Cover for underground structures

Abstract: A cover for underground structures has a large number of mutually independent projections of the same size and shape uniformly provided on its surface, while small protrusions of the same size and shape are formed on the upper surfaces of some or all of these projections. On top of these small protrusions are formed even smaller protrusions of the same size and shape as the topmost protrusions. Spacing between one large projection and another, between one small protrusion and another, or between one small protrusion and a projection having no small protrusion on its top is set at the optimum distance, and the size of the topmost protrusions is also designed in a shape to fit an optimum size to ensure good skid-prevention effect on tires of vehicles passing over the cover, especially in bad weather over a long period of time, no matter where the cover is installed.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to covers for underground structures, and more particularly to covers for underground structures which are designed to more efficiently prevent tires of vehicles such as motorcycles from slipping and skidding. Covers for underground structurest refers herein to large iron covers to close openings which connect buried materials as well as structural sewerage facilities to above ground, manhole covers, covers of house inlets, openable and closable iron covers for common-use tunnels which protect apparatus and equipment for underground power and communications facilities, iron covers for electric power transmission, iron covers for power distribution, fire hydrant covers functioning as openable and closable doors to connect underground conduits in waterworks systems, gas pipes and their accessories to above the ground, sluice valve covers, air-valve covers, covers for gas distributing pipes, and water-gauge valves. 2. Description of the Prior Art On the surfaces of the currently available bodies of covers for underground structures placed on sidewalks and roadways are formed protruding and recessed patterns which serve as decorations as well to prevent pedestrians from slipping and vehicles from skidding in inclement weather. Examples of the cover bodies with improved surface patterns for slip and skid prevention are available in Japanese Unexamined Utility Model Publication No. 42252/83 and Japanese Unexamined Utility Model Publication No. 86156/88. The models disclosed therein are concerned with a cover body on the surface of which is formed projections on top of which are formed small protrusions and recesses, or small protrusions being formed on recessed portions in areas other than the projections. In other words, these models seek to expand the surface coarseness of consecutive protrusions or depressions by adding small protrusions and recesses to the protruding and recessed patterns on top of the cover body and to improve anti-slip and anti-skid performance by increasing frictional resistance. Nonetheless, the small protrusions and recesses provided on top of the projections on the surface of the cover body come into contact with tires whenever vehicles pass and rub therewith, thereby wearing out with the passage of time after installation, with a resultant reduction of frictional resistance leading to diminished anti-slip and anti-skid effect. Even immediately after the installation, as dirt, rainwater, and other materials accumulate on the recessed portions of small projections and recesses, the frictional resistance will not effectively act upon the tires, and anti-slip and anti-skid effect will likewise deteriorate. In this manner, the conventional structure purports to enlarge the surface coarseness of the cover body and to increase frictional resistance, whereas the mere provision of small protrusions and recesses fails to maintain the anti-slip and anti-skid performance over a long period of time. Another problem is that when dirt, rainwater, and other materials are involved, it is impossible to bring the anti-slip and anti-skid performance into play at the outset. JP-U-58-42252 discloses projections and recesses formed in the unevenness portions of the conventional iron cover to increase the frictional resistance of a cover for underground structures by enlarging the surface roughness of the cover body. The small unevenness provided on the surface of the cover body contacts the tire whenever a car passes. FR-A-2252013 discloses a cover for underground structures which increases the adhesion of the tire on the surface of the cover. On the surface of the cover there are formed a plurality of identical projections having the shape of truncated pyramids whereby these projections are arranged in a regular manner on the surface. SUMMARY OF THE INVENTION It is the object of the invention to provide a cover for underground structures which more effectively prevents tires of vehicles from slipping and skidding. This object is fulfilled by a cover for underground structures having the features. disclosed in claims 1 and 5 respectively. Preferred embodiments are defined in the dependent