地下水透過節理向上滲流對邊坡穩定的影響

The influence of joints with upward groundwater on dip slope stability

黃紹宬(Shao-Cheng Huang)

指導教授 ： 林銘郎

國立臺灣大學/工學院/土木工程學研究所/碩士(2015年)

https://doi.org/10.6342/NTU.2015.00959

摘要

2010年4月25日台灣國道三號3.1k處發生大規模順向坡滑動，覆蓋面積約2萬多平方公尺。經事後調查報告得知該災害滑動面為砂頁岩互層，地層傾角為14度，而發生主因有三個，地質災害-順向坡、工程擾動-開挖、時間因素-地錨鏽蝕。然而有一個疑問待解決，就是國道三號事件發生於無豪雨的日子，根據以往歷史資料可知，地下水對邊坡穩定的影響甚大，若無豪雨發生，則可能是地下水對邊坡產生不穩定影響。在災後第一時間的現地調查發現，在部分滑動面有觀察到垂直節理，且有明顯的銹染現象。而依據地電阻探得知滑動面底部含有地下水，推測地下水由垂直節理與滑動面連通，產生向上水壓進而對邊坡產生不穩定影響。
本研究分為物理試驗與數值模擬兩部分，物理試驗為自行設計滲流傾斜儀，於傾斜儀平版上放置以石膏加砂製成的人造試體，給予人造試體向上的水壓力，本研究分別改變傾斜角度、試體節理位置、滲流面位置，來探討地下水向上對邊坡穩定的影響。根據物理試驗結果，於水閥全開下試體滑動時為臨界狀態，以靜力學公式反推出水壓力大小，並與施加水壓力比較，可得知水壓力消散為原來的1/15，並隨著節理的數量增加，向上的水壓力減小。再將向上的水壓力與試體重量比較，可得知水壓力需約為塊體重量的3.8倍才能促使塊體下滑。
數值模擬使用軟體FLAC模擬國道三號崩塌事件。崩塌地底部為受壓水層，地下水經由垂直節理進入滑動面，模型頂部為多節理之風化砂岩。比較長期受向上滲流及長期地錨衰減、摩擦角下降之塑性圖、最大剪應變率，藉以瞭解岩體裂隙水對邊坡穩定的影響。
根據數值分析結果在長期受水壓影響下，於最上方垂直節理會引致上方砂岩產生張裂縫。受到地錨衰減影響使覆蓋砂岩產生向下滑動趨勢，致使回填區剪應變增加，若施加114m水壓則回填區會產生大量變形。摩擦角由19度下降至17度時，坡面易沿著弱面滑動，且滑動面會延伸至回填區下方。

關鍵字

順向坡 ； 節理岩體 ； 向上滲流 ； 裂隙水 ； 物理模型

並列摘要

The massive forward slope sliding happened at Taiwan's National Highway No. 3 3.25k on February 25, 2010, which covers an area about more than 20,000 square meters. After investigating the cause of the disaster that the sliding surface on sandstone/shale interlayer and angle of 14° ,there are three reasons for the occurrence of an event of collapse, Geological disasters dip slope, engineering disturbance- excavation, the time factor- efficiency reduction of rock anchors. However, there is a problem to be resolved, the forward slope slided at National Highway No. 3 occurred in not a torrential rainy day. Based on historical data showing that the groundwater impacts on slope stability. If no heavy rain occured, it might be the groundwater impacts on slope unstable. There are vertical joints and the significant phenomenon of rust stained at partial siliding surface. According to the geophysical survey, the bottom of the sliding surface contains groundwater. Presume presumably the groundwater flow into rock layer by vertical joints ,cause upward water pressure effect slope stability.
This study has two parts, physical test and numerical simulation. The physical tests design water flow inclinometer, the specimen material is manufactured of gypsum sand that place on the inclinometer sheets, and give upward water pressure from the specimen bottom.
In this study, change inclinometer angle, joint position of specimen, position of seepage, to investigate the effect of upward water on slope stability. In accordance with the physical test, the specimen sliding when the fully open valve is critical state, using this critical state to figure out water pressure formula. And compared with the apply water pressure, the water pressure dissipate to the original 1/15. When the number of joints increase, the upward water pressure reduce. Therefore, compared with the upward water pressure and sample weight, finding out the water pressure required 3.8 times the weight of the block bringing about driving the block.
For the numerical simulation, simulating National Highway No. 3 event by FLAC. Top of the model is sandstone, bottom of the model is confined aquifer. The groundwater flow the vertical joints into the sliding surface. In order to understand the impact of cleft water on slope stability, compared with the influence of the efficiency reduction of rock anchors, friction angle decreases, compare to plastic chart and maximum shear strain rate.
According to the numerical analysis results, the National Highway No. 3 event has been influenced for a long time by water pressure causing weathered sandstone generate tension cracks on the top of the vertical joints. Due to anchor attenuation influence makes sandstone sliding down, resulted in increasing shear strain in backfill region. When water pressure increases to 114m, the backfilled region will produce large deformation. The friction angle decreases from 19° to 17°, the slope slides along the weak surface easily, and the sliding surface extends to the bottom of the backfill region.

並列關鍵字

dip slope ； jointed rock mass ； upward groundwater ； cleft water ； physical model

參考文獻

6. 黃玟菘(2012)，開挖引致順向坡坡體變形與破壞歷程之研究，碩士論文，臺灣大學土木工程學系。

3. 中華民國大地工程學會(2011)，國道三號3.1 公里崩塌事件原因調查工作總結報告，交通部。

1. 洪如江(2002)，順向坡之破壞與穩定，地工技術第94期，第5-18頁。

5. 青山工程顧問有限公司(2012) ，台 62 線順向坡地質鑽探、分析監測及量設計技術服務，公路總局第一區養護工程處。

16. Stimpson, B. (1970). Modeling materials of engineering rock mechanics. Int. J. Rock Mech. Min. Sci. Vol 7. pp. 77-121.