Parametric analysis of mixshield tunnelling in mixed ground containing mudstone and protection of adjacent buildings: case study in Nanning metro

Micro-disturbance construction in mixed ground is a major challenge in shield tunnelling construction. It has a high tendency to trigger clogging risks when mix shield tunnelling is performed in mixed ground containing mudstone, which has some undesired influences on the shield tunnelling performance and settlement control, especially when passing near buildings. This study focused on the characteristics of the tunnelling parameters of mix shield tunnelling in mixed ground and the settlement control measures for such a situation. A K-means clustering was conducted to investigate the relationship between the mixed ratio (the height ratio of mudstone in the excavation face) and the tunnelling parameters and silhouette evaluation was used to determine the optimal number of clusters. We also employed the K-means++ algorithm to select the initial centroids. The results of the analysis indicate that the slurry pressure in the excavation chamber (SPexcavation), total thrust and torque significantly increased while the penetration rate slowed down with the increase in λ. When 0.15 ≤ λ < 0.6, the fluctuation in the average value of SPexcavation is the greatest, which is unfavourable for settlement control. Based on studies regarding the protection of adjacent buildings in the Nanning metro line 1, it is beneficial to modify the slurry properties, slow down the advance speed and increase the cycle time when tunnelling in mudstone. The control of the SPexcavation is the first priority, and automatic monitoring along with sufficient grouting is essential for the protection of adjacent buildings.