Abstract:
A shock-absorbing fence comprises: a guard net provided in a stretched state between main posts provided to stand at a predetermined interval; a safety net attached between a skirt of the guard net and a ground; and a member joining an upper rim of the safety net to the skirt of the guard net as well as securing a lower rim of the safety net to the ground, the member including a horizontal support rope member provided along the lower rim of the safety net and a plurality of hillside support rope members connecting between a plurality of points of the horizontal support rope member and a plurality of hillside anchors, the safety net being allowed to deform in an extended manner following deflection and deformation of the guard net, while maintaining a gap below the guard net in a closed state, in case of rockfall.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a continuation application of International Patent Application No. PCT/JP2008/000943 filed Apr. 10, 2008 claiming priority upon Japanese Patent Application No. 2008-026345 filed Feb. 6, 2008, of which full contents are incorporated herein by reference. 
     BACKGROUND 
     1. Technical Field 
     The present invention relates to a shock-absorbing fence that absorbs impacts of falling objects such as falling rocks and soil, and in particular relates to a shock-absorbing fence that has the improved capability to capture falling rocks. 
     2. Description of the Related Art 
     In Japanese Patent Application Laid-Open Publication No. H7-197423, there is disclosed a shock-absorbing fence that includes: tubular main posts built in vertical holes bored at intervals on a hillside slope to be provided in a standing state; and a net formed of metallic mesh provided in a stretched state on each of the tubular main posts along with cables at a plurality of stages. 
     There is also proposed a shock-absorbing fence for absorbing impacts of falling rocks and the like by converting the impact energy into the friction energy, which includes as the basic configuration: main posts provided to stand at predetermined intervals; horizontal rope members anchored between the main posts in a horizontally slidable manner and secured at both ends thereof; and a wire net retained on the horizontal rope members to cover a space between the main posts. In addition to the basic configuration, the proposed shock-absorbing fence further includes as an absorbing section: an extra length section formed by overlapping rope members in the middle of the horizontal rope members; and a clump that clumps the extra length section with a certain strength. In case a tensile force beyond the predetermined level is applied to the horizontal rope members, the extra length section can extend to absorb the tensile force while a certain friction force is maintained on the horizontal rope members (see for example, Japanese Patent Application Laid-Open Publication No. H6-173221, and Japanese Patent Application Laid-Open Publication No. H6-336709). 
     (1) This is a type of shock-absorbing fence configured to stop falling rocks and the like in a band-shaped net stretched across main posts. When such a shock-absorbing fence catches falling rocks, the midsection of the net deflects and deforms by a large amount toward the foot of a slope, and at the same time, the height (effective height) of the fence changes by a large amount. 
     In other words, the net deflects and deforms by a large amount in and around the region hit by falling rocks, and due to such deformation, the length between the upper and lower rims of the net (net width in its height direction) becomes shorter. 
     As a result, in case that rockfalls occur in a repetitive manner, rocks from subsequent rockfalls can slip through a gap wide open between the bottom end of the net and the ground, and the rocks may jump over the net, posing problems in its capability to trap falling rocks. 
     (2) If cost is not taken into account, it is possible to enhance capability to trap falling rocks by reducing the amount of deformation by utilizing a large quantity of absorbers, increasing the number of ropes, and installing a multiple layer of shock-absorbing fences. 
     However, in light of the current severe economic environment, it is difficult to implement costly countermeasures as mentioned above, and for this reason, it is much needed to propose a shock-absorbing fence whose installation costs can be lowered without sacrificing the protection performance. 
     BRIEF SUMMARY 
     An object of the present invention is to solve the problems listed above, and to provide a shock-absorbing fence that meets at least one of the following requirements:
         (1) To reduce the change of the effective height of a guard net in case of rockfall;   (2) To enhance safety by improving the capability to trap falling rocks;   (3) To improve shock absorption capabilities while maintaining low costs; and   (4) To be constructed easily.       

     A shock-absorbing fence according to an aspect of the present invention comprises: a guard net provided in a stretched state between main posts provided to stand at a predetermined interval; a safety net attached between a skirt of the guard net and a ground; and a member joining an upper rim of the safety net to the skirt of the guard net as well as securing a lower rim of the safety net to the ground, the member including a horizontal support rope member provided along the lower rim of the safety net and a plurality of hillside support rope members connecting between a plurality of points of the horizontal support rope member and a plurality of hillside anchors, the safety net being allowed to deform in an extended manner following deflection and deformation of the guard net, while maintaining a gap below the guard net in a closed state, in case of rockfall. 