subclaims. The cover for underground structures according to this invention can maintain excellent anti-slip and anti-skid performance over a long period of time regardless of the installation environment, through optimization of arrangement patterns of projections to be provided on the surface of the cover body as well as the shape of the projections. Accordingly, the present invention provides a cover for underground structures wherein small protrusions are formed on top of some or all of the above-mentioned projections, and wherein size of the topmost surface of each topmost level of the above-mentioned small protrusions and projections on which no small protrusion is formed fits inside a circle of a diameter of 25mm, whereas at least one of the minimum pitches between the outlines of each of the above-mentioned projections and the outlines of the above-mentioned small protrusions is 10mm to 30mm long. Furthermore, the top surfaces of at least part of the projections and the small protrusions comprise corners at the outer rim thereof, and wherein the corners of the small protrusions of the topmost level and the corners of the small protrusions of the lower level or of the projections are so located to be positioned in a rotated manner with respect to each other when seen from above. BRIEF DESCRIPTION OF THE DRAWINGS Preferred embodiments of the invention are described in the following detailed description including the drawings in which: Fig. 1 is a schematic plan view of a cover for underground structures not covered by the appended claims; Fig. 2 illustrates a specific example of a projection and a small protrusion of the cover of Fig. 1; Fig. 2 (a) is a perspective view; Fig. 2 (b) is a longitudinal section of Fig. 2 (a); Fig. 3 illustrates conceptually shapes and a positional relationship of projections and small protrusions of the embodiment of Fig. 1; Fig. 4 is a longitudinal section showing pitches L relative to arrangements of projections for purposes of defining the pitches L between projections of varying shapes of longitudinal sections with; Fig. 5 is a plan view showing pitches L relative to arrangements of projections to define the pitch L between projections; Fig. 6 is a plan view of an embodiment of this invention; Fig. 7 shows a detail of an example of protruding parts of the embodiment of Fig. 6; Fig. 7 (a) shows a plan view of an arrangement example thereof; Fig. 7 (b) illustrates a sectional shape of the protruding parts of Fig. 7 (a) taken in the direction of the arrows substantially along line B-B; Fig. 8 illustrates a detail of another protruding parts of the embodiment of Fig. 6; Fig. 8 (a) shows a plan view of an arrangement example thereof; Fig. 8 (b) illustrates a sectional shape of the protruding parts of Fig. 8 (a) taken in the direction of the arrows substantially along line C-C; Fig. 9 is a plan view of another embodiment of this invention; Fig. 10 illustrates a detail of an example of the protruding parts of the embodiment of Fig. 9; Fig. 10 (a) is a plan view of an arrangement example thereof; Fig. 10 (b) shows a sectional shape of the protruding parts of Fig. 10 (a) taken in the direction of the arrows substantially along line D-D; Fig. 11 is a plan view of the protruding parts of Fig. 9 in a worn-out state; DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS In addition to the foregoing construction, it is possible to have another construction of the cover body in accordance with the present invention, wherein projections and/or small protrusions having sharp-angle edges are disposed on at least one some of the surface thereof. Moreover, the cover body for underground structures wherein the patterns of projections and depressions are formed on the surface thereof by means of a plurality of projections and depressions may be of such construction that projections having sharp-angle edges are arranged on at least one some of the surface thereof, and that small protrusions are further formed on the top surface of some or all of the projections with the projections and/or small protrusions having sharp-angle edges provided upon at least one some of the surface of the cover body. In regard to at least one of the projections with small protrusions formed thereon, the level of the above-mentioned projections may be higher than that of other projections on which small protrusions are formed. It is to be noted that the pitch of the outlines between projections and small protrusions independently formed on the surface of the cover body approximately corresponds to a pitch of the points of action of the engaging force subject to the repeated loads of tires, as vehicles pass over the cover body. Consequently, it is possible to determine the mode of action of the engaging force best suited to the tires by determining how the engaging force acts on the tires according to the length of pitches of the outlines between mutually adjacent projections or mutually adjacent small protrusions. The present invention specifies the pitch of the outlines between the projections and between small protrusions as well as the