layout of the materials.
FIG. 4 is an image diagram of the stress distribution generated in the grout by the tension method shown in the tension control chart of FIG.

As shown in FIG. 1, by arranging the boundary positions of the tendons mutually shifted by the dispersion intervals L ₁ and L ₂ to provide the stress distribution part L ₃ , the free length 1 of each tendon 1A, 1B, 1C will be different from 1a-1c, it is necessary to use the allochronous tension method disclosed in Patent Document 4.

The maximum load (Pmax) shown in FIG. 3 is the total value of the maximum tension generated in each tendon (1A, 1B, 1C). The maximum load (Pmax) is, for example, the yield load (Py) of the tendon, and the specifications and quantity of the tendon are determined so as to exceed the design maximum seismic load. The load at the time of fixing (Pt) is the total value (P1+P2+P3) of the tension force at the time of the completion of fixing introduced into each tendon. P1, P2, and P3 are 60% (0.6 Pu) of the tensile load of each tendon (1A, 1B, 1C).

When a plurality of tendons are used for one anchor, the tendons of the same specification are basically used, and the same applies to the present invention.
In conventional tension-type anchors that do not disperse the boundary position, the free length of multiple tendons is assumed to be the same, so the elongation due to the introduced tension is the same, and the multiple tendons are simultaneously tensioned to a predetermined anchoring load. In this case, the same tension force is induced in each prestressing tendon, so that simultaneous tensioning is possible. When an external force such as an earthquake load is generated, each tendon is evenly tensioned, so there is no particular need to pay attention to the method of tensioning the tendons of the anchor.

However, in the tension dispersion anchor, as shown in FIG. 1, the boundary positions of the tendons (1A, 1B, 1C) are shifted by dispersion intervals (L _1, L ₂ ), and the free length (unbonded Since the length of each tendon (1a, 1b _, 1c) is different from that of (1a, 1b, 1c), each tendon (1A, 1B, 1C) has a different elongation amount when the same tension is generated. In the illustrated embodiment, the maximum load (Pmax) is the sum (3×Tb) of the maximum tension (Tb=Py) of each tendon.
It is an essential design condition that the tendons (1A, 1B, 1C) have the same maximum tension Tb. That is, the design condition is that each tendon (1A, 1B, 1C) yields simultaneously.
In order to satisfy this condition, it is necessary to eliminate the difference in elongation of each tendon (1A, 1B, 1C), and it is necessary to use the so-called differential tensioning method disclosed in Patent Document 4.

The procedure will be specifically described below.
The elongation until the tension generated in the tendon 1C with the longest free length reaches Tb is δ ₁ ,
The elongation until the tension generated in the tendon 1B reaches Tb is δ ₂ ,
The elongation until the tension generated in the tendon 1A reaches Tb is δ ₃ ,
Calculate each.
Each elongation δ will be different depending on the dispersion spacing.
Next, the difference in elongation between the tendon 1C and the tendon 1B is Δ ₁ (Δ ₁ = δ ₁ - δ ₂ ), and the difference in elongation between the tendon 1B and the tendon 1A is Δ ₂ (Δ ₂ = δ ₂ - δ ₃ ), respectively.

Each tendon is then tensioned by the tensioning method described below.
First, the tendon _1C having the longest free length is tensioned with tension T1. The tension T1 is assumed to be the extension Δ1 _minute (the _first break point (first black circle) in the graph of FIG. 3). Thus, the elongation difference Δ1 between the tendon 1C and the tendon _1B is eliminated.
Next, the tendons 1C and 1B are tensioned simultaneously. Assuming that the tension force introduced into the tendon 1B is T2, tension is applied by an elongation Δ of ₂ minutes ( _second break point (second black circle) in the graph of FIG. 3). _The tension introduced into tendon ₁ is therefore the sum of the tension increased by T1 and the extension Δ2. In this way, all the differences in elongation between tendons 1A, 1B and 1C are eliminated.

