非彈性應變回復法評估花蓮和平地區及彰濱工業區之現地應力場

Abstract

現地應力場為地質科學與大地工程之重要資訊，對於地下結構物的設計、天然資源的開發與廢棄物的地下儲藏皆扮演重要的角色。現地應力場( in-situ stress )是由地質營力與重力場長期交互影響累積的結果。而影響現地應力的因素有很多，目前對於現地應力的成因尚無詳細完整的理論基礎，而實際量測現地應力為不失為有效的解決辦法。 台灣島為歐亞板塊和菲律賓海板塊碰撞聚合作用的造山帶，是板塊運動活躍的活動構造區。花蓮和平地區位於台灣東北部的脊樑山脈，正好處於菲律賓海板塊與歐亞大陸板塊隱沒反轉的轉折帶上，造山演化史複雜。台灣西部彰化縣濱海工業區的崙尾區則是屬於填海造陸區域，地下的地層以西部麓山帶的地層為主，地層由淺至深分別為：頭嵙山層、卓蘭層、錦水頁岩、桂竹林層、觀音山砂岩、打鹿頁岩和北寮層。 本研究使用花蓮和平地區的變質花崗岩及彰濱工業區沉積岩不同深度的岩芯以應力釋放產生的非彈性應變回復來評估這兩地區三維現地應力場隨深度的變化。實驗結果顯示這兩處的應力場皆是正斷層應力場，拉張方向大致是南北向拉張為主。和平地區的最大、次大與最小主應力以及軸差應力的應力梯度分別為26MPa/km、22MPa/km與19MPa/km，其回歸係數皆達0.9以上，其軸差應力梯度約為7MPa/km，至於彰濱工業區最大、次大與最小主應力的梯度分別為22MPa/km、18MPa/km與17MPa/km，其回歸係數皆達0.8以上，其軸差應力梯度約為5MPa/km。另外，利用ASR結果推導出側向應力係數與應力比值的經驗公式可用來評估實驗場址及其附近的應力場。同時藉由彰濱ASR量測結果與岩石力學資料可進一步推估造成岩體破裂時的最大孔隙水壓，以及二氧化碳的最大灌注壓力。 本研究的成果可以探討ASR技術於台灣現地應力量測的可能性與前瞻性，並期望未來重要深井工程與基礎工程能以鑽孔岩芯進行非彈性應變回復實驗來評估應力場資訊，提供學術界與工程界參考與使用。In-situ stress is an essential information for geological science and geotechnical engineering and plays an important role for the design of underground structures, natural resource exploration and underground storage of waste. In-situ stress is long-term interaction result among geological processes and the gravitational field. Although many factors can affect in-situ stress, so far the origin(s) of in-situ stress is not theoretically established yet. The measurement of in-situ stress can be a practically good solution. Taiwan is an orogenic belt of oblique collision between the Eurasian Plate and the Philippine Sea Plate, known as a mobile tectonic region. Heping of Hualien is located in the northeastern part of the Backbone Range, where the reversal of the Philippine Sea Plate subduction polarity take place and structural history is complex. The Chang-Hua Coastal Industrial Park is a part of land reclamation area in the western Taiwan. Underground formations from shallow to deep similar to that of the Western Foothills are consisted of the Toukoshan formation, Cholan formation, Chinshui shale, Kueichulin formation, Guanyinshang formation, Talu shale and Peliao formation. In this study, we used retrieved cores with different depths from meta-granite in the Hualien Heping and sedimentary rock in the Chang-Hua Coastal Industrial Park to evaluate the variation of three-dimensional in-situ stress with depth via anelastic strain recovery (ASR) method. Results show that the stress field is normal faulting with N-S extension in both places. In Hualien Heping, the maximum, intermedium and minimum principal stress gradient is 26MPa/km, 22MPa/km, and 19MPa/km, respectively. Their regression coefficients are all larger than 0.9. The gradient of differential stress is 7MPa/km. In the Chang-Hua Coastal Industrial Park, the maximum, intermedium and minimum principal stress gradient is 22MPa/km, 18MPa/km and 17MPa/km, respectively. The regression coefficients are over 0.8. The gradient of differential stress is 5 MPa/km. In addition, empirical equations of lateral stress coefficients and stress ratio deduced from ASR results can used to characterize the site aspects and evaluate the stress field nearby. Combining ASR results with rock mechanic data in the Chang-Hua Coastal Industrial Park can further estimate the maximum fluid pressure and the maximum injection pressure of carbon dioxide for rock fracturing. Results of this study can be used to evaluate the application and feasibility of ASR in Taiwan. Hopefully, the assessment of in-situ stress via ASR on retrieved cores from borehole and foundation engineering can provide information for academic and engineering reference and utility.