WO1991016505A1

WO1991016505A1 - Manhole inserts

Info

Publication number: WO1991016505A1
Application number: PCT/US1990/002219
Authority: WO
Inventors: David L. Neathery; Harold E. Owen, Iii
Original assignee: Neathery David L; Owen Harold E Iii
Priority date: 1988-12-15
Filing date: 1990-04-24
Publication date: 1991-10-31

Abstract

The watertight manhole inserts (1) have dish-shaped, square, rectangular and elliptical seals (14) or stoppers molded from a polymeric material to prevent rainwater from entering a sewer through round, square, rectangular or elliptical manholes (22) having similarly configured covers. The manhole inserts (1), located beneath the manhole covers, plug the manhole and prevent flooding of the underlying sewer. The dish-shaped, rounded or flat bowl or bottom manhole inserts (1) have a pressure-relief valve (12), optional vacuum relief valve (27), lift strap (9) and substantially perpendicular, upward-standing side wall (3) with an outwardly-extending top rim (5) engaging a shoulder (23) in the manhole supporting the manhole inserts (1). Optional multiple, upward-standing bottom (7) and side wall ribs (4) are provided to stiffen the bottom (6) and side walls (3). In a preferred embodiment, the insert rims (5) are sandwiched between two parallel elements of a ribbed rubber seal (14), the top of which seal receives the manhole cover and the bottom (20) of which includes spaced seal ribs (14) which engage the manhole shoulder (23) and act as a water barrier.

Description

MANHOLE INSERTS

Background of the Invention Field of the Invention

This invention relates to manholes for sewer systems and more particularly, to round square, rectangular and elliptical watertight manhole inserts which are designed to seal the manhole immediately beneath a manhole cover to prevent flooding of the sewer system. In a preferred embodiment of the invention a round manhole insert is characterized by a bowl-shaped dish having a rounded or cupped bottom provided with parallel or grid-type ribs and a pressure relief valve, as well as an optional vacuum relief valve, and having vertical sides which are terminated by an outwardly-projecting rim adapted for seating on a shoulder formed in the manhole beneath the manhole cove . In another preferred embodiment the manhole insert includes a square, rectangular or elliptical rim fitted with vertical sides and a flat or curved bottom. In a most preferred embodiment of the invention the outwardly-extending rim of the various inserts is sandwiched between a ribbed seal, the upper flat element of which seal receives the manhole cover and the ribbed bottom element resting on the shoulder shaped in the manhole, to prevent, or at least minimize, the flow of water from ground level around the manhole cover and through the manhole into the sewer system. The bottoms of the manhole inserts may be provided with concentrically spaced, parallel or grid-type ribs for strength and both ends of a lift strap may be attached to the insert bottom, in order to facilitate lifting and removing the manhole inserts from a mounted position in the manhole. In another preferred embodiment of the invention one or more side ribs, stiffeners or corrugations may be provided in the upward-standing side wall extending from the dish or bottom of the manhole inserts to provide additional side wall strength. The pressure relief valve is designed to relieve pressure which may be generated in the sewerage system served by the manhole, while the optional vacuum relief valve operates to relieve any vacuum which may develop in the sewerage system. The manhole inserts of this invention are most preferably molded from a synthetic polymeric material as hereinafter set forth.

One of the problems which is associated with sewerage systems is that of flooding during periods of high water due to prolonged or heavy rainfall. This problem is intensified in both urban and suburban areas where increased construction activity has resulted in large quantities of concrete being poured to construct parking lots, as well as foundations and other structures. This increase in non-permeable construction material reduces the amount of surface area which is available to absorb rainfall and increases the flow of water in storm drains and sewerage systems, as well as diversion canals, natural streams and other channels which must accept and dispose of the increased flow of water. These conditions of urban and suburban development result in flooding under circumstances where the water exceeds the capacity of storm drain and sewerage systems.- which systems are sized to receive a specified average flow of water. Moreover, it is customary to provide separate drainage systems for rainwater and sewage and flooding of the sewerage systems by rainwater runoff sometimes creates a contamination problem.