shapes of such which are best suited in terms of such mode of action of the engaging force with tires, and the desired object of the present invention is accomplished if at least one of the pitches of the outlines between the adjacent protruding parts including small protrusions is set to be in a range of 1 0mm to 30mm, while the planar shape of the topmost surfaces of the protruding parts is such as to be contained in a diameter of 25mm. Also, the engaging force with the cover body with sharp-angle edges provided on the outer periphery side of the projections and the small protrusions is reinforced due to the sharp-angle edges. The small protrusions and edges, when formed on all the projections established on the cover body, will offer the best slip and skid prevention effect over the entire surface of the cover body, and in the case of forming such portions on some of the projections, an arrangement of such portions uniformly over the surface thereof will enhance the effect of slip and skid prevention. It will be appreciated that as far as at least one of the above-mentioned projections on which small protrusions are formed on a level higher than that of the other projections on which small protrusions are formed, the small protrusions will wear out, so that the surfacing time of the above-mentioned projections can be used as a measure of when to replace the cover body. Namely, upon surfacing of the above-mentioned projections, the small protrusions formed on other projections are still remaining, whereby this time is used as a criterion of the period of replacement of the cover body, degradation of the anti-slip and anti-skid performance can be prevented. Fig. 1 is a plan view of a body of a cover for underground structures not covered by the appended claims, the description of which is useful for understanding the present invention and features of which can be used in combination with the present invention. A body cover 1 therein is conventionally taper-fit to a frame (not illustrated herein) set underground, and on the surface thereof are independently formed projections 2 of a planar regular hexagon with sides of the outlines of the projections 2 so disposed to be parallel to those of other adjacent projections, whereas small protrusions 2a in a reduced planar outline shape of a projection 2 stick out at a uniform level on the surface of all projections 2, as shown in Fig. 2. As clearly illustrated by Fig. 2, the projection 2 is formed at a uniform level in a planar regular hexagon, a small protrusion 2a being in a relationship of similarity to an outline shape of the projection 2 with the top surface thereof uniformly flat. It is to be noted that the level of each top surface of each small protrusion 2a is designed to be identical. Fig. 3 is a view equivalent to a longitudinal section in the direction of the arrows substantially along line A-A, conceptually illustrating a positional relationship of mutually adjacent projections 2 with small protrusions 2a, together with an outline of a tire 50. This cover is constructed to meet the requirements that a pitch L between the mutually adjacent outlines of one small protrusion 2a formed on one projection 2 and another be set in a range from 10mm to 30mm, and that the top surface in a planar shape of all small protrusions 2a be included in a circle of a diameter of 25mm. The projections 2 are arranged over the surface of the cover body 1 in the embodiment It is to be understood that when marks such as a municipal mark or characters are to be placed on the cover body 1 or when an edge is provided on the outer periphery, the projections 2 are disposed on the area excluding such items. It is also illustrated that the small protrusions are formed on all of the projections 2. But, a pattern wherein part of the projections 2 are dispersed is acceptable, although the levels of small protrusions 2a and the projections 2 on which no small protrusions are formed are preferably identical. Referring to Fig. 4 and Fig. 5, there are shown illustrations to define the pitch L between the outlines of one projection and another in accordance with this invention. For the sake of simplifying description, a pitch L between projections with no formation of small protrusions 2a is shown. In this cover, the length between points of engagement with a tire on a projection 2 and another projection 2 shall be a basic unit regardless of the shape of longitudinal section of the projections 2. That is, in the case of a rectangular shape in a longitudinal section as shown in Fig. 4 (a), the basic unit shall be the length between the corners forming upper edges, whereas in the case of a trapezoidal shape in the longitudinal section, the basic unit shall likewise be the length between the comers of upper edges. Further, in the case of a triangle or a spherical shape of longitudinal section in Fig. 4 (c), the basic unit shall be the length between the points on the respective highest levels. In addition to the length between points of engagement with the tire as shown in Fig. 4 as the definition of the pitch L between