西安地铁3号线通化门站—胡家庙站区间暗挖段工法比选

张 琨

(西安科技大学建筑与土木工程学院,710054,西安//工程师)

西安地铁3号线通化门站—胡家庙站区间暗挖段工法比选

张 琨

(西安科技大学建筑与土木工程学院,710054,西安//工程师)

根据西安地铁3号线通化门站—胡家庙站区间暗挖段工程特点,提出了台阶法+临时仰拱、中隔法(CD法)和交叉中隔法(CRD法)三种可行工法。运用FLAC数值模拟方法对三种工法引起的地表沉降进行了预测分析,确定台阶法+临时仰拱法为最优施工工法。实测结果表明,暗挖施工引起的地表沉降满足变形要求,提出的暗挖段施工工法合理有效。

地铁; 暗挖隧道; 施工工法; 数值模拟

Author′s address School of Architecture and Civil Engineering,Xi′an University of Science and Technology,710054,Xi′an,China

地铁隧道开挖的核心问题之一,是如何控制地层沉降。地表沉降作为地层沉降的重要反映,有着重要的工程意义[1-2]。地表沉降过大,可能引起管线开裂、邻近建构筑物变形损害,以及威胁上覆道路车辆的行驶安全,故施工中地表沉降的控制优劣,直接影响施工自身及邻近建构筑物安全[3]。实践中,地铁暗挖施工引起的地表沉降值大小除了受到地层条件、断面形状和尺寸以及支护结形式的影响外,更重要的是受施工工法的影响。不同施工工法对隧道周围地层的扰动程度不同,造成隧道上方地表沉降值差异较大[4-5]。在隧道暗挖段地层条件、断面形状和尺寸等影响因素一定的情况下,合理选择施工工法是控制地表沉降的有效途径。因此,分析不同工法的施工特点,选择一种适合特定地层暗挖施工的合理工法有着重要的工程应用价值。本文对所选区段B型断面进行不同工法地表沉降FLAC (有限差分法)模拟研究,旨在根据实际开挖条件,综合安全、效益等指标,选择最为合理的开挖工法。

1 工程概况

通化门站至胡家庙站区间(以下简为“通胡”区间)位于西安市金花北路地下,从通化门站起,连续下穿3栋建筑物后,沿金花北路西侧地下向北,沿线经东二环长乐桥、西北电力设计院、西玛机电有限公司属楼等建筑,在长缨路南侧到达胡家庙站。沿线建筑林立,管线众多,人流车流密集。

该区间设置盾构向南始发井一处,采用明挖法施工。其兼作区间联络通道及过f4地裂缝浅埋暗挖段施工竖井。浅埋暗挖段断面为单导洞单线马蹄形隧道。考虑到向南的盾构始发、过地裂缝净空预留、标准断面以及人防段的不同断面要求,暗挖段由南向北分别设置A、B、C、D共 4种断面。区间暗挖段平面图如图1所示。其中B断面为地裂缝设防段,隧道宽、高分别为8.08 m和8.67 m,左右线长度分别为132 m和117 m,为主暗挖段。为安全施工及提高综合效益,本文重点对B型断面进行工法比选研究,并以地表沉降为控制指标确定最优开挖方案。

1.1 工程地质

图1 区间暗挖段平面图

明挖盾构井开挖时实际地层与地质报告基本一致。通胡区间暗挖隧道主要穿越的地层为饱和软黄土、古土壤和老黄土。 该明挖段具体地质情况见图2。

1.2 水文地质

对本区间地铁施工有直接影响的是地下潜水。潜水层水位约为地下9.5～11.6 m。通胡区间场地西南侧约2.5 km处有兴庆湖(水深约2 m,水面高程约408 m),东南侧约1 km处有长乐公园人工湖,北侧约4 km处有太液池(水深约1.5 m,水面高程约398.8 m)。2010年10—12月勘察时钻探揭露,场地内地下潜水稳定水位埋深在10.4～15.6 m之间,相应高程为395.72～399.8 m。根据西安长期水位观测资料,勘察时该水位接近平水位期。西安地下水年变幅为2 m左右。拟建场地的地下水主要接受大气降水及侧向地下水径流补给,潜水排泄方式主要为侧向径流排泄。

