Effect of Excavation Direction on Stability of Tunnels in Transversely Isotropic Rock Mass

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Title:	Effect of Excavation Direction on Stability of Tunnels in Transversely Isotropic Rock Mass
Authors:	裴文斌;Bui, Van-Binh
Contributors:	土木工程學系
Keywords:	隧道穩定性;離散元素法;橫向等向性岩體;離散裂縫網絡;破壞模式;隧道開挖方向;tunnel stability;discrete element method;transversely isotropic rock mass;discrete fracture network;failure modes;tunneling direction
Date:	2022-02-25
Issue Date:	2022-07-13 17:59:43 (UTC+8)
Publisher:	國立中央大學
Abstract:	裂隙岩盤中的隧道穩定性問題是工程師們極大的挑戰。含一組或兩組不連續面的裂隙岩體，其工程行為具有異向性，隧道開挖方向對於隧道穩定性有顯著影響。隧道開挖方向與不連續位態間關係對穩定性影響雖已是被確認的工程課題，工程師們多以岩體分類法的經驗指引，加以評分調整。既有文獻中，針對裂隙對隧道穩定性的理論分析或模型實驗的研究數量頗為豐碩，然而，多侷限於少數特定條件下的研究結果，完整、系統性的研究結果尚付闕如，無法驗證經驗指引的正確性。本文採用三維離散元素針對不同位態之合成岩體，不同開挖方向情境下之隧道工程行為，進行一系列的數值模擬。合成岩體為一組不連續面，七種傾角，分別為為0, 15, 30, 45, 60, 75 及90。開挖情境有三種，分別為：情境1-開挖方向平行不連續面走向；情境2-開挖方向垂直不連續面走向，且與不連續面傾向同向及情境3-開挖方向垂直不連續面走向，且與不連續面傾向反向。針對隧道周圍的位移、應力集中、裂縫分佈和破壞情況進行分析探討。研究發現，對於低傾角（ = 0～15）的岩體，隧道穩定性與掘進方向無關。可評為“普通”。對於中傾角（45～60）的岩體，在開挖情境Ⅰ下，隧道發生嚴重的位移與破壞，而情境2及情境3的位移量較小，且情境3之位移量大於情境2。對於大傾角（ = 75-90）的岩體，情境2及情境3均相當穩定，可評為“非常好”，情景1為“良好”。本文比較數值分析結果與 Bieniawski 的經驗指引發現，多數情況兩者相符，仍有部分條件下，兩者有不小出入。本文也發現Bieniawski 的經驗指引本身出現不一致的情況。本文針對不一致的部分，蒐集既有文獻及本文分析結果，小幅修正Bieniawski 的經驗指引。本文分析隧道周圍岩塊的位移量，提出四種位移場類型，以描述隧道周圍岩體的損壞過程和損壞狀態。含弱面的拉伸位移場 (DF-I) 表示弱面周圍岩石層的脫層。完整岩石中的拉伸位移場（DF-II）表示隧道側壁板狀劈裂現象。剪切和拉伸位移場 (DF-III) 和剪切位移場類型 (DF-IV) 描述岩體沿弱面滑動進入隧道的趨勢。本研究還介紹了對應於每種情況的隧道周圍的故障模式。情境1包含四種破壞模式，例如“分離和屈曲”、“滑動”、“彎曲和剝離”以及“劈裂和剝離”。情景2及情景3中，隧道冠部和仰拱處觀察到“彎曲和剝落離”模式。“板狀劈裂和剝離”發生在側壁上。“脫層和屈曲”發生在具有垂直弱面的岩體的隧道面上。傾角30～75之岩體，在情境2下的破壞模式為“彎曲與剝離”模式，在情境3下則為“滑移”模式。本文為初步研究，結果存在一些局限性，本文提供分析洞察隧道開挖方向對裂隙岩體隧道穩定性影響的研究途徑。 ;The tunnel stability in fractured rock masses is a great challenge for tunnel engineers. Rock masses contains one set or two set of discontinuities behave anisotropically. The excavation direction of tunnels influences tunnel stability significantly. Empirical guidance regarding tunnel stability reported in the literature indicated that the tunneling direction relative to joint strike and dip angle could affect the stability of a tunnel. However, little scientific data were available to quantify such an effect. This study aims to investigate the effect of joint orientation and dip angle through a series of three-dimensional (3-D) discrete element method (DEM) analyses that are set up to cover various tunneling scenarios. The synthetic rock mass simulated the transversely isotropic rock mass is generated by combining the intact rock model and Discrete Fracture Network (DFN) model. The synthetic rock masses containing one joint set are generated with seven joint dip angles of 0, 15, 30, 45, 60, 75 and 90. Three scenarios of excavation direction were simulated in synthetic rock mass: (1) Scenario I – tunneling direction parallel to joint strike, (2) Scenario II – tunnel axis perpendicular to joint strike and tunneling along dip direction, and (3) Scenario III - tunnel axis perpendicular to joint strike and tunneling against the dip direction. The displacement, stress concentration, crack distribution and failure around the tunnel were measured. It is found that, for rock mass with low dip angles ( = 0-15), the tunnel stability is irrespective to the tunneling direction. It is rated “Fair” for the suitability for tunneling. For rock mass with dip angles of 45- 60, terrible displacement and severe failure is observed in the tunnel under Scenario I, while effects under Scenario II and Scenario III are less severe. The displacement around the tunnel under Scenario III is much more than that under Scenario II. For rock mass with high dip angles ( = 75- 90), the grade of tunnel stability under Scenario II and Scenario III is rated “very favorable,” whereas the grade under Scenario I is found “favorable.” Based on the results of numerical simulations, a slight modification of Bieniawski’s classification guidance is proposed. Four displacement field types are proposed to describe the damage process and damage state of rock mass around the tunnel. (1) Tensile displacement field with the joint (DF-I) represents the detachment of the rock layer around the joints. (2) Tensile displacement field in intact rock (DF-II) describes the slabbing phenomenon of rock mass in the slabs in the sidewall of the tunnel. (3) Shear and tensile displacement field (DF-III) and (4) shear displacement field type (DF-IV) delineates the tendency to the sliding movement of rock mass along the joint into the tunnel. In addition, failure modes around the tunnel corresponding to each scenario are presented in this study. Scenario Ireports four failure modes such as “Detaching and Buckling,” “Sliding,” “Bending and Spalling,” and “Slabbing and Spalling.” In Scenario II and Scenario III, “Bending and spalling” mode is observed in the crown and invert of the tunnel. “Slabbing and Spalling” occurs in the sidewall. “Detaching and Buckling” occurs in the tunnel face with rock mass containing vertical joint set. In the tunnel face under Scenario II, “Bending and Spalling” mode is observed with dip angles of 30- 75, whereas “Sliding” model is shown with dip angles of 30- 75 in the tunnel face under Scenario III. Although the findings of this study are considered preliminary and with several limitations, it provides insight into the effect of tunneling direction on tunnel stability in a transversely isotropic rock mass.
Appears in Collections:	[Graduate Institute of Civil Engineering] Electronic Thesis & Dissertation

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