Description of the Prior Art Various types of closures and covers, many of which are fitted with ventilating means, have been long known in the art. An early patent which details such a closure is the "Ventilating Vault Cover" detailed in U.S. Patent No. 606,554, dated June 28, 1898, to J. Jacobs. The ventilating vault cover is designed to close a vault opening in a watertight manner when in a closed position and vent the vault when in open configuration. U.S. Patent No. 3,046,853, dated July 31, 1962, to C. Legendre, discloses a "Manhole Closure" . The manhole closure includes a lid of one-piece construction which cooperates with a seat or manhole frame, also of one-piece construction, such that the two parts can be easily assembled for mounting in a manhole. A "Safety Replacement Cover for Catch Basins" is detailed in U.S. Patent No. 2,576,353, dated November 27, 1951, to O.W. Nelson. The cover detailed in this patent includes a dish-like, reticulate tray which is designed to provide adequate drainage into a catch basin and facilitates safety to prevent injury from stepping into the catch basin. A "Manhole Cover" is covered in U.S. Patent No.

3,920,347, dated November 18, 1975, to John T. Sauriol, et al. The manhole cover is constructed of a molded synthetic polymeric material and has a transversely intersecting structural rib network which is integrally joined with a top surface portion and encloses annular peripheral portions of the cover. The structural rib network is preferably dome-shaped, with the principal structural support disposed centrally of the cover at the apex of the dome. A "Drain Hole Seal With Bottom Bleeder" is detailed in U.S. Patent No. 4,649,674, dated March 17, 1987, to Craig S. Gaul, et al. The patent details a seal structure having a bottom bleeding feature such as a bottom bore and fitted with a flange to facilitate sealing engagement between a cover and a frame located around an access opening of the manhole. The structure includes a central portion which is downwardly depressed to avoid a path of spin of the cover. A "Watertight Manhole Insert" is set forth in U.S. Patent No. 4,650,365, dated March 17, 1987, to Franklin D. Runnels. The manhole insert is designed to prevent rainwater from entering a sanitary sewer system through manholes which are closed by manhole covers. The insert includes a shallow, dish-shaped body portion and a mounting flange designed to rest upon the supporting flange of a manhole frame. A recess is formed in the bottom of the body portion and includes a valve for releasing pressure which accumulates in the manhole beneath the insert. The valve is shielded to prevent damage by a manhole cover sliding across the insert.

Additionally, the valve can be used as a handle to remove the insert from the manhole. The Runnels insert has also been modified in application to include a circumferential rib near the rim for stiffening purposes. A "Manhole Closure Assembly" is detailed in U.S. Patent

No. 4,067,659, dated January 10, 1978, to Samuel A. Campagna, Jr., et al. The assembly is designed to prevent accumulated storm water from entering into a manhole opening and includes a cover section having a downwardly-depressed central portion. A circular mounting flange extends outwardly from the depressed central ϋortion and the flange is adapted to rest upon inwardly-directed manhole cover supporting flanges of the annular manhole frame. The cover section is depressed. such that the cover exceeds the path of spin when resting upon the supporting flange of the manhole frame. A valve is secured to the cover section for the purpose of relieving pressure inside the manhole when the sewer pressure exceeds a predetermined level. Other patents which detail manhole closure and seal devices are U.S. 3,969,847, dated July 20, 1976, entitled "Combination Manhole Closure Assembly and Separate Support Ring"; U.S. 3,973,856, dated August 10, 1976, entitled "Seal Construction"; U.S. 4,721,314, dated January 26, 1988, entitled "Lip Seal Device"; and U.S. 4,758,004, entitled "Casket Sealing Gasket Having Sealing Ridges of Different Heights" .

It is an object of this invention to provide new and improved manhole inserts which are designed to close and substantially seal a manhole of any shape and size to prevent rainwater from flooding a sewerage system communicating with the manhole.

Another object of the invention is to provide a manhole insert which is molded, stamped or otherwise shaped from a selected metal or synthetic polymeric material into a flanged member having upward-standing stiffening ribs, for installation on a shoulder provided in a manhole beneath the manhole cover, in order to prevent, or at least reduce, the flow of rainwater into the manhole and the underlying disposal system.

