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Alignment / Guidance

For bores with a straight alignment, the standard method of guidance for microtunnelling is the use of a laser beam set in the starting shaft and a target placed towards the rear of the microtunnelling machine. The target may be an "active" target responding to the laser light hitting the target or a "passive" target where the position of the laser beam on the target is observed by a camera. For curved microtunnelling, more sophisticated gyroscopic systems may be used - involving a north-seeking gyro-compass to track horizontal alignment and a remote water level gauge system for vertical alignment. As the diameter of the tunnel increases other options involving automated target-seeking theodolites can be used for positioning in curved alignments and hand surveying can also be used. Where possible, hand surveying is often used periodically for calibration or quality control checks of automated systems.

Based on the information received from the guidance system, the machine operator can make steering corrections as the machine advances. Most commonly, steering is accomplished by adjusting the relative thrusts on a set of jacks between the front portion of the microtunneling machine and the remainder of the machine.

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