It's the Pits: Pits and Excavations in a Trenchless Project, A Primer
Trenchless doesn't mean no digging. Here's a primer on the pits and excavations in a trenchless project you will need to be familiar with.
Trenchless technology is used for the construction and rehabilitation of buried pipelines. Owing to its non-disruptive nature, it has become the method of choice when it comes to rehabilitating or constructing under busy city streets, highways and railroads.
It is also the preferred method to drill under environmentally sensitive locations such as estuaries, lakes and lagoons. The reason is simple: it does not require excavating the surface. (Read also: Trenchless and the Environment: How No-Dig Construction Saves Ecosystems.)
While it’s true that trenchless technology does not require excavation, it does involve minor (compared to trenching) excavation at the start and end of the pipeline and sometimes along the route. Here we will take a look at a few of the types of excavations and pits required for trenchless methods such as pipe jacking, pipe ramming and microtunneling.
All trenchless operations require a launch and receiving pit but with different arrangements depending on the type of trenchless method used. For example, the pipe jacking method will require a jacking pit with a thrust wall, and intermediate jacking pits for longer drives. These excavations are made to make arrangements for placing machinery and auxiliary equipment and to reach the level of pipe installation. Since trenchless methods do not use surface excavation, they require the pipe to be installed at a pre-determined depth and grade.
Entrance or Launch Pit
It provides a safe working place for workers and machinery and also serves as the point where excavated material is retrieved. Slurry lines, drilling or cutter apparatus, jacking equipment and guide rails can all be installed in the entrance pit, depending on what is required for the method used.
The entrance pit is where equipment and associated ancillaries are installed, including the thrust wall, so that the tunnel can be bored from that point. The entrance pit must be of sufficient length to accommodate pipe lengths without bending, especially for pipes with large diameter. (Read also: Common Issues That Come With Larger Diameter Boreholes.)
Pits also require support structures to prevent them from caving in. Shoring and props are used to secure the excavation to ensure the safe working space that is required for people working and handling machinery in the excavated pit.
A receiving pit is excavated at the location where the pipeline is supposed to come out of the ground. If the project goes according to design, the pipeline will make an exit at the reception pit. The receiving pit also serves as the destination point for a pipe jacking or microtunneling operation where tunneling equipment is retrieved.
Jacking or Thrust Pits
A jacking pit (also known as a thrust pit) is an excavation on a planned pipeline or conduit installation site from which equipment needed for directional boring or tunneling is launched. Trenchless methods like pipe jacking, and microtunneling require jacking shafts in which powerful hydraulic jacks simultaneously excavate and install pipes providing a structurally sound, and watertight tunnel.
A metal shield is sometimes used to protect the pipe from damage during the jacking operation. A thrust wall or structural framework is constructed in the thrust pit to provide a reaction system for the jack.
The jacking shaft houses all the necessary equipment needed to complete the installation process including space for the crew and for the slurry separation chamber. Jacking pits vary in size depending on the diameter and length of the casing and the depth of the bore.
For drilling methods such as in auger boring, where there is no way to steer the auger, the jacking pit becomes critical to a successful and accurate bore as it serves to put the boring machine on the needed line and grade. The thrust pit helps spread the reaction loads evenly on the walls of the pit while the pipes are jacked into the ground.
Thrust pit and reception pits are mandatory in pipe jacking operations. Thrust pits are excavated depending on the excavation methods, requirements of the drive and the budget. A thrust wall is constructed at the back of the thrust pit to provide a reaction against which the jacking is done.
A thrust ring is provided around the pipe being jacked to ensure that the jacking forces are evenly distributed around the pipe circumference. The pipe jacks in the thrust pit are aligned by positioning guide rails on which the pipes are laid. Alignment accuracy is maintained by using a steerable shield that is periodically checked for line and level from a reference point.
Intermediate Jacking Stations
Intermediate jacking stations or shafts (IJS) are pits excavated in addition to the jacking and reception shafts in a pipe jacking operation. Where long lengths of pipe have to be installed using this method, IJS are installed at regular intervals to allow pipe jacking in sections. (Read also: Everything You Need to Know About Pipe Jacking.)
The frictional force developed between the pipe and soil is higher when pipeline length is greater. These forces are very difficult to overcome by the jacks in the jacking pit and high loads are experienced during driving. When the thrust force becomes insufficient to drive the pipe, the pipes before the IJS are held stressed back to the thrust wall in the launch pit.
The jacks in the IJS advance the forward pipe section and once the stroke is completed, the stress in the jacking pit is released and the pipeline is allowed to come back to its original position and the IJS is closed. The IJS can also be converted into manholes.
This term for a pit is used typically for a pipe ramming operation. The pipe and the ramming tool are lined up in the insertion pit in the desired direction. The pit for a pipe ramming operation does not require a thrust wall or blocks to push pipe segments. Ramming action is used on the pipe to drive the pipe into the soil. If there is an available slope at the worksite, pipe ramming does not even require a pit. The insertion pit however requires a solid base that is sloped and leveled to match the grade of the bore. Guide rails are also recommended in order to ensure a level grade at start of bore.