A still further object of this invention is to provide a manhole insert having a dish-shaped or flat bottom fitted with an upward-standing, substantially vertically-oriented side having at least one circumferential side rib for stiffening purposes and a flange or rim outwardly-extending from the side, which rim engages the shoulder of a manhole located beneath the manhole cover to retard water from flowing into the manhole and the underlying sewer or drainage system. Yet another object of the invention is to provide a manhole insert for retarding the flow of water into a sewerage system through a manhole, which insert is characterized by a square, elliptical or rectangular-shaped closure having an outwardly-projecting flange or rim that is fitted with a rubber seal and further including upward-standing, parallel or grid-type ribs spaced in the bottom thereof, which manhole insert is located in the manhole beneath the manhole cover. Still another object of this invention is to provide a new and improved manhole insert which includes a stopper or plug of desired configuration and having a pressure relief valve and a vacuum relief valve mounted therein and further including an outwardly-extending rim fitted with a rubber seal, multiple upward-standing, spaced ribs provided in the bottom of the insert and an optional lift strap attached to the insert at both ends, for removing the insert from a manhole located beneath the manhole cover. Another object of the invention is to provide new and improved, ribbed manhole inserts having a configuration adapted to fit in an existing manhole, which manhole inserts are each fitted with a rubber seal for mounting in the upper portion of the manhole beneath the manhole cover and which rubber seal includes a top element and a bottom element sandwiched on the manhole insert rim in order to reduce or prevent the flow of water into an underlying sewerage system, control manhole odor and prevent the accumulation of dirt and trash in the manhole and the underlying sewer.

Summary of the Invention

These and other objects of the invention are provided in manhole inserts for sewerage systems, which inserts are characterized by molded, stamped, pressed or otherwise formed, dish-shaped, square, elliptical and rectangular bottom portions provided with pressure and vacuum relief valves and an upward-standing, substantially vertically-oriented side wall having optional circumferential side ribs and an outwardly projecting rim for seating in a manhole beneath the manhole cover, in order to prevent rainwater from flowing around the edge of the manhole cover into the manhole and through the underlying sewerage system. In a preferred embodiment, at least one rib is provided in the insert bottom for stiffening and strengthening purposes and a rubber seal is secured to the rim by glue or ultrasonic weld and is provided with downwardly-extending projections for engaging a manhole ledge or shoulder built into the manhole. A lift strap may also be riveted or connected at both ends to the bottom portion to faciliate easy removal of the insert from the manhole.

Brief Description of the Drawings The invention will be better understood by reference to the accompanying drawings, wherein:

FIGURE 1 is a perspective view of a square manhole insert of this invention mounted in functional configuration in a manhole and provided with a side rib; FIGURE 2 is a sectional view taken along line 2-2 of the manhole insert illustrated in FIGURE 1, more particularly illustrating optional bottom ribs provided in the bottom of the manhole insert;

FIGURE 3 is a top view of a rectangular manhole insert;

FI RE 4 is a sectional view taken along line 4-4 of the rectangular manhole insert illustrated in FIGURE 3, more particularly illustrating spaced, upward-standing ribs located in the bottom of the insert; FIGURE 5 is an enlarged sectional view of the rim, side and bottom of the manhole insert illustrated in FIGURE 4;

FIGURE 6 is an enlarged sectional view of the rim, side and bottom of the manhole insert illustrated in FIGURE 4, more particularly illustrating optional side wall ribs;

FIGURE 7 is a top view of a round manhole insert featuring parallel bottom ribs; FIGURE 8 is a sectional view of the rim, side wall and bottom, taken along line 8-8 of the manhole insert illustrated in FIGURE 7;

FIGURE 9 is a top view of a round manhole insert having a rib grid provided in the bottom thereof; FIGURE 10 is a top view of a round manhole insert having concentric bottom ribs;

FIGURE 11 is a sectional view of the rim, side wall and bottom, taken along line 11-11 of the manhole insert illustrated in FIGURE 10; FIGURE 12 is a top view of a round manhole insert having a combination radial and concentric rib grid system;

FIGURE 13 is an enlarged sectional view of a rounded rib provided in the bottom of a manhole insert;

