Corrosion resistant leakproof plastic manhole system

A leakproof modular manhole system comprising a plurality of separate cooperating, plastic form units that snap fit together in a vertical stack. The deployed system provides a complete replacement for an existing manhole, or forms an entirely new corrosion resistant and leakproof manhole. Each of the seamless units is preferably rotationally molded from polyethylene plastic. Each double walled unit interiorly defines a material receptive annular cavity. The cavity may be filled with structural or non structural fill material. A generally tubular base unit is disposed at the bottom of the excavation for connection with the sewer line by an eccentric reducer coupling. A concrete invert is formed in the base in fluid flow communication with the sewer line. One or more tubular riser units extend serially upwardly from the base. The risers are available in varying lengths to accommodate different manhole depths. The system corbel is formed by a cone that structurally terminates alongside the pavement. Preferably the base, the risers, and the cone snap fit together. A segmented extender extends from the cone to a manhole frame. It adjusts the length of the system to raise the relative grade of the top if grade alterations are made near the manhole. The installed system is leakproof and corrosion resistant, and it encapsulates the invert at the manhole bottom to further isolate the manhole from surrounding ground water.

BACKGROUND OF THE INVENTION 
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. 
SUMMARY OF THE INVENTION 
My corrosion resistant leakproof plastic manhole system provides a complete 
replacement for an existing manhole. Alternatively one or more portions of 
the system may be used to construct a new manhole. My system facilitates 
the erection of a noncorroding, leakproof manhole in a desired excavation 
that readily interconnects with the sewer lines. Portions of the present 
system may be precast and then placed in an excavation for the new 
manhole. 
The system is modular, comprising a plurality of individual, stackable 
units. The units are constructed of seamless polyethylene plastic. Each is 
double-walled, interiorly defining a material receptive cavity. The cavity 
may be filled with structural or non structural fill material. The units 
are vertically stacked, preferably employing a system of spigot and bell 
joints sealed by gaskets. 
The units comprising the manhole system include a base unit supporting a 
cone unit. One or more riser units may be disposed between the cone and 
base as necessary. The base is connected to existing or newly installed 
sewer lines by an eccentric reducer. Extenders are employed to raise the 
relative grade of the top of the system if repaving or other grade 
alterations are made near the manhole. 
Structural fill material used to fill the cavity in the units can be 
cement, small aggregate concrete, cellular concrete, grout, plastic foam, 
or the like. The cavity can also contain preformed reinforcing steel wire 
to add to the structural integrity of the modular units. This reinforcing 
steel would be contained within the double walled cavity. Nonstructural 
material such as sand, gravel, or recycled materials may add mass to the 
system. Cavity fill material is placed or pumped into the cavities through 
holes cut or drilled in the upper or outer portion of each of the units, 
which are plugged after the cavity is filled. 
The base unit rests in the bottom of a manhole excavation. It is generally 
cylindrically shaped with an open top and a floor. The cavity is formed 
between inner and outer walls and inner and outer seamless floors. The 
upper portion of the unit is shaped to provide a "spigot" joint. The 
spigot mates with a "bell" formed in a riser or cone. The spigot comprises 
an outwardly projecting ring and an upwardly projecting lip. 
One or more tubular riser units maybe disposed upon the base. Each riser 
unit comprises an inner and outer seamless shell like the base unit. The 
risers employ structural ribs for rigidity to prevent torsional flex. The 
bottom portion of the risers defines a bell intended to captivate an 
upwardly projecting spigot. The bell comprises an outwardly projecting, 
inverted ledge defined in the inner wall and a downwardly projecting brim. 
A circular gasket secures the joint between the spigot and the bell. The 
upper portion of a riser defines a spigot similar to the one described 
above. The riser's spigot mates with the lower bell of another riser or a 
bell in the skirt of a cone. This joint is also sealed by a gasket. 
The cone unit preferably comprises an inner and outer seamless shell like 
the other units. The cone is truncated, and the bell is formed in the 
lower portion. The bell comprises an outwardly projecting inverted ledge 
and downwardly projecting brim, similar to the riser bell. The spigot of 
the unit directly below is received within the bell. A gasket is 
captivated between the spigot and bell to seal the joint. A generally 
circular tapered opening is established by an inwardly projecting shelf 
shaped in the upper portion of the cone. The opening receives an extender 
or manhole cover frame. 