projections 2, there is another condition of the pitch L as the length of a line portion equivalent to the mutually shortest distance. Namely, as shown in Fig. 5, when the projections 2 are in a planar shape of hexagon, for respective cases of a lattice-like arrangement of the projections 2 of Fig. 5 (a) and an offset arrangement thereof of Fig. 5 (b), the length equivalent to the line portion with the arrows in respective views shall be the pitch. For instance, when the lattice-like arrangement is employed, the length between the comers or the facing sides shall be the pitch L, while the length between the facing sides shall be the pitch L for all cases of the offset arrangement. At this point, as explained in connection with the cover of Fig. 1, there is the following basis of requirements which set the pitch L of the outlines between mutually adjacent small protrusions 2a as anywhere from 10mm to 30mm, and which stipulates that as far as all small protrusions 2a or the projections 2 on which no small protrusions 2a are formed are concerned, the topmost surface of such protrusions and projections in a planar shape should be of a size containable within a circle of a diameter of 25mm. Inasmuch as these requirements on the pitch L and the size of the planar shape including the projections 2 and the small protrusions 2a according to this invention concern the arrangement pattern and shape of the protruding parts, the following description will be made by referring to the projections and the small protrusions simply as the general term of "Protruding Part(s)." For the cover body 1 of manhole covers installed on general roads to effectively prevent slipping and skidding of tires of vehicles, one criterion is that the frictional resistance thereof should be at least equal to the frictional resistance of the road surface wet with rain and other matter. The frictional resistance of a road surface wet with rain and other matter of general roads is supposed to be within a range from 0.45 to 0.6, for example, according to the Safety Driving Traffic Manual (published by the Safety Driving Control Association of Home Delivery Businesses Including Pizza Delivery, p. 7). Consequently, this invention specified the shape of the protruding parts of the cover body 1 and the arrangement patterns of such parts so that the frictional resistance against the tires shall be in excess of 0.45. In other words, when a large frictional resistance exceeding 0.45 is designated, not only is it important to increase the number of points on which the tire comes to engage the protruding parts of the cover body 1 when the tire passes the surface thereof, but also to deform the tire sufficiently. The following phenomena were verified after the inventor researched the relationship of frictional resistance against tires and the pitch L, as well as the size of the topmost surfaces of the protruding parts. (1) When the pitch L is too small, tire deformation decreases and the gripping force is reduced. (2) When the pitch L is too large, the tire comes into contact with the bottom surface of the recess between the protruding parts of the cover body 1, with the result that the tire deformation decreases or the tire only passes the bottom surface so that tire deformation is not prompted. This phenomenon is particularly noticeable on narrow treads. (3) When the dimension of the topmost surface of the protruding parts is too large, the number of points on which the tire comes to engage decreases relative to the whole dimension of the cover body 1, and the repetition of tire deformation when the tire passes the surface of the cover body 1 also decreases. In view of the foregoing, it is apparent that proper settings are needed in regard to the pitch between the protruding parts and the size of the topmost surfaces of such parts if the necessary frictional resistance is to be obtained. After conducting due research and examination of the relationship among the pitch L, the size of the topmost surfaces of the protruding parts, and frictional resistance including relevancy of the types of tires, the inventor came to the conclusion that so long as the requirements of the pitch L to be in a range from 10mm to 30mm and the size of the topmost surfaces of the protruding parts to be such as to be fit within a circle of a diameter of 25mm are met, frictional resistance will be in excess of 0.45. It is apparent that the smaller the topmost surface shape of the protruding part is, the more engaging power with the tire is produced, and the more slip and skid prevention effect is obtained. From this standpoint, the most effective method is to shape the protruding part into, for example, a tip portion tapering off in the longitudinal section of the triangular shape shown in Fig. 4 (c). In this case, the part in engagement with the tire is a point or, to be specific, a flat area of naught, thus meeting one of the requirements that the size of the topmost surface be such as to fit within a circle of a diameter of 25mm. On the other hand, regarding the level of the protruding part, if such level should