FIGURE 14 is an enlarged sectional view of a V-shaped rib provided in the bottom of a manhole insert;

FIGURE 15 is a perspective view of a square manhole insert having parallel bottom ribs and no side rib;

FIGURE 16 is a sectional view taken along line 16-16 of the manhole insert illustrated in FIGURE 15; FIGURE 17 is a sectional view taken along line 17-17 of the manhole insert illustrated in FIGURE 15;

FIGURE 18 is an enlarged sectional view of the rim, side and bottom portions of the square manhole insert illustrated in FIGURE 17, more particularly illustrating a preferred rib seal;

FIGURE 19 is a side sectional view of a pressure relief valve and vacuum relief valve mounted in the bottom of a manhole insert;

FIGURE 20 is a top view of a rectangular manhole insert having a grid rib pattern in the bottom thereof;

FIGURE 21 is a top view of an elliptical manhole insert; and FIGURE 22 is a side sectional view of a rectangular manhole having no bottom or side ribs.

Description of the Preferred Embodiments

Referring initially to FIGURES 7-12, 18 and 19 of the drawings, round manhole inserts of this invention are generally illustrated by reference numeral 1. The round manhole inserts 1 are each characterized by a dish 2, having a curved bottom 6 which terminates in a circular, upward-standing side 3. A rim 5 projects outwardly from the top edge of the side 3 in substantially perpendicular relationship and seats in the manhole 22 beneath the street level 32, as hereinafter described. Parallel bottom ribs 7 are disposed in spaced relationship in the curved bottom 6, as illustrated in FIGURE 7, while intersecting ribs 7 form a grid pattern, as illustrated in FIGURE 9, concentric ribs 7 are indicated in FIGURE 10 and a combination of radial and concentric ribs 7 are provided in the curved bottom 6, as illustrated in FIGURE 12. In a most preferred embodiment the dish 2 is molded from a suitable moldable plastic material, as hereinafter described, and the bottom ribs 7 are vacuum molded or otherwise configured in the curved bottom 6, in each case, as illustrated. Alternatively, the dish 2 can be stamped, pressed, cast or otherwise formed from a suitable metal such as aluminum and stainless steel, in non-exclusive particular. A pressure relief valve 12 is seated substantially in the center of the curved bottom 6 as further illustrated in FIGURES 7, 9, 12 and 19, in order to release pressure which may build up in the manhole 22. In a preferred embodiment of the invention the manhole insert 1 is mounted in the manhole 22 with the rim 5 engaging a manhole shoulder 23, located in the manhole structure 26, as illustrated in FIGURE 18. In a most preferred embodiment of the invention a ribbed rubber seal 14 is fitted on the rim 5, with a flat top segment 15 receiving the conventional manhole cover 25 and the bottom segment 20 lying adjacent to and resting on the manhole shoulder 23 of the manhole structure 26, as further illustrated in FIGURE 18. The top segment 15 and the bottom segment 20 are typically formed integrally with an end segment 19, which extends around the edge of the rim 5. In still another preferred embodiment of the invention, the bottom segment 20 of the ribbed seal 14 is provided with spaced ribs 18, which are seated on the manhole shoulder 23, as still further illustrated in FIGURE 18. The spatial orientation of the ribs 18 is most preferably in close proximity to each other, wherein three such ribs 18 are provided, although it will be recognized that any number of ribs 18 may be provided, as illustrated in FIGURE 3. The ribs 18 serve to prevent, or at least minimize, the flow of water around the manhole cover 25 and across the manhole shoulder 23 into the manhole 22, as further illustrated in FIGURE 18. In a most preferred embodiment of the invention, the top segment 15 is ultrasonically welded, glued, or otherwise attached to the top surface of the rim 5.

