Slurry Shield Tunnelling in Clayey Soils: Typical Problems and Countermeasures

The distinguishing characteristics of clay are high cohesion and low permeability. For slurry shield tunnelling in clayey soils, favorable aspects are that slurry cake with low permeability can be established more easily and effectively at the excavation face thanks to clay’s high cohesion, which is advantageous for stability of excavation face, and long stand-up time of clay offers enough time for backfilling of the tail void before collapse of surrounding soils, thus lessening ground volume loss. However, some typical problems are encountered due to clay’s high cohesion and low permeability. One primary problem is the clogging of slurry pipeline, once in case of which, slurry pressure will fluctuate severely, thus inducing unstable condition at the excavation face. In extreme cases, the pipeline bursts and soils at the excavation face collapse towards the cutterhead for immediate drop of slurry pressure. Another common problem is clay’s adhesion to the cutterhead, which weakens the excavation efficiency of cutterhead and limits advance rate of the shield machine. Tunnelling will inevitably disturb surrounding soils and excess pore water pressure occurs. In clayey soils, due to clay’s low permeability, it usually takes quite a long time for the excess pore water pressure to disperse completely. The consolidation settlements associated with pore water dispersing account for a large percentage of the total settlements. Accumulated ground settlements threaten structures and pipelines nearby. For these problems encountered during slurry shield tunnelling in clayey soils, both preventive and counter measures are put forward in detail in this study. The proposed measures can be used as a reference to avoid, mitigate and deal with problems encountered during slurry shield tunnelling in clayey soils.