Pipe jacking is a term that describes a specific trenchless pipeline installation technique. It's also used in conjunction with other trenchless methods, such as horizontal auger boring and microtunneling, where the jacking process is used to push in a pipe into a prepared borehole. The pipe jacking described hereafter is the pipeline installation technique itself and not an accompanied process. Pipe jacking was first used at the end of the 19th century and was improved on by companies from Europe and Japan in the 1950’s and 1960’s by improving grade and line accuracy, improved shields for face stabilization and extended drive length.
Pipe Jacking in Brief
In a typical pipe jacking operation, the product pipe or casing is propelled using jacks located in the drive or launch shaft to the reception shaft. The jacking force is conducted through the pipe to the excavation face. Workers are required to assist in excavating as well as removing the excavated spoil in the pipe jacking process. This mandates the minimum pipe diameter for installation to be 42 inches, with a jacking shield at the excavation face to protect those working inside. The jacking force from the thrust plate is first transmitted to the pipe and then to the face of excavation. If the casing or pipe is not strong enough to withstand the jacking force, it will fail. (Read Trenchless Pipeline Installation Methods and Their Pros and Cons.)
Casing Material and Strength
The pipe or casing materials most suitable for a pipe jacking operation are vitrified clay, steel, glass reinforced plastic pipe (GFRP) and reinforced concrete pipe (RCP). The casing or pipe materials must withstand high compressive forces involved in the process. The materials used for the pipe jacking operation should adhere to American Society for Testing and Materials (ASTM) standards for a successful installation. For example; steel casing pipe should meet ASTM A53, Grade B, Types E or S, or other approved grade with ends prepared for butt welding and beveled at 37.5 degrees. RCP should meet requirement 706-02 RCP for Class V with smooth exterior barrier.
Accuracy and Alignment
The pipe jacking method is a very slow process compared to other trenchless methods. However; It creates accurate alignment and grade in any type of subsurface soil condition, provided appropriate equipment is available. The accuracy of alignment is achieved even with minimum steering capability and is possible to within one inch of proposed line and grade. A laser set at the bore pit is shot through the pipe to the excavation face and measures alignment continually as the excavation proceeds. This allows workers to detect any change in grade or alignment and take corrective measures. An automatic steering system is incorporated in the equipment and the steering is accomplished by adjusting the jacking forces at the jacking shaft.
Working Conditions and Applications
Pipe jacking is capable of boring through any type of subsurface soil condition with adequate precaution and appropriate equipment, but works best in consistent granular and cohesive soils. (Learn more in Balancing Soil Pressure During Microtunneling.) Unlimited pipe lengths can be installed using this method by using intermediate jacking stations at intervals along the pipe length. This allows pipe jacking to be conducted in sections than at a stretch. Pipe diameter is limited to 42 inches and greater; for longer installations, the minimum recommended size is 48 inches. Pressure pipelines and conduits that must confirm to tight tolerances such as culverts, sanitary sewers and storm drains that work on gravity are best installed using this method.
Components of Pipe Jacking Operation
There are many components of a pipe jacking set up. The control and steering desk, crane, jacking pipes and slurry separation and mixing plant are located at the ground surface. The jacking operation is monitored and controlled from the control desk and the jacking pipes are lowered into the jacking shaft using the crane. The excavated jacking shaft consists of a concrete thrust block at the back of the pit with the jacking equipment arranged on a jacking frame with rails. The supply pumps for bentonite slurry provide lubrication for the jacking operation. The shield machine at the excavation face provides a safe working area for the workers.
The Pipe Jacking Process
The Pipe jacking process basically involves jacking or pushing the casing or pipe into the ground, as the soil within the pipe is excavated either manually or by mechanical means such as augers. The setup is similar to that of auger boring with bore pits excavated at the beginning and end of the proposed bore. (Read An Unboring Yet Basic Guide to Boring.) The jacking pit is equipped with a thrust block and a jacking frame to support pipe and equipment to correct grade. To prevent ingress of water at excavation carried out below water table, a headwall and seal assembly is incorporated into the jacking and reception shaft.
A jacking shield is first pushed into the ground allowing workers to perform excavation safely. Sections of pipes are then pushed in behind the shield using interconnected hydraulic jacks placed in the jacking pit. A thrust ring is installed around the pipe circumference to ensure even distribution of jacking forces. The excavated soil is removed using battery powered carts, conveyor system or augers. When one pipe section is completely jacked into the ground, the hydraulic jacks are completely retrieved to the back of the thrust pit. Another pipe section is placed in between the hydraulic jacks and the installed pipe and the jacking process is repeated. This process is continued till the pipe reaches the reception shaft.
Pipe Jacking Excavation Methods
Workers use pneumatic equipment, picks or shovels to excavate the bore face. This process is slow and best suited for short drives. Workers can quickly address situations such as boulders, tree roots and mixed face conditions.
This method has an exposed face at the shield front. Excavation is carried out using a small backhoe or excavating equipment mounted in the shield. This method also allows access for workers to the excavation face to deal with obstacles and unexpected soil conditions.
This method employs a toothed sphere at the end of a boom mounted on a track. The sphere is rotated at the bore face for excavating soil. Non-circular tunnels employ this method.
Tunnel Boring Machines (TBM)
This method is very efficient and employs a full face machine but is limited to circular tunnels. The cutting head fully supports the excavation face. The rotating cutting head excavates the soil which is passed through the opening provided in the cutter head. The operator sits behind the cutting head to steer the bore and make necessary adjustments.