     In the shock-absorbing fence according to another aspect of the present invention, the guard net includes: upper and lower horizontal rope members extended horizontally between the main posts, the upper and lower horizontal rope members configured to serve as a damper; a diagonal rope member extended diagonally between the main posts, the diagonal rope member configured to serve as a damper; and a mesh member in a band shape attached to the upper and lower horizontal rope members and the diagonal rope member. 
     In the shock-absorbing fence according to still another aspect of the present invention, the mesh member and the safety net are formed of metallic mesh. 
     In the shock-absorbing fence according to still another aspect of the present invention, the safety net is rhombic metallic mesh capable of being rather extended in a longitudinal direction of a slope than extended in a transverse direction of the slope. 
     The present invention can provide at least one of the following advantageous effects: 
     (1) When hit by falling rocks, the safety net moves along with the guard net stretched between the main posts, thereby to ensure entrapment of the falling rocks. 
     (2) In case of rockfall, the safety net extends in a manner following deflection and deformation of the guard net while maintaining a gap below the guard net closed, thereby to reduce the change of the effective height of the guard net. In this way, falling rocks are prevented from slipping through or jumping over the safety net, and the capability to trap falling rocks is greatly improved. 
     (3) The section where the safety net is attached to the guard net is reinforced as the double-layered structure, thereby to enhance the safety reliability of the shock-absorbing fence. 
     (4) When hit by falling rocks, the amount of the impact energy absorbed accompanying the weight shift of the guard net increases by the weight of the safety net, and the amount of the impact energy absorbed also increases by the friction resistance between the hillside slope and safety net. 
     As a result, there is less deflection and deformation of the guard net than before. 
     In particular, it is suitable for installation in locations adjacent to road infrastructure, railroad facility, and housing, for example, where the amount of deflection and deformation of the guard net is strictly limited. 
     (5) Installation can be performed with simple additional work of attaching a safety net which exerts absorption of impact force to a shock-absorbing fence, and thus, a low-cost shock-absorbing fence with high shock-absorbing performance can be provided. 
     (6) The technology is very versatile, as it can be applied to a variety of well-known shock-absorbing fences equipped with main posts and guard nets. 
     (7) The installation merely involves: bringing in component members of the shock-absorbing fence into the site; providing tilted main posts to stand without laying concrete foundation; providing to stretch a plurality of rope members and a mesh member; and attaching a safety net to the skirt of the mesh member, and thus, it is suitable for installation in mountain areas and precipitous terrains to which heavy vehicles are less accessible. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       For more thorough understanding of the present invention and advantages thereof, the following description should be read in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a perspective view illustrating a partially omitted shock-absorbing fence according to an embodiment of the present invention. 
         FIG. 2  is a top view illustrating a shock-absorbing fence. 
         FIG. 3  is a section view illustrating the section III-III in  FIG. 1 . 
         FIG. 4  is an enlarged view illustrating the top part of a main post to indicate connection configuration of horizontal rope members. 
         FIG. 5  is an upper half view illustrating a shock-absorbing fence to indicate another connection configuration of horizontal rope members. 
         FIG. 6  is a model view illustrating a shock-absorbing fence to explain the mechanism for capturing a falling rock. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. 
     (1) Entire Configuration of a Shock-Absorbing Fence 
       FIGS. 1 and 2  illustrate one example of a shock-absorbing fence according to the embodiment of the present invention. 
     It is assumed that the present invention includes a shock-absorbing fence equipped with main posts  10  provided to stand at predetermined intervals and a guard net  20  stretched across the main posts  10 . Moreover, the present invention is provided with a safety net  30  installed between the skirt of the guard net  20  and a slope  40  as means for reducing the change of the effective height of the guard net  20 . 
     The shock-absorbing fence in this embodiment is only exemplary and it can be applied to a variety of well-known shock-absorbing fences as long as it is equipped with protective nets. 
     Hereinafter, the configuration of a shock-absorbing fence according to an embodiment of the present invention will be described in details. 
     (2) Main Post 
     Main posts  10  to be provided to stand at predetermined intervals on a slope  40  is each formed of a hollow tube member. Conventional, standard main posts are provided to stand on concrete foundation laid on the installation position. In this embodiment, however, installation of the concrete foundation is omitted, and the main posts  10  are installed in a tiltable manner with respect to the slope  40 . 
