應用分離元素法探討順向坡失穩歷程-以國道三號崩塌事件為例

Dip-slope failure assessment based on Discrete numerical simulation - example from Formosa highway

張賀翔(Ho-Shyang Chang)

指導教授 ： 張國楨

國立臺北科技大學/工程學院/土木與防災研究所/碩士(2012年)

https://doi.org/10.6841/NTUT.2012.00385

摘要

鑒於台灣板塊運動頻繁且劇烈，造成山多平原少，對於交通的拓展十分不便，迫於無奈，經常需要將道路建於邊坡上或貫穿邊坡，而當中很容易遭遇順向坡，因此在施工中，順向坡坡腳遭破壞，邊坡就失去原本穩定性，如果新建物無法負荷原本坡腳之載重，往往會發生重大災害。民國九十九年四月二十五日，國道三號3.1公里處發生地滑事件，造成四人罹難。對於地滑地點在設計規畫階段皆符合規範設計，但仍發生重大的公安災害令人十分震驚。
本研究運用數值地形模型(DTM)具有詳細三維座標的特點，並結合顆粒流離散元素分析軟體PFC^3D，基於其特性足以模擬大變形進行模擬，並分析地滑的行為及機制。首先，本研究利用災前、災後的航空照片分別建置成數值地形模型(DTM) ，由於本例不同以往之處為地滑塊體與滑動面並未分離，導致滑動面難以取得，因此，本研究利用上部露出之岩層滑動面進行內差，進而取得完整滑動面。之後，利用災前DTM與有完整滑動面之災後DTM來分離出滑動塊體，經計算後，地滑塊體約為十五萬立方公尺之土石。在PFC^3D建置地滑塊體方面，首先，生成由顆粒所匯集而成之矩形塊體，等向應力及孔隙率皆於此步驟先行設置完成，並配合本團隊所研發之程式，將災前、災後DTM所夾空間完全涵蓋矩形塊體，並由程式進行排除即得地滑塊體之顆粒。此外，程式可將矩形塊體任意縮放比例，達到套疊任一崩塌區之快速建模目的。滑動模型由災前DTM所轉換之地形面與顆粒所代表之地滑區塊所組成。
本研究進一步由地質圖判斷，將地滑塊體分為因含水弱化的砂頁岩互層及上部較完整的砂岩兩部分區塊，並在前端另設置一區顆粒來模擬地錨作用。考慮以上因素後，將參數一併輸入，完成模型準備。在模型變數上，本研究選擇滑動面弱化之摩擦角由10度弱化至2度，地錨剩餘效率70%、50%以及30%，將滑動面弱化程度及地錨折減率，以組合之方式設置於地滑模型來模擬真實情況下滑動面弱化情況，模擬結果為當滑動面弱化至4度、地錨剩餘效率為30%時與真實情況最為符合現地堆積狀況。山崩歷時約17秒，到達國道三號邊緣時間為4.36秒，掩埋南下路段時間為6.96秒，掩埋北上路段時間為10.39秒，崩落過程所產生之最高速度為15.34m/s，最遠水平滑動距離為102.84 m，最大落距為29.43 m。
使用數值地形模型(DTM)搭配顆粒流離散元素分析軟體PFC^3D其優點為將真實地形面完整呈現於模型上，以更符合災害發生實際狀況，並可模擬地滑發生之完整動態歷程。相較於一般常用之有限元素分析軟體，僅可求得破壞當時應力應變得知穩定性外，本方法還可將破壞後塊體持續發生之大變形行為，如運動軌跡、速度變化、滑動距離，以及停止運動後堆積型態，進一步掌握災害發生之影響範圍。

關鍵字

分離元素法 ； 數值地形模型 ； 國道三號 ； 順向坡

並列摘要

It is because that frequent and intense of the plate movement in Taiwan make it so inconvenient to construct transportation system, and thus transportation is restricted to develop on the dip-slope which would impair the toe of slope leading to instability during construction. Accordingly, if the new-built construction cannot bear the weight of toe of slope, tremendous disaster will occur.Four people passed away on April 25th 2010 as a result of a landslide in Formosa Freeway. It was astonishing that under legitimate regulation the design of the construction still caused great casualty of public security.
This research applied the Digital Terrain Model (DTM) featuring specific three dimensional coordinate, combines Particle Flow Code (PFC3D) which could simulate great deformation and analyze the landslide formation. First of all, builds up DTM based on the aerial photos before and after landslide. The different point from other cases is that there is no separation between slide block and slide slope result in difficult to acquire its slide slope. Using slide slop on upper part of exposed strata to obtain completed slide slop through interpolation method.Second, via calculation, the slide block is approximately 150 thousand cubic meters. In the PFC3D part, the topographic surface, based on pre-disaster and post-event DTM, serve as sliding surface of the numerical simulations models. To begin with, particle clusters production that the isotropic stress and porosity should take into account. To ensure the production range should cover the entire slid mass from the source area. Besides that, to transpose the particle clusters to the numerical model where the source is. Moreover, keep the particles which the positions are situated within the slide block confined by the upper and lower surface of the DTM before and after landslide, or eliminate from the clusters.
According to the geological map, the particles of slide block can be divided into two parts: 1) interbedded sandstone and shale which soften by water 2) upper part of sandstone. Furthermore, set up a layer of particles to simulate ground anchor. In the end, the model is ready for simulation. The advantages of DTM collocate PFC3d are that real terrain can be represented on the model, and can be simulated the complete landslide process dynamically.
Comparing with other finite element analysis software that only provides state of instability but by using this method we can know the dynamical process of sliding include trajectory, velocity change, sliding distance and also accumulation patterns after stopping and know the effect areas from the disaster event.

並列關鍵字

discrete element method ； digital terrain model ； Formosa highway ； dip-slope failure

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