Referring again to FIGURES 7, 9, 10 and 12 of the drawing, in another most preferred embodiment of the invention the manhole insert 1 is characterized by variously spaced and configured bottom ribs 7, in order to stiffen the curved bottom 6. As in the case of the manhole inserts 1 illustrated in FIGURES 7 and 12, the manhole insert 1 is designed such that certain ones of the rib ends 7a terminate in spaced relationship with respect to the valve 12, which is threadably inserted by means of a companion threaded nipple 13, in a bottom opening 8 located in the curved bottom 6 of the dish 2, as further illustrated in FIGURES 10 and 19. The valve shoulder 12a serves the dual function of stabilizing the pressure relief valve 12 against shock and facilitating sealing of the pressure relief valve 12 to allow a selected water flow rate therethrough. In the embodiments of the manhole inserts 1 illustrated in FIGURES 7, 9, 10 and 12, the bottom ribs 7 are preferably shaped from the curved bottom 6 by application of stamping, pressing, casting or vacuum molding techniques which are well known to those skilled in the art. Accordingly, it will be appreciated that the manhole insert 1 can be configured from molded synthetic polymeric materials such as polyethylene, polypropylene and acrylonitrile-butadiene-styrene, commonly called ABS, as well as metals such as aluminum and stainless steel, in non-exclusive particular. Furthermore, in a most preferred embodiment of the invention, the locked on ribbed seal 14, illustrated in FIGURE 18, is of the envelope style and is constructed of EPDM rubber which is welded, glued or otherwise secured or "locked" to the rim 5 by a suitable rubber cement, as heretofore described. In still another most preferred embodiment of the invention, the valve 12 is designed to relieve at a pressure of one-half p.ε.i. and to have a vat-er .leak-down rate not exceeding five gallons for 24 hours. As further illustrated in FIGURE 19, an optional vacuum relief valve 27 can be installed in an opening (not illustrated) provided in the flat bottom 16. A grommet 30 seats the mount stem 29 in this opening and a valve body 28 is attached to the hollow mount stem 29. An extension tube 31 is also secured to the upper end of the mount stem 29, in order to project above any trash, dirt or ice which may accumulate in the dish 2 and maintain effective function of the vacuum relief valve 27.

Referring again to FIGURE 18 of the drawing, in yet another preferred embodiment of the invention one or more of circumferential side ribs 4 are provided in the upward-standing side 3, in order to further stiffen the side 3. It will be appreciated by those skilled in the art that although one or more side ribs 4 can be provided, as desired, a pair of side ribs 4 is most preferred, in order to faciliate a desired stiffening of the side 3.

Referring now to FIGURES 1 and 2 of the drawings a square manhole insert 34 is illustrated seated in a square manhole (not illustrated) located at street level 32. The square manhole insert 34 is characterized by a continuous, downwardly-extending side wall 3, a flat bottom 16 shaped inwardly from the side wall 3 and a pressure relief valve 12, provided in the center of the flat bottom 16. A single side wall rib 4 is provided in the side wall 3 and a lift strap 9 is secured to one of the upward-standing parallel bottom ribs 7 by means of a stainless steel rivet 11, as further illustrated in FIGURE 1. FIGURE 2 further illustrates a preferred technique for mounting the pressure relief valve 12 in the flat bottom 16 using a threaded nipple 13 which is threadably seated in an opening (not illustrated) provided in the flat bottom 16.

Referring now tc FIGURES 3 and 4 and 18 of the drawings in another preferred embodiment of the invention a rectangular manhole insert 36 is illustrated. Like the square manhole insert 34 illustrated in FIGURE 1, the rectangular manhole insert 36 includes a continuous side wall 3, illustrated in FIGURE 4, and an outwardly-extending rim 5 projecting from the top edge of the side wall 3. The flat, recessed bottom 16 projects inwardly from the side wall 3 and multiple upward-standing horizontal and vertical bottom ribs 7 are provided in the flat bottom 16 for stiffening purposes, as further illustrated. The grid pattern formed by the bottom ribs 7 is designed to further strengthen and stiffen the flat bottom 16 of the rectangular manhole insert 36. As further illustrated in FIGURES 2 and 4, as well as FIGURE 18, the rim 5 of both the square manhole insert 34 and the rectangular manhole insert 36 is preferably fitted with a ribbed seal 14, which rests against the manhole shoulder 23 of the manhole 22 in order to seal the rim 5 in each case against water encroachment into the manhole.