     In advance, an anchor or a lock bolt is fixed in the ground at the installation position of each main post  10 , and a main post rope member  11  integrally connected to the anchor or lock bolt is exposed on the surface of the ground. 
     Then, as illustrated in  FIGS. 3 and 4 , the main post rope member  11  is inserted inside the main post  10  through from the lower opening, and then an absorber  12  is secured at the base of the main post rope member  11  projected from the upper opening of the main post  10 , so that the main post  10  is installed in a manner to maintain the standing attitude. 
     In this embodiment, there is provided an example that the main post rope member  11  pulled out of a main post cap  15  covering the top of the main post  10 , and that the base of the main post rope member  11  secured by an absorber  12  with a size unslippable through the opening of the main post cap  15 . 
     The absorber  12  allows the main post rope member  11  to slide when a tensile force beyond friction resistance is applied between the main post rope member  11  and the gripping section of the absorber  12 , and damps the impact energy when the main post  10  is tilted. 
     As an absorber  12 , well-known friction-resistance type absorbers such as those formed of two plate bodies clamped together by bolts and nuts can be used. 
     As appropriate, a supporting rope  13  may also be connected between each of top and bottom parts of the main post  10  and the anchor  41  on the slope  40 , and an absorber  17  may be provided in an appropriate manner at the end or in the middle of the supporting rope  13 . 
     In this embodiment, a tubular support  14  covers the exterior of the supporting rope  13  which is connected to the top part of each main post  10 , thereby to prevent a fence from falling toward the hillside in case of rockfall. 
     (3) Guard Net 
     A guard net  20  is a member to stop falling rocks and the like. In this embodiment, as illustrated in  FIG. 1 , the guard net includes: upper and lower horizontal rope members  21 ; diagonal rope members  22  extended across main posts  10 ; and a mesh member  23  attached to the rope members  21  and  22  via helical coils and the like. 
     The rope members  21 ,  22  and the mesh member  23  are combined in order to eventually transmit to the main posts  10  the impact force of falling rocks transmitted from the mesh member  23  as well as the impact force and loads acting directly on the rope members  21  and  22 , and also in order to absorb efficiently the impact energy in synergy with the mesh member  23 . 
     Also, in order to allow transmission of a large force between the rope members  21 ,  22  and the mesh member  23 , each of the rope members  21  and  22  is woven through each mesh of the mesh member  23 , or the rope members  21  and  22  placed one side of the mesh member  23  are bundled to be stitched onto the mesh member  23  by means of helical coils. 
     Hereinafter, component members of the guard net  20  will be described in details. 
     (3-1) Rope Member 
     An absorber  12  is installed near the end of each of the upper and lower horizontal rope members  21  stretched laterally across the main posts  10  to provide these rope members  21  with shock-absorbing function. 
     Each of the upper and lower horizontal rope members  21  is stretched laterally across the main posts  10  in a continuous manner. 
     In this embodiment, the upper and lower horizontal rope members  21  are provided in a stretched state between the main posts  10  adjacent to each other; however, it goes without saying that the upper and lower horizontal rope members  21  may be provided in a stretched state between the main posts  10  distant beyond the installation span thereof.  FIG. 4  is an enlarged view of connection configuration of the horizontal rope members  21  illustrated in  FIGS. 1 and 2 . In this embodiment, two adjacent horizontal ropes  21  are connected with each other via the main post  10  positioned therebetween in such a manner that the two horizontal rope members  21  are provided to penetrate respectively two parts of the main post  10  different in height, and that absorbers  16  in a laterally pairwise manner are installed to hold respectively the ends of the two rope members  21  penetrating and jutting out the two parts and to abut respectively on both sides of the main post  10 . 
       FIG. 5  illustrates a configuration where two horizontal rope members  21  are connected through an absorber  16  that does not abut on the main posts  10 . In this embodiment, the two horizontal rope members  21  penetrating respectively the main posts  10  are overlapped between the main posts  10 , and the overlapped section of the two horizontal rope members  21  are held by the absorber  16 , thereby connecting between the two adjacent horizontal rope members  21 . 
     The configuration of the horizontal rope members  21  stretched laterally at the lower parts of the main posts  10  is similar to the configuration of the horizontal rope members  21  stretched laterally at the upper parts of the main posts  10  shown in  FIGS. 4 and 5 , and thus, the illustration of the former configuration is omitted. 
