西安地鐵3號線通化門站—胡家廟站區間暗挖段工法比選

張 琨

(西安科技大學建筑與土木工程學院,710054,西安//工程師)

西安地鐵3號線通化門站—胡家廟站區間暗挖段工法比選

張 琨

(西安科技大學建筑與土木工程學院,710054,西安//工程師)

根據西安地鐵3號線通化門站—胡家廟站區間暗挖段工程特點,提出了臺階法+臨時仰拱、中隔法(CD法)和交叉中隔法(CRD法)三種可行工法。運用FLAC數值模擬方法對三種工法引起的地表沉降進行了預測分析,確定臺階法+臨時仰拱法為最優施工工法。實測結果表明,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]。地表沉降過大,可能引起管線開裂、鄰近建構筑物變形損害,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劣,直接影響施工自身及鄰近建構筑物安全[3]。實踐中,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結形式的影響外,更重要的是受施工工法的影響。不同施工工法對隧道周圍地層的擾動程度不同,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,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用價值。本文對所選區段B型斷面進行不同工法地表沉降FLAC (有限差分法)模擬研究,旨在根據實際開挖條件,綜合安全、效益等指標,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間(以下簡為“通胡”區間)位于西安市金花北路地下,從通化門站起,連續下穿3棟建筑物后,沿金花北路西側地下向北,沿線經東二環長樂橋、西北電力設計院、西瑪機電有限公司屬樓等建筑,在長纓路南側到達胡家廟站。沿線建筑林立,管線眾多,人流車流密集。

該區間設置盾構向南始發井一處,采用明挖法施工。其兼作區間聯絡通道及過f4地裂縫淺埋暗挖段施工豎井。淺埋暗挖段斷面為單導洞單線馬蹄形隧道。考慮到向南的盾構始發、過地裂縫凈空預留、標準斷面以及人防段的不同斷面要求,暗挖段由南向北分別設置A、B、C、D共 4種斷面。區間暗挖段平面圖如圖1所示。其中B斷面為地裂縫設防段,隧道寬、高分別為8.08 m和8.67 m,左右線長度分別為132 m和117 m,為主暗挖段。為安全施工及提高綜合效益,本文重點對B型斷面進行工法比選研究,并以地表沉降為控制指標確定最優開挖方案。

1.1 工程地質

圖1 區間暗挖段平面圖

明挖盾構井開挖時實際地層與地質報告基本一致。通胡區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。 該明挖段具體地質情況見圖2。

1.2 水文地質

對本區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。通胡區間場地西南側約2.5 km處有興慶湖(水深約2 m,水面高程約408 m),東南側約1 km處有長樂公園人工湖,北側約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月勘察時鉆探揭露,場地內地下潛水穩定水位埋深在10.4～15.6 m之間,相應高程為395.72～399.8 m。根據西安長期水位觀測資料,勘察時該水位接近平水位期。西安地下水年變幅為2 m左右。擬建場地的地下水主要接受大氣降水及側向地下水徑流補給,潛水排泄方式主要為側向徑流排泄。

2 FLAC工法比選建模

2.1 比選工法

通胡區間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

依據開挖斷面尺寸、埋深以及地層描述,現選擇臺階法(留核心土)+臨時仰拱、CRD法(交叉中隔法)、CD法(中隔法)等開挖方法進行FLAC平面模擬對比分析。

2.2 模型建立及計算參數

本文以地表沉降值為指標進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。計算采用摩爾-庫倫本構模型,支護結構選擇殼單元進行模擬。根據地質剖面圖,合并相似土層,將計算地層劃分為6組。各土層厚度及計算參數見表1。

2.3 開挖過程

圖4為三種比選工法的開挖模擬過程。淺埋暗挖施工應遵循短開挖、強支護、快封閉的原則,根據基本原理并結合相關文獻,本文對開挖應力釋放按照30%控制,以此反映開挖到支護前的圍巖應力應變變化過程。