The extender is segmented and generally tubular. It comprises a corrugated 
outer wall that may be readily cut by the installer to an appropriate 
length. Its lower portion mates with the opening in the cone. The extender 
raises the manhole frame associated with the present system. This may be 
necessary due to repaving over the manhole. An O-ring is located in a 
channel in the tapered base of the extender to provide a leakproof seal. 
Brick and mortar will be applied around the extender in compliance with 
local building codes. The brick and mortar structure is partially 
supported by the cone's shelf. The upper open end of the extender mounts 
the manhole frame. 
The tubular, eccentric reducers preferably comprise a central body and 
eccentric segments. A hole is cut through the walls of the base to receive 
the body portion. The segments have varying diameters to mate with various 
sizes of sewer pipe. However the segments are aligned to provide a smooth 
bottom. Each eccentric segment has a ridge to aid attaching a flexible 
pipe coupling. The coupling comprises a rubber sleeve with circular clamps 
on each end. A cutting groove is defined between the rib and the edge of 
the eccentric segment to match the diameter of the connecting sewer pipe. 
The groove also facilitates subsequent field removal of those reducer 
segments that are smaller than needed. 
A preferred method of sealing the reducer in place calls for cutting a hole 
smaller than the outer diameter of the eccentric reducer's body in the 
base. The area around the hole is heated, softening the plastic. The 
reducer is inserted into the heated hole from inside the base. Filling the 
cavity defined between the walls of the base with cement, concrete or the 
like will help complete the seal. A sealing ring or gasket may also be 
placed around the body of the reducer. Polyethylene welding is preferable 
in establishing the necessary seal. 
Therefore, a primary object of the present invention is to provide a 
Plastic Manhole Lining and Relining System that is corrosion resistant and 
leak resistant. 
A related object of the present invention is to provide a manhole system 
constructed of corrosion resistant parts that can be quickly assembled in 
the field. 
Another object of my invention is to provide a leakproof modular manhole 
system that is substantially more resistant to exfiltration and 
infiltration than conventional concrete manholes. 
A further related object of the present invention is to provide an 
economical and reliable modular manhole system that can be efficiently 
produced, for example, through rotational molding techniques. 
An object of the present invention is to provide a double walled, seamless 
manhole exhibiting stability and uniformity. 
Another important object is to provide a modular manhole system that may be 
custom assembled at the job site to adapt itself for installations of 
varying depth. 
Another important object is to provide a modular manhole system of the 
character described that will not crack or leak in response to moderate 
shifting forces experienced by the ground. 
A related object is to provide a manhole system exhibiting maximum 
flexibility. 
Still another object is to provide a manhole system that may be produced 
with varying thicknesses (i.e., of the double walls) as structurally 
necessary without changing the overall outside dimensions of the units. 
Another object of the present invention is to provide a double walled 
manhole capable of being assembled in the field. 
Another object of my manhole system is to provide a seamless, double walled 
manhole comprising an internal cavity that may optionally be filled with a 
variety of reinforcing materials as desired by the contractor. 
A related object of the present invention is to provide a seamless double 
walled manhole that may be filled with cement based products, recycled 
products, plastic foams or resins. 
An object of the present invention is to provide a manhole that may be 
either pre-cast or cast in place. 
An object of the present invention is to provide a manhole system that may 
be manufactured in a variety of colors. 
Another primary object is to provide seamless circular double wall plastic 
shells that form inner and outer structural walls separated by a filled 
structural cavity that, when assembled, form a leakproof circular manhole. 
Another object is to provide a modular manhole system of the character 
described that may enclose a reinforcement cage within the cavity of the 
double wall to meet structural demands. 
An object of the present invention is to provide a manhole system that 
minimizes the number of seals, seams and joints. 
Another object of the present invention is to provide a manhole constructed 
of a series of seamless, circular double wall units that can be assembled 
in a variety of configurations to readily adapt the system for varying 
applications. 
An object of my invention is to provide bell and spigot joints for a 
multi-piece manhole system that will establish water-tight structural 
connections. 
A further object of the present invention is to provide a manhole with an 
outside, waterproof barrier to handle hydrostatic loading occurring in 
manholes constructed under the ground water table. 
An object of the present invention is to provide a manhole that is more 
resistant to erosion and abrasion than typical conventional concrete 
manholes. 