     An absorber  16  has only to serve as holding one or two rope members  21 , and serve as the damping energy while allowing the rope members  21  to slide, when the rope members  21  are applied with a tensile force beyond the level of friction resistance of the gripping section, and therefore, a well-known friction resistance type of absorbers such as those formed of two plate bodies clamped together by bolts and nuts, for example, can be used for the absorber  16 . 
     Diagonal rope members  22  are extended across the upper and lower parts of the adjacent main posts  10  to cross each other. As shown in  FIG. 4 , the diagonal rope members  22  may be extended across hooks  10   a  protruded respectively from the upper and lower sides of the main posts  10 , and absorbers such as wire clips (not shown) may be arranged close to the end or in the middle of the diagonal rope members  22 . 
     (3-2) Mesh Member 
     A mesh member  23  is formed of a mesh net of reticulated high-strength fiber such as metallic mesh and aramid fiber, or of a plastic mesh net. In consideration of the workability and cost, a rhombic metallic mesh is preferred. 
     As illustrated in  FIG. 1 , the mesh member  23  is joined in a movable manner to the entire length or a part of the rope members  21 ,  22  by wrapping helical coils and the like around the mesh member  23  and rope members  21 ,  22 . 
     The rhombic metallic mesh used for the mesh member  23  inherently has difference in the degree of extensibility between vertical and horizontal directions due to the manufacturing process. For this reason, when fixing the mesh member  23  between main posts  10 , the rhombic metallic mesh should be so arranged as to be rather extended in a horizontal direction than extended in a vertical direction in  FIG. 1 , in order to achieve the improved capability of mesh member  23  to capture falling rocks. 
     (4) Safety Net 
     A safety net  30  has a length longer than the gap between the guard net  20  and the slope  40 , and is formed of a mesh net of reticulated high-strength fiber such as metallic mesh and aramid fiber. In consideration of the workability and cost, a rhombic metallic mesh is preferred. 
     One (top) end of the safety net  30  is integrally joined to the skirt of the mesh member  23  by wrapping helical coils and the like around the one end and the skirt, whereas the other (bottom) end of the safety net  30  is secured to the slope  40 , thereby covering the gap created between the skirt of the mesh member  23  and the slope  40 . 
     As shown in  FIG. 2 , in this embodiment, there are connected respectively between: a plurality of locations on a horizontal support rope member  31  integrally attached along the other (bottom) end of a plurality of safety nets  30 ; and a plurality of hillside support rope members  32  extended from anchors  41  installed on the hillside of the slope  40 , so that the anchors  41  can support the external force applied to the safety net  30 . 
     Anchor pins and the like may also be used to secure the other (bottom) end of the safety net  30  directly to the slope  40 . 
     The safety net  30  has a combination of: the function of closing a gap below the skirt of the mesh member  23  to prevent falling rocks from physically slipping through; and the function of reducing the change of the effective height of the guard net  20  in case of rockfall. 
     For that purpose, as illustrated in  FIG. 3 , the middle or lower portion of the safety net  30  should be slacked in a bellows shape, folded up or folded back, or the like to be provided with a sufficient length, thereby having such a margin as to expand toward the foot of a hill to follow the deflection and deformation of the safety net  20  in case of rockfall. 
     Therefore, the total length of the safety net  30  should be adopted appropriately in accordance with the conditions of the site, since such a total length effects on the amount of deflection/change of the guard net  20  toward the foot of the slope/the effective height of the guard net  20 , in case of rockfall. 
     The overlapped area where the safety net  30  is installed over the guard net  20  is reinforced by the double-layered structure; thus, the higher the joint position between the one (top) end of the safety net  30  and the guard net  20  is, the larger the reinforced area of the guard net  20  is. 
     One has only to adopt appropriately the joint position between the one (top) end of the safety net  30  and the guard net  20  in accordance with the site where the shock-absorbing fence is installed. 
     When a rhombic metallic mesh is used for the safety net  30 , such a rhombic metallic mesh should be so arranged as to be rather extended in a vertical direction (longitudinal direction of the slope  40 ) than extended in a horizontal direction (transverse direction of the slope  40 ) in  FIG. 1 . 
     Also, when the safety net  30  is arranged along the mesh member  23  as illustrated in  FIG. 2 , overlapping a plurality of rhombic metallic meshes is desirable. 