2 FLAC工法比选建模

2.1 比选工法

通胡区间B断面宽8.08 m,高8.67 m,埋深约10 m。断面尺寸如图3所示。

图3 B型断面尺寸图

依据开挖断面尺寸、埋深以及地层描述,现选择台阶法(留核心土)+临时仰拱、CRD法(交叉中隔法)、CD法(中隔法)等开挖方法进行FLAC平面模拟对比分析。

2.2 模型建立及计算参数

本文以地表沉降值为指标进行单线平面比选研究,基于圣维南原理及隧道埋深和尺寸,最终确定模型尺寸为63 m×44.6 m×1 m。计算采用摩尔-库伦本构模型,支护结构选择壳单元进行模拟。根据地质剖面图,合并相似土层,将计算地层划分为6组。各土层厚度及计算参数见表1。

2.3 开挖过程

图4为三种比选工法的开挖模拟过程。浅埋暗挖施工应遵循短开挖、强支护、快封闭的原则,根据基本原理并结合相关文献,本文对开挖应力释放按照30%控制,以此反映开挖到支护前的围岩应力应变变化过程。

2.4 计算结果分析

选取三种开挖工法的地表沉降最值进行对比分析。图5为三种工法的沉降对比图。

表1 模型参数

图4 三种工法开挖支护过程

从图5可以看出,在地表沉降控制效果的优劣上,依次为CRD、CD及台阶法,其地表沉降最大值分别为10.74 mm、17.28 mm及21.15 mm。通过及时支护以及一定的辅助工法,三种开挖方式引发的地表沉降值均可控制在限定范围。对比具体数值可以发现,在沉降最大值上,CD法相比台阶法(加临时仰拱)可减小约4 mm,CRD法相比台阶法和CD法可减小约10 mm和7 mm,可见CRD法施工对地表沉降的控制显著。另外,台阶法和CD法沉降曲线在对称分布性上存在较为明显差异。这是因为台阶法属于对称开挖,地表沉降槽关于隧道中线对称分布;而CRD法和CD法各导洞的开挖和支护顺序存在差异,右导洞开挖时其拱顶对应地表已有左导洞开挖时产生的沉降,加之左侧先行导洞开挖后会对右侧导洞围岩造成损伤,因此最终的地表沉降曲线最大值点会向右侧移动。

2.5 工法确定

模拟对比分析台阶法+临时仰拱、CRD法及CD法引发的地表沉降可知,对于洞径在8～9 m、埋深9 m左右的马蹄形黄土隧道开挖,在采用超前小导管注浆加固等辅助工法后,三种工法均可保证沉降量在限定范围,且CRD法相比另两种工法沉降控制显著。

然而,CRD法及CD法在施工过程中须严格遵守正台阶法的施工要点,尤其要考虑时空效应,每一步开挖必须快速、及时成环,施工工艺复杂、要求高,施工进度较慢,且综合造价远超台阶法。因此,在综合造价、工期以及工艺复杂度等指标后,采用台阶法(留核心土)+临时仰拱的开挖工法进行施工,从而达到有效控制地表沉降和工程造价的目的。

图5 三种工法地表沉降曲线对比图

3 现场实测分析

(1) 测点布设。通胡区间地表沉降监测点主要沿隧道轴线和垂直隧道轴线两个方向进行布置。本文只分析隧道开挖后的横向沉降,其测点布置情况如图6。

(2) 实测结果分析。取单线开挖地表沉降稳定后的沉降值进行分析,以评价实际开挖工法地表沉降控制的效果。图7为地表横向沉降曲线。从图7可以看到,实测台阶法于地表隧中达到沉降最大值27.68 mm,较模拟值大约6 mm。这是因为模拟值未考虑路面其他偶然荷载、土体蠕变以及本身岩土体材料的非均一性。在横向沉降范围上,二者基本一致,主沉降区约为两侧15 m,实际沉降最大值符合控制标准,达到了安全经济的目的。对于暗挖施工的西安地铁隧道,当洞顶埋深在10 m左右、洞径为8 m左右时,可考虑采用台阶法+临时仰拱并配以必要的辅助工法进行开挖。在整个施工过程中,暗挖隧道上方的道路变形很小,路面未遭到破坏,交通运输正常,邻近建筑物安全稳定。

图6 地表沉降监测横断面布置图

图7 台阶法实测与模拟对比曲线

4 结语

本文根据西安地铁3号线通胡区间暗挖隧道工程特点,拟选了台阶法+临时仰拱、CRD法和CD法三种施工工法。运用FLAC数值模拟方法对三种不同施工工法引起的暗挖隧道上方地表沉降进行了预测分析,并综合造价、工期以及工艺复杂度等指标,确定了台阶法(留核心土)+临时仰拱的施工工法。同时,进行地表沉降测点布置,并完成了现场监测。监测结果表明,暗挖隧道上方的道路变形很小,地表最大沉降值位于隧道中线,为27.68 mm,路面未遭到破坏,交通运输正常,满足地表沉降控制要求,确保了邻近建筑物安全稳定。这说明,本文提出的台阶法+临时仰拱的工法合理有效,对同类地质条件下暗挖隧道施工工法的选择具有工程指导意义。

