Microtunnelling is an efficient trenchless technique for the construction of small diameter tunnels. The method is especially useful for laying pipelines beneath highways, railroads, and other sensitive areas with minimum disruption to businesses and traffic on the surface.
It offers precise line and level installations in various ground conditions. The main applications of microtunnelling are in sewerage, drinking water, and communication and power supply networks.
Due to the small diameter of these tunnels (approximately 500mm to 4,000mm in diameter), it is not practical for an operator to drive the tunneling machine; therefore, a remotely controlled microtunnel boring machine (MTBM) combined with the pipe jack-and-bore method is employed for construction. Pipe jacking includes lining a tunnel bore formed by a shield or other means by pushing specially-designed jacking pipes into the tunnel bore from one shaft to another.
Microtunnelling and Pipe Jacking System
A microtunnelling and pipe jacking system comprises of the following parts:
It is a mechanized, steerable mini boring machine equipped with a suitable cutter head in the front to excavate smaller diameter tunnels. It employs a laser guidance system to transmit real-time feedback and projected position to the operator. The operator uses steering jack to steer the machine during drives.
2. Jacking System
It comprises high thrust hydraulic jacks mounted on a jacking frame capable of exerting the required jacking force against a purpose-built thrust wall to push the pipes and the shield forward through the ground.
3. Automated Spoil Removal System
Discharge pumps carry the slurry through the circuit to the separation plant and up to the vibrating screen, where the coarse material is screened out and deposited in a container. The screened suspension is then transported by pumps into cyclones where the fine material is separated from the suspension.
4. Guidance System
It includes a laser beam device installed on the jacking shaft, and the beam is set to the desired level, gradient, and alignment. Some machines have photosensitive cells on the target panel located at the rear of the shield to convert the laser portion into digital data.
5. Remote-Control System
The remote-control system is used to operate the shield and other equipment. It measures, monitors, and records important data, while displaying any faults on the monitor.
Planning Process for Microtunnelling
The planning phase of microtunnelling includes soil/rock testing and collection and presentation of geotechnical data. Based on this, an appropriate MTBM system and slurry separation equipment are selected for the project. The steps are described below:
- Determine the pipeline position and grade: In order to locate the position of the pipeline, it is required to understand the ground conditions and capabilities of the microtunnelling equipment. This can be achieved by analyzing past construction projects and identifying obstructions along the proposed tunnel route. The length of individual drives and the location of shafts should be planned in advance to save costs and avoid issues during construction.
- Data Collection: The project owner should undertake an excessive number of borings, test pits, and lab tests for successful planning, bidding, and building of the project. On sites with cobbles and boulders, test pits or large diameter bucket borings should be used.
- Laboratory Testing: Reliable information can be obtained from standard index testing. The fraction of clay can be determined from grain size analysis with hydrometers. This data can be used to ascertain screen sizes, number of hydrocyclones, and the probable use of a centrifuge system.
Based on the results of the planning process, the appropriate microtunnelling system should be selected to excavate the ground with optimum productivity and minimum risk. The construction process can be divided into the following steps:
- Excavate the launching and reception shafts on opposite ends of the tunneling drive.
- Hydraulic jacks in the launch shaft should be employed to push the MTBM into the earth.
- The excavated spoils should be transported to the surface via pipes containing slurry water.
- The jacks should be retracted, and the slurry lines and control cables should be disconnected.
- A pipe or casing should be lowered into the shaft and inserted between the jacking frame and the MTBM.
- The slurry lines and control cables should be reconnected, and the MTBM should be advanced to another drive.
- The process should be repeated until the MTBM reaches the reception shaft.
- The MTBM and trailing equipment should be retrieved.
Microtunnelling is an efficient trenchless technique for the construction of small diameter tunnels. It is especially useful for laying pipelines beneath highways, railroads, and other sensitive areas with minimum disruption to businesses and traffic on the surface.
MTBM is a mechanized, steerable mini boring machine equipped with a suitable cutter head in the front to excavate smaller diameter tunnels.
Pipe jacking includes lining a tunnel bore formed by a shield or other means by pushing specially-designed jacking pipes into the tunnel bore from one shaft to another.
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