Classic Manhole Problems Solved with Technology
By Eric Dickson Mar 01, 2013
All underground utilities — sewers, storm drains, buried electric and telecommunications — require places to access them for maintenance and repair. This is typically done with manholes.
Manholes are typically built of brick or concrete, designed to be rigid structures from the access point to the utility. For many of these manholes, the access point is in a roadway, which creates the need finesse the final height and slope of the opening to match a surface grade. Traditional techniques for this adjustment include the use of brick and mortar, or precast grade rings with shims and mortar, accomplishing a continuation of the rigid structure into the final roadway surface.
The interface between a roadway and a manhole, however, is not a static environment, and many times the rigid construction of the final adjustment fails over time due to dynamic forces that the brick and mortar, or concrete rings, were not designed to accommodate. Thermal expansion, freeze-thaw cycles, traffic impact and other forces are constantly at work creating failures in the manhole chimney section.
Failures start out as cracks, but even small cracks can have big consequences. Rain water leaking into manholes through the chimney has been show to be a major source of sewer overflows, as well as an ongoing expense at the treatment plants. As that rain water leaks in, it can take soil and sediment with it, which eventually causes cracks and potholes in the roads above.
Additionally, the installation of concrete rings for adjustment is typically an unfriendly process. Moving heavy objects below the knees creates high risk of injuries. Crushed fingers and toes, strained backs, herniated disks are not uncommon when working with concrete rings.
During the past 15 years, several technologies have been developed to attempt to address the shortcomings of the traditional construction of the adjusting section of manholes. Each technology has attempted to introduce an alternate material that can meet the performance expectations of both the utility owners (corrosion resistance, water tight sealing, longevity, etc.), as well as the roadway owners (loading requirements, thermal cycling, etc.).
SolutionsThere are four main technologies currently available as alternatives for concrete grade rings. They are: Injection Molded HDPE, Solid Rubber, Expanded Polystyrene (EPS) with a Polyurea Coating and Expanded Polypropylene (EPP). These products share some common attributes relative to their concrete precursors, including weight reduction, smaller increments of height adjustment, as well as some elastomeric properties specific to each.
The use of solid rubber is a natural evolution of a material that has been used in sanitary sewer systems for several decades. Rubber is a lighter material than concrete, and additionally has demonstrated capability to provide water-tight sealing in application. It offers excellent corrosion resistance, and therefore durability in sanitary systems, which are generally considered to be the harshest environments for buried utilities.
The use of Injection Molded HDPE was also a natural evolution of a material that has been used in pipe for buried utilities as well. HDPE rings do not require a solid structure to achieve loading capability for roadways, so through engineering the design, the hollow structure can achieve excellent weight reduction and provide excellent resistance to corrosion.
Expanded Polystyrene (EPS) was introduced more recently than rubber and HDPE. EPS is a foamed plastic that achieves high-compressive strength at very low density. This material provides a lighter, more rigid ring that rubber, and has less hollow space than the HDPE. Since Expanded Polystyrene itself is not corrosion resistant, these rings incorporate a Polyurea coating to protect them from the environment of the buried utility, which come pre-applied as well as post-applied to the final structure to insure complete coating and sealing.
The most recent material to be introduced is Expanded Polypropylene (EPP), which is a foamed plastic similar to EPS. Unlike EPS, however, EPP is corrosion resistant, so the rings are constructed of consistent material throughout, which allows for drilling, minor intrusions, offsetting and a number of other installation related modifications without compromising the loading capability and the structural integrity of the product. The structural capability of EPP rings has an engineering safety factor that means that if a ring is accidentally cut by a shovel or pick, or pierced by a protruding rock during backfill, the loading capability, as well as the sealing capability, is not compromised. EPP brings some of the flexibility of rubber at a lighter weight. EPP is impact resistant, developed specifically to absorb energy of impact without transmitting it through to the supporting structure or rebounding it at the loading structure, making it ideal for the dynamic forces of traffic loading. The EPP material will actually serve to protect the rest of a manhole structure from the shock waves of impacting tires, prolonging the expected life of the structure.
Although each has its own functional weight, all of these products are lightweight relative to concrete. A typical 6-in. concrete adjusting ring can weigh 300 lbs or more, the replacement technologies can range from 8 to 35 lbs for similar adjustments. The design of a grade ring is actually less than ideal for concrete because of its weight: concrete is extremely strong in compression, but tends to perform poorly in flex and tension. The limited thickness of the adjusting rings relative to their weight makes them break during handling and installation.
All the rings offer smaller increments of adjustment than concrete rings. Concrete rings typically offer 2-in. increments. The remaining adjustment is accomplished with shims and mortar. The performance of shims and mortar are often worse than the concrete rings, sometimes because of installation, sometimes because of the material selected. The concrete alternative materials are able to accommodate typical handling and installation at much smaller thicknesses (as low as Ľ in.) and can eliminate the need for shims and mortar.
The technology for making final adjustments in a manhole has continued to evolve for a number of reasons. The desire to save money from wastewater treatment plants or to eliminate sewer overflows and the EPA fines associated are a few. The desire to reduce the degradation of roadways and reduce cracking, settling, potholes and ongoing repair is another. Saving the expense of having to dig up the top of the manholes and rebuild them is also a big driver. These technologies have all attempted to find the best function and value, but all have made significant improvement over the traditional brick or concrete approach.
Eric Dickson is general manager of Cretex Specialty Products.