| Daping tunnel in Chongqing light-rail construction is located in downtown Chongqing, where there has a dense population as well as numerous buildings. Daping station tunnel enjoys a span in the clear of 19.8 meters, whereas its excavation span and area are 26.3 meters and 430.3 square meters respectively in the context of mere 4-meter overburden and with a surrounding rock category of III. In view of complex and unfavorable surface conditions over the station tunnel, such as house buildings and city roads both of which can't resist severe shocks or vibrations, it necessitates as small vibrations as possible induced in tunnel construction process and a strict requirement for the surface subsidence amount. So success of the technology will be a milestone indicating this technology of large section tunnel construction in our country has improved into a new phrase and reached a leading international level.Special emphasis is laid upon the construction operation process of flat tunnels with outsized span at even shallow depth, meanwhile, how to build large span flat tunnel is briefed in this article. Measures taken to reduce vibrations caused by explosion in driving tunnel and surface subsidence and methods of monitoring and surveying in the course of construction are detailed in this article. Furthermore, analysis of data from monitoring and surveying shows that it can serve as a guide to construction operation. In addition, the effect of surrounding rock's stability on such civil engineering project is succinctly analyzed. In a word, research results will undoubtedly benefit and guide later like projects.Main conclusions are as follows.1. Construction of flat tunnel of large span at extra-shallow depth must be conducted on the principle that advance grouting should be strictly performed before excavation and length of pull at one time should be short, together with reinforced support, immediate closing and constant monitoring and surveying. That preceding pilot tunnels, large-volume excavation divided into small ones, excavation of flank before that of center, upper before lower, arch before wall etc is a good method which could effectively reduce surface subsidence and its adverse effect on adjacent buildings, in addition, some other measures are also taken to lessen blast-induced vibration in the ground and surrounding structures. In a word, all means and steps mentioned above have already proved effective and practicable in practice.2. Due to exceedingly flat arched vault of the flat tunnel with outsized span and renewed stress allocation in surrounding rock after excavation, which would inevitably induce some excessively large load on the base of arch, therefore, larger load-bearing capability in theIS"foundation is a must. On the other hand, smooth face explosion must be applied to excavation of the arch base to further ensure the integrity and continuity of country rock around tunnel. Immediate closing after excavation would also effectively prevent pooling water from softening surrounding rock. Tensile stress field would appear in the proximity of arched vault after excavation, slacking of surrounding rock and subsequent dropping of blocks of rock from such area would possibly occur at any time in the context of extra-shallow depth. So I-steel arched support must be erected to provide against possible block dropping and be integrated into a ring with that of pilot tunnels on both sides.3. Immediate and constant monitoring and surveying must be conducted during construction. Special attention should be paid to the flat tunnel with outsized span at shallow depth. Apart from some common surveys, it is quite necessary to survey initial support stress, which is conducive to directly and accurately understanding changes of stress in surrounding rock and can provide helpful guidelines for governing tunnel construction in an importantly practical way. |
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