Pipe jacking (PJ) is a trenchless technology method for installing a prefabricated pipe through the ground from a drive shaft to a reception shaft. The pipe is propelled by jacks located in the drive shaft. The jacking force is transmitted through the pipe to the face of the PJ excavation. The spoil is transported out of the jacking pipe and shaft manually or mechanically. Both the excavation and spoil removal processes require workers to be inside the pipe during the jacking operation. Therefore, the minimum recommended diameter for pipes installed by PJ is 1,075 mm (42 in). However, it is feasible to install Reinforced Concrete Pipes (RCPs) with 900 mm (36 in) (I.D.) and 1,100 mm (44 in) (O.D.).
The cyclic procedure uses the thrust power of the hydraulic jacks to force the pipe forward through the ground as the PJ face is excavated. The spoil is transported through the inside of the pipe to the drive shaft, where it is removed and disposed of. After each pipe segment has been installed, the rams of the jacks are retracted so that another pipe segment can be placed in position for the jacking cycle to begin again.
Because of the large jacking forces required to push large diameter pipe through the ground, the design and construction of the jacking shaft are critical to the success of the project. The shaft floor and thrust reaction structure must be designed to withstand the weight of heavy pipe segments being placed on them repeatedly.
One of the important optional equipment available for the PJ method is the intermediate jacking stations (IJSs). IJSs are used for pipes, 1.2 m (36 in) in diameter or larger, between the drive shaft jacking plate and the jacking shield or Tunnel Boring Machine (TBM) to redistribute the total required jacking force on the pipe. IJSs consist of a steel cylinder installed between two pipe segments in the pipeline being jacked. Hydraulic jacks are then placed around the internal periphery of the steel cylinder. The IJS is pushed forward through the ground with the pipeline until its operation is necessary.
The basic PJ procedure is described as follows:
1. Excavate and prepare the driving shaft.
2. Set up the jacking frame and the hydraulic jacks to adjust to the proposed design line and grade.
3. Install laser guidance system in the driving shaft. When the laser is switched on, the laser points to the designated position as it is set up (picture). During the drilling operation, the operator who stays inside the boring machine continuously checks the mark on the steering head and the laser point. If the operator detects a deviation, the operator will articulate the steering head back to the correct alignment.
5. Mate the jacking push plate (thrust ring) to the shield or the TBM. The thrust ring is the frame that the main cylinders push against to advance the boring head and pipe. The ring provides a 360-degree surface against the pipe to minimize point pressure and reduce the chance of breakage (Akkerman Inc. 2002).
6. Advance the shield or the TBM through the prepared opening in the forward shaft support structure. Begin the excavation and spoil removal process. Continue excavation, spoil removal, and forward advancement until the shield or the TBM is installed. The movement of the jacking machine is controlled by the control panel outside of boring machine, whereas the drilling operation is controlled by the control levers inside the boring machine.
7. Retract the jacks and push plate to provide a space for the pipe segment.
8. Place the first pipe segment on the jacking tracks. For smooth boring operations, IJSs can be installed next to the shield or TBM (movie (This wmv file is 2.6 MB. Windows Media Player is required to run this file.)).
9. Mate the push plate to the pipe and pipe to the shield or the TBM (picture).
11. Repeat pipe jacking cycles until the complete line is installed.
12. Remove the shield or the TBM from reception shaft.
13. Remove the jacking equipment, IJS, and the tracks from drive shaft.
14. Restore the site as required.
Since pipe jacking requires people working inside the jacking pipe, the method is limited to person entry pipes. The minimum recommended diameter for pipe installed by PJ is 1,075 mm (42 in). Theoretically, there is no limit to the size of pipe that can be jacked. However, the largest pipe is approximately 3.7 m (12 ft) in diameter. The most common pipes installed by PJ range from 1,220 mm (48 in) to 1,830 mm (72 in) in diameter.
The length of the PJ drive is determined by the amount of available jacking thrust and the compressive strength of the pipe. The jacking thrust can be minimized or managed by providing an adequate over cut, applying adequate lubrication between the outside surface of the pipe and the bore hole, maintaining accurate line and grade control, using high-quality pipe products, and using IJSs. The longest PJ project in the United States had a continuous jacking length from drive shaft to reception shaft of approximately 1,050 m (3,500 ft) (Roe 1995). The most common drive lengths range from 150 m (500 ft) to 305 m (1,000 ft).
The type of pipe used for the PJ method must be capable of transmitting the required jacking forces from the thrust plate in the jacking shaft to the jacking shield or the TBM. Steel pipe, reinforced concrete pipe (RCP), and glass-fiber reinforced plastic pipe (GFRP) are the most common types of pipes used in PJ. Polymer concrete pipe (PCP) is commonly used in Europe for PJ and microtunneling.
A cushioning material should be used between the pipe segments to assist with distributing the jacking loads evenly over the cross section of the pipe. The most common type of material used as a cushion material is plywood and particle board.
The site must provide space for storage and handling of pipe and spoil and adequate space for the shaft. The size of the jacking shaft is determined by the pipe diameter, pipe segment length, jacking shield dimensions, jacking system dimensions, thrust wall design, pressure rings, and guide rail system. For example, the drive shaft size for a pipe jacking project using pipe 1,525 mm (60 in) in diameter with segments 3.3 m (10 ft) in length would require a 3.6 m (12 ft) wide and 7.5 to 9.6 m (25 to 32 ft) long shaft, depending on selection of jacking and excavation equipment.
Cohesive soils are the most favorable soil conditions for pipe jacking. It is possible to use pipe jacking in unstable soil conditions as long as special precautions are taken, such as dewatering and using closed-face machines and earth pressure balance machines to counterbalance the ground pressure.
A reasonable productivity range for pipe jacking projects is 10 m (33 ft) to 18 m (60 ft) per shift with a four or five person crew. Factors that can affect productivity include the presence of groundwater, unanticipated obstructions such as boulders or other utilities, and changed conditions such as encountering wet silty sand after selecting equipment for stable sandy clay (Iseley and Gokhale 1997).
Iseley, T. and Gokhale, S. (1997). “Trenchless installation of conduits beneath roadways.” NCHRP Synthesis 242. Transportation Research Board/National Research Council, Washington, D.C., 36.
M.R. “Guide to Best Practice for the Installation of Pipe Jacks and
Microtunnels.” ISBN 0 9525982 0 5. Pipe Jacking Association, London, U.K.