2.4 計算結果分析

選取三種開挖工法的地表沉降最值進行對比分析。圖5為三種工法的沉降對比圖。

表1 模型參數

圖4 三種工法開挖支護過程

從圖5可以看出,在地表沉降控制效果的優劣上,依次為CRD、CD及臺階法,其地表沉降最大值分別為10.74 mm、17.28 mm及21.15 mm。通過及時支護以及一定的輔助工法,三種開挖方式引發的地表沉降值均可控制在限定范圍。對比具體數值可以發現,在沉降最大值上,CD法相比臺階法(加臨時仰拱)可減小約4 mm,CRD法相比臺階法和CD法可減小約10 mm和7 mm,可見CRD法施工對地表沉降的控制顯著。另外,臺階法和CD法沉降曲線在對稱分布性上存在較為明顯差異。這是因為臺階法屬于對稱開挖,地表沉降槽關于隧道中線對稱分布;而CRD法和CD法各導洞的開挖和支護順序存在差異,右導洞開挖時其拱頂對應地表已有左導洞開挖時產生的沉降,加之左側先行導洞開挖后會對右側導洞圍巖造成損傷,因此最終的地表沉降曲線最大值點會向右側移動。

2.5 工法確定

模擬對比分析臺階法+臨時仰拱、CRD法及CD法引發的地表沉降可知,對于洞徑在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法在施工過程中須嚴格遵守正臺階法的施工要點,尤其要考慮時空效應,每一步開挖必須快速、及時成環,施工工藝復雜、要求高,施工進度較慢,且綜合造價遠超臺階法。因此,在綜合造價、工期以及工藝復雜度等指標后,采用臺階法(留核心土)+臨時仰拱的開挖工法進行施工,從而達到有效控制地表沉降和工程造價的目的。

圖5 三種工法地表沉降曲線對比圖

3 現場實測分析

(1) 測點布設。通胡區間地表沉降監測點主要沿隧道軸線和垂直隧道軸線兩個方向進行布置。本文只分析隧道開挖后的橫向沉降,其測點布置情況如圖6。

(2) 實測結果分析。取單線開挖地表沉降穩定后的沉降值進行分析,以評價實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,實測臺階法于地表隧中達到沉降最大值27.68 mm,較模擬值大約6 mm。這是因為模擬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,主沉降區約為兩側15 m,實際沉降最大值符合控制標準,達到了安全經濟的目的。對于暗挖施工的西安地鐵隧道,當洞頂埋深在10 m左右、洞徑為8 m左右時,可考慮采用臺階法+臨時仰拱并配以必要的輔助工法進行開挖。在整個施工過程中,暗挖隧道上方的道路變形很小,路面未遭到破壞,交通運輸正常,鄰近建筑物安全穩定。

圖6 地表沉降監測橫斷面布置圖

圖7 臺階法實測與模擬對比曲線

4 結語

本文根據西安地鐵3號線通胡區間暗挖隧道工程特點,擬選了臺階法+臨時仰拱、CRD法和CD法三種施工工法。運用FLAC數值模擬方法對三種不同施工工法引起的暗挖隧道上方地表沉降進行了預測分析,并綜合造價、工期以及工藝復雜度等指標,確定了臺階法(留核心土)+臨時仰拱的施工工法。同時,進行地表沉降測點布置,并完成了現場監測。監測結果表明,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,交通運輸正常,滿足地表沉降控制要求,確保了鄰近建筑物安全穩定。這說明,本文提出的臺階法+臨時仰拱的工法合理有效,對同類地質條件下暗挖隧道施工工法的選擇具有工程指導意義。

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

[2] 施成華.城市隧道施工地層變形時空統一預測理論及應用研究[D].長沙:中南大學,2008.

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

[4] 施仲衡.淺埋暗挖設計理論論述[J].現代隧道技術,2005,42(2):37-40.

[5] 王夢恕.隧道工程淺埋暗挖法施工要點[J].隧道建設,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)