Manhole chambers are one of the major components in every sewer system and occur, on average, every 300 lf in every gravity sewer pipe. These “windows” provide access for CCTV inspection, as well as entry for repairs to the main sewer pipe. While each buried component has its own set of problems, manholes are remarkably more complex in size, shape, components and material. Benches, channels, pipe penetrations, walls, cones, chimneys, grade rings, frames and covers make for unique problems that require innovative methods and materials to make no-dig renovations possible.
The EPA estimates that there are more than 30 million manholes in North America’s sewer systems and half of these were installed more than 40 years ago. The need for repair and replacement this huge volume of defective manholes exceeds city budgets. Priorities, therefore, must be established to address each problem in the most cost-effective means possible.
Our industry has learned that the lowest cost method is not always the best long-term solution. Critical investigation and sound engineering is imperative; and, of course, early diagnosis is critically important for choosing least cost solutions.
Over the past 30 years our industry has witnessed development and implementation of a wide variety of innovative technologies that address the primary problems found in manholes, which are erosion from inflow and infiltration and decay from corrosion, both of which can cause major structural damage.
Manhole Rehab Due to Erosion
Mechanical chimney seals and sewer dishes have a long and successful record of eliminating inflow at the cover, frame-joint and grade rings. Also available are cured-in-place vertical liners from the frame through the chimney and flexible sealants applied internally and/or externally have also grown in popularity as surface preparation has improved and installations can be made without any damage to or replacement of the street.
Infiltration control has benefited by using improved chemical grouts to stop major leaks in manhole walls, channels and pipe penetrations whereby a grout-curtain completely encapsulates the old manhole. Chemical grouting is also a frequent first step prior to installation of a manhole liner to stop leaks so the interior surfaces can be properly prepared. Early attention to small leaks avoids greater costs later.
Manhole walls, especially when constructed of brick and mortar, can fail catastrophically if repairs are ignored. Full structural replacement is the best option in these cases and no-dig solutions are available, as well as replacement methods that require only partial excavation.
If cutting and removal of pavement sections is selected, fiberglass and plastic pipe inserts can be placed and then backfilled with standard concrete. Special care must be taken where the insert fits into the bench and around pipe penetrations at the channels and drop inlets. Pavement removal and replacement can be avoided with other methods that assemble steel or plastic forms inside the old manhole equipped with eccentric and concentric forms for cone placement at grade. Again, the space between the existing wall and the forms is filled with rapid setting, high-strength concrete for same day return to service.
Alternative methods at lower costs are hand-sprayed or centrifugally cast liners that also provide a fully structural replacement with cement-based materials imparting high-strength and impermeability compacted in-place at the appropriate design thickness. The thickness design was developed at Iowa State University in 1990 under a grant from the National Science Foundation whereby liners were centrifugally compacted onto cardboard tubes, cured and then tested under compression in the soil cell at Spangler Underground Lab. These tests provided minimum material performance criteria at different thicknesses to replicate the pressures encountered at various depths and from dynamic traffic loads.
The ISU design was based on minimum material strengths of 3,000 psi in 24 hours (10,000 psi at full cure) and application made by precision placement and centrifugal compaction producing values not consistently possible with spraying and troweling by hand. This design has been used by engineers since it was first presented at the 1995 NASTT No-Dig Conference in Toronto, Canada.
Other structural liner options include cured-in-place liners applied vertically, hand laid fiberglass cloth and thick polymer liners. Since external pressures vary by depth and diameters, the F-1216 thickness guide for buried pipe with uniform depth and diameter is not appropriate for manholes. Hand-sprayed polymer liners depend entirely upon the skill of the applicator in the hole where hazardous conditions exist.
Manhole Rehab Due to Biogenic Corrosion
When biogenic corrosion is the problem, which is common at some level in sewer systems regardless of climate, protection must be used. Polymer coatings, when properly prepared and applied, provide an excellent barrier between the manhole wall and biogenic acid. Polymer coatings may be applied to existing walls when they are placed in service or undamaged or when applied to a newly installed cement-based liner.
Thorough coverage is necessary to avoid pinholes, bubbles, thin spots and exposed areas which can cause delamination of the coating. Post installation testing for proper thickness can be performed by a trained tester with high-voltage equipment. Surface preparation prior to coating is also important. Removal or limitation of moisture on or in the material surface is also important.
Many cement-based liners contain moisture until fully cured at 28 days. Since manhole walls are wet from use and condensation, special care must be exercised to enable bonding/adhesion. Adhesion is tested with a pull-tester at representative areas after the coating is fully cured.
An alternative method of protection against biogenic corrosion in manholes is acid-resistant materials. Materials such as high alumina cements and geopolymer cements offer a level of protection against mild biogenic acids. Once the biogenic mechanism starts, however, acid production from the conversion of hydrogen sulfide gas continues to greater and greater concentrations achieving levels of pH 0.10. In such cases, resistance is usually overcome and corrosion will accelerate. Using these types of cements in mild environments can extend their useful life. Alternatively, the production process of biogenic acid generation can be eliminated entirely by antimicrobial additives mixed into the cements. This prevents colonization of the acid-producing bacteria and acid cannot be produced where the bacteria cannot grow as evidenced by more than 25 years of successful use. Cement-based liners are not threatened by pinholes, thin spots or bonding problems.
Four Links of Manhole Renewal
Manhole renewal is like a chain with four links. And, if any link fails, the solution fails. The four links are:
- Sound engineering – proper assessment of the problem
- Proper design and method selection
- Proven technology – third party verification of materials and methods
- Factory trained contractors who are reliable and experienced.
Over this past year, ASCE and NASSCO collaborated on the third edition of Manual of Practice 92 – Manhole Rehabilitation and it is scheduled for publication in 2022. This MOP provides a comprehensive list of trenchless renovation methods and materials with specific details of benefits and limitations. It is an excellent reference.