By Mike Miller
Gasketing pipe is relatively straightforward, but one area that is difficult to seal well is the intersection of the horizontal pipe and vertical manhole. The historical method of sealing the joint used to be a rigid mortar ring around the pipe to fill the void. This method offered a familiar joint, but became the Achilles heel of the system.
Flexibility by design
Rigid mortar has a long history of use for sealing concrete products. It is almost always around the job site, can fill any size or shape opening and doesn’t require precision. But what cannot be controlled or changed is the rigidity of the connection. The ideal field connection has the adaptability of a mortar joint and the permanent flexibility of a conventional connector. Although it sounds simple, achieving this has been difficult.
Pipe-to-manhole connections offer some of the most challenging sealing problems in underground construction. Unlike pipe joints, differences in material properties between the connecting pipe and concrete manholes frequently leads to pipe complications. Consolidating backfill soil creates differential vertical loads and shear forces at the joint interface, which increases with bury depth. Without some connector flexibility, the shear load is transferred directly to the pipe, which may not be able to withstand the additional loading. Angular deflection is limited by the requirements of the connector geometry. Design changes can be problematic.
Adaptation over time
An additional factor that has changed for manhole connectors is the implementation of a vacuum test prior to being accepted. When manhole vacuum testing was introduced in the 1990s, component failures were not uncommon. Precasters and suppliers responded with better products and improved processes. Contractors also improved installation practices. While it created short-term challenges for the industry, manhole vacuum testing is credited with improving manhole construction quality.
Connector manufacturers have come forth with a wide variety of products to respond to these requirements. From simple, resilient rubber compression seals to multi-adjuster stainless steel boots, each one has its proponents, and almost every connector manufacturer offers its own version of each to accommodate the precaster’s preference.
Consult with the manufacturer
The variety of sizes, styles and applications of manhole connectors requires the precaster to work closely with the connector manufacturer. Connector specification may change when manhole diameters change. Different products accommodate different pipe types. Some connectors are more tolerant of variation in pipe surface finish or diametric variation. In any event, the precaster and its connector manufacturer(s) must function as a team to provide the best success for the installing contractor. When precasters and contractors work together, the outcome is always favorable.
The future of pipe connectors
Flexible connectors are being applied in areas of underground construction other than sanitary applications.
During the early 2000s, septic systems underwent a dramatic change in cost, complexity and functionality. Many government health agencies continue to follow customarily accepted on-site treatment methods. However, concerns with enhanced effluent quality or unacceptable drain field soil characteristics have led some agencies to incorporate on-site sanitary sewage treatment systems traditionally associated with larger municipal treatment plants. Depending on design, these systems can use many advanced treatments, including aeration, pumping, filtration, UV treatment and other techniques that rely on complex components. The system also needs to strictly control infiltration of groundwater and exfiltration of effluent at entry and exit points throughout so that proper treatment can be performed.
The movement by municipal owners toward sealing stormwater collection systems is growing. This closely follows the expansion and adoption of stormwater treatment products and techniques. These vary dramatically from landscaping and vegetation plans to separation and settlement products and high-volume filtration systems. Like sanitary sewage treatment, controlling the entry and exit points into these systems is critical for them to work as designed.
Another reason for sealing stormwater systems is to improve the life span of pavement. Since the majority of stormwater systems are constructed in association with street and highway projects, the performance of each is closely linked. By maintaining this soil support, especially at pavement penetrations such as inlets and manholes, the roadway maintains its surface integrity. Perhaps the most progressive proponent of sealing stormwater systems is the Florida Department of Transportation. With the state’s high groundwater and erodible fine grain sandy soils, potential mitigation of backfill is a critical design issue to be addressed. While it’s still in development, with many more challenges to be met, FDOT’s standards provide invaluable real-world experience with stormwater management.
By securing a quality pipe-to-manhole connection, you protect communities from contamination and costly damage. It is a crucial preventative step. For more resources about pipe-to-manhole connectors, visit precast.org/precast-products/manholes.
Mike Miller is the owner of Large Caliber Design, LLC, and remains active in sealing product design and development, as well as standards work with ASTM.