Risk analysis on cutter head failure of shield in composite ground

All kinds of accident examples and their potential reasons were analyzed and summarized for cutter head failure of shield in composite ground. In order to research the mechanism of failure risk mode on cutter head, cutters and other different cutting components in shield, Fault tree analysis (FTA) was applied in establishing an analytical model on cutter head failure risk. Firstly, potential failure risk modes and effect factors were all analyzed and calculated by set theory in a qualitative way. Then, relevant control measures were proposed to reduce the cutter head failure risk of shield. The paper results would give some useful references for cutter head design and construction in composite ground by shield in China. With the manufacture technique development of shield, the strata scope in which shield works is more and more wide. Shield technology is not only used in relatively uniformity or single ground, but also used in composite ground that concludes alternant soft and hard rock. During the advance process in composite ground, the accidents rate of cutter head is one half of the total shield risk accidents. The main form accidents are cutter abrasion, cutter disc abrasion, cutter disc distortion and so on (L. Guo 2006). These accidents made the advance rate and cutter head life largely depressed. According to statistics, in domestic compound EPB (earth pressure balance) shield construction, the failure of cutter abrasion came forth in engineering at differently degree (W. B. Zhu, S. J. Ju, etc. 2006). In the 23 cutter head accident cases of non-cutter failure, the frequency of cutter disc accident was highly 69.7% (cake 34.8%, cutter disc deformation 17.4%, cutter disc blocked 8.7%, cutter disc abrasion or rupture 8.7%); the frequencies of bearing mangle, muck mend system accident, falling across underground eye-winker were separately 21.7%, 4.4% and 4.4%. these engineering risk accidents also occurred in other countries (J. Zhao, Q.M. Gong, Z. Eisensten 2007). Therefore, it is very important to research the failure risk of shield cutter head in composite ground. Adopting the FTA (Fault Tree Analysis) method, this paper would analysis the cutter head failure problem of shield in the composite ground, study its mechanism and influence factor, and evaluate the importance degree of every influence factor. Then the measures were put forward for reducing the occurrence of cutter head failure. The works of this paper could provide references for developing shield technology in composite ground.