‘One size fits most’ manhole rehab
Manholes that get corroded with hydrogen sulphide present a thorny problem for municipalities, which need a cost-effective and efficient method of rehabilitation.
While the majority of H2S corrosion occurs at the base of the manhole, the upper section, or chimney, can also deteriorate, with cracks becoming weak points that let in rainwater. The top of the manhole, above the cementitious portion, can be especially problematic. As the concrete or bricks and mortar adjustment rings deteriorate or shift, they create potential for rain-induced infiltration. Adjustment ring movement at the top of the manhole can push up the manhole cover, resulting in popped tires for motorists who drive over offset covers that are no longer flush to the pavement surface.
On top of that, manholes lack uniformity, with each city’s manhole inventory coming in different shapes and sizes. That makes rehabilitation a real quandary.
LMK Technologies came up with a designed and engineered solution to address the challenge of manhole rehabilitation, and especially to provide an alternative to existing cured-in-place systems. The Cured in Place Manhole (CIPMH) system is described as a “one size fits most” technology that rehabilitates, prevents infiltration and inflow, and structurally renews the manhole.
“We wanted to give more options to rehabilitation,” says Rick Gage, vice president, sales with LMK, explaining why the system was developed. In an interview with CUI, Gage said that other cured-inplace systems require a technician to go out and accurately measure the manhole in several locations; ordering and delivering a custom-designed liner order could take up to three weeks.
“Here, we’ve got a roll of tubed material sitting on the surface. The technicians prep the surface of the manhole and you take your the depth measurement, and then they cut the liner off to the proper length. After that the technicians weigh out the proper amount of resin by using an Excel-based calculator. Then they vacuum-impregnate and pour it in and install it. Literally, it can be that fast.”
Using CIPMH, a municipality can rehabilitate a defective manhole in less than two hours, according to Illinois-based LMK. Crews can typically perform three to four installs per day, with minimal disruption and cost. The equipment investment for CIPMH runs about US$12,000.
The specialized cured-in-place manhole product comes in two variations: the CIPMH Chimney and the CIPMH Full Depth.
The CIPMH Chimney rehabilitates the chimney portion of the manhole extending from the rim of the manhole cover to above the barrel section, using a coated knit tube and ambient-cure silicate resin. The CIPMH Full Depth rehabs the full depth of the manhole extending from the manhole cover rim to the bench or crown of the mainline pipe.
This process is installed under pressure and cured ambiently with or without steam. The manhole liner is engineered to resist freeze-thaw cycles and create a corrosion-resistant lining. The Chimney or Full-Depth system can rehabilitate the surface of most types and configurations of manholes including barrel sections, eccentric and concentric cones constructed of brick and mortar, pre-cast or block. The rehabilitation is accomplished using a stretchable non-woven textile tube of particular length and a thermo-set resin with physical and chemical properties appropriate for the application.
The liner is vacuum impregnated onsite. The saturated liner is then lowered into the manhole and is temporarily held in position. The installation device is then lowered and properly positioned inside the liner. The installation device is then pressurized so that the liner is pressed against the existing structure. Once the resin-saturated liner is cured, the installation device is removed. The liner is then trimmed flush with the manhole cover seat.
“Whether it’s eccentric, concentric or just a barrel section, the one size fits most is flexible in nature,” says Gage. “The beauty is when we’re going vertical. We don’t have to so much worry about wrinkling and flow, like you would if you’re working in a horizontal pipe.
“We’re going to get little bit of wrinkling, a little bit of fold-over towards the top where it’s the smallest diameter, but that is purely cosmetic. The idea is to take the cured-in-place and make a more structural manhole that can hold back water, so we can help with infiltration and inflow reduction.”
While there are cheaper alternatives to cured-in-place methods of manhole rehab, including cementitious linings and sprayon epoxies, these methods require more surface preparation than CIPMH, which doesn’t need to bond to the manhole wall. “Our liner is going to migrate into those open joints, voids and cracks. It’s going to mechanically lock itself in place,” says Gage.
The benefits of the product have certainly borne fruit for LMK, which has seen several U.S. cities use it for rehabilitating their manhole inventories. Gage said Wichita, Kansas recently purchased the system and Chicago has signed on with LMK to rehab half of their manholes with CIPMH, and the other half using a cementitious product offered by AP/M Permaform. So far the city has installed about 9,000 manholes using CIPMH.