摘要
重庆轻轨大坪车站隧道位于重庆主城区内，地面上人口密集，建筑物林立，大坪车站隧道净跨19.8米，开挖宽度26.3米，开挖断面430.3m~2，覆盖层厚仅4米，围岩类别为Ⅲ类围岩。地表处情况复杂，多为抗震性能差的楼房和交通干道公路路面，对爆破振动和地表沉降要求严格。因此，该项技术研究成功将使我国隧道施工技术上升到一个新台阶，达到国际领先水平。
本文扼要介绍了大跨扁平隧道的施工方法，重点阐述了特大跨超浅埋扁平隧道的施工工艺过程，对爆破振动和地表沉降的控制方法，以及施工监控量测的过程和方法，并通过对量测结果的分析，说明监控量测对施工的指导作用，并对围岩稳定性进行了一些简要分析。研究成果不仅成功应用在目前我国城市地铁隧道开挖跨度及断面最大、拱部结构最为扁平的重庆轻轨大坪车站隧道上，而且对以后特大跨超浅埋扁平隧道的施工具有非常重要的指导意义。
主要结论如下：
1．软弱大跨超浅埋扁平隧道必须按“管超前，严注浆，多分部、短开挖、强支护、快封闭，勤量测”的施工原则进行施工，采用“眼镜超前，化大为小，先侧后中，先上后下，先拱后墙”的施工方法控制地表沉降和对周边建筑物的影响；采用“多分部，化大为小，短进尺，密布眼，弱装药，设减振槽”，控制爆破振动，经过实践证明，这些方法都是行之有效的。
2．特大跨超浅埋隧道拱部结构过于扁平，开挖后发生二次应力分布，拱脚处的应力过大，要求有较大的地基承载力，拱脚开挖时必须采用光面爆破，保证围岩的完整性和不受扰动，开挖后及时封闭，防止该处积水软化围岩。拱顶在核心土开挖后，在一定范围内出现拉应力区，在超浅埋情况下极容易掉块和发生围岩松驰，因此核心土必须边开挖边架设工字钢拱架，使之与左右导洞拱顶工字钢形成整体。
3．施工中必须进行监控量测，浅埋特大跨隧道施工除需进行常规项目量测外，必须进行初期支护应力量测，这样可准确、直观了解围岩应力变化情况，对指导隧道施工、预防险情有着非常重要的现实意义。
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 the
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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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