Why Reinforced Concrete Pipe Is Ideal for Pipe Jacking
Reinforced concrete pipe is a strong, durable, resilient material, which makes it a great match for the process of pipe jacking.
It was in 1842 in Mohawk, New York that the oldest recorded modern concrete pipe was installed, and it lasted over 100 years. The concept of steel bars in concrete pipes was first introduced in France in 1896 and was known as the Monier patent. It was brought to the United States in 1905 and since then has been extensively used in the installation of culverts, sewer and stormwater pipes and pressure pipe applications. The long service life of reinforced concrete pipe (RCP), its strength and durability, has made it an economical choice for contractors and asset owners.
Precast reinforced concrete pipe is the favored pipe material for trenchless pipe jacking operations, involving deep installations where conventional methods of trenching are not viable – environmentally or economically. (For a comparison of pipe materials, see The Lifespan of Steel, Clay, Plastic & Composite Pipes.)
Reinforced Concrete Pipe (RCP)
RCP is basically a concrete pipe, but reinforced with steel, lending the additional strength and flexibility to withstand soil pressure and other forces acting on the pipe. The materials required for manufacturing RCP are cement, aggregate, reinforcing steel and additives, mixed as per standard specifications. Precast RCP is considered one of the strongest pipes available, and can be designed for specific loading conditions.
The inherent strength of RCP makes it independently strong, thus requiring less reliance on surrounding soil support, though that is a necessary part of any pipe installation.
RCP is capable of withstanding the considerable amount of jacking force that is required to push it into the ground during the pipe jacking operation. The first concrete pipe jacking was carried out in North America in 1896 by the Northern Pacific Railroad for installing drainage systems under the railroad.
Since then the advancement in technology and addition of steel in concrete has enabled installation of concrete pipes as large as 3,350 mm in diameter using the pipe jacking method.
Pipe Jacking Process
Pipe jacking requires two shafts: one at the beginning and the other at the end of the designated route. The launching shaft acts as a platform for the placement of the jacking apparatus and its associated parts. A thrust block at the back of the shaft provides the necessary impact surface needed to help jack the pipe forward.
The soil at the front end of the pipeline is excavated using manual excavation, a microtunneling machine (MTBM) or a tunnel boring machine (TBM) depending on the diameter of the pipe to be jacked. Intermediate jacking shafts are sometimes used when long runs of pipeline have to be installed. This reduces the necessary axial thrust required for pushing the pipe.
The pipeline trails behind a cutting shield of required dimensions, which also protects workers from falling debris. The installed pipe exits at the reception shaft at the end of the drive length, finishing the jacking process.
Types of Loads on Jacked Pipe
Axial loads on the pipeline are affected by factors such as excavation method, type of lubricant, pipe diameter, soil friction and pipeline length. The load should be uniformly distributed around the pipe circumference to prevent concentration of stress in certain areas.
The cross-sectional area (CSA) of the pipe is usually sufficient to withstand pressure during jacking operations, but the expected jacking forces that the pipe will encounter should be ascertained prior to jacking. For excessive jacking pressure, concrete pipes with higher compressive strength may be required. Cushion material such as hardboard or solid core plywood will help distribute the force evenly around the pipe circumference.
If the jacking frame is not flush to the jacking pipe, lateral loads can come into play. It can also occur when the pipe has lost its line and grade and the pipe is adjusted to realign it. This action produces shear loads on the bell and spigot ends of the pipe.
Earth and Live Loads
A buried pipe is subject to load from the soil above it and also from moving vehicles that may pass over the pipeline. The required pipe strength is dependent on factors such as depth of soil cover, mass of the soil, overcut and live load, if any. The overcut is usually grouted after the jacking operation is complete. (To learn more, see How Loads Affect Buried Pipes.)
The pipe required for a jacking operation should meet material and manufacturing standards outlined in American Society for Testing and Materials (ASTM) and American Society of Civil Engineers (ASCE) guidelines. The jacking pipe contains two cages of circular reinforcement in the pipe barrel. It should be ensured that the reinforcement cages are circular and not elliptical. Some tolerances are required for pipes used in jacking operations, such as:
- The compressive strength of concrete used should never be less than 40 MPa.
- The outer cage of reinforcing steel should extend into the pipe groove and the inner reinforcement cage should extend into the tongue of the pipe.
- Pipes with internal and external diameter varying from 1,200 mm to 3,000 mm should not vary from the designed diameter more than +/-1% or +/-10 mm, whichever is less.
- The wall thickness of the pipe at any point should not vary more than +/-5% or 5 mm, whichever is greater.
- The outside diameter of the pipe should be a true circle and should not vary from it by more than 1%.
- The taper of the pipe barrel should not vary more than 3 mm from the bell to the spigot end.
- The pipe length should not vary from the designated length by more than +/-5 mm to a maximum variation of +/-10 mm.
Trenchless methods such as pipe jacking and microtunneling have eliminated the need to dig a trench for pipeline installation. As widely known, trenching takes up a lot of space and affects the economy and businesses in the area. Pipe jacking can be used to install RCP pipes when surface disruption is to be avoided, such as when pipes have to be installed under embankments, airports or highways, and also when the depth of installation is great.
The high strength of RCP is suitable for jacking operations as it is able to withstand the huge amount of jacking forces required to push the pipe forward. Another advantage is that RCP is not easily deflected from line and grade because of the smooth exterior surface that offers much less frictional resistance.
The inherent strength of RCP is better able to withstand the dead load acting on it from the soil cover, and the live load from moving vehicles that pass over the pipeline.