Six Design Considerations When Rehabilitating an Existing Pipe
Condition of America's Wastewater Infrastructure
America’s Wastewater Infrastructure is currently rated at a D+, according to the 2025 ASCE Report Card for America’s Infrastructure. The Wastewater category includes both piping for the collection systems and the wastewater treatment plants the piping leads to. The collection system alone experiences 3.3 failures per 100 miles of pipe every year, putting pressure on maintenance teams to identify suspect pipes for either complete replacement or pipeline rehabilitation. This article will focus on the design and construction considerations necessary when a pipeline owner determines they will be rehabilitating an existing pipe.
WHY REHABILITATE?
The first question most owners ask is, “Why would I rehab a pipe instead of replacing it?” There are many site-specific conditions which affect this decision, but the primary considerations are economic, hydraulic, and political pain points. For example, if the pipeline in question is located in a field, it’s going to be relatively easy and inexpensive to replace that line. Conversely, if the same line is now located in a busy intersection, the cost for replacement and the political pain points grow to a point where rehabilitation becomes more feasible. In all cases, the rehabilitated pipe needs to be able to carry the peak design flow, otherwise full replacement is necessary.
SIX DESIGN CONSIDERATIONS FOR PIPELINE REHABILITATION
Once the decision for rehabilitation is made, the primary design and construction considerations are:
- Condition Assessment of the Existing Pipe
The condition of the existing pipe needs to be known to determine the appropriate repair material. The necessary information depends on the type of system being repaired. Culverts, for example, are generally easily accessible, so visual inspection may provide enough information to move on to the other six considerations. Storm sewers or sanitary sewers will likely require more information such as as-built drawings of the existing pipe, CCTV (Closed Circuit TV), and even laser profiling if there are more than a few changes in direction, pipe deflections, and alignment issues suspected in the host pipe. - Hydraulic Capacity
Hydraulic capacity is the largest consideration when considering pipeline rehabilitation. If a culvert is being repaired, then the engineer needs to determine if the pipe is operating under inlet control or in outlet control. A few of the tools available to help a design engineer determine whether a pipe is operating under inlet control or outlet control are:
• USACE HEC-RAS
• FHWA HDS #5 (nomographs, free download)
• FHWA HY8 (free download)
• Many commercial software systems
Pipes operating under inlet control have two main hydraulic concerns: cross sectional end area, and end area geometry. Simply using a repair system which results in a lower manning’s n in a pipe operating under inlet control will not improve the hydraulic capacity of that pipe. For example, sliplining a pipe in inlet control reduces the open-end area of that pipe, which decreases the flow capability. There are ways to eliminate or reduce the decreased flow rates created by sliplining: improved inlets such as a beveled headwall, larger diameter pipe inlets, and end sections may help increase the end area to effectively carry the required flow.
Pipes operating under outlet control, and closed systems such as sanitary sewers and storm sewers, have many more hydraulic concerns: cross sectional end area, end area geometry, pipe length, slope, and manning’s n of the pipe. In many cases, using a rehabilitation system with a lower manning’s n will likely improve the pipe’s flow capacity. It must be pointed out that the flow rate is increased because the water velocity in the pipe is also increased. For most systems, this increased velocity is a good thing, but the new velocities still need to be calculated and reviewed. - Structural Capacity
There are two structural design approaches for repairing existing pipes: partially deteriorated existing pipes and fully deteriorated existing pipes.
Partially deteriorated pipe assumes that the existing pipe has some structural capacity left. In this case, only the forces created by the ground water are considered for the new pipe.
Fully deteriorated pipe assumes that the lining system must be capable of carrying 100% of the dead and live loads. Assuming the existing pipe is fully deteriorated is the most conservative approach. Because of this, all of Contech’s slipline pipe materials are also direct bury pipe materials, meaning that the slipline pipe is expected to carry all the dead loads and the live loads acting on it. - Material/System Selection
Material selection depends on many factors: diameter, shape, hydraulic requirements, and installation technique. For example, sliplining in smaller diameter pipes, such as pipes smaller than 15”, usually doesn’t work well because too much of the cross sectional area is being taken up, reducing the flow capacity the new system.
However, in larger diameter pipe systems, and pipe systems where other rehabilitation approaches require bypass pumping, sliplining very quickly becomes the best option due to additional costs associated with bypass pumping. Examples of these systems are typically sanitary sewers and some storm sewers.
Contech’s slipline products range across many shapes, sizes, and available design lives starting at 75 years. - Constructability
Constructability is often a means and methods consideration taken on by the installer, but is also a design consideration for the owner and engineer. Contractors come up with creative and innovative approaches which may differ from those considered by the engineer, and that’s great. But prior to contractors seeing the project in typical design/bid/build applications, the owner and engineer must feel comfortable that the project can be built. This may usher in other project delivery methods such as Construction Manager At Risk (CMAR) or design/build, but that’s another topic for another day.
It is strongly encouraged that contractors create a mandrel the size of the slipline pipe being used and pull it through the existing pipe prior to ordering the slipliner pipe. This is especially true when laser profiling of the existing pipe does not exist. Doing so will make sure that the proposed liner pipe fits through the existing pipe.
Pushing or pulling the slipliner material is also determined based on the contractor’s equipment, length of pipe to be rehabilitated, alignment of the existing pipe, and type of slipliner pipe being used. - Grouting/Flotation
The last consideration to be determined is the grout placement. All slipline materials should be grouted in place. The grout accomplishes a couple things. It holds the liner pipe in place and transfers loads between the old pipe and the new pipe. In most cases, the grout compressive strength only needs to be from 50psi for smaller diameter systems to 750psi for larger diameter systems.
The largest concern with grouting is buoyancy of the liner pipe. This is controlled by a combination of:
• Low grout density – A grout unit weight comparable to that of water (62.4pcf), or less, is ideal. The grout should be measured by spread, not slump.
• Temporary hold down techniques – Water in the liner pipe while grouting will help hold it down. Otherwise, temporary bracing can be utilized.
• Grouting in lifts – The lift heights are a function of the grout unit density and hold down capacity.
Another area of concern when grouting is the maximum pressure being felt at the pipe wall. Let the grout flow downhill via gravity instead of trying to push the grout uphill. Pushing the grout uphill will increase the pressure felt by the liner pipe.
Monitor the pressure of the grout as it leaves the grout tube near the liner pipe, not as it leaves the pump. In many cases, the grout pump is located well above the liner pipe, and the additional head pressure as the grout makes its way down to the liner pipe is mistakenly not considered.
Maximum allowable grout pressures vary by pipe material and diameter, so make sure to work with your supplier to determine what the maximum allowable grout pressure is for your specific project.
Pipeline rehabilitation, particularly sliplining, allows the owner and engineer to design over 75 years of additional life into their sewer system, if designed with the six considerations listed above in mind. Working with Contech early during the planning and design phases of a project will further increase the chances of a successful project because Contech has many different products to consider and the experience to match.