裂隙岩體滲流離散模式之研究

Study on discrete seepage model for fractured rock masses

江誌偉(Chih-Wei Chiang)

指導教授 ： 黃燦輝

共同指導教授 ： 王泰典(Tai-Tien Wang)

國立臺灣大學/工學院/土木工程學研究所/碩士(2008年)

https://doi.org/10.6342/NTU.2008.02062

摘要

地下開挖工程，諸如隧道、放射性廢料貯置場及液化天然氣儲存槽等，皆需建構適當的岩體行為描述模式，以評估、分析、預測或監控其工程行為。傳統上岩體工程行為可依處理問題的特性，大多於溫度場(thermal field)、滲流場(flow field)、應力場(mechanical field)或化學場(chemical field)個別加以描述，然而，滲流場及應力場的問題常為主導岩體工程安全、穩定的關鍵，因此，適當地描述岩體滲流－應力－應變相互關係的水力－力學耦合行為，不僅為探討岩體行為之基礎，更為完整評估岩體工程的重要課題。
本研究利用MATLAB程式語言撰寫裂隙岩體水力－力學耦合模式，首先透過序率途徑(stochastic approach)，依破裂面空間分佈與幾何特性參數（破裂面中心點位、破裂面跡線長度、破裂面位態及破裂面內寬）生成裂隙網絡(discrete fractured networks, DFN)後，繼而求得流體有效的通道－滲流網絡，再採用定率途徑(deterministic approach)，依循質量守恆與達西定律，求解通道中各節理之水頭，並據以計算裂隙岩體之滲透係數張量。
本研究程式可由使用者自行選擇水力行為或水力－力學耦合行為之理論基礎作分析。利用水力行為模式，將節理面內寬以水力內寬代替計算；利用水力－力學耦合行為模式，不僅藉由節理面力學內寬轉換為水力內寬提高準確性，更能探討因應力變化而造成岩體滲流行為改變之影響。而分析過程亦可依使用者自行選擇分析區域大小及空間位置，求得分析區域裂隙岩體之滲透係數。利用本模式自由調整觀測尺度及區域位置的方式，即可較確切地探求代表性體積元素(RVE)之尺寸。此外，裂隙岩體的滲透係數又會隨應力的影響而引致滲透係數異向性的變化，因此本研究亦探討裂隙岩體隨覆蓋深度引致滲透係數異向性的變化及參數對於滲透係數異向性之影響。
研究結果顯示：節理面受正向應力作用時，JRC及JCS將會影響滲透係數，但JRC估計的準確性將扮演計算上的關鍵因素。節理面受剪力作用時，殘餘摩擦角對於受剪作用節理面之滲透行為影響極低。破裂面幾何參數將會影響岩體之滲流行為，節理內寬為滲透係數大小最主要的影響參數，而破裂面位態對岩體滲流異向性具有高度影響。岩體覆蓋層至一固定深度，其滲透係數變趨於穩定。分析區域尺寸越小，滲透係數變異越大，選取尺寸越大，所求滲透係數變越趨於穩定，對於相同的選取尺度下，所求滲透係數變會隨著觀測位置的不同而具有一定的變異程度。間距密度越低，穩定之尺寸範圍越大；跡線長度越長，穩定之尺寸範圍越小；跡線長度越短，穩定之尺寸範圍越大。側向岩壓力係數及粗糙度係數皆會造成隨深度變化而引致滲透係數的異向性，且又以節理面粗糙度係數影響為大，此結果和初始力學內寬有關，另對於裂隙位態而言，其隨深度變化對於滲透係數的異向性影響則較小。

關鍵字

破裂岩體 ； 離散裂隙網絡模式(DFN) ； 水力－力學耦合 ； 代表性體積元素(RVE) ； 滲透係數異向性

並列摘要

It is crucial to well know on the behaviors of rock masses for underground utilization, such as the tunnel, radioactive waste material store and the liquefied natural
gas stores etc., all need to build the appropriate model for the description of behaviors of rock masses, so as to estimate, analyze, predict or control all sorts of project
behaviors. Traditionally, the behavior of rock masses can be described specifically in accordance with the characteristic of handling problem such as thermal field, flow field, mechanical field or chemical field. However, the problems of flow field and mechanical field always lead the security of fractured rock engineering. Therefore, the appropriate prediction on the flow rate, stress and deformation of fractured rock masses, i.e., the
hydro-mechanical coupled behavior, is not only the base for engineering design, but also the key point for its performance assessment.
In this study, the 2-D discrete fractured networks (DFN) model was performed through MATLAB programming language. The discrete fractured networks model was generated according to geometric parameters of fractures, such as the locations of centers of fractures, trace lengths, orientations and apertures, and they also follow their
own distribution type. Furthermore, the flow field in the rock was solved using Darcy’s Law and mass-balance equation with specified boundary conditions. Then, it based on the concept of equivalent continuum to obtain the hydraulic conductivity of a zone which was selected by users. On the basis of this model, this research considers the permeability of rock mass and to probe into their influences on the mechanical and hydraulic behaviors.
Users can select the theories of hydraulic behavior or hydro-mechanical coupled behavior free in this 2-D discrete fractured networks model. Using the theory of
hydraulic behavior, it will calculate with hydraulic apertures instead of mechanical apertures. Using the theory of hydro-mechanical coupled behavior, it not only employs mechanical apertures to advance the accuracy compared with using the theory of hydraulic behavior, but also probe into the variations of permeability of rock mass
caused by stresses changing. Under analytic process, users also can freely choose the zone and size which they want to observe and obtain the hydraulic conductivity of rock
mass in the zone. Utilizing this way which freely choose the zone and size, we can obtain the size of representative volume element (RVE) exactly. Additionally, the permeability of fractured rock mass would cause the permeability anisotropy with the change of stresses. Thus this research also probe into the change of permeability
anisotropy caused by overburdenend and caused by parameters.
The results of study show that when the compressed normal to the joint, JRC and JCS would affect the hydraulic conductivity. And the accuracy of JRC will be a key
factor. Under shear loading, the residual friction angle affects the hydraulic behavior of VII joints hardly. The hydraulic conductivity tends toward stability when the overburden is deep to certain depth. Utilizing the way which the observed zone’s size and position cooperate each other, the size of representative volume element (RVE) would be obtained exactly. The observed zone’s size chooses smaller, the more variable hydraulic conductivity is. The observed zone’s size chooses larger, the more steady hydraulic conductivity tends toward. The density of spacing is lower, the larger range of steady size is. The density of spacing is higher, the higher k value is. The trace length is longer, the smaller range of steady size is. The trace length is shorter, the larger range of steady
size is. The coefficient of lateral earth pressue and JRC could cause the permeability anisotropy with the change of overburdened, but the orientation of fractured rock mass
influence the permeability anisotropy smaller.

並列關鍵字

fractured rock masses ； 2-D discrete fractured networks (DFN) ； hydro-mechanical coupled behavior ； representative volume element (RVE) ； anisotropic permeability

參考文獻

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