Patent Grant
12077929
This application is a U.S. 371 Application of International Patent Application No. PCT/JP2019/012769, filed on 26 Mar. 2019, which application claims priority to and the benefit of JP Application No. 2018-078732, filed on 16 Apr. 2018, the disclosures of which are hereby incorporated herein by reference in their entireties.
The present invention relates to a manhole reinforcing method and a manhole reinforcing structure, and particularly to a reinforcing method and a reinforcing structure for a manhole having a neck part provided in a shifted position from the center part of an upper slab.
Facilities exposed to the external environment degrade over time. Manholes which are concrete structures buried underground are no exception. Therefore, there has been a demand for manhole reinforcing techniques.
PTL 1 discloses a manhole including a body having a lower slab, a side wall, and an upper slab and a neck part as an opening connected to the upper slab, trapezoidal hunch blocks are arranged horizontally at a corner part including four corners of the sidewall, and strip-shaped reinforcing members of fiber-reinforced plastic are adhered to the inner surfaces of the lower slab, the sidewall, and the upper slab.
According to the disclosure of PTL 2, the upper part of a sidewall and an upper slab and the lower part of the sidewall and a lower slab are fixed by reinforcing members in the longitudinal center of the sidewall of a rectangular manhole.
Using the techniques disclosed in PTL 1 and PTL 2 as a manhole reinforcing method, some reinforcement effects may probably be obtained. However, the inventors have found as a result of study that when a manhole made of a different material and having a different wall thickness such as a resin manhole is buried underground, the sidewall is subjected to stress directed from the inside to the outside because of a load from the upper part of the body. Simply combining these techniques does not provide sufficient reinforcement against the stress directed from the inside to the outside. In particular, there has been a demand for reduction in the operation and material costs involved in reinforcement of manholes. From this point of view, there was still room for further improvement in the reinforcement of manholes.
Furthermore, the manholes disclosed in PTL 1 and PTL 2 have a rectangular parallelepiped such as a box shape and a rectangular shape. The neck part of the manhole each disclosed in PTL 1 and PTL 2 is located in the center of the upper slab. In such a manhole, strain is generated uniformly on the upper slab and local strain is not generated.
In contrast, when a manhole has a neck part in a position shifted from the middle part of the upper slab, the distance from the outer edge of the upper slab to the opening of the upper slab continuous with the neck part greatly varies depending on the location. As a result of intensive study, the inventors have found that when a manhole has a neck part in a position shifted from the middle part of the upper slab, local strain increases and cracks are easily generated in a location where the distance from the outer edge of the upper slab to the opening is long.
In view of the foregoing, it is an object of the present invention to provide a reinforcing method and a reinforcing structure for a manhole having a neck part positioned shifted from the center part of the upper slab which allow the buried manhole to be reinforced sufficiently against stress inflicted thereon while reducing the operation and material costs involved in the reinforcement.
A method for reinforcing a manhole according to a first aspect of the invention is a method for reinforcing a manhole including a rectangular trunk part having an upper slab, a sidewall, and a lower slab and a neck part provided in a position shifted from a middle part of the upper slab of the trunk part, and the method includes providing a strip-shaped reinforcing member of reinforced fiber at the upper slab and the lower slab while refraining from providing the strip-shaped reinforcing member at the sidewall, and fixing an upper part of the sidewall and the upper slab and a lower part of the sidewall and the lower slab each by a reinforcing member at a middle part of the sidewall in a longitudinal direction of the trunk part.
According to one embodiment of the invention, the strip-shaped reinforcing member provided at the upper slab has a smaller area than the strip-shaped reinforcing member provided at the lower slab.
According to one embodiment of the invention, the neck part has a center position shifted to one end side from a middle position of the trunk part in the longitudinal direction, the strip-shaped reinforcing member provided at the upper slab has a greater area on the other end side in the longitudinal direction with respect to the neck part than on the one end side in the longitudinal direction with respect to the neck part.
According to one embodiment of the invention, the strip-shaped reinforcing member at the upper slab is provided in an in-plane direction of the upper slab and in a direction orthogonal to the longitudinal direction.
