張 琨

(西安科技大學(xué)建筑與土木工程學(xué)院,710054,西安//工程師)

西安地鐵3號(hào)線通化門站—胡家廟站區(qū)間暗挖段工法比選

張 琨

(西安科技大學(xué)建筑與土木工程學(xué)院,710054,西安//工程師)

根據(jù)西安地鐵3號(hào)線通化門站—胡家廟站區(qū)間暗挖段工程特點(diǎn),提出了臺(tái)階法+臨時(shí)仰拱、中隔法(CD法)和交叉中隔法(CRD法)三種可行工法。運(yùn)用FLAC數(shù)值模擬方法對(duì)三種工法引起的地表沉降進(jìn)行了預(yù)測(cè)分析,確定臺(tái)階法+臨時(shí)仰拱法為最優(yōu)施工工法。實(shí)測(cè)結(jié)果表明,暗挖施工引起的地表沉降滿足變形要求,提出的暗挖段施工工法合理有效。

地鐵; 暗挖隧道; 施工工法; 數(shù)值模擬

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

地鐵隧道開挖的核心問(wèn)題之一,是如何控制地層沉降。地表沉降作為地層沉降的重要反映,有著重要的工程意義[1-2]。地表沉降過(guò)大,可能引起管線開裂、鄰近建構(gòu)筑物變形損害,以及威脅上覆道路車輛的行駛安全,故施工中地表沉降的控制優(yōu)劣,直接影響施工自身及鄰近建構(gòu)筑物安全[3]。實(shí)踐中,地鐵暗挖施工引起的地表沉降值大小除了受到地層條件、斷面形狀和尺寸以及支護(hù)結(jié)形式的影響外,更重要的是受施工工法的影響。不同施工工法對(duì)隧道周圍地層的擾動(dòng)程度不同,造成隧道上方地表沉降值差異較大[4-5]。在隧道暗挖段地層條件、斷面形狀和尺寸等影響因素一定的情況下,合理選擇施工工法是控制地表沉降的有效途徑。因此,分析不同工法的施工特點(diǎn),選擇一種適合特定地層暗挖施工的合理工法有著重要的工程應(yīng)用價(jià)值。本文對(duì)所選區(qū)段B型斷面進(jìn)行不同工法地表沉降FLAC (有限差分法)模擬研究,旨在根據(jù)實(shí)際開挖條件,綜合安全、效益等指標(biāo),選擇最為合理的開挖工法。

1 工程概況

通化門站至胡家廟站區(qū)間(以下簡(jiǎn)為“通胡”區(qū)間)位于西安市金花北路地下,從通化門站起,連續(xù)下穿3棟建筑物后,沿金花北路西側(cè)地下向北,沿線經(jīng)東二環(huán)長(zhǎng)樂(lè)橋、西北電力設(shè)計(jì)院、西瑪機(jī)電有限公司屬樓等建筑,在長(zhǎng)纓路南側(cè)到達(dá)胡家廟站。沿線建筑林立,管線眾多,人流車流密集。

該區(qū)間設(shè)置盾構(gòu)向南始發(fā)井一處,采用明挖法施工。其兼作區(qū)間聯(lián)絡(luò)通道及過(guò)f4地裂縫淺埋暗挖段施工豎井。淺埋暗挖段斷面為單導(dǎo)洞單線馬蹄形隧道?？紤]到向南的盾構(gòu)始發(fā)、過(guò)地裂縫凈空預(yù)留、標(biāo)準(zhǔn)斷面以及人防段的不同斷面要求,暗挖段由南向北分別設(shè)置A、B、C、D共 4種斷面。區(qū)間暗挖段平面圖如圖1所示。其中B斷面為地裂縫設(shè)防段,隧道寬、高分別為8.08 m和8.67 m,左右線長(zhǎng)度分別為132 m和117 m,為主暗挖段。為安全施工及提高綜合效益,本文重點(diǎn)對(duì)B型斷面進(jìn)行工法比選研究,并以地表沉降為控制指標(biāo)確定最優(yōu)開挖方案。

1.1 工程地質(zhì)

圖1 區(qū)間暗挖段平面圖

明挖盾構(gòu)井開挖時(shí)實(shí)際地層與地質(zhì)報(bào)告基本一致。通胡區(qū)間暗挖隧道主要穿越的地層為飽和軟黃土、古土壤和老黃土。 該明挖段具體地質(zhì)情況見圖2。

1.2 水文地質(zhì)

