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  Use of bentonite and biodegradable slurry for cobble -rich soil conditioning

  Cobble-rich soils are always a difficult challenge for EPB-TBM tunnelling and the traditional soil conditioning through water and foam may not lead to suitable results. The addition of bentonite slurries is helpful to solve this problem but may be impaired by environmental regulations, in such cases the use of a biodegradable polymeric slurry may be a viable solution. This paper makes a comparison between the result achievable with foam conditioning alone, with the addition of bentonite slurry and, as innovative alternative, an expressly designed organic polymeric slurry. A slump test campaign has been performed on a cobble-rich reference soil, chosen in order to put in evidence the real performance of conditioning slurries. Tests showed that the adoption of slurries improves the behaviour of the conditioned soil. Comparing polymeric and low-density bentonite slurries for the achievement of a similar results in terms of slump and consistency, a lower quantity of slurry was needed using the polymer slurry. For high-density bentonite slurry, the polymeric one was not able to meet the performance previously achieved. In conclusion, for difficult but not critical conditions the use of polymeric slurry may be a better solution while for critical conditions the use of bentonite slurry may be needed to successfully accomplish the soil conditioning.

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