南化水庫加高工程材料試驗以及壩體動態分析

Abstract

1994年完工之南化水庫位於台南縣南化鄉北寮村附近的後堀溪上，現有南化水庫壩體（原壩體）為一分 區滾壓土石壩，壩高87.5公尺，壩頂長550公尺，大壩上游壩面坡度1V:3H，下游壩面坡度1V:2.5H。南化 水庫現有蓄水量約一億五仟萬立方公尺，為解決南部地區用水問題計畫將現有壩高增加二十公尺，以增 加約一億三仟萬立方公尺之蓄水量。南化水庫位於地震活動頻繁之嘉南地區。300餘年之地震歷史記錄 顯示，壩址50公里半徑範圍內至少發生22次規模大於5.5之地震，其中白河地震距離壩址僅14公里。 影響南化壩址之可能震源包括壩址西北側觸口至新化一帶之斷層系列，及壩址東南側旗山斷層至寶來 一帶之微震活躍區。原壩體在規劃階段曾進行初步之地震分析，並建議設計地震規模為7.2級，設計加 速度為0.4g，延時為40秒。研判壩址的最大加速度應介於0.36g~0.45g之間。原壩體在規劃、設計及施工 階段已累積相當數量之資料可供本計畫參考。現有之調查與試驗報告敘述之內容，包括築壩材料之靜態 剪力強度，動態三軸試驗，以及高礫石含量之上游殼層築壩材料的靜態強度試驗。 本計畫針對南化水庫增加壩高二十公尺時所面臨之「工程材料試驗以及壩體動態分析」專題進行分析 與檢討，以研究南化水庫加高設計斷面在地震力作用下之安全性，並作為南化水庫加高工程可行性之 評估項目之一。計畫之目的為： (一)確認南化水庫壩材靜態抗剪強度。 (二)量測南化水庫壩材現地波速。 (三)量測南化水庫壩材抗液化強度。 (四)量測南化水庫壩材動態非線性應力應變曲線。 (五)量測南化水庫加高築壩材料強度。 (六)決定南化水庫設計地震強度。 (七)進行壩體靜態與動態有限元素分析。 (八)確認南化水庫設計加高斷面在地震力作用下之安全性。 為達到上述之研究目的，本計畫之工作範圍將涵蓋以下項目： (一) 前期作業與成果整理。 (二) 地震危害度分析以。 (三) 設計地震之決定。 (四) 現地波速試驗。 (五) 試驗室試驗與土壤設計參數之決定。 (六) 壩體動態分析。

Nan-Hwa reservoir, completed in 1994, was located on the Hou-Chueh reek at Bay-Liao village, Nan-Hwa township of Tainan county. The Nan-Hwa dam (existing earth dam) was a zoned, roller compacted earth dam. The height of the existing earth dam was 87.5 m and the length was 550 m. The upstream slope of the earth dam was 1V:3H and the down stream slope was 1V:2.5H. The existing Nan-Hwa reservoir had a storage capacity of approximately 150 million cubic meters. In order to alleviate water shortage problems of southern Taiwan, it was decided to raise the Nan-Hwa dam by 20 m in order to increase its storage capacity by approximately 130 million cubic meters. Nan-Hwa reservoir was situated in a seismically active area commonly referred to as the Chia-Nan territory (the territory of Chia Yii and Tinan). According to seismic records collected in the past 300 years, there have been more than 22 earthquakes with a minimum magnitude of 5.5, within a distance of 50 km from the existing dam site. Out of these 22 earthquakes, the Bai-Ho earthquake was centered at merely 14 km from the dam site. The potential epicenters that may affect the seismic activities at the dam site include a series of fault zones from Tsu-Kou to Shin-Hwa, located at the northwest side of the reservoir, and areas to the south east of the dam site, from Chi-Shan to Bao-Lai where minor earthquakes occurred frequently. The preliminary seismic hazard analysis conducted during the planning stage for the existing dam suggested that the earth dam should be designed for earthquakes with a magnitude of 7.2, ground acceleration of 0.4 g, and a duration of 40 seconds. The studies concluded that the peak ground acceleration at the dam site should be in a range of 0.36 to 0.45 g. A considerable amount of engineering and research reports had been compiled through various stages of the planning, design and construction of the existing earth dam. These reports provided static shear strength parameters and cyclic triaxial test results of the earth dam construction materials. Results of quasi-static shear tests on the upstream slope construction material with high gravel contents were also available. The aim of this research project titled "Material Characterization and Dynamic Analysis for the Proposed Raising of Nan-Hwa Earth Dam" is to evaluate the safety of Nan-Hwa dam under earthquake loading conditions, after the proposed dam raise of 20 m. Results of this research will also serve as one of the items to be evaluated in the feasibility study of the proposed dam raise project. The objectives of this research include: 1. Determine the static shear strength parameters of the Nan-Hwa dam construction materials. 2. Measure the in situ shear wave velocity of the existing earth dam materials. 3. Evaluate the liquefaction potential of the Nan-Hwa earth dam. 4. Determine the dynamic stress-strain relationship of the Nan-Hwa earth dam materials. 5. Determine the shear strength parameters of the construction materials to be used in the raising of Nan-Hwa dam. 6. Determine the strength of potential earthquakes to be used in the design of Nan-Hwa dam with its new configuration. 7. Conduct static and dynamic finite element analysis for the revised cross section of Nan-Hwa dam. 8. Evaluate the safety of Nan-Hwa dam under earthquake loading conditions after the raising. The following tasks have been identified in order to accomplish the objectives of this research: 1. Compilation of results from previous studies. 2. Earthquake hazard analysis. 3. Determination of the design earthquake magnitude. 4. Field shear wave velocity measurements of the existing earth dam materials. 5. Laboratory experiments and the determination of the engineering design parameters for the materials to be used in the earth dam raising project. 6. Dynamic analysis of the earth dam.