A reinforcing structure for a manhole according to a second aspect of the invention is a structure for reinforcing a manhole including a rectangular trunk part having an upper slab, a sidewall, and a lower slab and a neck part provided in a position shifted from a middle part of the upper slab of the trunk part, and the structure includes a strip-shaped reinforcing member of reinforced fiber provided at the upper slab part and the lower slab part and refrained from being provided at the sidewall, and a reinforcing member each fixing an upper part of the sidewall and the upper slab and a lower part of the sidewall and the lower slab at a middle part of the sidewall in a longitudinal direction of the trunk part.
According to one embodiment of the invention, the strip-shaped reinforcing member provided at the upper slab has a smaller area than the strip-shaped reinforcing member provided at the lower slab.
According to one embodiment of the invention, the neck part has a center position shifted to one end side from a middle position of the trunk part in the longitudinal direction, and the strip-shaped reinforcing member provided at the upper slab has a greater area on the other end side in the longitudinal direction with respect to the neck part than on the one end side in the longitudinal direction with respect to the neck part.
According to one embodiment of the invention, the strip-shaped reinforcing member at the upper slab is provided in an in-plane direction of the upper slab and in a direction orthogonal to the longitudinal direction.
According to the present invention, a reinforcing method and a reinforcing structure for a manhole having a neck part provided in a shifted position from the middle part of an upper slab can be provided, and the method and the structure allow the buried manhole to be sufficiently reinforced against stress inflicted thereon while reducing the operation and material costs involved in the reinforcement.
Hereinafter, a manhole reinforcing method and a manhole reinforcing structure according to the present invention will be described with reference to
As shown in
The trunk part 2 is a rectangular body. More specifically, the rectangular trunk part 2 includes an upper slab 11, a sidewall 12, and a lower slab 13. The protrusion 3 according to the present embodiment is provided to protrude from the sidewall 12 of the trunk part 2. In this way, the manhole 1 according to the present embodiment has an L-shape formed by the trunk part 2 and the protrusion 3.
The protrusion 3 according to the present embodiment is also formed to have a substantially rectangular shape and includes a protrusion upper slab 14, a protrusion sidewall 15, and a protrusion lower slab 16. According to the present embodiment, the upper slab 11 of the trunk part 2 and the protrusion upper slab 14 of the protrusion 3 are made of an integrally formed L-shaped slab. According to the present embodiment, the lower slab 13 of the trunk part 2 and the protrusion lower slab 16 of the protrusion 3 are made of an integrally formed L-shaped slab. The protrusion sidewall 15 is provided with an opening 15a through which a communication cable or the like can be extended in and out of the protrusion 3.
The upper slab 11 and the lower slab 13 of the trunk part 2 have a rectangular plate shape. The sidewall 12 of the trunk part 2 has a rectangular tube shape which connects the upper slab 11 and the lower slab 13.
An opening 11a connected with the neck part 4 is formed at the upper slab 11 of the trunk part 2. The manhole 1 buried underground can be entered/exited to/from the trunk part 2 through the neck part 4 and the opening 11a from the upper end of the neck part 4 positioned on the ground. As shown in
The lower slab 13 of the trunk part 2 is provided with a basin 13a (see
The neck part 4 is provided in a shifted position from the middle part of the upper slab 11 of the trunk part 2. More specifically, the neck part 4 is provided to project upwardly from the position of the opening 11a of the upper slab 11. Therefore, similarly to the opening 11a, the neck part 4 according to the present embodiment is provided in such a position that its center position does not match the center position of the upper slab 11. More specifically, the neck part 4 according to the present embodiment is localized on one end side rather than the middle position of the upper slab 11 in the longitudinal direction A of the trunk part 2.
The manhole 1 according to the present embodiment may be an L-shaped manhole such as “L-3R type (W1600×L2500×H1500)” manufactured by SUNREC CO., LTD. disclosed in NPL 1 but the manhole is not particularly limited if the manhole has a neck part provided in a shifted position from the middle part of the upper slab of the rectangular trunk part.
As shown in
The strip-shaped reinforcing members 18 are strip-shaped (sheet-shaped) members made of reinforced fiber. As shown in
As shown in
As described above, the center position of the neck part 4 is provided in a shifted position on one end side (the left side in
According to the present embodiment, the strip-shaped reinforcing members 18 at the upper slab 11 are provided in the in-plane direction of the upper slab 11 and orthogonal to the longitudinal direction A (hereinafter simply referred to as “the transverse direction B”). The strip-shaped reinforcing members 18 at the upper slab 11 are also provided entirely in the transverse direction B of the upper slab 11. Note that the whole area in the transverse direction B of the upper slab 11 according to the present embodiment refers to the entire area of the part sandwiched between slope parts 17a (which will be described) in the transverse direction B. Effects of the arrangement by the arrangement will be described below in more detail.
