Sliplining in Urban Areas

HDPE pipe moved to sliplining location
By Todd Grafenauer

With cities needing to replace a vast amount of aging water and sewer infrastructure, many find value in one of the oldest and simplest trenchless technologies of sliplining.

The method installs a new pipe inside the existing failing pipeline. The new pipe is typically installed either by pushing or pulling the new pipe into the host pipe with installation distances of a few hundred feet up to one mile at a time.

The sliplining technology is advantageous for replacement as the method is cost-effective, requiring only a few access pits. This reduces restoration efforts including environmental impact. The method also follows the existing utility path that can be critical as many cities do not have an available easement to relocate the new pipeline, especially for large diameter transmission mains located in congested utility corridors. Following the existing utility path also affords the city to reduce upfront design time and cost.

While cities may ultimately select the sliplining method due to the many advantages the method offers, another consideration is constructability. Installing or replacing water and sewer infrastructure in mature urbanized areas presents its own set of challenges for any construction method, including sliplining. An important part of the evaluation of selecting the best construction method for a given project is not only in finding value, but one that can be constructed within the constraints of mature residential neighborhoods and business districts.

The trend of cities preferring trenchless methods in mature urban areas continue due to the reduction in social impact. These include limiting road and intersection closures, reducing the impact to businesses, homeowners and pedestrians and preference for expeditated construction schedules to return the area back to normal. With the reduction in social impact that trenchless methods afford, cities still need to understand the constructability of the methods during the planning and design phase of the project.

sliplining in urban areas

Sliplining Constructability

Sliplining has generally utilized two methods for installing the new pipe within the failing host pipe:
The first is pre-assembling the new pipe before installation. Continuous or pre-assembling the new pipe before installation, such as using the butt fusion method with HDPE pipe, has often been the preferred method of installation by the contractor.

By pre-assembling the new pipe to correspond to the pull in distance, allows the contractor to focus on the proper procedures for butt fusion in advance of the pull. It also allows the contractor to focus solely on installation on the day of the pull, which helps to expedite pulling in the pipe.

However, many cities have limited laydown areas in which it is not feasible to pre-assemble more than 500 to 1,000 ft of pipe in one continuous length. Many cities cannot block major intersections or approaches to businesses.

In these situations, there are alternatives that can be utilized to ensure the constructability of continuous sliplining. These include pre-assembling the new pipe in an area that has room to assemble the pipe in the predetermined installation distance of 2,000 ft or more. In this case, the new pipe would be moved the morning of the installation to the sliplining location. This would limit the pipe blocking any intersections or business approaches to only a few hours as the new pipe is pulled into place.

It must be noted that the new pipe material selected for installation plays a critical factor if this is an option. One reason why HDPE pipe has become a preferred option for sliplining is the pipe material has the characteristics required to move the pipe from the pre-assemble area to the sliplining location. These characteristics include the material properties of being light weight, extremely flexible with a scratch resistance property.

sliplining in urban areas

Another option is the fuse-pull-fuse-pull method. This option includes pre-assembling the new pipe at the sliplining installation location, however, only to assemble lengths of new pipe that fit within the existing landscape that minimize impact to the area.

An example of this would be if the sliplining installation distance is 3,000 ft, but a major intersection is located 1,000 ft away and cannot be shut down, then to pre-assemble three sections of new pipe at 1,000 ft each.

With this option, sliplining activity would first install the first 1,000-ft section, then operations would pause to connect the next 1,000-ft section, continue sliplining until the last 1,000-ft section is connected, and then finish the pull.

The second option for sliplining the new pipe includes the cartridge method or segmental installation.
With this method, each section of new pipe is assembled in the insertion pit. Once the connection is made, the new pipe section is either pushed or pulled in. Many cartridge sliplining installations will utilize the pushing method and can be assisted with equipment such as a jacking machine.

With the cartridge method, after the first section of pipe is installed, the pull is then stopped, and another section is lowered into the insertion pit. The next joint is then connected and sliplining operations resume.
This method continues until then entire section for replacement is completed. There are multiple new pipe materials that can be utilized with the cartridge method including steel, ductile iron, concrete, PVC and HDPE pipe.

With the trend of cities preferring trenchless methods in mature urban areas continuing to grow due to the need to reduce traffic and pedestrian disruption, resident and commercial inconvenience – there are multiple solutions with sliplining that can address constructability concerns of limited laydown areas that exist within mature urban areas.

Todd Grafenauer is vice president and education director at Murphy Pipelines.

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