Guidance systems are a crucial part of any trenchless project irrespective of the method employed.
Excavating without a proper guidance system is just as bad as driving through a crowded unknown city without a GPS to guide you; you may reach the destination, but end with lots of wasted time and money.
Microtunneling is a trenchless method used to install pipelines that cross infrastructures such as highways, railway lines and major rivers. The microtunnel boring machine (MTBM) uses high end guided systems with live monitoring for real-time correction capability, giving a high degree of accuracy compared to other trenchless methods.
For guidance systems to work properly they should be properly set up, calibrated, maintained in a controlled atmosphere, and checked periodically.
Microtunneling Guidance System
A reliable and sophisticated guidance system is key to completing a microtunneling project successfully. The guidance system essentially provides control over the line and grade of the excavation.
A laser-controlled guidance system is commonly used for microtunneling projects and provides real-time data to the operator for controlled steering operations.
The motorized laser target unit helps determine the horizontal and vertical position and the pitch and roll of the MTBM.
Working of the Laser Guidance System
The laser guidance system consists of an active laser, active target, control console, and data logger. A laser station is fixed firmly inside the tunnel and projects a laser beam onto a laser target board mounted on the MTBM. An aiming unit equipped with high precision motorized prisms and sensitive to a red laser is mounted on the MTBM on the same vertical axis as the center of the target board and measures the distance.
As the laser beam hits the target board, the offsets of the laser spots are used to infer the line and grade of the tunnel alignment.
Another laser emitter is located at the backside of the tunnel on a fixed platform. The laser should be able to hit the sensitive target without any obstacles in its path.
The laser emitter can also be replaced by a Theodolite total laser station and prism for further referencing.
The distance of the target from the platform is a minimum of 30 meters and a maximum of 100 meters. A lesser distance can lead to a lack of precision due to movements in the concrete segments generated by thrust forces from the MTBM. A larger distance can lead to refraction errors in the laser beam.
In brief, the guidance system utilizes an active laser as a fixed reference point directed along the centerline of the tunnel at line and grade. This laser beam is projected onto the active target in the MTBM that contains photosensitive cells that read and convert the information to digital data.
This data is transmitted to the control console where the operator interprets it.
Proper Use of the Guidance System
Set-up and Calibration
Before the MTBM is launched, the guidance system including the camera should be thoroughly calibrated and set-up as the guidance system camera is the primary reference for the MTBM operator.
The operator uses it to check the target location at the back end of the MTBM and ascertains whether the alignment is in line and grade.
The inclinometers and roll sensors are checked to confirm they are functioning well, and then calibrated.
The theodolite is also checked for tilt offset and collimation, and the same is also done during the operation if the MTBM experiences a strong vibration, bump or disturbance.
Installing and Protecting the Guidance System Camera
The guidance system camera should be shielded from MTBM operations and debris. Hydraulic jacking frames have a thrust block with a target space in the reaction wall to allow the laser to sight down the centerline of the pipe.
The camera reference point can be distorted by any movement and care has to be taken to prevent that. The mounting apparatus for the laser is constructed behind the jacking frame after it is positioned in the launch pit.
The mounting apparatus should not experience any disturbance by the jacking forces, neither by any other operation above and below the ground.
Frequent System Check
To ensure that the MTBM is on track and going on the planned course, the guidance system should be periodically checked during the operation.
These periodic checks can help re-calibrate any instruments that would have been disturbed during the operation. Working on a disturbed system can lead to significant errors in the long run.
Sometimes system errors in the guidance system can occur, and operators and engineers should be trained to identify these problems and resolve them before it becomes a major issue.
The atmosphere within the tunnel can impact the clarity of the laser on the target.
The software is designed to read the location of the most intense light, and therefore heat, dust, and humidity can disturb the pointedness of the laser beam and impact the reading.
This reading is transmitted to the operator to ascertain the position of the MTBM. The uncontrolled atmosphere in the tunnel can lead to laser refraction and dispersion and poor clarity, leading to an error in data.
The proper ventilation systems in the tunnel can help circulate air, regulate temperature and humidity, reduce laser refraction and dispersion, and create a consistent atmosphere for a pointed laser beam.