裂隙延伸於岩楔穩定性之影響-以分離元素法探討

Abstract

岩體內所存在之不連續面可以視為岩體中的預裂隙，對於岩體強度有很大的影響。若能掌握並預測在不同應力條件下，裂隙的發展趨勢，對於岩石邊坡穩定工程有相當大的幫助。因含預裂隙之天然岩材不易取得、岩材組成具變異性、實驗條件控制不易且裂隙觀察困難等，故本研究採用分離元素法為基礎之PFC3D軟體進行模擬及分析，相較於連續體分析模式，分離元素法具有元素可分離的優勢，符合本研究對於裂隙延伸行為探討之需求。 本研究包含兩個部分，第一部分係以數值模擬與實驗室基本力學實驗所得之結果進行比對與驗證，確認微觀參數與數值模型之合理性。首先模擬貫穿巴西圓盤試驗(Central cracked Brazilian disk test, CCBD)，驗證以平滑節理模式模擬預裂隙，可以準確的模擬裂隙的產生及破壞時的作用力。進一步，分析雙預裂隙單軸拉伸試驗，探討不同預裂隙幾何配置下，對於裂隙延伸行為之影響。分析結果發現，兩預裂隙傾角為負值時，裂隙容易發展並聚結，而裂隙的聚結是導致岩體強度降低的一個重要因素。最低的張力強度，常發生在預裂隙傾角-30度至10度這個區間。第二部份為案例分析，案例為位於台二線83.6K處之岩坡，其破壞除了由於兩相交節理面導致岩楔滑落之外，在破壞面頂端發現有岩橋貫穿的痕跡。此狀況與上述雙預裂隙受到拉應力下，裂隙延伸導致破壞的情況相似。利用攝影測量方式進行節理判斷，並建立一含雙預裂隙之岩坡模型，進而探討不同預裂隙傾角下，岩體強度之變化。分析結果可繪製出一強度倒U形曲線，岩橋強度落於曲線上半部為安全區域，曲線下半部為破壞區域。當預裂隙傾角介於-20度到20度之間時，維持穩定所需之岩橋強度最高，表示此時的預裂隙位態最不利。

The discontinuities in the rock mass can be regarded as pre-cracks, which have a great influence on the strength of the rock mass. If we can predict and control the propagation of fracture under different stress conditions. It will have great help to the rock slope stabilization. Because of the natural rock materials with pre-cracks are difficult to obtain, the experimental conditions are difficult to control and the fractures are difficult to observe. Therefore, PFC3D based on the distinct element method is used for simulation and analysis. Distinct element method has the advantage of separable elements, which is the advantage of study fracture propagation. This study consists of two parts. The first part is to compare and verify the results obtained by numerical simulation and laboratory experiments in order to confirm the rationality of microscopic parameters and numerical models. The Central cracked Brazilian disk test (CCBD) was simulated to verify the pre-cracks which simulated in smooth-joint mode. It can accurately simulate the failure force and fracture. Then, the double pre-crack uniaxial tensile test was analyzed to investigate the effect of different pre-crack geometry on the fracture propagation. The analysis results show that when the angle is negative, the fractures are easy to propagation and coalescence. The coalescence of the fracture has a great impact on the strength of rock mass. The lowest tensile strength often occurs in the interval of pre-crack inclination -30 degrees to 10 degrees. The second part is a case study. The case is a rock slope located at 83.6K on the Provincial Highway No. 2. The damage is not only due to the slip of the rock wedge due to the two intersecting joints, but also traces of rock bridge penetration at the top of the failure surface. This condition is similar to the case where the double pre-crack rock mass is subjected to tensile stress and the fracture propagation causes damage. A rock slope model with double pre-cracks is established to investigate the variation of rock mass strength under different pre-crack dip angles. The analysis results can draw an inverted U-shaped curve. If the strength of the rock bridge falls in the upper half of the curve, it will not be destroyed. It called the safety zone. On the contrast, the lower half of the curve is the failure zone. When the pre-crack dip angle is between -20 degrees and 20 degrees, the strength of the rock bridge required to maintain stability is the highest, indicating that the pre-crack at this position is the most unfavorable.