be extremely low, the tire would contact the bottom surface of the recessed part as in the case of the excess pitch L, so that the slip and skid prevention effect due to tire deformation becomes insufficient. Accordingly, even empirically speaking, it is proper to set the level of the protruding part in excess of 1 mm to maintain at least a minimum of slip and skid prevention effect. In actuality, should the level thereof be exceeding 6mm, there is a disadvantage in that the protruding part tends to break. Likewise, from an empirically standpoint, it is desirable to set approximately 6mm as the maximum level thereof. Hence, it is preferable according to this invention that the level of the protruding part be set at a range from 1 mm to 6mm, regardless of the planar shape thereof. When a small protrusion 2a is to be formed on the top surface of a projection 2 as in this embodiment, it is preferable to set the respective levels of the projection 2 and the small protrusion 2a in the range from 1 mm to 6mm. In other words, when the protruding part comprises a plurality of steps, the respective levels of the steps are preferably set in the range from 1 mm to 6mm. As mentioned above, as far as the protruding part is concerned, in addition to the conditions stipulating that the size of the topmost surface thereof in a planar shape is to fit within a circle of a diameter of 25mm and that the pitch L as defined above is to be provided between the adjacent protruding parts, meeting a further condition of 1 mm to 6mm for the level thereof will bring about the best effect on slip and skid prevention. Moreover, it is to be understood in this cover that as Fig. 3 illustrates, each ratio of the level of the small protrusion 2a to the level up to the top surface of the projection 2 and that of the small protrusion 2a's planar area to the projection 2's planar area are approximately to an extent that the tire 50 is in engagement with only the small protrusion 2a with the small protrusion 2a not in a worn-out state as illustrated and that the tire 50 is not in contact with the top surface of the projection 2. In view of the foregoing, even if the small protrusion 2a gradually wears out, the projection 2 positioned at the step thereunder comes into contact with the tire 50. Therefore, even if the engaging force with the tire 50 diminishes as a result of the wearing out of the small protrusion 2a to cause the comers thereof to be rounded, a new engaging force due to the comers of the projection 2 will come to action, thereby checking any drop in slip and skid prevention effect and enabling the service life of the cover body 1 to be extended. As a result, even in an installation environment subject to a large number of passing vehicles, the effect of slip and skid prevention can be maintained over a long period of time. In this case, it goes without saying that the optimum procedures are to set not only the pitch between the outlines of two adjacent small protrusions 2a but also that of the outlines of two adjacent projections 2 positioned at the next lower level at a range from 10mm to 30mm. Fig. 6 is a plan view showing an embodiment of this invention. A cover body 3 has the periphery thereof as a highest-level edge 3a and the part excluding this edge 3a is formed as a uniformly flat base 3b having a surface shape, wherein two types of protruding parts 4 and 5 are dispersed. In Fig. 6, a mark seat 3c and character seats 3d are provided on the central part of the cover body 3, so that a mark such as a municipal seal or characters and symbols indicating use of the cover body 3 and other matters are to be respectively put into the mark seat 3c and the character seats 3d. As shown by the detail view of Fig. 7, a protruding part 4 is formed of a planar shape directly rising from a base 3b on top of a hexagonal projection 4a with additional planar shapes forming respectively two levels of small protrusions 4b and 4c in hexagonal shapes. The projection 4a and the small protrusion 4b thereon are respectively in agreement with a hexagonal posture, the small protrusion 4c of the topmost level being in a posture rotated 30 degrees relative to the small protrusion 4b thereunder and the projection 4a to position the corners thereof differently. By making the corners of a small protrusion 4c of the topmost level take a different position than the corners of the small protrusion 4b thereunder and the projection 4a, despite a variety of advancing directions of tires, the engaging force by the protruding part 4 as a whole can be increased, thus holding the slip and skid prevention effect at high levels. In the protruding part 4 of such a shape, the planar shape of the small protrusion 4c of the topmost level is of a size to be contained within a circle of a diameter of 25mm, and the pitch thereof may be such that either one of the dimensions L1 to L6 shown in Fig. 7 (b) only needs to be in the range from 10mm to 30mm. However, the pitch L6 between the adjacent small protrusions 4c of the topmost level should preferably be in the range from 10mm to 30mm. It is to be noted