Referring now to FIGURE 6 of the drawing in a most preferred embodiment of the invention one or more side wall ribs 4 are provided in the side wall 3 of the square manhole insert 34 and the rectangular manhole insert 36, in order to further stiffen the side wall 3 and increase the structural integrity of the square manhole insert 34 and the rectangular manhole insert 36. As illustrated in FIGURES 13 and 14 the bottom ribs 7 may be characterized by a rounded rib 17, as illustrated in FIGURE 13 or a V-shaped rib 21 as illustrated in FIGURE 14. Other rib configurations, such as the box configuration illustrated in FIGURES 1-4, may also be used in order to stiffen the curved bottom 6, flat bottom 16 and side wall 3 of the various manhole inserts of this ^•in finfϊon.

Referring now to FIGURES 15-17 in another preferred embodiment of the invention a square manhole insert 34 is illustrated, with a smooth continuous side wall 3 which curves to define a flat bottom 16, provided with upward-standing, spaced parallel bottom ribs 7. In a preferred embodiment of the invention the bottom ribs 7 extend into and connect with the side wall 3 to further stiffen the square manhole insert 34. In an alternative preferred embodiment of the invention as illustrated in FIGURE 17, corrugated side wall ribs 4 may be provided in the side wall 3 for additional stiffening purposes. It will be appreciated that the rim 5 and ribbed seal 14 configuration of the square manhole insert 34 is the same as that for the round manhole insert 1, heretofore described, as well as the rectangular manhole insert 36 and elliptical manhole insert 38, hereinafter described. Referring now to FIGURE 20 of the drawings in yet another preferred embodiment of the invention a rectangular manhole insert 36, having a grid pattern of bottom ribs 7, is illustrated such that the bottom ribs 7 are disposed in both vertical and horizontal configuration to further stiffen and strengthen the flat bottom 16. As further illustrated in FIGURE 21 an elliptical manhole insert 38 is detailed, which includes multiple fan-spaced bottom ribs 7 at the ends thereof which follow the radius of the ends and a grid pattern of bottom ribs 7 in the center thereof. A pressure relief valve 12 is provided in one end of the elliptical manhole insert 38, while a vacuum relief valve 27 is provided in the opposite end thereof.

Referring now to FIGURE 22 another rectangular manhole insert 36 is detailed, and is characterized by a smooth, continuous side wall 3, terminating at the upper end in a rim 5 and a smooth, flat, recessed bottom 16 extending from the side wall 3 with no ribs provided therein. In another preferred embodiment of the invention and referring again to FIGURES 1, 3, 4 and 15 of the drawing, a lift strap 9 is secured at the strap ends 10 to a bottom rib 7 shaped in the curved bottom 6 and flat bottom 16, respectively, in spaced relationship, by means of stainless steel rivets 11. Since it is common practice for workmen to engage the lift strap 9 with a pick or other tool in order to remove the manhole insert 1 from the manhole, it has been found that the lift strap 9 must be secured at both of the strap ends 10 to the curved bottom 6 and flat bottom 16 or to a bottom rib 7 of the dish 2, to insure that the lift strap 9 does not tear away from the curved bottom 6 and flat bottom 16 or bottom rib 7. It has been found that when the lift strap 9 is attached to the dish 2 by the two stainless steel rivets 11 in this manner, a force in excess of 500 pounds is necessary to tear the strap away from the manhole insert.

It will be appreciated by those skilled in the art that the manhole inserts of this invention are characterized by convenience and flexibility and operate to stop the flow of rainwater into sewerage systems, control manhole odors, prevent dirt and trash from accumulating in the manhole and the sewerage system and help to prevent manhole rattling and "flipping" due to street traffic. The manhole inserts 1 can be constructed of extremely durable polyethylene, polypropylene or ABS plastic, as well as a metal such as aluminum or stainless steel, and are configured to be maintained in position on the manhole shoulder in a manhole without dropping from that position into the manhole or sewer underlying the manhole. Furthermore, although only the round manhole insert 1, square manhole insert 34, rectangular manhole insert 36 and elliptical manhole insert 38 are illustrated in the drawing and described herein, it will be recognized that other manhole insert shapes, such as polygonal and triangular manhole inserts, in non-exclusive particular, may be constructed, depending upon the shape of the manhole in which the manhole insert is to be used. Moreover, although certain bottom rib 7 and side wall rib 4 configurations are noted for use with specific manhole inserts, it is understood that each rib configuration can be used in any desired manhole insert.