A still further object is to provide a manhole system that isolates the 
sewer invert from the surrounding water table. It is a feature of my 
system that the resultant concrete invert is encapsulated and isolated 
within a double walled plastic base. 
Another object is to provide a manhole exhibiting the qualities of 
composite construction. 
An object of the present invention is to provide a manhole constructed of a 
series of units employing a resilient profile gasket to maintain a 
watertight seal. 
Another fundamental object of the present invention is to provide a manhole 
replacement system. 
A further object is to provide a modular manhole system that can be made 
with enclosed reinforcement (i.e., "rebar") within the cavity of the 
manhole components. The latter feature is facilitated by the rotational 
molding process. Such reinforcement meets high strength needs in deep 
installations. 
Another object is to provide a modular manhole system that avoids 
limitations arising from depth specifications. 
Yet another object is to provide an electrically non-conductive manhole 
structure for the electric power and telecommunication industries. 
A still further object is to provide a manhole system that can be hand 
carried to a job site, without using major construction lifting equipment. 
Another object is to provide a system that can withstand high hydrostatic 
loading created by installation in high water table locations in wetlands, 
marshes, and water bearing soils. 
Another fundamental object is to provide a manhole system that is 
absolutely water tight, unlike any other manhole system, and to do so 
without coatings or liners on the inside or outside of the structure. 
A related object is to absolutely prohibit manhole contamination of the 
water table, and infiltration or exfiltration. 
Another important object is to insure that the manhole system is locked in 
place during the placement of backfill soil to minimize any settlement or 
upward movements of the manhole. 
These and other objects and advantages of the present invention, along with 
features of novelty appurtenant thereto, will appear or become apparent in 
the course of the following descriptive sections.

DETAILED DESCRIPTION 
With initial attention directed to FIGS. 1-3 of the accompanying drawings, 
the best mode of the herein disclosed manhole system is generally 
designated by the reference numeral 50. My modular system 50 is ideal 
where a preexisting concrete manhole must be excavated and replaced, or it 
may be used as an original manhole in a new or refurbished sewer system. 
The manhole system 50 can be custom assembled at a desired job site 51, 
wherein a selected number of its individual, independent units can be snap 
fitted together to function together harmoniously while custom adapting 
the system to the desired application. Further, the contractor can custom 
select desired structural properties without changing the dimensions or 
assembly technique of the system. 
System 50 is installed in the ground 44 (FIG. 1) and roadbed 43. Base 55 is 
disposed in the bottom floor 85 of the excavation and anchors the system. 
The system extends vertically upwardly through ground 44, compacted sand 
and gravel layer 45, subgrade 46, and surface pavement 47. While roadbed 
43 may of course vary in construction from that illustrated, system 50 can 
be easily customized to fit the application. After system 50 is assembled, 
the individual units are left in place (i.e., they are not removed), 
unlike conventional form systems. However, the resulting corrosion 
resistant structure duplicates one function of forms, in that it 
inherently allows for concrete filling both internally (in its cavities) 
and externally (around and against its external walls) to provide lateral 
and subjacent support to the ground 44 and roadbed 43. 
In the best mode system 50 comprises a base unit 55, one or more 
cooperating riser units 60, an upper cone unit 65, and various 
accessories. The risers and cones extend through the subterranean ground 
44, while the cone 65 enters the compacted sand and gravel layer 45. The 
cone unit 65 forms the corbel of the manhole system 50 while the riser 
units 60 and base unit 55 together form a stable, generally vertical 
structure of substantially uniform diameter. A reducer coupling, generally 
indicated by the reference numeral 70, connects the system's base 55 to 
existing or newly installed sewer lines 72. Extenders 75 to be described 
in detail hereinafter are employed to adapt for grade variations in 
pavement 45 at the top of the excavation; the extender forms a transition 
between the cone and the road layers 45-47. By varying extender length, as 
explained below, system 50 facilitates repavement or other grade 
alterations in the surface. A conventional manhole frame 80 mated to the 
cone unit 65 via the extender 75 is captivated during pavement surfacing 
within resilient pavement 47. 