However, since the free lengths of the tendon 1A and the tendon _1B are different, even if tensioning is performed with the same tension force that generates the same elongation Δ2, the tension force T2 introduced into the tendon _1B is applied to the tendon 1A. Note that the tension applied to
After the elongation difference Δ2 is eliminated, the prestressing tendons 1, ₂ and 3 are simultaneously tensioned to elongation δ and fixed. Thus, the total tension P (P ₁ +P ₂ +P ₃ ) introduced into each tendon (1A, 1B, 1C) becomes the predetermined load (Pt) during fixation.
However, as already explained, since the maximum tension Tb generated in each tendon reaches the yield load Py at the same time, each tendon (1A, 1B, 1C) has a different free length. The tension forces introduced in the tendons (1A, 1B, 1C) are different, in the order P ₁ >P ₂ >P ₃ .

FIG. 4 shows an image of the stress distribution generated in the grout G at each tensioning stage based on the heterochronous tensioning method.
First, the stress distribution generated in the grout G becomes a wavy stress distribution (1) in a state in which the tendon 1C having the longest free length is tensioned by _an elongation difference Δ1. The stress is maximized at the boundary position of the tendon 1C as indicated by the stress crest (peak), and is attenuated in the longitudinal (depth) direction from the crest to gradually decrease.
Next, stress distribution (2) is the stress distribution newly generated in the grout after the tendons 1C and _1B are simultaneously tensioned by the difference Δ2 in elongation. At the boundary position of the tendon 1B, the stress degree σ2 (a) becomes maximum like a peak, and at the boundary position of the tendon 1C, the size of the mountain portion increases from the previous peak portion to the current portion (the corresponding amount during attenuation). position) is accumulated, and as a result, the peak becomes even larger, resulting in σ1(b). Its magnitude varies with the dispersion interval _L2 .
In other words, if the dispersion interval is large, the cumulatively formed ridges become smaller, the sizes of the ridges are averaged, and there is no need to reinforce the ridges. However, the anchor body length portion 12 of the entire anchor tends to be long.

Finally, after all prestressing tendons (1A, 1B, 1C) are simultaneously tensioned until elongation δ occurs, the stress distribution (3) similarly newly generated in the grout becomes stress distribution (3).
The stresses (σ1, σ2, σ3) are maximized at the boundary positions of the tendons (1A, 1B, 1C) and appear like waves, and the size of each peak varies depending on the dispersion interval. Become.

In the reinforcing method of the present invention, the dispersion interval depends on the number of tendons and the anchoring force (tension force) to be introduced, but is preferably 200 mm to 1000 mm. If it is too short, the dispersion effect cannot be obtained, and conversely, if it is too long, the entire anchor body length portion 12 becomes long, resulting in a lack of economic efficiency.

If it is necessary to reinforce multiple peaks (maximum stress σmax) exceeding the allowable value, reinforcing materials may be individually arranged for each peak, but considering the simplicity and practicality of construction Alternatively, one continuous reinforcement member 4 may be arranged across a plurality of peaks.

Moreover, the stress distribution shown in FIG. 1 and the above description is that at the time of fixing before reinforcement. The stress distribution after reinforcement is not shown, but in the reinforcement range, the stress is averaged by the restraint effect of the reinforcing member 4, and the value of peaks becomes small.
Further, the predetermined permissible value used in the tensile dispersion type ground anchor reinforcement method of the present invention is added as an extra coefficient γ in consideration of the following actual usage conditions.

The stress distribution generated in the grout is wavy, and the stress attenuates and decreases from the maximum stress level (wavy ridges) toward the depth direction. σmax is a value only for peaks of stress distribution, and is a so-called locally concentrated stress. In other words, the stress distribution is not distributed at a constant value in the longitudinal direction, but the maximum stress σmax (mountain) is a locally concentrated stress, and the stress gradually decreases away from that position. To go. However, the design standard strength (fck) of grout is the average value of the grout base material strength, and the base material strength also increases against localized stress, and is generally about 1.5 to 2.0 times higher than the average value. doubles as high. Therefore, since it is not reasonable to design using the design standard strength (fck) of grout against localized stress, it was considered necessary to correct it.
The peripheral frictional resistance between the ground and the anchor body, which consists of grout and tendons, differs depending on the type and condition of the ground. (fck) is higher.
Therefore, in the present invention, the predetermined permissible value Fa of the grout is obtained by multiplying the design basis strength (fck) of the grout to be used by the extra coefficient γ. however. Since there are various types of ground, it is unreasonable to categorically determine the additional coefficient. However, it is preferable to set it to 2.0 or less so as not to overestimate.
In other words, it is desirable to set the additional coefficient γ=1.0 to 2.0. Therefore, the predetermined tolerance is Fa=γfck.