While the preferred embodiments of the invention have been described above, it will be recognized and understood that various modifications may be made therein and the appended claims are intended to cover all such modifications which may fall within the spirit and scope of the invention.

Claims

WHAT IS CLAIMED IS:

1. A manhole insert for location in a manhole beneath a manhole cover, said manhole insert comprising a generally dish-shaped body of selected configuration having a bottom terminated by an upward-standing side wall; a plurality of bottom ribs provided in said bottom for strengthening said bottom; a pressure relief valve provided in said bottom for releasing pressure in the manhole; and a rim shaped in the top edge of said side wall for supporting said manhole insert in the manhole.

2. The manhole insert of claim l wherein said at least one bottom rib further comprises a plurality of bottom ribs provided in said bottom in spaced, substantially parallel relationship.

3. The manhole insert of claim 2 wherein said dish is round and said bottom ribs include radial ribs and concentric ribs disposed in said bottom, with one end of said radial ribs facing said pressure relief valve.

4. The manhole insert of claim 1 further comprising a lift strap attached to said bottom for lifting said manhole insert from the manhole when the manhole cover is removed.

5. The manhole insert of claim 4 wherein both ends of said lift strap are attached to said bottom.

6. The manhole insert of claim 1 wherein said bottom ribs are provided in said bottom in spaced, substantially parallel relationship and further comprising a lift strap attached to one of said bottom ribs for lifting said manhole insert from the manhole when the manhole cover is removed.

7. The manhole insert of claim 6 wherein said body is round and said bottom ribs include radial ribs and concentric ribs disposed in said bottom, with one end of said radial ribs facing said pressure relief valve.

8. The manhole insert of claim 7 wherein both ends of said lift strap are attached to said one of said bottom ribs.

9. The manhole insert of claim 1 further comprising seal means provided on said rim for sealing said manhole insert in the manhole.

10. The manhole insert of claim 9 wherein said bottom ribs are provided in said bottom in spaced, substantially parallel relationship.

11. The manhole insert of claim 9 further comprising a lift strap attached to said bottom for lifting said manhole insert from the manhole when the manhole cover is removed.

12. The manhole insert of claim 9 wherein said bottom ribs are provided in said bottom in spaced, substantially parallel relationship and further comprising a lift strap attached to said one of said bottom ribs for lifting said manhole insert from the manhole when the manhole cover is removed.

13. The manhole insert of claim 11 wherein said body is round and said bottom ribs are radially disposed in said bottom, with one end of said radially disposed bottom ribs facing said pressure relief valve.

14. The manhole insert of claim 13 wherein both ends of said lift strap are attached to said bottom.

15. The manhole insert of claim 1 further comprising at least one side rib provided in said side wall for strengthening said side wall.

16. The manhole insert of claim 15 wherein said at least one bottom rib further comprises a plurality of bottom ribs provided in said bottom in spaced, substantially parallel relationship.

17. The manhole insert of claim 16 wherein said body is round and said bottom ribs are radially and concentrically disposed in said bottom, with one end of said radially disposed bottom ribs facing said pressure relief valve.

18. The manhole insert of claim 17 further comprising a lift strap attached to said one of said bottom ribs for lifting said manhole insert from the manhole when the manhole cover is removed.

19. The manhole insert of claim 18 wherein both ends of said lift strap are attached to said one of said bottom ribs.

20. The manhole insert of claim 15 further comprising seal means provided on said rim for sealing said manhole insert in the manhole.

21. The manhole insert of claim 20 further comprising a lift strap attached to said bottom for lifting said manhole insert from the manhole when the manhole cover is removed.

22. The manhole insert of claim 21 wherein both ends of said lift strap are attached to said bottom.

23. The manhole insert of claim 9 wherein said seal means further comprises a top segment disposed on the top surface of said rim for receiving the manhole cover, a bottom segment disposed on the bottom surface of said rim for seating said rim in the manhole and an end segment spanning the edge of said rim and connecting said top segment and said bottom segment.