that in this embodiment, each of L2 to L6 excluding the pitch L1 between the projections 4a is set at a value in the range from 10mm to 30mm. Also, the levels of the projection 4a and the prominent portions of the small protrusions 4b and 4c are respectively 2mm, the level of the entire protruding part 4 from the base 3b up is 6mm. Another type of protruding part 5, as shown by a detail view of Fig. 8, has a shape of two levels of small protrusions 5b and 5c laid over a projection 5a in a planar shaped hexagon. As shown in Fig. 6, these projections 5a are dispersed in a one-sided way near the central part side of the cover body 3, each having a set of seven small protrusions 5b and 5c being formed thereon. These small protrusions 5b and 5c are in the same planar hexagon as other projections 4, the top level small protrusion 5c taking a posture rotating 30 degrees relative to the small protrusion 5b thereunder. In this protruding part 5, the level of the prominent portion from the base 3b is 6mm, the level of the prominent portions of the small protrusions 5b and 5c is 2mm. The planar shape of the top level small protrusion 5c is of a size containable within a circle of a diameter of 25mm in the same way as the protruding part 4. The pitches L1 to L6 shown in Fig. 8 (b) can be respectively set in a range from 10mm to 30mm, and this same setting can be established for one of the respective sets of L1 to L3 and L4 to L6. However, the setting of 1 0mm to 30mm is preferable for the pitches L2 and L5 between the mutually adjacent topmost level small protrusions 5c. Note that the illustrated embodiment is based on the relationships of L1=L4=12mm, L2=L5=19mm, and L3=L6=16mm. As described above, even those projections 4a and 5a with the small protrusions 4b, 4c, 5b, and 5c respectively formed on them can permit an engaging force due to tire deformation, to act effectively by meeting the condition of the pitch L; and since the engaging force due to the corners of the small protrusions 4b, 4c, 5b, and 5c or the projections 4a and 5a can be made to act, slip and skid prevention can be enhanced. It will be appreciated that the region enclosed by the long and dotted demarcation lines in Fig. 6 can be treated as a base pattern 3e to be formed at a level of about 1 mm from the base 3b. This base pattern 3e is established also, for example, for the purpose of prompting discharge of water toward the edge 3a of the cover body 3, although the protruding parts 4 and 5 to be disposed on the top surface of the base 3b and the base pattern 3e have the same level of respective topmost surfaces. In the above-mentioned embodiment, the pitch L between the mutually adjacent projections and that of the mutually adjacent small protrusions as well as the size of the topmost surfaces are designated to improve the anti-slip and anti-skid performance. In addition to this construction, another procedure may be used, wherein fine protrusions and recesses which are conventionally employed are provided on the top surface of the projections and the small protrusions to enlarge the surface coarseness of the projections and the small protrusions. Fig. 9 shows still another embodiment, wherein some of the protruding parts 4 shown in the previous embodiment from Fig. 6 to Fig. 8 are replaced with different protruding parts 6. The parts or portions of this embodiment which are the same as those shown in the previous embodiment are shown in the same numbers and symbols, a detailed description of which will be omitted. As shown in Fig. 9, in this embodiment, six protruding parts 6 are uniformly arranged at positions which are substantially in the middle between the central part of the cover body 3 and the edge 3a. As shown in the detail view of Fig. 10, a protruding part 6 is formed of a projection 6a in a circular planar shape as well as a small protrusion 6b in a planar hexagon, the top surface of the small protrusion 6b being formed on the same level as that of the top surfaces of the topmost small protrusions 4c and 5c of the other protruding parts 4 and 5. The prominent portion of the small protrusion 6b is made longer than that of the small protrusions 4c and 5c of the other protruding parts 4 and 5, and the level of the top surface of the projection 6a thereunder is formed lower than that of the small protrusions 4b and 5b of the other protruding parts 4 and 5 but higher than that of the protrusion 4a of the protruding part 4 and the protrusion 5a of the protruding part 5. In this embodiment, the pitches L (L1, L2, L3, and L4 of Fig. 10) between the outlines of the mutually adjacent protruding parts are respectively set in the range from 10mm to 30mm. Since the planar shape of the topmost level small protrusions 4c, 5c, and 6b is such as to be contained within a circle of a diameter of 25mm, such protrusions are endowed with the slip and skid prevention effect in the same way as the previous embodiment. Fig. 11 is a plan view showing the worn-out state of the protruding parts 4, 5, and 6 of Fig. 9 which have lost the length of the prominent portions of the small protrusions 6b of