地層內水壓分佈監測技術之研發與應用

Abstract

從土壤力學觀點，在總應力不變情況下，孔隙水壓的增加導致有效應力降低，乃至於土壤之破壞。與此土壤力學現象相關之實務問題可以包括因降雨而導致之邊坡崩塌以及橋樑基礎之沖刷。現有之現場水位或水壓量測不容易以自動化，即時的方法，有效的量測地層內孔隙水壓的分佈。雖然理論上，孔隙水壓量測應該可以做為邊坡崩塌以及橋樑基礎沖刷預警量測之重要參數，但受限於現有技術之缺陷，還無法在現地做實際應用。本研究利用光纖光柵感應之優勢，研發串列式光纖光柵水壓計以及現場安裝技術。為驗證此技術之實用性，作者在阿里山五彎仔邊坡，60m單一鑽孔內以間隔5m的方式安裝光纖光柵水壓計串列，來監測孔隙水壓分佈與變化做為邊坡穩定監測之一部份。使用類似的方法，作者於台1線大甲溪橋P27橋墩、國道3號P27L(P30R)橋墩與東勢大橋P5橋墩安裝一系列光纖光柵水壓計串列，並以橋墩室內模型沖刷實驗，建立動態水壓與沖刷深度關係。使用即時自動化水壓分佈監測數據，結合應力路徑觀念，作者提出根據應力狀態做邊坡穩定或橋樑基礎沖刷預警之方法。本論文敘述串列式光纖光柵水壓監測技術之研發以及此技術在邊坡崩塌與橋樑基礎沖刷監測預警之應用實例。

From soil mechanics point of view, under a constant total stress, the increase of pore water pressure causes reduction in effective stress and eventually soil failure. Practical problems that relate to this soil mechanics phenomenon can include rainfall induced landslides and bridge foundation scouring. The currently available technologies in field water level or pressure monitoring are difficult to apply for real-time automated measurements. Hence, even though theoretically, pore water pressure measurement can be an important parameter of a warning system in case of a potential landslide or bridge foundation scouring, it is rarely used in practice due to these deficiencies. Taking advantage of the optical fiber Bragg grating (FBG), this research developed the technology to install FBG piezometer array in the field for pore water pressure profile momnitoring. To verify its practicality, the author installed an FBG piezometer array as part of slope stability monitoring at Five Turn Point of Alishan. In this case, piezometers were spaced at 5m intervals in a single 60m borehole. Using similar techniques, FBG piezometer arrays were installed at three bridge sites along Tachia River. These installations included Pier 27 of Taiwan Highway 1 bridge, pier P27L (P30R) of National Highway No. 3 bridge and pier P5 of Tungshih bridge. Laboratory bridge model scouring tests were carried concurrent with the field installation to establish correlations between dynamic pore water pressure and scouring depth. The author developed a stress based warning method to predict the occurrence of a slope failure or bridge foundation scouring, based on real-time pore water pressure profile measurements from the field and the concept of stress path. The thesis describes the technology of FBG piezometer array, its development and cases of field applications in slope stability and bridge foundation scouring monitoring.