Assessment method for clay clogging and disintegration of fines in mechanised tunnelling

• We evaluate approaches to indicate the clogging potential of clayey soils.
• We develop a classification scheme for clogging for all types of soft ground TBMs.
• Water content is identified to be decisive for consistency and stickiness.
• Soil-dependent changes in water content leading to sticky behaviour are assessed.
• The ability of clayey soil to dissolve in water is also examined.

Shield tunnelling in fine-grained soils is frequently obstructed by clogging. The clogging risk of soils depends on multiple mineralogical and soil-mechanical parameters. The existing approach of estimating the clogging potential, which is derived from the indices of plasticity and consistency, has proven to be beneficial for practical purposes. Furthermore, the disintegration of fines from a cohesive soil within free water, such as slurry, has also shown to have a major negative impact on shield tunnelling. Both risks depend on the properties of the soils as well as the hydrogeological and operational conditions. Soils can be transformed to critical consistencies for clogging depending on the extent of water being available in the excavation process (ground water or support slurry). Reverse transformation into the natural consistency during tunnelling is not possible. Measures to reduce clogging often involve artificial increase of the water content to obtain less critical consistencies. A new diagram allows for the quantification of changes in the water content and thus, estimating changes in the consistency of fine-grained soils under different boundary conditions (availability of water).

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