24. The manhole insert of claim 23 wherein said body is shaped in the configuration of a parallelogram.

25. The manhole insert of claim 23 wherein said body is substantially elliptical.

26. The manhole insert of claim 23 further comprising at least one seal rib projecting from said bottom segment for engaging the manhole and removably sealing said manhole insert in the manhole.

27. The manhole insert of claim 1 further comprising a vacuum relief valve provided in said bottom for relieving a vacuum in the manhole.

28. The manhole insert of claim 27 wherein said bottom ribs are provided in said bottom in spaced relationship.

29. The manhole insert of claim 27 further comprising a lift strap attached to one of said bottom ribs for lifting said manhole insert from the manhole when the manhole cover is removed.

30. The manhole insert of claim 27 further comprising seal means provided on said rim for sealing said manhole insert in the manhole.

31. The manhole insert of claim 27 wherein said bottom ribs are provided in said bottom in spaced relationship and further comprising:

(a) a lift strap attached to said one of said bottom ribs for lifting said manhole insert from the manhole when the manhole cover is removed;

(b) a seal characterized by a top segment disposed on the top surface of said rim for receiving the manhole cover, a bottom segment disposed on the bottom surface of said rim for seating said rim in the manhole and an end segment spanning the edge of said rim and connecting said top segment and said bottom segment; and

(c) at least one side rib provided in said side wall for strengthening said side well.

32. The manhole insert of claim 31 wherein said body is round.

33. The manhole insert of claim 31 wherein said body is shaped in the configuration of a parallelogram.

34. A manhole insert for location in a manhole beneath a manhole cover, said manhole insert comprising a recessed body having a bottom terminated by a continuous, upward-standing, substantially vertically oriented side wall; a plurality of bottom ribs provided in said bottom for strengthening said bottom; a pressure relief valve provided substantially in the center of said bottom for releasing pressure in the manhole; a rim shaped in the top edge of said side wall for supporting said manhole insert in the manhole; and seal means provided on said rim for sealing said manhole insert in the manhole.

35. The manhole insert of claim 34 wherein said body is round and said bottom ribs are disposed in said bottom in spaced, substantially parallel relationship.

36. The manhole insert of claim 34 wherein said body is shaped in the configuration of a parallelogram.

37. The manhole insert of claim 34 wherein said body is substantially elliptical.

38. The manhole insert of claim 34 wherein said body is round.

39. The manhole insert of claim 34 wherein said seal means further comprises a top segment disposed on the top surface of said rim for receiving the manhole cover, a bottom segment disposed on the bottom surface of said rim for seating said rim in the manhole and an end segment spanning the edge of said rim and connecting said top segmeiil and said bottom segment.

40. The manhole insert of claim 34 further comprising at least one side rib provided in said side wall for strengthening said side wall.

41. The manhole insert of claim 34 further comprising a lift strap having both ends secured to one of said bottom ribs for lifting said manhole insert from the manhole when the manhole cover is removed.

42. A manhole insert for location in a manhole beneath a manhole cover, said manhole insert comprising a recessed, one-piece body shaped substantially in the configuration of the manhole and having a bottom bordered by an upward-standing, substantially vertically oriented side wall; a plurality of bottom ribs oriented in said bottom for strengthening said bottom; a pressure relief valve provided substantially in the center of said bottom for releasing pressure in the manhole; a vacuum relief valve provided in said bottom for relieving a vacuum in the manhole; a rim shaped in the top edge of said side wall for supporting said manhole insert in the manhole; and seal means provided on said rim for sealing said manhole insert in the manhole.

43. The manhole insert of claim 42 wherein said seal means further comprises a top segment disposed on the top surface of said rim for receiving the manhole cover, a bottom segment disposed on the bottom surface of said rim for seating said rim in the manhole and an end segment spanning the edge of said rim and connecting said top segment and said bottom segment.

44. The manhole insert of claim 42 further comprising at least one side rib provided in said side wall for strengthening said side wall.

45. The manhole insert of claim 44 wherein said body aped in the configuration of a parallelogram.

46. The manhole insert of claim 44 wherein said body bstantially elliptical.

47. The manhole insert of claim 44 wherein said body und.