Precast concrete segments used in shield tunnel linings are prone to damage in many situations. These damages can occur at different stages such as fabrication in segment factory, transportation to tunneling site, during tunneling process, and at serviceability stage. The aim of the present article is to study the damages inflicted on concrete segments during production and transportation, and to present a new classification of these damages throughout the two stages. The developed classification is based on field observations and examinations of major subway and water conveyance mechanized shield tunnels of Iran, located in Tehran, Tabriz, Mashhad, Kermanshah (Nosood) and Isfahan (Golab). The quality of tunnel lining suffers from what, as a direct consequence of any damage to concrete segments, during production and transportation, which will be also discussed in this article. For further investigation, more than 250 concrete segments from Tehran subway line 3 and 350 segments of concrete segments from Tehran subway line 7 were selected and studied for a statistical analysis of chipping and crack, consecutively. Absence of preventive measures to limit segment damages in precast segment factories is one of the main reasons for increased number of damaged concrete segments, and as a result, increased costs of tunnel construction at later stages. In this paper, production phase damages and factors contributing to these damages are studied. According to the findings of the study, the human (operator) error was the most important cause for chipping, and, time-dependent behavior of concrete was the essential reason in crack of precast segments. Eventually, final section of the article presents practical solutions for reduction of damages during fabrication and transportation of concrete segments.
Some of the most frequent damages of concrete segments in shield tunnels are chipping and cracking, which are followed by degradation of lining system. In this paper, these types of damages are studied in four subway and two water conveyance tunnels. More than 2100 concrete rings are examined for chipping inspection and another 3000 for determination of the cracking. Statistical analysis of the research data showed that corners of the key and counter-key segments carry the highest number of chipping, while most of the cracking occur in the middle zones and shape of the segments and number of trust jacks affect the cracking pattern. Two kinds of numerical models are used to examine the underlying damages, which are based on geometrical characteristics of tunnel lining and boring machine besides operational mistakes. Findings of the numerical simulation revealed that installation of segmental lining with the least amount of erection tolerances results in low amount of chipping, this is while using key-segments with 12–17 degrees of insertion angle reduces total magnitude of damage due to tensile and compressive stresses. Furthermore, the deviation angle of TBM’s jack and segment’s axis should never be more than 5 degrees; otherwise even high-quality concrete segments wouldn’t remain undamaged. Employment of boring machines with articulated system is proposed in this case.
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