Abstract:

 The present study aims to investigate the mud cake problem in a tunnel boring machine in cohesive stratum. For this, a case study is conducted on a rectangular pipe jacking project on Wuxi metro line 4 in China. Different ratios of polymer are added to soil samples with different moisture contents in order to examine their adhesion strength, undrained shear strength, and the water retaining capacity in hightemperature environment before and after soil conditioning. The mechanism of soil conditioning is analyzed by changing the liquid plastic limit. The risk of formation of mud cake and effects of soil conditioning are judged using the risk zoning map of mud cake formation. The results reveal the following: (1) An increase in water content significantly reduces the adhesion strength of soil; when the water content of soil is 35%~45%, the polymer can effectively reduce its adhesion strength. (2) An increase in water content can reduce undrained shear strength, while an increase in polymer content can effectively strengthen undrained shear strength. When the addition ratio of polymer is higher than 0.15%, its strengthening effect is significant. (3) Polymers can improve the liquid limit and water retaining capacity of the soil and also ensure fluidity. (4) The risk of mud cake formation and the conditioning effect of soil can be comprehensively examined based on the risk zoning map of mud cake formation, the adhesion strength of claymetal interface, and undrained shear strength of excavated soil.

Key words: tunnel boring machine, cohesive soil, mud cake, soil conditioning, polymer