1. Field of the Invention
The present invention relates generally to manhole structures, and to the construction or replacement of manholes. More particularly, the present invention relates to plastic, corrosion resistant, leakproof components for manhole repair or assembly. Art pertinent to the present invention may be found in U.S. Class 52, Subclasses 20 and 71; Class 156, Subclass 71; and Class 264, Subclass 32.
2. Discussion of the Prior Art
Modern sewage systems are overloaded. The factors contributing to this overload include the growth of our cities, the inevitable aging of sewer systems, and the poor quality associated with installation of new systems. The deterioration of associated manholes is a major cause for alarm. It has been mandated by the Environmental Protection Agency that cities must stop the flow of sewage into streams, lakes, rivers and oceans.
Modern, sanitary sewer systems comprise of a variety of interconnected lines, pumping stations, conduits and the like. Municipal sewers typically comprise a plurality of networked, generally horizontally extending underground lines that are generally, but not always, adjacent and beneath the street network. These sewers include horizontal, subterranean lines formed of longitudinally aligned sections of slightly inclined pipes, that terminate periodically within manholes. A manhole is essentially a vertical passageway, typically beginning at ground level at or near the street surface, that extends downwardly into the ground. Typically a manhole receives one or more sewer line junctions. Eventually the sanitary sewer lines run to a sewage treatment facility, so that the waste may be properly processed. Storm sewer lines, on the other hand, may be discharged directly into rivers and streams.
Manholes enable human access to line junctions and installations for system inspections, maintenance and repairs. Typical manholes are formed through various construction techniques of bricks, tiles or concrete blocks bonded together with cement mortar. Pre-cast and "cast in place" concrete manholes are also common.
While it will be recognized that numerous problems are associated with conventional sewer systems, water seepage through conventional manhole structures is a primary cause of system overloading. Inflow and infiltration through conventional manholes cause flooding of the sanitary sewer system and overloading of "downstream" treatment plants. As a result, raw sewage can be discharged directly into the environment by way of the drainage system, much of which is above ground. Such inflow and infiltration will increase the flow in the system as much as ten times in some instances. An increase of three to four times is not uncommon. Studies have shown that as much as seventy-five percent of the inflow during rainy periods occurs through defects in the manholes. The remaining twenty-five percent occurs through the transport lines between each manhole. Of course exfiltration through leaking manholes is dangerous as well.
The deterioration associated with older brick and mortar manholes is obvious. Concrete manholes allow infiltration and/or exfiltration as a result of honeycombing, cold joints, or improperly sealed joints. Unstable ground promotes cracking and deterioration, so concrete manholes installed in earthquake-prone areas are particularly vulnerable.
Conventional concrete manhole assemblies typically experience significant interior corrosion and deterioration with age. This deterioration occurs even where acidic effluents, known to be harmful to sewers and sewer treatment systems, are prohibited from entering or are first dissipated or neutralized. Hydrogen sulfide is inherent in sewage. It is developed due to the presence of sulfur compounds, such as sulfate, sulfite, or other inorganic or organic sulfur. The above-mentioned compounds are reduced to sulfide by sulfate-reducing bacteria normally found in the effluent. The generation of hydrogen sulfide is accelerated in the presence of moderate temperatures and low flow rates.
The useful life of concrete is determined by dividing the available effective thickness of the concrete by the corrosion rate. The corrosion rate can be calculated when all factors are known. The effective thickness of the concrete is the amount covering the steel reinforcement typically embedded within the manhole assembly.
Coatings have been applied to manhole interiors, but they have a poor track record. For example, although coal tar, or epoxy provides effective protection against hydrogen sulfide, such coatings have provided poor field performance due application difficulties. However, it is recognized in the industry that coatings are not the same as "linings." Cementitious linings may be sprayed on in place as disclosed in U.S. Pat. No. 5,002,438 issued to Strong on Mar. 26, 1991.
It is also known to insert a pre-formed structural liner inside an existing manhole. The liner must conform to the configuration of existing manhole as closely as possible, and it must usually be custom designed and made. Other means of rehabilitating include the use of a sleeve or cylinder disposed within the manhole that forms an annulus between itself and the existing brick structure. The annulus is filled with grout to form a lining. However, since the confines of the manhole are extremely irregular, the temporary liner is difficult to properly configure, and the operation is haphazard and unreliable at best.
Previously, linings of plastic material have provided excellent performance for interior corrosion protection against hydrogen sulfide and sulfuric acid. Such plastic linings are further compatible with plastic pipe now being used extensively in sanitary systems. However, to date, it is extremely difficult to fabricate interior linings and integrate such interior linings into manhole assemblies. Flexible type linings are presently used in pipes to protect the upper portions attacked by sulfurous compounds. Integrating known plastic tubular liner sections within the existing concrete structure has proven difficult. Known prior art assemblies lack suitable structural strength, and they present additional problems in handling and assembly. In addition, the linear seams are difficult to seal. Further, these seams have been found to degrade, resulting in leaks through the manhole structure.
