A comparison of laboratory tests for the evaluation of clogging risk in mechanized tunnelling

• An experimental investigation aimed at quantifying clogging potential was conducted. • Mixing tests and pull-out tests were performed on eight natural soils. • The effects of conditioning were evaluated by testing four foaming agents. • The results emphasize advantages and limitations of both tests. • The relationships between clogging risk and soil properties are discussed. Clogging is one of the most relevant issues in mechanized tunnelling with EPB-TBM in fine-grained soils. This phenomenon, caused by the interaction between soil particles and between soil particles and metallic surfaces of the TBM’s cutterhead and working chamber, is influenced by several properties of the soil, mainly grain size distribution, plasticity characteristics and water content. To quantify the clogging effects, and ultimately to provide general indications for proper soil conditioning, classification systems based on laboratory test results were proposed in the last decade. The most adopted systems are based on the pull-out test, developed and used in different versions in several studies, and the mixing test, widespread both in the academia and among chemical suppliers and field engineers. In this work, these two kinds of tests were performed on eight fine-grained natural soils. On one soil, the tests were also performed after its conditioning with four foaming agents of different class and different dosages. The analysis of the results enables to: i ) highlight the advantages and limitation of each test and provide a general overview of the relationship between clogging phenomena and soil properties; ii ) find the correlations between the results of the two tests; iii) prove the usefulness of these tests in quantifying the efficacy of soil conditioning in reducing the clogging risk.