March 28, 2002
Avoiding pitfalls in pipeline restoration
By WESLEY SALEIRA
Garry Struthers Associates
As in other parts of the country, the Northwest has its portion of aging infrastructure, particularly its sewer systems.
Sewer pipelines are generally “out of sight, out of mind.” They grab their share of public attention with catastrophic cave-ins, blockages, backups, flooded streets and basements or overflowing wet wells.
It is now recognized that a comprehensive program of on-going investigation and preventive maintenance of sewer systems is not only economical but also more cost effective than having to attend to emergency repairs.
Municipalities, under pressure more than ever to maintain sewer rates at current levels, are taking this approach for formulating their sewer maintenance programs.
Aging sewer pipelines can be rehabilitated to extend their service life.
The oldest sewer pipelines tend to be either of masonry construction or cast-in place monolithic construction. This type of construction was subsequently replaced by the traditional open-cut pipeline method with individual sections of pipes placed and jointed in trenches. The earlier pipe joints were poor and not tested, unlike modern pipeline construction that require routinely stringent joint tests before acceptance.
Sewer flows are conveyed to treatment plants for treatment. Aging sewers tend to carry significant amounts of infiltration and inflow. Hence eliminating external flows to the sewer system translates to lower treatment costs. Thus the need for rehabilitation of the sewer system, as the structural condition of an aging sewer impacts the infiltration and inflow entering the sewer, and vice versa.
Infiltration and inflow
Infiltration is groundwater or stormwater which enters the sanitary sewer system through cracks, holes and open pipe joints. Inflow, on the other hand, is stormwater which enters the sanitary sewer system directly from cross-connection with the storm sewer, from stormwater collectors such as roof drains or catch basins, or from ponding over manhole covers.
Inflow enters the sanitary sewer after rain begins and stops after the rain stops. Infiltration enters the sanitary sewer after the stormwater has seeped through the ground to the level of the sewer, and continues until the groundwater table falls below the level of the sewer line.
In comparison to older sewer systems, newer systems usually have lower infiltration and inflow. However, with passage of time, the structural integrity of sewer pipelines is reduced by biological, chemical and physical action.
In the Northwest, earthquakes cause pipes to become misaligned, opening gaps at joints or causing breaks. Such effects are also caused by uncontrolled construction activities near or above the sewer line. Underground springs undermine the sewer causing the line to sag and joints to open. Hydrogen sulfide produced in the sewer attacks the crown of concrete pipe. Root intrusion at joints and cracks can wedge open pipelines. Poorly made lateral connections can weaken a pipe or leave a gap around the connection.
Inflow sources are added to sewers at times intentionally but most often unintentionally. Storm sewers or stormwater carrying laterals may be tied into sanitary sewers by mistake due to inaccurate or non-existent drawings.
Sometimes roof or parking lot drains are tied to the sanitary sewer because a project was developed in an area lacking a storm sewer system. In rare cases, potable and fire protection water lines may be mistakenly tied into the sanitary sewer system.
Several investigative and rehabilitation techniques are available for evaluating infiltration and inflow (I/I), as well as structural integrity of a sewer system which includes sewers, manholes and laterals.
A systematic I/I or sewer system evaluation study can identify the specific causes and quantify the I/I entering the sewer system. This information can then be used for developing cost effective corrective actions.
Elements of an I/I or sewer system evaluation study consist of the following: flow monitoring, interviews with maintenance personnel and review of repair records, visual inspections of lines and manholes, smoke and dye testing, and TV inspection of the lines.
Usually, a low cost technique is used to gather information for assessing the need for and planning costlier additional investigations. Based on current and future capacity needs and the results from I/I, structural and corrosion evaluations, appropriate pipe rehabilitation alternatives can be selected.
Structural repairs may be accomplished with trenchless and less-trench technologies, such as sliplining, inversion lining, and fold and form methods. Manholes may be structurally repaired by complete replacement or by varying degrees of rebuilding.
I/I reduction needs to include correcting sources of direct inflow into the sewer system. Inflow is excluded from the sanitary sewer by correcting cross-connections with the storm sewer, raising manhole rims, and installing solid manhole covers.
For infiltration exclusion, it is necessary to repair every significant defect in a particular segment. If only the larger cracks or holes are fixed, water will find its way to the small cracks and likely to make it worse. Defect repairs include grouting of joints, sliplining, inversion lining, applied-in-place concrete lining, coatings, lateral repair or replacement, and manhole repair.
Not every sewer rehabilitation project is a success in spite of the correct pipe rehabilitation method, good specifications and the right contractor.
Not recognizing the fact that pipe rehabilitation projects involve processes and products to manufacture the final pipe is a major contributor for this failure. In cases where matured methods such as inversion lining are selected, several vendors providing the same process, but with minor variations on either the process or product, could bid on the project. Evaluating and selecting the right product and process, as well as inspections must be made to ensure the products supplied at the site are compatible for the process and also meet the specifications.
In spite of the wealth of knowledge and expertise available, most of the time sewer rehabilitation projects are dictated by the input from vendors and suppliers. Each supplier recommends his method over the other, a case of fitting the problem to the solution.
Project inspectors also need to be trained and aware of the many pitfalls of pipeline rehabilitation projects. Rehabilitated sewer pipes are composite pipes that are easily damaged by construction equipment, particularly if they get too close. It is important that accurate maps are kept of the rehabilitated sewers to ensure future construction does not cause any damage.
Also of importance is that the leaky aging sewers operate as a relief system for the local groundwater. When these leaks are eliminated local groundwater may rise and cause flooding, landslides, and slope stability problems.
Wesley E. Saleira, Ph.D., P.E., is a licensed senior civil engineer with Garry Struthers Associates.
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