Filter cake formation for slurry shield tunneling in highly permeable sand

• Three slurry infiltration types correspond to three modalities of filter cakes well in the tests.
• The particle size and density of slurries affect the filter-cake formation greatly.
• The pore size of the soils is an important factor that influences the slurry infiltration.
• The filter-cake types can be well defined by the ratio of the average pore size (D0¯) and the slurry d85.

For slurry shield tunneling projects excavated in the highly permeable soils, the slurry usually passes through the ground without forming a filter cake. As a result it may be impossible (depending on the yield strength of slurry) to built up the required support pressure which would lead to a face instability. This kind of face instability is influenced by the factors, such as the permeability of ground, the slurry pressure and the properties of slurry. How to form a thin and lowly permeable filter cake efficiently and quickly on the tunnel face becomes a crucial engineering problem for the slurry shield tunneling in the coarse-grained and highly permeable grounds. In this paper, a new apparatus was developed to study the filter cake formation on the surface of coarse-grained sands under elevated pressure. A series of pressure filtration tests were carried out on nine different slurries and five different soils. By measuring the quantities of discharged water during the test and observing the state of filter cakes formed on the soil surface, the criteria of filter cake formation was identified. The test results indicated three different types of slurry infiltration. Correspondingly, the following three different types of filter cakes were observed: a filter cake, a filter cake with an infiltrated zone, and infiltrated zone without filter cake. It was found that the ratio between the average pore size of soil (D0¯) and the particle size for which 85% by weight of particles in the slurry (d85) may be used to classify the types of filter cake formation.