Steering characteristics of microtunnelling in various deposits

To keep the sewer line functioning well, it needs to steer Microtunnelling Boring Machine (MTBM) properly and maintain driving within line and grade tolerances. An experienced operator can avoid rapid or erratic changes in direction when steering a MTBM. In addition, the steering ability of MTBM and the type of geological medium are also important factors to influence the alignment behaviour. This study developed a simple model to assess the characteristics of steering MTBM in various deposits by analysing both the alignment deviations and steering angles recorded from pipe to pipe. The steering results from three brands of MTBM involved with nine projects indicate that the relationship of the average adjustment and the average deviation remains linear for a certain type of geological medium. The ratio of average adjustment-to-average deviation changes from 1 for tunnelling in hard cobble formation and sandstone bedrock to 0.2 for tunnelling in soft clay. The ratio of average adjustment and average deviation is dependent only on the subgrade modulus if geologic medium is homogeneous for a certain type of MTBM. The steering behaviour of a poor drive due to inexperience operator will follow the same relationship of average adjustment and average deviation but with larger average adjustment and average deviation. However, the steering behaviours in a non-homogeneous soil deposit will not follow the steering linear relationship determined by subgrade modulus but show the smaller ratio of average adjustment-to-average deviation. As a result, the proposed model can provide a simple way to evaluate the cause of a poor steering result is from either non-homogeneous nature of geologic medium or error of operator or both.

► The model identifying the cause to lead to a poor steering result is proposed.
► The relation of average adjustment and deviation is a series of straight lines.
► In non-homogeneous soil, the steering behaviour will not follow the linear relation.