Concrete Pressure Pipe in Tunnel Installations

American Concrete Pressure Pipe Associationneling is a necessary aspect of water pipeline construction. Tunnels are used to avoid obstacles such as mountains and rivers. Tunneling is also used to avoid open excavation which would affect traffic filled city streets or residential neighborhoods. In many urban areas, open excavation is costly and disruptive.

Because Concrete Pressure Pipe is so rugged and maintains its joint configuration, even when transmitting substantial forces, it is an ideal pipe material for tunnel installations.

All four types of Concrete Pressure Pipe namely; Prestressed Concrete Lined Cvlinder Pine. Prestressed Concrete Embedded Cylinder Pipe, Bar-Wrapped Concrete Cylinder Pipe and Reinforced Concrete Cylinder Pipe assure leak-proof systems and are well suited for use in these tunnel applications.

Construction Methods

Construction methods for installing concrete pressure pipe in tunnels varies with the size of the pipe, the length of the tunnel and the type of primary liner casing system used.

The liner casings are usually on of four types: steel liner plate, steel ribs and timber lagging, precast concrete segments or concrete pipe itself. There are two construction methods used to install the tunnel casing -tunneling or jacking. Each project requires individual evaluation by experienced engineers to determine the appropriate installation method to follow.

For smaller diameter pipe in short tunnels, such as those under a highway or a railroad, it is common practice to slide the concrete pressure pipe through the liner.

Transporting or moving large pipe into or through the tunnel varies from a simple push-pull skidding arrangement, using hydraulic jacks or winches and cables, to special pipe carrying machines that not only transport the pipe but are capable of positioning and joining a section of pipe to the previously placed section.

Joint Testing

No pipe laying job is complete without testing the joint for leaks. Other pipe materials require filling the line with water and applying pressure to test for leaks. Typically, that is accomplished after a considerable amount of pipeline has been placed into the tunnel. Should a leak occur, finding it is a major problem and correcting it is a larger one. With concrete pressure pipe a double O-ring testable joint alternative gives the pipe laying contractor the assurance of watertight joints immediately after make-up, and it takes only minutes to perform. In practice, the joint employs dual O-ring gaskets with an air test fitting installed between them, accessible from the interior of the pipe. Immediately after making the joint, the contractor connects an air line from a small air tank or pump to the test fitting and pressurizes the space between the gaskets. Any leakage is detected immediately. However the pressure is usually maintained for approximately five minutes. On the rare occasion that leakage takes place, the joint is disassembled and the gaskets are replaced.

Pipe Protection

If concrete pressure pipe is to be pushed or pulled into the tunnel liner, care must be taken to protect the pipe from damage. Provision must also be made to protect the steel bell and spigot joint rings.

• Timber sections can be strapped to the exterior of each pipe to act as runners, with the liner surface lubricated, possibly with bentonite.
  Timber Skid Method

• For mortar-coated concrete pressure pipe, an additional band of mortar can be applied to tile exterior of the pipe, during manufacture, to act as a wearing surface.
  Raised Coating Collar Method

• Exterior segmented bands, with plastic tipped steel ribs, called casing spacers, can be bolted together around the pipe at the job site. The ribs act as runners, reducing friction and protecting the pipe exterior from damage.

• In any push-pull installation, a lubricant, such as bentonite, may also be used on the tunnel casing system to reduce the sliding resistance.

• If pipe is to be pushed or pulled into the tunnel casing, the pipe ends must be protected from damage. That can be accomplished with timber cushions placed in the joints. Consult with the pipe manufacturer for details.
  Casing Spacer Method

  Regardless of the pipe installation method, if the annular space between the tunnel liner and the concrete pressure pipe is not filled with Portland cement or grout, then joint protection must be applied, either by grouting the joints, painting or metalizing the steel joint rings, or by another joint protection system.

Long-Term Pipe Support

Typically, the annular space between the concrete pressure pipe and the tunnel liner is filled with Portland cement grout. A bulkhead is used to retain the mortar or grout. It is placed in the annular space at each end of the section that is to be grouted. Provision must be made to prevent floatation of the pipe and to vent trapped air.

Grout may be delivered to the annular space through ports installed in the pipe walls during manufacturing, or by use of a grout placement line extended back to the remote bulkhead. The grout placement line is withdrawn as the grouting progresses.

Alternatively, the annual space may be filled with sand, or the pipe may be blocked, top and bottom as well as sides, to prevent its shifting in the tunnel.

Applications

There are many successful applications of concrete pressure pipe installed in tunnels. It is manufactured with concrete bearing the compressive loads, and reinforcing steel taking the tensile loads.

Whether the pipeline must traverse a river, pass under city streets or be placed under railroad or highway embankments, this type of pipe can be designed and manufactured to take the loads of installation, and still perform for a lifetime.

Editor's Note: This article was adapted from a technical information paper prepared by the <%=company%>. It was modified slightly to conform to the journalistic style used by Water Online and Public Works Online.