Preferably the cone 65, the riser 60, and the base 55 are seamless and 
double walled. Preferably they are rotationally molded from a 
corrosion-resistant material such as a polyethylene resin plastic. The 
cavity between the walls of the units may be filled with structural or 
nonstructural fill material. They allow the user to obtain a variety of 
strength and weight combinations. Structural fill material such as cement 
or concrete 82 (FIGS. 34, 35) may be pumped in through a hole drilled in 
the upper portion of each of the units. Nonstructural ballast material 
such as sand or gravel can further weight the units to facilitate their 
use in a particular application. 
The base 55 is installed at the bottom 85 of an excavation. With primary 
reference directed to FIGS. 4-6, base unit 55 is generally cylindrical 
with an open top 87 and a floor surface 89. Reinforcing ribs 88 are 
defined in the floor 89. Base 55 comprises an inner seamless shell 91 and 
an integral seamless outer shell 93. Inner shell 91 comprises a 
peripheral, cylindrical wall 91A surrounding a floor 91B. Outer shell 93 
has peripheral wall 93A encircling integral, lower floor 93B. Floor 93B is 
reinforced by ribs 90 that are complementary to ribs 88 (FIG. 5). Ribs 88 
and 90 help resist structural deformations such as ballooning during 
cavity filling. The shells meet around the upper ledge 95 of the base unit 
55. Hence, a continuous generally annular cavity 97 is formed between the 
inner and outer seamless shells 91, 93. Cavity 97 may be filled with 
structural or non structural material as necessary. Holes may be drilled 
in circumferential ledges 95 or 103 for filling with conventional 
materials. Afterwards the fill-holes are plugged. 
The upper peripheral portion of the base 55 forms a spigot 99 (FIG. 5) that 
mates with the bell of a riser or cone 60 or 65 to form a joint. The 
spigot 99 comprises an outwardly projecting, peripheral ring 101 formed in 
the outer wall 93A (FIGS. 4-6, 28, 29). Ring 101 forms a ledge 103 that 
may structurally support the riser or cone unit 60, 65 above. A lip 105 
projects upwardly from the ledge 103. A notch 107 is defined in the lip 
105 to support the rubber gasket 132 (FIG. 3). The lip 105 and the inner 
wall 91A form the upper ledge 95. In assembly, the upwardly projecting lip 
105 is inserted into a bell portion 110 of a riser 60 or cone unit 65 
disposed above the base. 
One or more riser units 60 or 60B (FIG. 3) may be stacked upon the base 
unit 55. As illustrated in FIGS. 3 and 7-11, the riser units 60, 60B are 
tubular, seamless and double walled. Riser 60 is longer than riser 60B. 
Since the risers of different lengths are provided, the system can easily 
be adapted for different depths. Each riser has an inner and an outer 
shell 112, 114 respectively. Inner shell 112 comprises a peripheral wall 
112A spaced apart from concentric peripheral wall 114A of shell 114, with 
annular cavity 120 (FIGS. 10-11) formed therebetween. Walls 112A and 114A 
meet along the top and bottom ledges 116, 118 of the riser 60. Cavity 120 
between the walls 112A, 114A may be filled with structural or 
nonstructural material. 
The risers 60, 60B include external rectangular recesses 123 (FIG. 8) 
formed in radially spaced apart locations in the external periphery of 
wall 114A. The structural ribs 122 formed between adjacent recesses 123 in 
the outer walls provide rigidity and prevent torsional flexing. (The ribs 
122 of the different stacked risers 60 in FIGS. 1-3 do not necessarily 
need to be aligned as illustrated.) 
The bottom of each riser defines a bell 110 that mates with the upwardly 
projecting spigot associated with a lower unit, such as base 55 or another 
riser. The bell 110 comprises an outwardly projecting ledge 124 defined in 
the inner wall 112A near the bottom of the unit 60 (FIGS. 10, 11, 28, 29). 
A brim 126 projects downwardly from ledge 124 at a slight angle to 
facilitate mating with a lower spigot. The cavity 120 is bounded at its 
bottom by brim 126, ledge 118 and ledge 124. The spigot 99, 128 of the 
unit below a riser 60 is received within the hollow volume 130 defined by 
the downwardly projecting brim 126 and the ledge 124. A circular L-shaped 
gasket 132 (FIG. 3) is placed between the spigot of the base unit 55 and 
the bell of the riser unit 60 to seal the joint. 
