How does the orientation of joints in a rock mass impact TBM steering and guidance?

The orientation of joints significantly impacts TBM steering and guidance because joints represent planes of weakness within the rock mass. Joints oriented parallel or sub-parallel to the tunnel axis can cause the TBM to deviate from its intended path, as the machine tends to follow the path of least resistance. This can lead to deviations in both the horizontal and vertical alignment of the tunnel. Joints that are steeply dipping (nearly vertical) and perpendicular to the tunnel axis can cause instability at the tunnel face, making it difficult for the TBM to maintain a stable excavation. In this situation, the TBM may experience face collapse or excessive ground deformation, requiring corrective steering actions. Joints that are oriented obliquely (at an angle) to the tunnel axis can cause the TBM to steer towards one side or the other, depending on the joint's orientation and the rock mass's overall structure. The TBM's steering system must constantly compensate for these forces to maintain the correct alignment. The frequency and spacing of joints also influence steering. Closely spaced joints can create a highly fractured rock mass that is more prone to instability and deviation. Widely spaced joints may have less of an impact on steering, but can still pose a risk if they are unfavorably oriented. Real-time monitoring of joint orientation and spacing, using techniques such as borehole logging and face mapping, is essential for effective TBM steering and guidance. This information allows the TBM operator to anticipate and correct for potential deviations before they become significant. The term 'joints' refers to fractures or discontinuities in a rock mass. 'TBM steering and guidance' refers to the process of controlling the TBM's direction to maintain the correct alignment of the tunnel.