Unlike CIPP, modified cross section methods do not make use of resins to secure the liner in-place. Lacking resin-coated lining, these methods do not have the curing time requirement of CIPP. A tight fit is obtained when the folded pipe expands to the host pipe's inside diameter under applied heat and pressure. As with the CIPP method, dimples are formed at lateral junctions and similar methods of reconnecting the laterals can be employed. Materials typically used for modified cross section
linings include Polyvinyl Chloride (PVC) and High Density Polyethylene (HDPE).
Trenchless sewer rehabilitation methods are now routinely applied to wastewater collection system improvement projects in the United States and many other countries. Trenchless sewer rehabilitation has been successfully applied by both large municipalities such as New York, NY; Los Angeles, CA; Boston, MA; Miami, FL; and Houston, TX; and smaller municipalities such as Baton Rouge, LA; Madison, WI; and Amarillo, TX. Kramer and Thomson (1997) estimate that the market value for sewer and pressure pipe rehabilitation projects will be $5 billion dollars world-wide in the year 2000.
In many municipalities, sewer rehabilitation projects are an essential part of operation andmaintenance (O&M) programs for the collection system. For example, as part of an O&M program focused on pro-active maintenance, Fairfax County, Virginia, has identified two older sewersheds for rehabilitation. All trunk and main lines within each sewershed are television inspected. Results of the TV inspection are used to prioritize cleaning needs and to help determine appropriate rehabilitation measures. Projects within the targeted sewersheds have utilized the CIPP and fold-and-form rehabilitation methods.
In an effort to monitor the effectiveness of the rehabilitation efforts, the department installed permanent and temporary meters in these two sewersheds. Fairfax County's focused approach to maintenance has reduced average flows to the wastewater treatment plant (WWTP) despite several years of above- normal rainfall.
APPLICABILITY
While trenchless techniques may be applied to rehabilitate existing pipelines in a variety of conditions, they are particularly valuable in urban environments where construction impacts are particularly disruptive to businesses, homeowners, and automotive and pedestrian traffic. Other underground utilities and existing infrastructure are an obstacle in the traditional dig and replace method, and trenchless techniques are widelyapplied where these are present. Most trenchless techniques are applicable to both gravity and pressure pipelines. Many trenchless methods are capable of performing spot repairs as well as manhole to manhole lining.
For most applications, trenchless sewer rehabilitation techniques require less installation time-and therefore less pump-around time- than traditional dig-and-replace methods. Installation time can be critical in deciding between trenchless sewer rehabilitation methods and dig- and-replace methods. For example, when considering sewer repair or replacement options for a critical force main crossing the Elbe River in Heidenau, Germany, city officials determined that the line could not be out of service for more than 12 days (Saccogna, 1998). As a result of this time constraint, as well as reduced disruption to riverboat traffic, city officials chose to rehabilitate the sewer using the swagelining™ process. The successfully rehabilitated sewer was out of service only eight days.
Trenchless sewer rehabilitation can be performed to increase the hydraulic capacity of the collection system. While pipe bursting typically yields the largest increase in hydraulic capacity, rehabilitation by other trenchless methods may also increase hydraulic capacity, by reducing friction. A hydraulic analysis of the pre- and post-rehabilitation conditions can be performed to evaluate the impact on collection system capacity. In general, the hydraulic analysis is performed by municipal engineers and/or consultants who prepare specifications for contractors.