The reinforcing members 19 fix the upper part of the sidewall 12 and the upper slab 11 and the lower part of the sidewall 12 and the lower slab 13 at the middle part of the sidewall 12 in the longitudinal direction A. More specifically, for example as illustrated in
The reinforcing members 19 which fix the upper slab 11 and the sidewall 12 are provided to be in contact with the region from the upper part of the sidewall 12 to the upper slab 11 via a corner part 17 at which the sidewall 12 and the upper slab 11 cross each other. The reinforcing members 19 which fix the lower slab 13 and the sidewall 12 are provided to be in contact with the region from the lower part of the sidewall 12 to the lower slab 13 via the corner part 17 at which the sidewall 12 and the lower slab 13 cross each other. The reinforcing member 19 may be fixed, for example, by using an anchor bolt pre-installed in the wall of the trunk part 2.
Note that the shape of the reinforcing member 19 is not particularly limited if the member can fix the upper part of the sidewall 12 and the upper slab 11 and the lower part of the sidewall 12 and the lower slab 13.
Next, a method for reinforcing the manhole 1 according to the present embodiment will be described.
First, the strip-shaped reinforcing members 18 are adhered to the upper slab 11 and the lower slab 13 of the manhole 1. As described above, according to the present embodiment, the strip-shaped reinforcing members 18 are not provided at the sidewall 12 and are provided at the upper slab 11 and the lower slab 13. More specifically, the strip-shaped reinforcing members 18 according to the present embodiment are provided only at the upper slab 11 and the lower slab 13.
Then, the upper part of the sidewall 12 and the upper slab 11 and the lower part of the sidewall 12 and the lower slab 13 are fixed by the reinforcing members 19 at the middle part of the sidewall 12 of manhole 1 in the longitudinal direction A. Note that at the corner part 17 where the sidewall 12 and the upper slab 11 cross each other, a slope part 17a inclined at a prescribed angle (for example 45°) with respect to the sidewall 12 and the upper slab 11 is provided. The inner surface of the sidewall 12 is connected to one end of the slope part 17a, and the inner surface of the upper slab 11 is connected to the other end of the slope part 17a. Similarly, a slope part 17a is provided at the corner part 17 between the sidewall 12 and the lower slab 13. The reinforcing members 19 provided to fix the upper part of the sidewall 12 and the upper slab 11 are in contact with the upper part of the inner surface of the sidewall 12, the slope part 17a between the sidewall 12 and the upper slab 11, and the inner surface of the upper slab 11. The reinforcing members 19 provided to fix the lower part of the sidewall 12 and the lower slab 13 are in contact the lower part of the inner surface of the sidewall 12, the slope part 17a between the sidewall 12 and the lower slab 13, and the inner surface of the lower slab 13.
Note that the middle part of the sidewall 12 of the manhole 1 in the longitudinal direction A refers not only to the middle position in the longitudinal direction A where the tensile stress inside the trunk part 2 is maximized but also to the middle region in the longitudinal direction A where tensile stress to the inside of the trunk part 2 is generated. As shown in
According to the present embodiment, the area of the strip-shaped reinforcing members 18 provided at the upper slab 11 is smaller than the area of the strip-shaped reinforcing members 18 provided at the lower slab 13. More specifically, the upper slab 11 according to the present embodiment is adhered with a smaller number of strip-shaped reinforcing members 18 than those adhered to the lower slab 13. Effects of the arrangement will be described below in more detail.
According to the present embodiment, the strip-shaped reinforcing members 18 at the upper slab 11 are provided so that the area for providing the members on the other end side (the right side in
According to the present embodiment, the strip-shaped reinforcing members 18 at the upper slab 11 are provided in the transverse direction B. More specifically, the strip-shaped reinforcing members 18 at the upper slab 11 are provided entirely along the upper slab 11 in the transverse direction B. Effects of the arrangement will be described below in more detail.
Next, effects of reinforcement using the reinforcing structure 100 for the manhole 1 according to the present embodiment will be described.