對(duì)本區(qū)間地鐵施工有直接影響的是地下潛水。潛水層水位約為地下9.5～11.6 m。通胡區(qū)間場(chǎng)地西南側(cè)約2.5 km處有興慶湖(水深約2 m,水面高程約408 m),東南側(cè)約1 km處有長(zhǎng)樂(lè)公園人工湖,北側(cè)約4 km處有太液池(水深約1.5 m,水面高程約398.8 m)。2010年10—12月勘察時(shí)鉆探揭露,場(chǎng)地內(nèi)地下潛水穩(wěn)定水位埋深在10.4～15.6 m之間,相應(yīng)高程為395.72～399.8 m。根據(jù)西安長(zhǎng)期水位觀測(cè)資料,勘察時(shí)該水位接近平水位期。西安地下水年變幅為2 m左右。擬建場(chǎng)地的地下水主要接受大氣降水及側(cè)向地下水徑流補(bǔ)給,潛水排泄方式主要為側(cè)向徑流排泄。

2 FLAC工法比選建模

2.1 比選工法

通胡區(qū)間B斷面寬8.08 m,高8.67 m,埋深約10 m。斷面尺寸如圖3所示。

圖3 B型斷面尺寸圖

依據(jù)開挖斷面尺寸、埋深以及地層描述,現(xiàn)選擇臺(tái)階法(留核心土)+臨時(shí)仰拱、CRD法(交叉中隔法)、CD法(中隔法)等開挖方法進(jìn)行FLAC平面模擬對(duì)比分析。

2.2 模型建立及計(jì)算參數(shù)

本文以地表沉降值為指標(biāo)進(jìn)行單線平面比選研究,基于圣維南原理及隧道埋深和尺寸,最終確定模型尺寸為63 m×44.6 m×1 m。計(jì)算采用摩爾-庫(kù)倫本構(gòu)模型,支護(hù)結(jié)構(gòu)選擇殼單元進(jìn)行模擬。根據(jù)地質(zhì)剖面圖,合并相似土層,將計(jì)算地層劃分為6組。各土層厚度及計(jì)算參數(shù)見表1。

2.3 開挖過(guò)程

圖4為三種比選工法的開挖模擬過(guò)程。淺埋暗挖施工應(yīng)遵循短開挖、強(qiáng)支護(hù)、快封閉的原則,根據(jù)基本原理并結(jié)合相關(guān)文獻(xiàn),本文對(duì)開挖應(yīng)力釋放按照30%控制,以此反映開挖到支護(hù)前的圍巖應(yīng)力應(yīng)變變化過(guò)程。

2.4 計(jì)算結(jié)果分析

選取三種開挖工法的地表沉降最值進(jìn)行對(duì)比分析。圖5為三種工法的沉降對(duì)比圖。

表1 模型參數(shù)

圖4 三種工法開挖支護(hù)過(guò)程

從圖5可以看出,在地表沉降控制效果的優(yōu)劣上,依次為CRD、CD及臺(tái)階法,其地表沉降最大值分別為10.74 mm、17.28 mm及21.15 mm。通過(guò)及時(shí)支護(hù)以及一定的輔助工法,三種開挖方式引發(fā)的地表沉降值均可控制在限定范圍。對(duì)比具體數(shù)值可以發(fā)現(xiàn),在沉降最大值上,CD法相比臺(tái)階法(加臨時(shí)仰拱)可減小約4 mm,CRD法相比臺(tái)階法和CD法可減小約10 mm和7 mm,可見CRD法施工對(duì)地表沉降的控制顯著。另外,臺(tái)階法和CD法沉降曲線在對(duì)稱分布性上存在較為明顯差異。這是因?yàn)榕_(tái)階法屬于對(duì)稱開挖,地表沉降槽關(guān)于隧道中線對(duì)稱分布;而CRD法和CD法各導(dǎo)洞的開挖和支護(hù)順序存在差異,右導(dǎo)洞開挖時(shí)其拱頂對(duì)應(yīng)地表已有左導(dǎo)洞開挖時(shí)產(chǎn)生的沉降,加之左側(cè)先行導(dǎo)洞開挖后會(huì)對(duì)右側(cè)導(dǎo)洞圍巖造成損傷,因此最終的地表沉降曲線最大值點(diǎn)會(huì)向右側(cè)移動(dòng)。

2.5 工法確定

模擬對(duì)比分析臺(tái)階法+臨時(shí)仰拱、CRD法及CD法引發(fā)的地表沉降可知,對(duì)于洞徑在8～9 m、埋深9 m左右的馬蹄形黃土隧道開挖,在采用超前小導(dǎo)管注漿加固等輔助工法后,三種工法均可保證沉降量在限定范圍,且CRD法相比另兩種工法沉降控制顯著。