FIG. 5 shows an example of the stress distribution in the grout due to the heterochronous tension shown in FIG. 4, and the stress distribution will be described in detail.
The tension distribution anchor shown in FIG. 1 is shown in FIG. 5 as an example.
In addition, the same thing is expressed with the same code|symbol.
The stress distribution in the grout due to the tension of the tendon can be obtained by analysis means used for design, such as FEM analysis. As a customary means of design, the actual stress distribution obtained from the analysis can be used for design modeling, and can be represented by an approximate calculation formula to proceed to the next section calculation.
Here, as an example of the stress distribution model, the stress intensity is maximum at the boundary position and decays in a quadratic curve toward the tip of the tendon, and the reinforcement for a triangular distribution that decays linearly from the boundary position. Explain how to find the range.
First, the stress distribution in the case of quadratic damping shown in FIG. 5(a) will be described.
As described with reference to FIG. 4, first, after tensioning the tendon 1C with the longest free length by the difference in elongation Δ1, stress distribution ( ₁ ) is obtained from the boundary position of the tendon 1C, and the cross section at the boundary position Let σ1(a) be the maximum stress degree (mountain portion) generated at .
Next, the tendons 1C and 1B are simultaneously tensioned, and after being tensioned enough to generate the difference in elongation Δ ₂ , the stress distribution from the boundary position of the tendon 1B becomes (2)-1, and the maximum stress distribution generated in the cross section at the boundary position Let σ2(a) be the stress level (mountain), and σ2( _b ) be the stress level generated in the section of the tendon 1C at a distance of L2 from that position. Stress distribution (1) and stress distribution (2)-1 are cumulatively combined to obtain stress distribution (2), and the degree of stress generated in the cross section at the boundary position of tendon 1C is +σ1(a). Finally, after simultaneously tensioning all the tendons (1A, 1B, 1C) until elongation δ occurs, if the stress distribution is accumulated in the same way, the stress distribution becomes (3)-1, and the stress degree generated in the cross section at each boundary position is , from left to right, σ3, σ3(a), σ3(b), and further cumulative with the previous stress distribution (2) to finally become the stress distribution (3). The maximum stress degrees generated in the cross sections at the boundary positions in the order of 1A, 1B, and 1C are σ3, σ2, and σ1, respectively.
As a result, for example, when σ1 exceeds a predetermined allowable value Fa, the reinforcing member 4 is arranged with the exceeding range a as the reinforcement range.
The value σ ₀ = σ1−Fa exceeds, and the effective cross-sectional area (boundary position of the tendon 1C) A ₀ at the position where σ ₀ is generated is calculated, the material of the reinforcing material is determined, and the reinforcement is performed by the following formula (1) The specification and quantity of materials can be determined.
fs×As≧σ ₀ ×A ₀ (1)

In addition, the stress distribution can be a linear function in consideration of simplicity in design. FIG. 5B shows a case where the stress distribution is a linear function (triangular) distribution.
The stress distribution generated in each tension stage and the accumulated stress distribution are the same as the quadratic curve distribution, so the explanation is omitted.
Assume that σ1 obtained in the same manner finally exceeds a predetermined allowable value Fa.
Comparing the two, the triangular distribution has larger values for both σ2(a) and σ3(b). It will be a little more, but it will be on the safe side.
In other words, it can be applied as a triangular distribution in order to achieve design practicality and simplicity.