First, among the components of the reinforcing structure 100 for the manhole 1 according to the present embodiment, effects of reinforcement using the strip-shaped reinforcing members 18 provided at the lower slab 13 and the reinforcing members 19 provided to fix the upper part of the sidewall 12 and the upper slab 11 and the lower part of the sidewall 12 and the lower slab 13 at the middle part of the sidewall 12 in the longitudinal direction A will be described.
In order to determine the effects of reinforcement using the strip-shaped reinforcing members 18 provided at the lower slab 13 and the reinforcing members 19 provided in the above-described position, the inventors conducted a load test and the upper part of the reinforced body was loaded. First, with reference to
As shown in
The structure tester 200 may be a large structure tester used to perform compressive strength testing for concrete under JIS A 1108. The hydraulic device 30 which applies a compressive load on the test specimen T may be configured to be capable of pressing the loading plate 20 with a prescribed load by driving a hydraulic cylinder. The hydraulic device 30 can be configured to adjust the flow rate or displacement rate by controlling a flow control valve which adjusts the flow rate to the hydraulic cylinder. Instead of the hydraulic device 30, a drive device using anything other than hydraulic pressure such as an electric motor may be configured to press the test specimen T.
Although a control circuit for controlling the structure tester 200 is not shown, the tester may include an input unit for receiving an operation input from the operator, a storage unit for storing test results, a display unit for displaying test results, and a control unit for controlling the operation of the entire structure tester 200. The control unit adjusts the pressurizing rate or displacement rate of the hydraulic device 30 for example by controlling the flow control valve described above. The control unit obtains an output from a strain gauge attached to the test specimen T. This allows the relation between the compression load value and the strain of the test specimen T to be obtained as data.
The loading plate 20 is a flat steel plate having a flat lower surface and transmits pressing force from the hydraulic device 30 to the test specimen T. As shown in
The top plate 20T is made of styrene foam (EPS: Expanded Poly-Styrene). Expanded polystyrene is a material obtained by foaming polystyrene into fine foams followed by curing, and the material is lightweight and elastic and has high impact absorption. The expanded polystyrene can deform as the size of the bubbles changes with respect to external force and can absorb the external force. Therefore, as shown in
The bottom plate 20B is made of expanded polystyrene similarly to the top plate 20T. The bottom plate 20B made of expanded polystyrene is placed between the floor F and the test specimen T, so that even when the test specimen T is deformed because of the load for example from the loading plate 20, the bottom plate 20B is deformed in conformity with the deformation of the lower surface of the test specimen T, and the bottom plate 20B can always be in contact with both the lower surface of the test specimen T and the floor F in a large area. Therefore, the test specimen T can receive reaction force against the pressing force from the loading plate 20 at the entire surface of the test specimen T.
When the hydraulic device 30 presses the test specimen T downward, upward force acts on the reaction force member 40 which supports the hydraulic device 30 in response to reaction force from the floor F. This upward force is transmitted to the reaction force support 50 formed on each side of the reaction force member 40.
Now, a loading test using the structure tester 200 will be described.
First, two kinds of test specimens T were prepared. The test specimens T were each a resin concrete structure having a size of 3000×1405×1690 (mm3) and a mass of 3.2 (t). One of the test specimens T was reinforced with the strip-shaped reinforcing member 18 and the reinforcing members 19. The test specimen T includes an upper slab 81 corresponding to the upper slab 11, a sidewall 82 corresponding to the sidewall 12, and a lower slab 83 corresponding to the lower slab 13. The strip-shaped reinforcing member 18 was provided only at the lower slab 83. The reinforcing members 19 were provided to fix the upper part of the sidewall 82 and the upper slab 81 and the lower part of the sidewall 82 and the lower slab 83 at the middle part of the sidewall 82 in the longitudinal direction A. The other test specimen T was not reinforced. A carbon fiber sheet made of carbon fiber was used for the strip-shaped reinforcing member 18.
A strain gauge was attached to the test specimen T.
Then, the test specimen T was installed at the structure tester 200. A 10 MN large structure tester manufactured by JFE Techno-Research Corporation was used as the structure tester 200. Styrene foam (DX-35 having a plate thickness of 100 mm from SEKISUI PLASTICS CO., Ltd.) was inserted between the floor F and the test specimen T and between the test specimen T and the loading plate 20. The loading plate 20 was then moved downward at a speed of 0.01 mm/sec to put a compressive load on the test specimen T by monotonic uniaxial loading.