the protruding parts 6. When abrasion proceeds to the extent that the small protrusions 6b are lost, the position of the topmost surface of each of the protruding parts 4, 5, and 6 becomes the position shown in a broken line of Fig. 10, whereupon the second level small protrusions 4b and 5b on top of the protruding parts 4 and 5 as well as the projection 6a of the protruding part 6 surface. Upon surfacing of the projection 6a, the small protrusions 4b and 5b which are formed on the other projections 4a and 5a are still remaining, wherefore some degree of anti-slip and anti-skid performance is maintained. However, because abrasion exceeding this degree would result in removal of the small protrusions 4b and 5b, thus considerably diminishing anti-slip and anti-skid performance, the surfacing time of the projection 6a serves as a criterion of the replacement period of the cover body 3. When the level of the projection 6a of the protruding part 6 is higher than that of the projections 4a and 5a of the other protruding parts 4 and 5, the surfacing time of the projection 6a shows a measure of time for replacing the cover body 3, so that when this point in time is used as the criterion and the cover body 3 is replaced, degradation of anti-slip and anti-skid performance can be prevented. Also, since in the embodiment, the planar shape of the projection 6a of the protruding part bears no similarity to that of the projections 4a and 5a of the other protruding parts 4 and 5 as well as that of the small protrusions 4b and 5b, the projection 6a, when surfacing due to abrasion, becomes noticeable enough to make it possible to discover the surfacing thereof with ease. Further, the pitches L3 and L4 between the small protrusions 4b, 5b and between the projections 6a which are on the topmost level in the surfacing state of the projections 6a in Fig. 11 are, as mentioned above, in the range from 10mm to 30mm, and the planar shape of such protrusions and projection is in such a size as to be included in a circle of a diameter of 25mm. Consequently, the effect of slip and skid prevention continues to be sufficiently maintained. Although six protruding parts 6 are uniformly placed on the surface of the cover body 3 in this embodiment, the number and method of arrangement of the protruding parts 6 may not be confined to this example. Arranging a plurality of the protruding parts 6 uniformly is preferable, however, to determine the degree of abrasion or the degree of one-sided abrasion on the entire surface of the cover body 3. Cover bodies with a round shape were described above in reference to the embodiments. Naturally, angular cover bodies are equally acceptable. According to this invention, by setting one of the minimum pitches between the outlines of the independently formed projections as well as between those of the small protrusions provided on the top surfaces of such projections to be within the range of 1 0mm to 30mm as well as the size of the topmost surface in a planar shape to be included within a circle of a diameter of 25mm, the engaging force due to tire deformation can be effectively put to operation to inhibit the generation of slipping and skidding. Also, by setting up small protrusions on top of the projections, the engaging force due to the angular portions of the small protrusions or the projections, in addition to the engaging force due to tire deformation, can be rendered to act, thus enhancing the slip and skid prevention effect even more and inhibiting a decrease in the slip and skid prevention effect due to abrasion of the small protrusions as well. Furthermore, those cover bodies having the formation of sharp-angle edges on the projections and small protrusions, since the sharp-angle portions increase the engaging force with tires, can maintain the slip and skid prevention effect on high levels as compared to those cover bodies on the surface of which are merely arranged flat projections and small protrusions. In addition, if the minimum pitches between the outlines of the mutually adjacent projections and between those of the mutually adjacent small protrusions forming such edges are set within the range of 10mm to 30mm, the slip and skid prevention effect will increase even further. As for at least one of the projections on which small protrusions are formed, when the level of the above-mentioned projections is formed higher than that of the other projections on which small protrusions are formed, surfacing of the above-mentioned projections due to abrasion will serve as a criterion of when to replace the cover body. As result, if this is used as an indication for replacing the cover body, a drop in anti-slip and anti-skid performance can be prevented. It will be apparent to those skilled in the art that many variations and modifications may be made to the preferred embodiments as described above without substantially departing from the principles of the present invention. All such variations and modifications are intended to be included herein and within the scope of the present invention, as set forth in the following claims.