The upper portion of a typical riser defines a spigot 128 similar to spigot 
99 defined in the upper portion of base 55. The spigot of the riser 128 is 
intended to mate with the lower bell 110 of another riser unit 60 or a 
lower bell 134 defined in a cone unit 65. A gasket 132A (FIG. 3) is also 
disposed between this spigot 128 and the bell of the additional riser or 
cone unit 99, 134. The riser spigot 128 is formed by an outwardly 
projecting ring 136 meeting with the upwardly extending inner wall 112A 
(FIGS. 10, 11, 30, 31). The ring 136 is integral with the ribs 122 and is 
formed in the outer wall 114A as are the ribs 122. The outwardly 
projecting ring 136 defines a ledge 138 that can structurally support a 
riser 60 or cone unit 65. A lip 140 projects upwardly from the ledge. A 
notch 142 defined in the lip 140 receives the aforementioned gasket 132A. 
The lip 140 and the inner wall 112A meet to form the upper edge 116 of the 
riser unit 60. 
A preferred cone unit 65 (FIGS. 12-16) is snap fitted to the highest riser 
60, 60B or base unit 55. The cone 65 comprises concentric inner and outer 
seamless shell 144, 146 respectively that define a hollow cavity 148 
therebetween. The cone 65 is truncated in shape, and it defines the 
portion of the manhole often referred to as the corbel. The inner wall 
144A of the cone 65 defines evenly spaced, radially spaced apart stiles 
150 (FIG. 16) that provide structural integrity to the cone unit 65. 
Radially spaced apart internal slots 151 border stiles 150. The structure 
of stiles 150 is illustrated best in FIG. 16. A bell 134 similar to the 
one formed in the lower portion of a riser unit 110 is defined in the 
lower portion of the cone 65. The bell 134 mates with the spigot of one of 
the riser units 128 or the base unit 99. 
The bell portion 134 of the cone unit 65 is principally defined by an 
outwardly projecting ledge 153 formed in the inner wall 144A (FIGS. 12-16, 
30, 31). A brim 155 projects downwardly at a slight angle from the ledge 
153. This angle facilitates mating with the spigot of a base or riser unit 
99, 128. The cavity 148 between the walls 144A, 146A is closed due to the 
brim 155 and the outer wall 146A meeting to form the lower ledge 157 of 
the cone unit 65. The bell 134 of the cone unit 65 captivates an L-shaped, 
circular rubber gasket 132A between the downwardly projecting brim 155 and 
the upwardly projecting lip 105, 140 of the lower spigot 99, 128. The 
gasket 132A seals the joint of the spigot 99, 128 and the bell 134. 
The upper portion of the cone unit 65 defines a generally circular opening 
160 that receives either a manhole cover frame 80 or an extender 75. The 
generally circular opening 160 is concentric with the manhole units 55, 
60, 65 but it is of a gradually reduced diameter. As best illustrated in 
FIG. 15, the opening 160 is bordered by a peripheral shelf 167 formed by 
the upper extreme 165 of the cone unit 65 and an inwardly projecting curb 
170. The curb 170 and shelf 167 are joined by a slightly angled, generally 
vertical face 172 that circumscribes opening 160. Hence, the inner border 
of the circular opening 160 is slightly tapered to facilitate reception 
and seal of an extender 75, manhole cover frame 80 or manhole cover 175 
(FIGS. 32, 33). 
A typical extender 75 (FIGS. 17-20) is employed to elevate the grade of a 
manhole made in conformance with the present system 50. In other words it 
adapts the resultant corbel to the proper height relative to pavement 47 
(FIG. 1). Raising the manhole frame 80 optionally associated with the 
present system 50 may be necessitated by repaving operations or other 
surface construction that raise the grade in the area of the manhole 50. 
With attention directed to FIGS. 17 through 20, it will be seen that a 
preferred extender 75 is tubular, solid, and single walled. Each extender 
is segmented, so that it may be custom cut to fit the desired system 
height. Thus each extender defines multiple radial corrugations 177 
separated from one another by adjacent, reduced diameter, integral tubular 
portions 178. The extender mates (i.e., snap fits) with the tapered 
circular opening 160 in the cone unit 65 (FIGS. 32, 33). The lower stump 
portion 190 of the extender 75 is tapered to mate with the generally 
circular upper opening 160 of the cone section 65. A positive seal is 
formed by rubber gasket 180 (FIG. 3) disposed in a peripheral circular 
channel 185 (FIG. 19) circumscribing stump 190. 
