開挖引致順向坡坡體變形與破壞歷程之研究

A Study of Toe-excavation Induced Failure Process for a Dip Slope with Rock Anchorage

黃玟菘(Wen-Sung 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(2012年)

https://doi.org/10.6342/NTU.2012.02716

摘要

在2010年4月25日無風無雨的情況下，位於台灣基隆市七堵區國道旁人工地錨邊坡，突然發生大規模的順向坡破壞。經由事後調查與本研究數值分析得知，早期於此順向坡坡趾處的橋台降挖工程，係引致滑動層強度大幅下降，造成坡體變形與地錨荷重紀錄增加最關鍵的因素。當時在橋台降挖區域進行地錨加固與回填後，坡體已暫時趨於穩定。然而，完工後至今，上邊坡岩體強度之長期弱化是影響邊坡穩定狀態的關鍵性因素。相較之下，護坡用的預力地錨因鋼鍵鏽蝕所造成之地錨效能衰減，對坡體穩定之影響程度，則不若橋台降挖與岩體強度弱化對坡體所造成之影響。
事件處為第三紀沉積岩所構成之順向坡，岩層走向為北20~48度東，傾向東南12~18度。此一順向坡上曾進行挖填方工程，並於下邊坡處打設預力地錨加固，係一人工錨碇邊坡。首先由LEM(STABL)分析結果得知，若開挖時期的施工擾動讓滑動層強度已降至殘餘強度，坡體將趨於臨界狀態(F.S≈1)。回填後，無論是岩體受長期風化而強度降低，或是地錨因鏽蝕而效能減弱，都足以使坡體趨於不穩定狀態(F.S.<1)。
本研究進一步利用鑽探試驗資料、配合岩體評分法(GSI)評估當地岩體的強度參數，並使用FDM(FLAC 5.0)進行橋台降挖階段之逆分析(back analysis)，藉由數值模型中因坡體變形所引致之地錨荷重增量與實際監測資料之比對，證明橋台降挖期間，滑動層強度因工程擾動有大幅度下降之情況。
利用校核過之數值模型分別討論岩體強度弱化(長期受風化、地下水入滲影響)與地錨效能損失(鋼鍵鏽蝕)對坡體長期穩定性之影響。數值結果說明，坡體因降挖區域進行預力地錨加固和緊急回填後，暫時趨於穩定。但完工後，在地錨效能衰減影響，滑動面的發展僅於坡趾處；受到上覆砂岩層與滑動層岩體強度持續弱化影響，才得以造成這幾十年間滑動面逐步由坡趾處向坡頂處發展，引致邊坡漸趨於不穩，終至破壞發生。

關鍵字

地質強度因子(GSI) ； 順向坡 ； 地錨 ； FLAC ； 極限平衡法、逆算分析

並列摘要

On April 25, 2010, without rainfall and earthquake triggering a massive landslide (200000 m3) covered a 200m stretch of Taiwan’s National Freeway No. 3, killing 4 people, burying three cars and destroying a bridge. The failure mode appears to be a dip-slope type failure occurred on a rock anchorage cut slope. Based on the investigations and the numerical analysis in this study, the toe-excavation at this dip-slope in 1998 was the most crucial factor. Although the excavated area had stabilized soon with rock anchors and backfills. The strength of rock mass had great effect on the stability of dip-slope after backfills.

The strike of Tertiary sedimentary strata is northeast-southwest and dip 12˚ ~15˚ toward southeast. Considering the strength of sliding layer had reduced from peak to residual strength which was caused by the disturbance of excavation, the limit equilibrium method (LEM) analysis was utilized in the back analysis at first. The results showed the stability condition of slope approached the critical state (F.S.≈1). The efficiency reduction of rock anchors and strength reduction of overlying stratum (sandstone) had been considered in following analysis. The results showed the unstable condition (F.S. <1).

This study also utilized the result of laboratory test, geological strength index(GSI) and finite difference method (FDM, FLAC 5.0) to discuss the failure process The analysis indicated that the incremental load of anchors have similar tendency comparing to the monitoring records in toe-excavation stages. This result showed that the strength of the sliding layer was significantly influenced by toe-excavation.

The numerical model which calibrated with monitoring records in toe-excavation stage was used to discuss the failure process after backfilling. The efficiency reduction of rock anchors had less impact on the development of sliding surfaces. But the development of sliding surfaces were significantly influenced by the strength reduction of the rock mass。The sliding surface gradually developed from the toe of the dip-slope to the top of the dip-slope and the stability of the slope became more unstable in the process of rock mass strength reduction .

並列關鍵字

GSI ； Dip-slope ； Anchor ； FLAC ； Limit equilibrium method ； back analysis

參考文獻

1. 洪如江、林美聆、陳天健、王國隆(2000)，921集集大地震相關之坡地災害、坡地破壞特性、與案例分析，地工技術雜誌第81期，第17-32頁。

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

3. 紀宗吉、林朝宗、劉桓吉(1998)，林肯大郡地層滑動災變之原因，地工技術雜誌第68期，第67-74頁。

4. 陳堯中、廖洪鈞、林宏達、陳志南(1998)，汐止林肯大郡災變原因，工技術雜誌第68期，第29-40頁。

5. 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。