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(Continued from Commentary)

and Line 19 under planning for the passengers at the airport going to the Beijing city centre to provide an easy access.This line only sets three stops.It only takes 30 min to run from the New Airport North Terminal to Caoqiao Station.It is planned to synchronize putting into use with the Beijing New Airport in 2019.And the rail transit express line will also connect Beijing New Airport with existing Beijing Capital International Airport.There is a 67 km distance between these two airports (similar to the 68.1 km rail transit linking line under planning between Pudong Airport and Hongqiao airport in Shanghai).In addition,there is also a Langfang-zhuozhou intercity railway line and a Beijing-Kowloon passenger dedicated railway line that could get access to Beijing New Airport.These large-capacity transport facilities will provide a strong guarantee for those new airport passengers to travel.

Compared with the rail transit planning of Beijing New Airport,at present,there are still big gaps in the supporting rail transport services of Shanghai′s two airports.On December 12,2016,Shanghai Pudong Airport ushered in the 100 millionth passenger of Shanghai Airports in 2016,making Shanghai become the fifth place of the hundred-million-passenger class aviation cities in the world after London,New York,Tokyo,and Atlanta.With the expansion of the airport size,the throughputs of the two airports in Shanghai will continue to increase.By 2020,only Pudong Airport′s throughputs will have reached 120 million person-times,entering the ranks of the world′s busiest airports.However,it could not be adapted to the continuous expansion of airport size and passenger throughputs′ continuously increasing that the public transport of the airports and the urban areas has not been a corresponding development.They can not meet the needs of broad passengers.

First of all,there is no express main route between Pudong Airport and the urban areas and between Pudong Airport and Hongqiao Airport.Although Metro Line 2 connects the two major airports,and also runs through downtown areas,there are 30 stations to stop one by one and passengers must transfer at Guanglan Road Station.After transferring,there are only 4 carriages per train leading to Pudong Airport,and inner the carriages,passengers are very crowded.The author once started off from Pudong Airport to go to Hongqiao Airport by Metro Line 2.I even transferred twice at Guanglan Road Station and Songhong Road Station.The maglev demonstration line only gets access to Long Yang Road from Pudong Airport,and does not enter the city center.Its potential ability couldn′t take a full play.Passengers rarely take it.It only plays a role of a tourist line.Therefore,it gets heavy losses every year.Secondly,up to now,the Pudong area in which Pudong Airport locates does not have a high-speed rail station yet.

To this end,it is suggested that Pudong Airport should include the public transport of urban areas into its overall planning of the airport development.The situation that the public transportation severely lags behind the passenger throughput increases and the expansion of airport size must be changed as soon as possible.As for the maglev line that runs from Longyang Road to Pudong Airport,the current practical application for the airport transportation does not achieve the desired effect,and suffers heavy losses.It is suggested that experts should be organized to consult on this maglev line,so as to make its value and effectiveness fully play.The city field express line of “Hongqiao Airport—Pujiang Town—Pudong Airport” planned by Shanghai Shentong Metro Group is also only “hearing the stairway creak,but seeing no one come down”.

On March 21,2017,when Shanghai Party Secretary Han Zheng conducted his investigation and survey in Shanghai Shentong Metro Group,he stressed that “It is necessary to fix short boards and optimize long boards conscientiously,embodying security,convenience and on-time of Shanghai rail transit.It is the expectation of the general public,and is also requirements of meticulous management in the super-city′s development processes.”In order to fix short boards,we must have a sense of urgency.To achieve the strategic blueprint for building an excellent global city,the public transport development of Pudong Airport still need to carry forward the “seizing the day” spirit.

(Translated by SUN Zheng)

Selection of Tunnel Construction Method with Deep Excavation on Xi′an Metro Line 3

ZHANG Kun

According to the characteristics of tunnel construction with deep excavated section on Xi′an metro Line 3,three methods are put forward: step method+temporary invert arch,CD method and CRD method.By using the FLAC numerical simulation to forecast the surface subsidence on the tunnel, the best construction method——step method+temporary invert arch is selected.The data of site monitoring indicate that the surface subsidence could meet the control standards, thus the construction method proves to be reasonable and effective.

metro; bored tunnel; construction method; numerical simulation

U455.4

10.16037/j.1007-869x.2017.04.027

2016-05-02)