Manhole extension

Claim 1

In a manhole extension for use in manhole casings of the type having an upwardly-facing annular seat recessed beneath a rim bordering the upper marginal edge of an upstanding wall therebetween wherein said extension includes a lower annular flange adopted to rest atop the casing seat, an upper annular flange spaced above the lower one, a ring connecting the inner edges of the upper and lower flanges cooperating therewith to define an outwardly-facing channel, and an inflatable gasket positioned within said channel for the purpose of sealing the joint between said extension and casing, the improvement which comprises a circular fence depending from the upper annular flange defining a gasket receiving seat which extends from the upper flange to a location spaced apart from the lower flange to span a gap between the top of the casing and the upper flange, said inflatable gasket being positioned outside of said gasket receiving seat to expand into said gasket receiving area and against the inside of said fence, said fence being positioned and adapted to cooperate with the channel and with the upstanding casing wall when the upper flange is placed thereagainst to define an enclosure substantially completely enveloping the gasket upon inflation thereof while leaving a portion of the inflated gasket exposed to engage said casing wall as well as the inside of said fence to form a continuous annular fluid-tight seal therewith while preventing extrusion of the inflated gasket out of the gap.