Survey and Analysis of Potential Disasters at Dongyang Landslide area, Fengyuan District, Taichung

Abstract

臺中市豐原區東陽地區當地民眾反應民國88 年921 地震後，常因颱風、豪雨出現邊坡崩滑等情形，影響周遭居民生命財產及出入安全。經現場勘查發現區內地表有數公尺至數十公尺長的裂隙斷續出現，裂隙大多呈東北-西南走向，其東南側下陷1~11cm，地表有1m 至12m 長的裂隙斷續出現，初步確認本區具崩滑徵兆，危及下方道路、民宅、農田等保全對象安全。為求瞭解地滑發生原因與研提治理對策，故進行該地滑地之調查、測量及監測。根據現場調查結果分析、滑動機制分析以及邊坡穩定分析，本研究區為順向坡地形，其岩性屬厚層之泥岩或砂岩，地層屬軟岩質且強度較低，易於強地震時產生局部弧形錯動。於汛期時，雨水漫流沖刷入滲表土層，造成表土層浸潤，使得土壤凝聚力驟減，降低土體強度，因而引發本區部分表層土體崩壞，是以降雨可能造成本區表土土體沖刷破壞。且依據地下水位變化與地滑位移量，其地表滑動深度較為明顯處，由此判斷，依賦存狀況應屬於自由水類型，富集方式屬於為斷層型富集，顯示出地下水為計畫區內發生地滑變位之潛在因子，其首要問題應為地下水之排除。結果顯示降雨發生至地表逕流產生時，已有部分水體滲入地層內，為了將水份入滲量降低，需規劃設計地表排水，加強地表截排水設施，以及應重新檢核保全住戶後方之擋土設施，以防範災害發生之可能。

After the 921 earthquake, landslides due to the typhoon, heavy rainfall events were found by local people at Dongyang area, Fengyuan District, Taichung City, which may endanger the surrounding life and property. The field inspections showed several to ten meters long fissure intermittently appear on region surface. The fissure was mostly northeast-southwest direction. It was initial recognition of the area with landslide indication. It has endangered the safety of the surrounding roads, homes, and the security of preservation object. Therefore, field survey, monitoring, and data analysis were applied to the Dongyang landslide area. It is expected to understand the causes of landslide and help the future countermeasures planning. According to the survey results, the area is a dip slope terrain. The lithology is the genus thick layer of mudstone or sandstone. In the flood season, the overland flow erosion sharply reduces soil strength, and thus triggered some of topsoil collapse. In accordance with the groundwater levels, the depth of surface sliding are located within the groundwater areas. As a result, it is suggested to re-examine the drainage facilities (such as the slope of the drainage ditches, vertical and horizontal drainage facilities, and groundwater drainage) in order to achieve effective interception of surface water and reduce the potential of landslide.