As shown in
As described above, in the reinforcing structure 100 according to the present embodiment, the strip-shaped reinforcing member 18 is not provided at the sidewall 12. In order to reduce cracks in the sidewall 12, it may be possible to provide the sidewall 12 with the strip-shaped reinforcing member 18 as in the conventional structure. However, it is probably unnecessary to provide the strip-shaped reinforcing member 18 at the sidewall 12 for the following reasons.
First, a resin manhole has a thinner wall thickness than that of a manhole of reinforced concrete, and since stress is generated from the inside to the outside against the sidewall 12, the strip-shaped reinforcing member 18 provided on the sidewall 12 if any does not function as a rigid body, and the reinforcement effect is small.
In the loading test described above, earth pressure from the outside to the inside to be applied on the manhole buried underground is not generated, and excessive stress is generated against the sidewall 82 corresponding to the sidewall 12. Therefore, the stress on the sidewall 12 generated underground is less than the value obtained in the loading test described above.
As a result of measurement by the inventors, it was determined on the basis of the displacement amount that the lower slab 83 corresponding to the lower slab 13 having the strip-shaped reinforcing member 18 is loaded with greater stress than the sidewall 82 corresponding to the sidewall 12 without the strip-shaped reinforcing member 18. Therefore, the reinforcement of the sidewall 12 probably has lower priority than further reinforcement of the lower slab 13.
Note that since the strip-shaped reinforcing member 18 provided at the lower slab 13 functions as a rigid body against the stress from the outside to the inside of the lower slab 13, higher strength can be probably obtained by providing the strip-shaped reinforcing member 18 at the lower slab 13 rather than providing the reinforcing member 19 alone.
As shown in
On the other hand, in the Y-direction, the strain amount is smaller for the reinforced specimen immediately after the loading. As opposed to the case in the X-direction, compressive force caused by the collapse of the body does not act, and therefore the amount of strain was probably smaller for the reinforced test specimen. The change in the strain of a member corresponds to external force that the member has been subjected to. Therefore, it has been experimentally determined that partial reinforcement of the corner part (reinforcement by the reinforcing members 19) can reduce external force from the inside to the outside caused by the load from the upper part of the body.
As described above, it can be understood that as the strip-shaped reinforcing members 18 are adhered to the lower slab 13, and the upper part of the sidewall 12 and the upper slab 11 and the lower part of the sidewall 12 and the lower slab 13 are fixed by the reinforcing members 19, so that external force acting from the inside to the outside resulting from the load from the upper part of the body can be reduced. When the sidewall 12 is not provided with the strip-shaped reinforcing members 18, the operation and material costs involved in the reinforcement can be reduced as compared to the case in which the strip-shaped reinforcing members 18 are provided at the sidewall 12.
Next, among the components of the reinforcing structure 100 for the manhole 1 according to the present embodiment, effects of reinforcement by the strip-shaped reinforcing members 18 provided at the upper slab 11 will be described.
As a result of intensive study, the inventors have found out that when the neck part 4 (see
In contrast, in the reinforcing structure 100 for the manhole 1 according to the present embodiment, the upper slab 11 is also provided with the strip-shaped reinforcing member 18 made of reinforced fiber. This can reduce the local concentration of strain in a location where the distance from the outer edge of the upper slab 11 to the opening 11a is long. As a result, cracks in the upper slab 11 can be prevented.
As described above, according to the present embodiment, the area of the strip-shaped reinforcing members 18 provided at the upper slab 11 is smaller than the area of the strip-shaped reinforcing members 18 provided at the lower slab 13. As described above, the strip-shaped reinforcing members 18 are not excessively provided at the upper slab 11, so that the operation and material costs involved in the reinforcement can be reduced as compared to the case in which the amount of the strip-shaped reinforcing members 18 provided at the upper slab 11 is equal to or more than that of the strip-shaped reinforcing member 18 provided at the lower slab 13.
