Microtunnelling is a highly efficient trenchless construction technique used for creating small-diameter tunnels. This method is particularly advantageous for laying pipelines beneath highways, railroads, or other sensitive areas, as it minimizes surface disruptions to businesses and traffic.
This technique ensures precise alignment and level installation across various ground conditions. Common applications of microtunnelling include sewerage systems, potable water pipelines, and communication or power supply networks.
Due to the small diameter of these tunnels (ranging from approximately 500mm to 4,000mm), it is impractical for an operator to manually drive the tunneling machine. Instead, the process relies on a remotely operated microtunnel boring machine (MTBM) combined with a pipe jack-and-bore method. This involves pushing specially designed jacking pipes through a tunnel bore, which is created by a shield or similar apparatus.
Microtunnelling and Pipe Jacking System
A typical microtunnelling and pipe jacking system consists of several essential components:
1. Microtunnel Boring Machines (MTBMs)
MTBMs are mechanized, steerable mini boring machines equipped with a suitable cutter head to excavate small-diameter tunnels. They feature a laser guidance system that provides real-time feedback on position and alignment, enabling precise control. Operators use steering jacks to guide the machine during tunneling operations.
2. Jacking System
The jacking system consists of powerful hydraulic jacks mounted on a jacking frame. These jacks exert the necessary thrust against a purpose-built thrust wall to push the pipes and the shield forward through the ground.
3. Automated Spoil Removal System
Excavated material is transported through discharge pumps as slurry to a separation plant. The slurry passes through vibrating screens to remove coarse material, which is deposited into containers. The remaining suspension is processed by hydrocyclones, separating finer materials before the suspension is recycled or discharged.
4. Guidance System
A laser beam device installed on the jacking shaft sets the desired level, gradient, and alignment. Photosensitive cells on a target panel in the shield convert the laser data into digital information, enabling precise guidance.
5. Remote-Control System
The remote-control system operates the shield and other equipment, measuring and monitoring critical parameters. It records data and displays any faults on a monitor, ensuring efficient and safe operation.
Planning Process for Microtunnelling
The planning phase is crucial for the success of a microtunnelling project. It involves thorough soil and rock testing, as well as the collection and analysis of geotechnical data. These steps determine the appropriate MTBM and slurry separation equipment required for the project.
Step 1: Determine Pipeline Position and Grade
Establishing the position and grade of the pipeline requires a detailed understanding of ground conditions and equipment capabilities. This involves analyzing past projects, identifying potential obstructions, and planning the length of individual drives and shaft locations to minimize costs and construction challenges.
Step 2: Data Collection
Comprehensive data collection is essential for effective planning and execution. This includes conducting borings, test pits, and laboratory tests. In areas with cobbles or boulders, test pits or large-diameter bucket borings provide valuable insights.
Step 3: Laboratory Testing
Laboratory testing provides reliable data for project planning. Standard index testing determines factors such as clay content through grain size analysis. This information helps in selecting appropriate screen sizes, hydrocyclones, and centrifuge systems for slurry separation.
Microtunnelling Construction Process
The construction process builds on the insights gained during the planning phase to ensure optimum productivity and minimal risk. Key steps in the construction process include:
- Excavation of Launching and Reception Shafts: These shafts are excavated at opposite ends of the tunneling drive to provide access for the MTBM and other equipment.
- Insertion of the MTBM: Hydraulic jacks in the launch shaft push the MTBM into the ground, initiating the tunneling process.
- Spoil Transportation: Excavated material is transported to the surface via slurry pipelines. The material is separated and processed for disposal or recycling.
- Pipe Insertion and Jacking: After completing each drive, the hydraulic jacks are retracted, and a pipe or casing is lowered into the shaft. This pipe is inserted between the jacking frame and the MTBM.
- Reconnection and Advancement: The slurry lines and control cables are reconnected, and the MTBM advances to the next drive. This process is repeated until the MTBM reaches the reception shaft.
- MTBM Retrieval: Once the tunneling operation is complete, the MTBM and trailing equipment are retrieved from the reception shaft.
Conclusion
Microtunnelling is a cutting-edge technique that combines precision, efficiency, and minimal surface disruption. By leveraging advanced equipment such as MTBMs, hydraulic jacks, and automated spoil removal systems, microtunnelling ensures successful execution of projects in challenging conditions.
The detailed planning and construction processes underscore the importance of accurate geotechnical data and careful equipment selection. With its wide range of applications, microtunnelling continues to be a transformative solution for modern infrastructure development.
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