然而,CRD法及CD法在施工過(guò)程中須嚴(yán)格遵守正臺(tái)階法的施工要點(diǎn),尤其要考慮時(shí)空效應(yīng),每一步開挖必須快速、及時(shí)成環(huán),施工工藝復(fù)雜、要求高,施工進(jìn)度較慢,且綜合造價(jià)遠(yuǎn)超臺(tái)階法。因此,在綜合造價(jià)、工期以及工藝復(fù)雜度等指標(biāo)后,采用臺(tái)階法(留核心土)+臨時(shí)仰拱的開挖工法進(jìn)行施工,從而達(dá)到有效控制地表沉降和工程造價(jià)的目的。

圖5 三種工法地表沉降曲線對(duì)比圖

3 現(xiàn)場(chǎng)實(shí)測(cè)分析

(1) 測(cè)點(diǎn)布設(shè)。通胡區(qū)間地表沉降監(jiān)測(cè)點(diǎn)主要沿隧道軸線和垂直隧道軸線兩個(gè)方向進(jìn)行布置。本文只分析隧道開挖后的橫向沉降,其測(cè)點(diǎn)布置情況如圖6。

(2) 實(shí)測(cè)結(jié)果分析。取單線開挖地表沉降穩(wěn)定后的沉降值進(jìn)行分析,以評(píng)價(jià)實(shí)際開挖工法地表沉降控制的效果。圖7為地表橫向沉降曲線。從圖7可以看到,實(shí)測(cè)臺(tái)階法于地表隧中達(dá)到沉降最大值27.68 mm,較模擬值大約6 mm。這是因?yàn)槟M值未考慮路面其他偶然荷載、土體蠕變以及本身巖土體材料的非均一性。在橫向沉降范圍上,二者基本一致,主沉降區(qū)約為兩側(cè)15 m,實(shí)際沉降最大值符合控制標(biāo)準(zhǔn),達(dá)到了安全經(jīng)濟(jì)的目的。對(duì)于暗挖施工的西安地鐵隧道,當(dāng)洞頂埋深在10 m左右、洞徑為8 m左右時(shí),可考慮采用臺(tái)階法+臨時(shí)仰拱并配以必要的輔助工法進(jìn)行開挖。在整個(gè)施工過(guò)程中,暗挖隧道上方的道路變形很小,路面未遭到破壞,交通運(yùn)輸正常,鄰近建筑物安全穩(wěn)定。

圖6 地表沉降監(jiān)測(cè)橫斷面布置圖

圖7 臺(tái)階法實(shí)測(cè)與模擬對(duì)比曲線

4 結(jié)語(yǔ)

本文根據(jù)西安地鐵3號(hào)線通胡區(qū)間暗挖隧道工程特點(diǎn),擬選了臺(tái)階法+臨時(shí)仰拱、CRD法和CD法三種施工工法。運(yùn)用FLAC數(shù)值模擬方法對(duì)三種不同施工工法引起的暗挖隧道上方地表沉降進(jìn)行了預(yù)測(cè)分析,并綜合造價(jià)、工期以及工藝復(fù)雜度等指標(biāo),確定了臺(tái)階法(留核心土)+臨時(shí)仰拱的施工工法。同時(shí),進(jìn)行地表沉降測(cè)點(diǎn)布置,并完成了現(xiàn)場(chǎng)監(jiān)測(cè)。監(jiān)測(cè)結(jié)果表明,暗挖隧道上方的道路變形很小,地表最大沉降值位于隧道中線,為27.68 mm,路面未遭到破壞,交通運(yùn)輸正常,滿足地表沉降控制要求,確保了鄰近建筑物安全穩(wěn)定。這說(shuō)明,本文提出的臺(tái)階法+臨時(shí)仰拱的工法合理有效,對(duì)同類地質(zhì)條件下暗挖隧道施工工法的選擇具有工程指導(dǎo)意義。

[1] 孔恒.城市地鐵淺埋暗挖法隧道臨近施工理論與關(guān)鍵控制技術(shù)[J].市政技術(shù),2011,29(1):17-24.

[2] 施成華.城市隧道施工地層變形時(shí)空統(tǒng)一預(yù)測(cè)理論及應(yīng)用研究[D].長(zhǎng)沙:中南大學(xué),2008.

[3] 漆泰岳.地鐵施工引起地層和建筑物沉降特征研究[J].巖土工程學(xué)報(bào),2012,34(7):1283-1290.

[4] 施仲衡.淺埋暗挖設(shè)計(jì)理論論述[J].現(xiàn)代隧道技術(shù),2005,42(2):37-40.

[5] 王夢(mèng)恕.隧道工程淺埋暗挖法施工要點(diǎn)[J].隧道建設(shè),2006,26(5):1-4.

(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)