Claims: A cover for underground structures including a cover body (1; 3), mutually independent projections (2; 4a, 5a, 6a; 15; 16) arranged uniformly over the surface of the cover body (1; 3), said projections (2; 4a, 5a, 6a; 15; 16) having top surfaces and small protrusions (2a; 4b, 4c, 5b, 5c, 6b; 15a; 16a) formed on the top surfaces of at least some of the projections (2; 4a, 5a, 6a; 15; 16), wherein the size of the topmost surfaces of the small protrusions (2a; 4b, 4c, 5b, 5c, 6b; 15a; 16a) and the size of the topmost surfaces or points of the projections (2; 4a, 5a, 6a; 15; 16) on which no small protrusions (2a; 4b, 4c, 5b, 5c, 6b; 15a; 16a) are formed is set to be contained within a circle of a diameter of 25mm and the pitch between the adjoining projections (2; 4a, 5a, 6a; 15; 16), or between adjoining small protrusions (2a; 4b, 4c, 5b, 5c, 6b; 15a; 16a), or between a projection (2; 4a, 5a, 6a; 15; 16) and a small protrusion (2a; 4b, 4c, 5b, 5c, 6b; 15a; 16a) adjoining to the projection is set to 10-30mm, wherein the top surfaces of at least part of the projections (2; 4a, 5a, 6a; 15; 16) and of the small protrusions (2a; 4b, 4c, 5b, 5c, 6b; 15a; 16a) comprise corners at the outer rim thereof, and wherein the corners of the small protrusions (2a; 4c, 5c, 6b; 15a; 16a) of a topmost level and the corners of the small protrusions (4b, 5b; 15a; 16a) of a lower level or the corners of the projections (2; 4a, 5a, 6a; 15; 16) are so located to be positioned in a rotated manner with respect to each other when seen from above. A cover for underground structures according to claim 1 wherein small protrusions (2a; 4b, 4c, 5b, 5c, 6b; 15a; 16a) are formed on the top surfaces of all the projections (2; 4a, 5a, 6a; 15; 16) on the surface of the cover body (1; 3). A cover for underground structures according to claim 1 or 2 wherein the at least some of the projections (2; 4a, 5a, 6a; 15; 16) and/or protrusions (2a; 4b, 4c, 5b, 5c, 6b; 15a; 16a) have sharp-angle-like edge portions. A cover for underground structures according to one of the claims 1 to 3 wherein at least one of said projections (2; 4a, 5a, 6a; 15; 16) upon which said small protrusions (2a; 4b, 4c, 5b, 5c, 6b; 15a; 16a) are formed has a higher level than other projections (2; 4a, 5a, 6a; 15; 16) on which said small protrusions (2a; 4b, 4, 5b, 5c, 6b; 15a; 16a) are formed. A cover for underground structures including a cover body (1; 3), projections (2; 4a, 5a, 6a; 15; 16) and depressions on the surface of the cover body (1; 3) formed into protruding and recessed patterns, wherein said projections (2; 4a, 5a, 6a; 15; 16) have top surfaces and small protrusions (2a; 4b, 4, 5b, 5c, 6b; 15a; 16a) are formed on at least some of said projections (2; 4a, 5a, 6a; 15; 16), and wherein sharp-angle edges are formed on at least some of the small protrusions (2a; 4b, 4, 5b, 5c, 6b; 15a; 16a) or on at least some of the projections (2; 4a, 5a, 6a; 15; 16) and small protrusions (2a; 4b, 4, 5b, 5c, 6b; 15a; 16a).

Inventors: SAHARA KYOZO, TAKADA HIROYOSHI, WADA JUNJI, SAHARA, KYOZO, TAKADA, HIROYOSHI, WADA, JUNJI
Applicants: HINODE LTD, HINODE, LTD.
Application year: 1997
Main classification: E02D29
IPC codes: E02D29, E03F5, E03B9