Manhole Inspection and Rehabilitation

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In the world of manhole construction today, materials such as HDPE, concrete with corrosion reducing additives and plastics are used. Newly constructed concrete manholes are rarely seen without liners. However, the lasting power of T-lock and other liners depends on the contractor — a poorly installed T-lock liner can fail within the first 10 years. HDPE has its merits; it is a stout material that is manufactured to be thicker than T-lock. Concrete manholes with properly installed HDPE liners can last years without deterioration.

There are also some manholes built out of HDPE rings. It is most likely that HDPE ring manholes can last years. With over 20 million manholes in the nation and many of them in poor condition, what can be done? The first step is to inspect the manholes to determine their condition. The best practice for the manhole inspection is to use a combination of technology and getting down into the trenches. An initial investigation can be conducted with the use of a CCTV camera. If a particular area appears to be in good shape, further investigation is not required until the next round of maintenance checks.

If the area appears to be in poor or questionable condition, then a confined space entry into the manhole is performed. However, technology is extremely helpful in this part of an initial manhole inspection. Viewing the manhole via camera limits the number of confined space entries, keeping workers safe and costs down.

Manhole inspectors are like detectives — consistency is necessary in all aspects of investigation in order to make the most of the clues. This program allows for consistency of documentation and a repeatable process for evaluation. In this way, a long-term approach to manhole rehabilitation can be undertaken rather than simply reacting when problems arise. This standardized method for reporting the results of condition assessments keeps everyone on the same page, promotes cost efficiency and avoids unnecessary rehabilitation work.

In most cases, rehabilitation of the manhole is chosen over replacement due to cost and logistics. When determining what rehab methods to use for a particular manhole, it is important to note that a one-size-fits-all approach is not the best idea. No two manholes are identical. Site specific considerations such as physical location, access and right of way can be different for two manholes suffering from the same internal corrosion issues. Fully understanding the challenges involved will help guide the rehab process.

For example, where is the manhole located? Poor access to a manhole can greatly impair the ability to rehab that particular manhole. Are there any traffic control measures that need to be considered? What about flow issues, elevated H 2 S levels or moisture? Will it be necessary to ventilate and dehumidify? What are the cost constraints? How long does the rehab need to last? Being armed with the answers to these questions will help ensure that the rehab process goes smoothly. Depending on the answers to the above questions and the specific defects of the manhole, coatings, liners or a combination of both may be used to fix the damage.

When it comes to coatings, spray applied epoxies, urethanes and cementitious coatings that are calcium aluminate based are all used in rehab work. Other options include repairing existing liners with a similar material. At times, a combination of methods may be appropriate. If using a combination method such as this, the two rehab methods need to work well together, with a proper interface occurring between the coated concrete and the remaining liner. The old liner is replaced with a new structurally reinforced cure-in-place liner.

Regardless of the rehabilitation method chosen for a particular manhole, the goal is to create a corrosion barrier to prevent manhole deterioration. Pollak has more than 35 years experience in project management on water and wastewater projects. Investments will benefit , residents in rural communities. EPA receives 51 letters of interest, highlighting critical need. The partnership will develop, acquire and optimize water, desalination, wastewater treatment and water reuse facilities across the country.

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Thirty projects were selected for laboratory-scale and pilot-scale research to determine viability of a novel process or determine the technical, practical and economic viability of a process. Partner Agencies to continue funding program to develop new, reliable, locally controlled and drought-proof drinking water supply. If the manhole is precast concrete, structurally sound but leaking at the riser joints, internal seals or grouting might be options. If the manhole is precast concrete and the wall mortar has deteriorated and the steel cage is visible, then a wall build-back and a polymer top-coat or a cured-in-place liner might be the answer.

Many answers There are a multitude of other scenarios that can be further illustrated but the point is, each individual manhole or group of manholes may require a specific selection of products, to accomplish a long-term rehabilitation of the structure.

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Further, it must be determined whether or not the manhole is in jeopardy of structural failure. Unlike a pipe which may go through various stages of failure leading to total collapse, a manhole is either extensively deteriorated but structurally okay, or it has collapsed. Also unlike a pipeline, a manhole is a vertical structure where the hydrostatic pressure, earth loading and live loading from the surface is constant at any specific elevation.

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Loads on the manhole will, however, increase at the deeper elevations. So what formula should be used? Or should the formula be specific to manholes and incorporate the compressive strength of the material to calculate the buckling load and thereby determine the required wall thickness of the new product?

When installed, the new product should be equal to the full loading characteristic provided by the existing manhole. In either event, the formula should be standardized for the industry and require the same criteria for all coating and lining technologies.

To further explore this topic, Underground Construction sought input from a municipal manager, product manufacturer and a consulting engineer for their responses to several questions. Edward Carpenetti, P.

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  4. The commission allows installation of product approved by its Material Evaluation Committee. Chip Johnson, P. Ed Kampbell, P. What is your definition of full structural rehabilitation, and what is the role of the existing manhole to continue functioning as the structural component after rehabilitation? The structural repair should be able to function even if the original structure crumbles.

    Any reinforcement from the existing structure offers additional strength in the finished rehab installation, but is not dependent on that. Any remaining competent material becomes a part of the new soil-structure interaction system once rehabilitation has taken place.

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    Therefore, no single design approach is applicable — each existing structure and surrounding area should be evaluated on a case by case basis to determine the best way to rehabilitate a manhole. The forces acting on a manhole are different than the forces acting on a sewer main since a manhole is orientated perpendicular to the ground surface as opposed to parallel. This standard has been the industry wide accepted protocol for buried circular geometries for conduit and manhole design for plus years. Further, the close-fit liner can be subject to varying dynamic loadings in a vertical end orientation.

    To validly use the classical buckling theory in its F embodiment, a inch diameter manhole would have to be feet deep for a 0.

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    So the design appendix of F is not a viable design tool. In my engineering judgment, a close-fit liner system installed in a manhole must be designed to resist hydrostatic buckling only.

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    For proper insight into the analysis of buried silos, I recommend looking at Reynold Watkins book, Structural Mechanics of Buried Pipes. In a reasonably circular shape, the performance limit is ring compression yield strength.

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    It takes into consideration the exact geometry of the lining curve radii and the beneficial interaction with the host structure that may be attenuated by an annular void space. Therefore they need to be flexible so that over time, they do not crack, fracture or break. Standards such as ASTM F do not assume adhesion and while conservative, minimize the human installation application factor. Further, the design analysis method I recommended in the previous question demonstrates that the liner is going into a compressive state that ultimately results in a buckling failure.