聯絡通道及潛盾隧道施工造成之地盤變位

Abstract

本研究依據內湖線CB420標，國內首宗潛盾隧道穿越機場下方施工案例，以此工程施工造成地面沉陷及隆起之監測資料，與潛盾施工控制之土艙壓力係數、背填灌漿壓力及注入體積、盾首加泥壓力及注入體積、二次灌漿壓力及注入體積、切刃盤扭矩、千斤頂總推力及潛盾機推進速度等施工管理參數整合探討，獲得以下各項結論。

1.當土艙壓力係數介於0.6~0.7之間，沉陷量呈穩定狀態(5 mm ~ 10 mm)。與過去學者Jaky(1944)及Brooker & Ireland(1965)所提供之靜止土壓係數Ko比較，潛盾機土艙壓力係數Kch約為上述學者提出靜止土壓係數之1.3 ~ 1.4倍。依據此觀察，本研究建議土壓平衡式潛盾機於深度20 ~ 30 m之黏土層中進行鑽掘時，潛盾機土艙土壓係數Kch設定為Jaky靜止土壓係數之1.3 ~ 1.4倍。

2.本工程控制潛盾機排土率(SDR)自1.0下降至0.91，使地表沉陷量自較大沉陷漸趨減少，終於使地表沉陷量小於機場建物下方之沉陷警戒值20 mm。每環開挖與排土體積不相等可能原因為，排土量量測方式不夠準確，及正常壓密黏土受剪至破壞，其體積會發生收縮現象。

3.依據本工程所觀察結果，以高注入率灌注背填漿液並不能保證產生小沉陷量，本研究建議維持背填灌漿壓力σbg高於現地水壓加100 kPa (1.0 kgf/cm2)應能確保灌注效果；且背填灌注壓力不宜高於現地水壓力加200 kPa (2.0 kgf/cm2)，以避免造成漿液受壓向上推擠造成問題。依據台北捷運局之規範，本研究建議背填灌漿壓力pbg低於覆土壓力σv。

4.本研究建議，將盾首注泥壓力pmi控制在1.0 ~ 1.2倍覆土壓力σv之間，以確保注泥效果，且可避免灌注壓力造成土層水力破裂之情形。

5.本標未施作二次灌漿處測得之地表沉陷量，與有施作二次灌漿處測得之沉陷量並無明顯差異，二次灌漿對本工程之沉陷量改善成效並不明顯。

6.本工程潛盾機以20支推力1715 kN之千斤頂推進，在C區段維持10 ~ 15 mm小沉陷量。本研究建議在深度20 m ~ 30 m黏土層中施工時，千斤頂推力Fjack以1.5至2.0倍開挖面靜止土壓合力（1.5 Po至2.0 Po）進行掘進。

7.本工程由於穿越機場下方，以15 ~ 35 mm/min推進速度謹慎施作，以較慢推進速度施作所產生之地表沉陷量僅為5 mm ~ 17 mm，與學者Fujita (1982) 所歸納出，黏土層中土壓平衡式潛盾機造成地面沉陷範圍為35 ~ 85 mm比較，潛盾機以較慢速度推進有助於控制地盤沉陷。

This paper studies ground settlement and heaving due to shield tunnel for Taipei MRT, Lot CB 420 the Sung-Shan. Excavate round settlement was offered at the initial stage of tunneling, and heaving was offered at the runway of airport. Relationships between 100-day ground settlement and construction parameters were investigeted. The parameters studied include： chamber pressure coefficient, backfill grouting pressure, mud injection pressure, secondary grouting pressure, torque, to total jack thrust, and speed of shield. Based on the field date of Lot CB420, the following conclusions are drawn.

1.For this project, the controlling chamber coefficient (Kch) used was between 1.3 ~ 1.4 Ko, where Ko is the coefficient of earth pressure at rest proposed by Jaky (1944) and Brooker & Ireland (1965).

2.The measured soil discharge rate (SDR) is less than 1.0. This is probably due to discharge-rate measuring error and volume reduction of normally consolidated clay during shearing.

3.High injection volume of backfill grout does not ensure a small settlement. For this project, 100~200 kPa over water pressure was used as backfill grouting pressure.

4.Based on field data, it is found that the mud injecting pressure over 1.2σv (σv = over burden pressure)at the face caused hydraulic failure of the soil and runway heaving. It is suggested to control the mud injection pressure between 1.0 ~ 1.2σv.

5.The improvement of ground settlement with secondary grouting is not significant.

6.For this project, the total jack thrust Fjack was controlled between 1.5 ~ 2 times of at rest, Soil thrust Po acting at the face.

7.Low shield speed may improve the ground settlement due to tunneling.