節理岩體微觀力學模式之研究

Abstract

節理岩體由岩石材料（岩材）與節理面組成，其力學行為受岩材破壞與節理面滑動的影響而呈現非線性與異向性之特性，尤其是岩材中局部的微小破裂與節理面剪動時節瘤的破斷，對整體的受力行為影響甚鉅。因此，基於微觀角度，採用非連續體分析－個別元素法－探討岩體力學行為是有效的研究途徑與重要課題。然而目前岩石力學領域最廣泛應用的個別元素分析軟體－PFC中，對於描述岩材顆粒膠結之「平行鍵結模式」以及節理面滑動的「平滑節理模式」皆未臻完善，無法適當呈現節理岩體力學行為，致使模擬可信度不佳。據此，本研究以個別元素法PFC2D軟體作為模擬節理岩體的主軸，自微觀角度出發，分別改善岩石材料及節理面力學行為之模擬方式，並將之耦合，進而提出更可靠之節理岩體力學行為數值模式。 在節理面力學行為模擬的改良方面，本研究以Barton剪力強度模式做為模擬之力學基準，將節理面的粗糙度特性納入考量，使其可反映粗糙度及節瘤造成的影響，同時針對個別元素法於模擬節理面時之缺失進行改良，分別進行以下修正：(1)節理面接觸面積之剪力勁度修正；(2)節理面剪動狀態之剪力勁度修正；(3)漏失剪力增量之補償及(4)正向力之重新分配。透過上述修正，本研究提出可精準描述Barton剪力強度模式之個別元素法節理面模式－「粗糙節理模式」。 在岩材力學行為模擬的改良方面，透過觀察岩石薄片的微觀組構，本研究假設岩材為不同尺寸顆粒聚合而成之材料，且顆粒間膠結近似於雙凹形狀；以此為基礎，經由Dvorkin理論之彈性解，及本研究提出之三項修正方式：(1)運移拆解；(2)應力調整及(3)演算改良，藉由一系列不同幾何形狀的顆粒膠結彈性分析，可探討其在壓力、張力、剪力及彎矩作用下微觀的力學行為，進而發展能合理描述岩材微觀性質的「雙凹鍵結模式」。 最後，本研究結合「粗糙節理模式」與「雙凹鍵結模式」，提出具有微觀力學理論基礎的「節理岩體微觀力學模式」，可藉由節理面及岩石材料之微觀參數完整描述節理岩體的巨觀力學行為，並能觀察其受力時岩材局部破裂及節理面節瘤破壞等微觀演變過程。

The mechanical behaviors of jointed rock masses are affected by rock material failure and joint surface sliding, showing nonlinear and anisotropic characteristics. Among the influencing factors of rock mass mechanical behaviors, the micro-cracks in rock materials and asperity ruptures under joint shearing are two major factors. Therefore, it is important to investigate the mechanical behaviors of jointed rock masses from microscopic perspective using discontinuum analysis – distinct element method. However, in the widely-used software distinct element method –Particle Flow Code (PFC), the “parallel bond model” designed to describe grain cementation and the “smooth-joint model” to handle joint surface sliding are too simplified and unreliable to represent the mechanical behaviors of rock mass appropriately. Thus, this study used PFC as the main subject to simulate jointed rock masses, to improve the simulation of mechanical behavior of rock material and joint surface in microscopic behavior. Finally, this study proposed a more reliable numerical model of jointed rock mass. To improve mechanical behavior of joint plane, this study considered the characteristics of joint roughness and used Barton’s shear strength model as the basis of mechanism in, thus it can reflect the influence of roughness and asperities. On the other hand, the limits of distinct element method has been ameliorated, and following modification has been adopted: (1)The modification of shear stiffness based on joint contact area; (2)The modification of shear stiffness based on joint sliding state; (3)The compensation of shear force increment and (4)The redistribution of normal force. Based on above modifications, this study proposed a joint model in PFC – rough-joint model – which can reflect Barton’s shear strength criterion precisely. To improve mechanical behavior of rock material, this study assumes a rock material is composed of particles with different sizes based on the observation of rock thin section, and the shape of cement between particles can be treated as biconcave shape. Based on this assumption, from the elastic solution of Dvorkin theory and three modifications proposed by this study: (1)Motion decomposition (2)Stress redistribution and (3)Algorithm improvement, the mechanical behavior of biconcave shape bond can be described by a series of particle-cementation analysis with different geometries under compression, tension, shearing and bending situations, agree well with the proposed “biconcave bond model”. Finally, this study combined the “rough-joint model” and the “biconcave bond model” to propose a “jointed rock masses microscopic mechanical model” that is able to well describe the macroscopic mechanical behaviors of jointed rock masses based on the micro parameters of joint surface and rock material, and to observe the microscopic evolutions such as local cracks in rock material and asperity ruptures in joint surface during loading.