以經驗方法評估軟岩地盤開挖潛盾隧道引致之地表沉陷

根據現地監測資料，本研究探討潛盾隧道施工遭遇軟岩地盤造成之地表沉陷距離曲線及地表沉陷槽。本研究探討利用雙曲線模式模擬地表沉陷距離曲線之適用性。本研究探討利用常態分佈模式模擬地表沉陷槽之適用性。本研究探討隨著年代演進，EPB潛盾機於四類地盤掘進引致之沉陷量差異。本研究探討潛盾隧道於四類地盤施工引致之地表沉陷槽寬度i值及最大地表沉陷量Smax之比較，並獲得以下各項結論。 1.本研究蒐集自1982年至1998年及1998年至2018年黏土、砂土及卵礫石地盤施工案例。隨著年代推進，在同樣採用土壓平衡式潛盾機的情況下，於此3類地盤開挖隧道，造成之最大沉陷量呈逐漸減少之趨勢，其原因可能為施工機具品質的精進及施工技術的提升所致。 2.本研究蒐集4筆桃園機場捷運中壢延伸段CM01標之EPB潛盾機於軟岩地盤施工案例，其最大沉陷量Smax為0.9  0.3 mm，此數據與Fujita (1982)提出於黏土地盤施工造成之沉陷Smax = 60  25 mm，相去甚遠。推論是由於軟岩與黏土之單壓強度qu差距甚大，軟岩qu大於黏土qu數倍，導致隧道開挖軟岩地盤引致之最大沉陷量遠小於黏土地盤者。 3.本研究將CM01標4筆軟岩地盤開挖潛盾隧道之現地監測數據，依常態分佈模式分析後，求出沉陷槽寬度參數i值。與Peck (1969)建議之i值範圍進行比較，發現在相同深徑比的情況下，潛盾隧道於軟岩地盤掘進引致之沉陷槽寬度參數i值，明顯大於軟弱至堅硬黏土及地下水位以下砂土隧道施工造成之i值。 4.依據於1998至2018年發表之8個研究性案例，EPB潛盾機於黏土、砂土、卵礫石、軟岩地盤造成之最大地表沉陷量Smax平均值之大小依序為：27.2 mm、11.9 mm、4.5 mm、1.0 mm；而引致沉陷槽寬度參數i值則相反：最小到最大依序為7.59 m、8.35 m、13.0 m、15.9 m。由此可知，潛盾隧道開挖之地盤越軟弱，造成之沉陷量Smax越大，其沉陷槽越窄。

關鍵字

經驗方法；監測數據；沉陷量；潛盾隧道；軟岩；沉陷槽寬度參數

並列摘要

Bases on field monitored data, this study reported the logitudinal and transverse surface settlement trough caused by shield tunneling in soft rock. This study used the hyperbolic model to simulate the logitudinal settlement trough, and used the normal distribution model to simulate the transverse settlement trough. The amounts of maximum surface settlement caused by EPB shields tunneling in four types of grounds at different periods of time were compared. The width parameter i of the surface settlement trough caused by shield tunneling in the four types of grounds was also investigated.Based on this study the following conclusions were obtained, regarding surface settlement due to EPB shield tunneling. 1.In clay, the mean value plus and minus one standard deviation of Smax reported between 1982 and 1998 was 30.0  11.5 mm. For cases reported between 1998 and 2018, the measured Smax range was 24.0  12.0 mm. These values were significantly lower than the Smax range 60.0  25.0 mm reported by Fujita(1982).The decrease of Smax value with the advancement of time was probably due to the improvement of the quality of the shield machine and the improvement of construction technology. 2.This study collected field settlement data from 4 monitored sections for the construction site of the Taoyuan International Airport Mass Rapid Transit project in soft rock sites. The range of measured maximum surface settlement was 0.9  0.3 mm. These data were far greater than the settlement caused by the EPB tunneling in clay Smax = 60  25 mm, as reported by Fujita(1982). The main reason is that the compressive strength of the soft rock encountered (qu = 1029 to 3127 kPa) was many times larger than that of the medium clay (qu = 48 ~ 96 kPa), and soft clay (qu = 24 ~ 48 kPa). The rock strength caused a strong ground arching above the tunnel excavation, and that caused a smaller settlement in soft rock. 3.Based on the field monitored data from the Taoyuan International Airport MRT project, the normal distribution pattern analysis was conducted, and the settlement width parameter i value was determined. The i parameters obtained were compared with the range of i values recommended by Peck (1969). It is found that at same depth/diameter (Z/2R) ratio, the settlement trough width parameter i caused by shield tunneling in soft rock was apparently larger than those due to EPB shield tunneling in soft to stiff clays, and sands below the groundwater level. 4.Baesd on the eight surface settlement trough published between 1998-2018, the average Smax caused by EPB shield machine in clay, sand, gravel and soft rock sites was: 27.2 mm, 11.9. mm, 4.5 mm, 1.0 mm respectively. The value of the settlement trough width parameter i was: 7.59 m, 8.35 m, 13.0 m, 15.9 m respectively. It was found that in a weaker site of shield tunnel excavation a larger settlement amount Smax would be induce, and a narrower settlement trough was observed.