Furthermore, as described above, the center position of the neck part 4 is provided in a shifted position to one end side (the left side in
Therefore, according to the present embodiment, the area of the strip-shaped reinforcing members 18 provided at the upper slab 11 on the end side described above (the right side in
In this way, reinforcement can be efficiently provided in a position where the distance from the outer edge of the upper slab 11 to the opening 11a is long, in other words in a location where local strain is likely to occur. As a result, local strain can be prevented from increasing in a location where the distance from the outer edge of the upper slab 11 to the opening 11a is large, so that cracks can be reduced.
Also according to the present embodiment, the strip-shaped reinforcing members 18 at the upper slab 11 are provided in the transverse direction B. More specifically, the strip-shaped reinforcing members 18 at the upper slab 11 are provided entirely along the upper slab 11 in the transverse direction B. In this way, stress concentration from the outer edge of the upper slab 11 to the opening 11a can efficiently be relaxed.
Preferably, the strip-shaped reinforcing member 18 at the upper slab 11 is provided to cross the vicinity of the opening 11a. As shown in
In order to determine the reinforcement effect of the reinforcing structure 100 according to the present embodiment, FEM (Finite Element Method) analysis was performed. In the FEM analysis, it was assumed that the material strength of the manhole 1 was lowered, and it was examined at which time point cracks were generated. For the FEM analysis, nonlinear structural analysis software “ATENA ver.5.1.1” manufactured by Research Center of Computational Mechanics, Inc. was used.
In the FEM analysis, two analysis models were produced for the manhole reinforcement structure. In the manholes in the two produced analysis models, the shape and reinforcement arrangement of the manholes were faithfully reproduced from the structural drawing and the reinforcement arrangement drawing. One of the analysis models was a reproduction of the reinforcing structure 100. The other analysis model was substantially equal to the above analysis model as a reproduction of the reinforcing structure 100 except that this model is removed of the strip-shaped reinforcing members 18 from the upper slab 11. Therefore, the difference between the two analysis models is only the presence or absence of the strip-shaped reinforcing member 18 provided at the upper slab 11. Hereinafter, for the sake of illustration, one of the analysis models reproduced as the reinforcing structure 100 will be referred to as “example model” and the other analysis model reproduced as the reinforcing structure 100 removed of the strip-shaped reinforcing member 18 at the upper slab 11 will be referred to as “comparative example model”.
The mesh of the analysis model was a tetrahedral, one side of which is 10 cm and the mesh number was 26000. The following Table 1 and Table 2 show the parameters of the manholes and strip-shaped reinforcing members used in the two analysis models, respectively. The inner diameter of the opening 11a of the upper slab 11 is 0.850 m.
The following Table 3 shows design load parameters used in the FEM analysis.
A lateral soil pressure “QsL” by the live load and a static soil pressure “Qsd” by the relaid soil (the weight of the soil) were assumed to be applied on the outer surface of the sidewall 12, and these pressures were assumed to vary in the depth-wise direction of the soil as shown in the loading states in
The upper load (the live load and the static load) on the upper slab 11, the weight of cables to be accommodated, and the dead weight of the manhole excluding the weight of the lower slab 13 were assumed to act uniformly/equally on the lower slab 13 as a uniformly distributed load (subgrade reaction).
As can be seen from
As in the foregoing, the reinforcing structure 100 for the manhole 1 according to the present embodiment can reduce the cracks C at the upper slab 11 when the neck part 4 (see
The manhole reinforcing method and the reinforcing structure according to the present invention are not limited to the specific method and structure illustrated in the above description of the present embodiment, and various modifications, substitutions, and variations may be made without departing from the scope of claims.
The present invention relates to a reinforcing method and a reinforcing structure for a manhole.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2018-078732 | Apr 2018 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2019/012769 | 3/26/2019 | WO |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2019/202925 | 10/24/2019 | WO | A |
| Number | Date | Country |
|---|---|---|
| 113668612 | Nov 2021 | CN |
| 20082175 | Jan 2008 | JP |
| 4264095 | Feb 2009 | JP |
| 2012229562 | Nov 2012 | JP |
| 5377563 | Oct 2013 | JP |
| Entry |
|---|
| Manhole Handhole for Communication, Mar. 26, 2018, reading day, Sunrec Co., LTD, http://www.sunrec.co.jp/products_detail02/. |
| Category of lid (T-25, T-14) and basis of design), Japan Ground Manhold Association, Mar. 26, 2018, reading day. |
| Number | Date | Country | |
|---|---|---|---|
| 20210148079 A1 | May 2021 | US |