The prior art reflects numerous patents that teach the relining or repair of sewer conduits with add-on, sleeve-like liners. U.S. Pat. No. 4,796,669 Issued to St. Onge, Jan. 10, 1989 discloses a method for relining buried pipeline by coaxially inserting interconnected plastic sections of tubing within the pipeline. These sections are glued together until the entire pipeline has been relined. U.S. Pat. No. 4,245,970, issued Jan. 20, 1981, also discloses plastic structure for relining a sewer pipe. Britain, patent No. 4,818,314 issued Apr. 4, 1989 discloses a similar system including a plurality of liner segments for relining pipelines. U.S. Pat. No. 4,846,147 issued Jul, 11, 1989 discloses a chimney liner system wherein a sleeve formed from a fiberglass cloth is inserted interiorly to reline the chimney.
U.S. Pat. No. 4,456,401 issued Jun. 26, 1984 employs a felt liner impregnated with a liquid resin material inserted within the sewer line for repair. U.S. Pat. No. 4,386,628 Issued Jun. 7, 1983 teaches the maintenance lining passageways by inserting into it a flexible tubular material of a lower diameter. The tubular material is a laminate having an outer contiguous layer of a composition foamable to form an expanded cellular structure. The pipe is expanded and solidifies in place within the pipe.
Another popular method is to provide a segmented series of pipes or liner sections inserted into the pipe to be repaired. An annulus results between the pipe and the "liner," and grout or cementitious material may be pumped into the annulus to form an interior lining. U.S. Pat. No. 4,751,799 issued Jun. 21, 1988 employs liners comprising a plurality of individual liner sections to define the inner surface of the manhole member to be "relined." The resultant annulus thereafter receives grout. U.S. Pat. No. 4,728,223, issued Mar. 1, 1988; U.S. Pat. No. 4,602,659, Issued to Parkyn Jul. 29, 1986, Parkyn patent No. 4,601,312 issued Jul. 22, 1986, and U.S. Pat. No. 4,350,548 issued Sep. 21, 1982 all depict systems in that a resultant annulus is filled with grout.
U.S. Pat. No. 4,325,772 issued Apr. 20, 1982, shows the use of a flexible liner tube within an installed pipe. A liquid adhesive agent is forced into the annulus formed therebetween. Allen patent No. 4,678,370 issued Jul. 7, 1987 discloses a system of helically wound internal liners that define an annulus within the sewer pipe for receiving cementitious grout. A related invention is seen in Telford patent No. 3,269,421 issued Aug. 30, 1966. U.S. Pat. No. 3,834,433 issued to Larson on Sep. 10, 1974 discloses a sewer repair apparatus adapted to be moved within a pipe and centered upon a leaking area. Ends of the apparatus thereafter expand to form a seal, centered over the leaking pipe area. Subsequent pressurization of this area forces grout outwardly through the annulus, through the ends of the pipe, and forms an internal and external cover for patching the leak.
Other patents disclose various methods to cast a manhole in place. For example, U.S. Pat. No. 4,995,584 shows how form structure may be installed in place at the manhole for subsequent application of cementitious material to the annulus. However, the panels therein disclosed are difficult to use and they are complex. U.S. Pat. Nos. 4,997,602, 3,729,165 and 5,017,313 are similar.
Trimble, U.S. Pat. No. 5,032,197 uses removable and temporary protective plastic liner molds to form a liner for a manhole. Cementitious materials are applied within the annulus.
Neathery patent No. 4,957,389, issued Sep. 18, 1990, also discloses a seal type structure that is disposed within the manhole to define an annulus. Concrete is poured in the annulus between the form base and the chimney wall to seal the structure.
Singer, U.S. Pat. No. 3,745,738 discloses a corrosion resistant manhole. It is formed by placing concrete about a plastic liner within a form. The form is removed but the plastic liner is left in place to protect the interior of the newly formed manhole.
Also pertinent to the present invention is U.S. Pat. No. 5,081,802, issued Jan. 21, 1992 to Westhoff. It discloses a liner assembly formed from a plurality of flanges that enables the pouring of grout or cementitious materials in the annulus.
It is desirous to provide for the complete replacement of an existing manhole, or the construction of a new manhole, with a system capable of being precast or cast in place that preserves leak-proof integrity. Ideally such a manhole system should be modular, seamless, flexible, leakproof, and corrosion resistant. Each modular section should have an outside and an inside wall separated by a cavity that can be filled with structural or non structural fill material to create a manhole section. It would also be advantageous to adapt these sections to be stacked vertically through a reliable, leak proof system.
Such a modular manhole system must be capable of field customizing to conform to structural and weight requirements of the application, while maintaining required physical dimensions. The system structural cavity must be easily filled in the field to meet custom strength applications. Thus the manhole must be capable of "cast in place" installation. Alternatively, the manhole system must meet construction industry "precast" standards. And, when cavities are prefilled with material prior to shipment, the manhole units must not collapse or deform.