Conventional brick and mortar are conventionally employed to construct an 
annular reinforcing ring 195 (FIG. 1). Ring 195 is erected around the 
outer periphery of the extender 75 to facilitate structural support in 
compliance with local building codes. Additionally, this brick and mortar 
structure 195 may be supported by the upper shelf 167 in cone unit 65. The 
newly elevated opening 198 in the upper portion 200 of the extender 75 
will mount the manhole frame 80. 
The eccentric reducer couplings 70 that interface the manhole system 50 
with sewer lines 72 (FIG. 1) have a tubular, single wall body 210. The 
body 210 is intended to be inserted into a hole 255 (FIG. 3) cut through 
the walls 91A, 93A of the base unit 55 at a level appropriate to 
facilitate sewage flow. As illustrated in FIGS. 21 through 27, the 
eccentric reducer 70 further comprises multiple hollow, cylindrical, 
eccentric segments 215. The segments 215 are sized to mate with various 
sizes of conventional sewer pipe 72. Each segment 215 is tubular and 
shares a tangency point 220 (FIG. 22) along its diameter with the other 
segments 215, thereby providing a smooth bottom 225 to facilitate sewage 
flow. Each eccentric segment 215 defines a ridge 230 near the end opposite 
the tubular body 210. This ridge 230 facilitates attachment of a flexible 
pipe coupling 235 (FIG. 1). 
The preferred pipe couplings 235 (FIG. 1) comprise a rubber sleeve 240 with 
a circular clamp 242 on each end of the sleeve 240. Between the ridge 230 
and the edge 245 of the eccentric segment 215 is a cutting groove 250, 
depicted in detail in FIG. 27. This groove 250 acts as a guide to aid in 
cutting the eccentric reducer 70 off to eliminate the eccentric segments 
215 that are smaller than the sewer pipe 72 to which it is mated. 
Polyethylene welding is preferably employed to seal the tubular body 210 
of the eccentric reducer 70 in place through the walls of the base unit 
55. 
Turning to FIGS. 34 and 35, a hole 255 smaller than the outer diameter of 
the eccentric reducer's tubular body 210 is cut in the base unit 55. The 
area around the hole 255 is heated, softening the plastic of which the 
base unit 55 is constructed. The reducer segments 215 are then inserted 
into the heated hole 255 from inside the base unit 55 as polyethylene 
welding continues. The tubular body portion 210 is heated and pressed into 
place, sealing the reducer 70 with O-ring 231. To obtain a satisfactory 
seal, the plastic of the base unit walls 91A, 93A and the plastic of the 
tubular body 70 are preferably heat welded together or glued. Regardless, 
subsequent filling of the base unit 55 with structural material such as 
cement or small aggregate concrete 82 will help complete the seal. 
The preferred method for sealing the tubular body 210 within the walls 91A, 
93A of the base unit 55 includes an O-ring seal 231 about the periphery of 
the tubular body 210 (FIG. 35). Once again the use of structural fill 
material 82 completes the installation. Additionally, sealants such as 
silicone caulking, polyethylene resin based plastic glue, or grout may be 
employed to facilitate sealing the installation regardless of the method 
used. 
The concrete invert 241 is installed within the base after the reducers are 
fitted. It includes the conventional bench portions 243 about the sides of 
conventional flow channel 247. The invert forms a sewage flow pathway with 
its channel 247 that is substantially aligned with reducer couplings 70, 
71 (FIG. 34). 
For reinforcement purposes, it should be understood that with each of the 
aforedescribed modular units, structural steel reinforcement 271 (FIG. 
35), of a generally tubular configuration, can be enclosed within a cavity 
97 between the double walls of the base to provide additional strength if 
required by the user. Although rebar is illustrated in the base unit, it 
may be employed in any or all of the modular units. The optional rebar 
will be inserted during the rotational molding process. 
From the foregoing, it will be seen that this invention is one well adapted 
to obtain all the ends and objects herein set forth, together with other 
advantages that are inherent to the structure. 
It will be understood that certain features and subcombinations are of 
utility and may be employed without reference to other features and 
subcombinations. This is contemplated by and is within the scope of the 
claims. 
As many possible embodiments may be made of the invention without departing 
from the scope thereof, it is to be understood that all matter herein set 
forth or shown in the accompanying drawings is to be interpreted as 
illustrative and not in a limiting sense.