台灣北部未固結砂岩山岳隧道開挖破壞模式之案例研究

The Case Study Of Tunnel Excavation For Un-consolidated Sandstone In Northern Taiwan

郭武峰(wu-Feng Kuo)

指導教授 ： 鄭光炎

國立臺北科技大學/工程學院/土木與防災研究所/碩士(2007年)

https://doi.org/10.6841/NTUT.2007.00016

摘要

軟弱地盤在工程地質學上並無一致性的定義,一般而言大致可對應於ISRM地質材料強度分級中之極弱岩石(R0)與弱岩(R2)區域之間，並以岩心單壓強度小於25Mpa做為劃分參考基準，因此它可以是西部麓山帶新第三紀上新世之低固結或未固結之砂岩，泥岩，或是第四紀洪積世之砂層、粘土層以及其他沖積層、表土、火山灰質砂礫、風化岩等，本文所探討的軟岩地層是以位於台灣北部麓山帶之未固結砂泥互層、膠結鬆散砂層為主之大南灣層為標的，並以位於該地層區域之某山岳隧道為例(以下以G隧道簡稱)，探討其中依據『台灣地區岩體分類與隧道支撐系統』所建置之砂質含量大於50%，岩類級別為CⅡ(S)之未固結或膠結甚差之砂層與泥層、礫石互層之甚弱岩石地層，在隧道開挖施工過程中因地盤膠結鬆散、遇水易軟化之工程地質特性所導致之各種開挖破壞模式、規模及對應之地質與計測變形特性，藉由筆者及其他單位之研究資料以供工程界進一步瞭解此類型未固結軟弱岩盤之山岳隧道設計及施工應考量之因素,以降低施工期間之災害風險。
經本文分析結果，CⅡ(S)特殊未固結砂層夾雜泥層、礫石互層反算參數約為E=125~275 MPa；C=50~70 KPa；φ=35°，此岩類之隧道施工破壞模式乃以開挖抽坍為主，而其中頂拱抽坍發生率最高其次是側壁部位，唯地層抽坍行為對於已完成之支撐保護工鮮少造成破壞或大量變形；另抽坍破壞機制主要受控於層面複合材料之固結與膠結程度，尤其是細粒料，因此如何較準確的預先掌握隧道開挖面區域附近地層及所對應之含、滲水量之高低與分佈位置之工程地質特性，採取預防性之排水、止水工法及快速有效之固結灌漿材料及配套之保護工以提高岩體強度，並減少對土體之開挖擾動以營造快挖快撐之有利施工條件將是影響CⅡ(S) 岩類隧道開挖施工進度重要關鍵。

關鍵字

軟岩 ； 未固結 ； 軟化 ； 抽坍 ； 回饋分析

並列摘要

Title：The Case Study Of Tunnel Excavation For Un-Consolidated Sandstone
In Northern Taiwan Pages：123
School：National Taipei University of Technology
Department：Institute of Civil and Prevention Engineering
Time：November, 2007 Degree：Master
Researcher：Wu-Feng Kuo Advisor：Kuang-Yen Cheng

Keyword: Soft-Rock, Un-consolidated, Soften, Collapse, Back analysis

There’s no specific definition of un-consolidated rock in engineering geological scope in general. It can be correlated to extremely weak rock as specificed by ISRM with characteristic strength less than 25 MPa.
A tunnel (Named as G Tunnel hereafter) which discussed is located at northern hill and ridge area of northern Taiwan, driven through Dananwan Formation soft rock, composite by unconsolidated interbreeded sandstone and mudstone, as the research object of this paper is to analyze the possibility of Demolition Model while encountering low-strength, poorly consolidate capability, and easily soften material during the process of excavation in CⅡ(S) rock mass, wihch defineed as the weight percentage of loose sandstone is more than 50%, by Public Construction Commission Rock Mass Classification System (PCCR-System). It further provides the construction filed to have some general idea to design and construct weak rock in tunnels and decrease the risk during construction period.
As result of Feedback analysis from this article, the material parameter of CⅡ(S) is E=125~275 MPa；C=50~70 KPa；φ=35°, This main damageed model is collapse during tunnel excavation. The possibilities of collapse location could be top arch and sidewall. But the finished supporting arches are rarely damaged or heavily deformed by unstable soil stratum.
The failure mechanism is controlled by bonding different particle size material specially the fines.Therefore, the key ways of keeping the tunnel excavation under control are shown below:
a). Eexact pre-survey the ground water distribution and correlated action for drainage,
dewatering method and efficient.
b). Strengthen the hardness of rockmass by fast consolidate grouting policy.
c). Reduce the disturbance during excavation to meet the fast-support scope of tunnel.

並列關鍵字

Soft rock ； Un-consolidated ； Soften ； Collapse ； Back analysis

參考文獻

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[1] Dobereiner, L. and D. Freitas, M. H. “Geotechnical properties of weak sandstones,” Geotechnique, Vol. 36, No.1,1986, pp. 79-94.