     [The Operation of the Shock-Absorbing Fence] 
     Hereinafter, the operation of the shock-absorbing fence is described based on  FIG. 6 . 
     (1) Shock Absorption by the Guard Net and Main Posts 
     When falling rocks and the like hit a shock-absorbing fence including the main posts  10  and the guard net  20  described above, the impact energy is damped by the sliding of wire members constituting the mesh member  23  of the guard net  20  as well as by the deflection and deformation thereof toward the foot of the slope. 
     Further, the impact energy applied to the guard net  20  is also applied to the rope members  21  and  23 . 
     When the rope members  21  and  23  are applied with a tensile force beyond the level of friction resistance of the gripping section of the absorber, each of the rope members  21  and  23  slides inside the absorber resisting against the grip force of the absorber. The impact energy is damped by the friction force generated at such occasion. 
     The impact energy is transmitted through the mesh members  23 , rope members  21  and  23 , and then, ultimately to the main posts  10 . 
     When the impact energy applied to the main posts  10  exceeds the energy corresponding to the grip force of the absorber  12  installed on top of the main posts  10 , the main post rope members  11  slide inside the absorber  12 , and the friction force caused by such sliding serves as a braking force against the load to absorb the impact energy, while allowing the main posts  10  to be tilted. 
     When the support ropes  13  are connected to the main posts  10  and the absorbers  17  are provided on these support ropes  13 , the impact energy is damped by the sliding resistance between the support ropes  13  and the absorbers  17 . 
     (2) Shock Absorption by the Safety Net 
     As described above, when falling rocks hit the guard net  20  of the shock-absorbing fence, the guard net  20  deflects and deforms toward the foot of the slope while the main posts  10  become tilted. 
     With the guard net  20  deflecting and deforming toward the foot of the slope, the safety net  30  connected to the skirt of the mesh members  23  also expand toward the foot of the hill following the deflection and deformation of the guard net  20 . 
     The impact energy is also damped by sliding resistance, weight-shift resistance, and deformation resistance, of the safety net  30 , generated at such occasion. 
     (3) Reduction of the Change of the Effective Height 
     The gap between the skirt of the guard net  20  and the slope  40  is closed by the safety net  30 , thereby assuring to prevent falling rocks from slipping through the gap below the skirt of the guard net  20 . 
     The falling rocks which hit the skirt of the guard net  20  and the safety net  30  are guided into the guard net  20  and captured therein. 
     The safety net  30  installed between the guard net  20  and the slope  40  not only serves as a shielding member to prevent falling rocks from slipping through, but also serves as reducing the change of the effective height of the guard net  20  as described below. 
     In other words, the degree of extension of the safety net  30  toward the foot of the slope in case of rockfall is greater in relation to that of the guard net  20 . 
     Therefore, compared with the case the safety net  30  is not installed, when the safety net  30  is extended to expand toward the foot of the hill in compliance with the guard net  20  deflecting and deforming toward the foot of the slope, the tensile force applied to the upper part of the guard net  20  in the direction toward the center of the net is absorbed by a large amount. 
     As a result, even if the guard net  20  deflects and deforms by a large amount toward the foot of the slope, the actual effective height (fence height) of the guard net  20  does not change by a large amount. 
     As described above, by attaching the safety net  30  along the guard net  20 , subsequent falling rocks can be not only prevented from slipping through the guard net  20 , but also restrained from jumping over the guard net  20 , thereby dramatically improving the capability to capture the falling rocks. 
     (4) Reduction of the Deflection of the Guard Net 
     When hit by falling rocks, the amount of the impact energy absorbed accompanying the weight shift of the guard net  20  increases by the weight of the safety net  30 . Furthermore, the amount of impact energy absorbed also increases by the friction resistance between the slope  40  and the safety net  30 . 
     As a result, the amount of deflection and deformation of the guard net  20  can be adjusted to be less than before. 
     (5) Other Embodiments 
     Described above is an embodiment where the safety net  30  is a member separable from the guard net  20 , by which the gap below the skirt of the guard net  20  is closed. However, the guard net  20  may also be formed with an extended skirt section, and the extended section of the guard net  20  may be utilized as a safety net. 
     The above embodiments of the present invention are simply for facilitating the understanding of the present invention and are not in any way to be construed as limiting the present invention. The present invention may variously be changed or altered without departing from its spirit and encompass equivalents thereof.