FIG. 6 shows an example in which the reinforcement method of the present invention is applied to a ground anchor for stabilizing a slope with a three-stage tension dispersion type ground anchor.
In the tension distribution anchor for slope stability installed on the slope, this is an example of reinforcement in which a reinforcing member 4 is arranged continuously between peaks of the stress distribution of the grout of the tension distribution anchor A1 on the top stage. . It is a ground anchor in which the boundary position of the tendon is shifted by two dispersion intervals, and although illustration is omitted, three peaks are formed at the three boundary positions as stress distribution, and all three are within the allowable value. This figure shows an example in which one reinforcing member 4 is arranged continuously over three mountain portions.
In the tension distribution anchor A2 in the middle row, although illustration is omitted, three peaks are formed at three boundary positions as a stress distribution. This is an example in which the reinforcing material 4 is arranged only for reinforcement.
The tension distribution anchor A3 at the bottom has no reinforcing material because no stress distribution exceeds a predetermined allowable value at the three boundary positions, so reinforcement is unnecessary.

1A, 1B, 1C Tendon 1 (1a, 1b, 1c) Free length 2 (2a, 2b, 2c) Fixed length 2u U-turn part 3 (3A, 3B, 3C) Coating part
4 Reinforcing material 40 Continuous reinforcing material 5 Wire 5C Core wire 5S Side wire 5P Synthetic resin antirust layer 5Pc Synthetic resin antirust layer 50 PC steel stranded wire (bare wire)
500 PC steel stranded wire with antirust layer 6 Polyethylene coating (PE coating)
7 Filling material 10 Anchor head 11 Anchor free length 12 Anchor body length Hx Surplus drilling length L ₁ , L ₂ Dispersion interval L ₃ Stress distribution part G: Grout a: Reinforcement range fs: Allowable stress level of reinforcing material As: Cross-sectional area of reinforcing material σ ₀ : Value exceeding a predetermined allowable value σ ₀ = σmax - Fa
σmax (σ1, σ2, σ3): stress degree (peak) of peaks in stress distribution
Fa: Predetermined allowable value A ₀ : Effective cross-sectional area at the position where σ ₀ occurs A ₀ = Ah - nAp
Ah: drilling cross-sectional area at the position where σ ₀ occurs Ah = drilling diameter φ ² × π/4
Ap : Cross-sectional area of tendons (PC steel strands) n : Quantity of tendons (PC steel strands) in the cross section at the position where σ ₀ occurs

Claims (3)

In a tension distribution ground anchor formed by an anchor head, an anchor free length, and an anchor body length,
A plurality of tendons made of PC steel strands are inserted into the drilled hole, and the unbonded portion of each tendon is covered with PE on the anchor head side, and the back side of the drilled hole of the tendon is PE. The uncoated bond portion is defined as the fixed length, and the free lengths of the respective tendons are different, and the boundary position between the free length and the fixed length of the plurality of prestressing tendons is located in the depth direction of the drilled hole from the long portion of the anchor body. The stress distribution generated in the grout in the long portion of the anchor body, which is staggered from each other at a predetermined dispersed interval, and is generated in the grout in the long portion of the anchor body by fixing the respective tendons under tension after the grout injected into the drilled hole hardens. A reinforcement method for a tension dispersion type ground anchor, characterized by arranging a reinforcing material in a reinforcement range of a portion exceeding a predetermined allowable value (Fa). In the reinforcing method of the tension distribution ground anchor of claim 1,
A reinforcing method for a tension dispersion ground anchor, wherein the reinforcing material is made of steel and the specifications and quantity of the reinforcing material are determined by the following formula (1).
fs×As≧σ ₀ ×A ₀ (1)
In formula (1),
fs: allowable stress level of reinforcing material As: cross-sectional area of reinforcing material σ ₀ : value exceeding a predetermined allowable value σ ₀ = σmax - Fa
σmax : Maximum value of stress in the above stress distribution (peak value)
Fa: Predetermined allowable value A ₀ : Effective cross-sectional area at the position where σ ₀ is generated A ₀ =Ah−nAp
Ah: drilling cross-sectional area at the position where σ ₀ occurs Ah = drilling diameter φ ² × π/4
Ap : Cross-sectional area of tendons (PC steel strands) n : Quantity of tendons (PC steel strands) in the cross section at the position where σ ₀ occurs In the method of reinforcing a tension dispersion type ground anchor according to claim 1 or 2, the predetermined allowable value (Fa) is obtained by multiplying the design basis strength (fck) of the grout by an additional coefficient (γ), and the additional coefficient (γ ) is determined according to the type of ground, and is 2.0 or less.

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