裂隙岩體水力特性代表性單元體積研究

Representative Element Volume for Hydraulic Characteristics of Fractured Rock Mass

曹家銘(Chia-Min Tsao)

指導教授 ： 王泰典

國立臺灣大學/工學院/土木工程學研究所/碩士(2020年)

https://doi.org/10.6342/NTU202002557

摘要

近年來因地熱資源開發、地下水庫的建置以及深層核廢料處置、二氧化碳封存等議題，水文地質結合了岩體工程領域發展如火如荼。相關工程設計規劃及施工需對於水文地質以及各水力特性參數詳加了解，而岩體中的不連續面為其滲流行為具異質、異向性的主要因素，不僅造成場址的滲透特性調查評估困難，而所得參數亦常有顯著變異。Wang等人(2015)於和社井場進行現地水文地質試驗，其中屬於小尺度的單井雙封塞試驗之水力傳導係數變異達9000倍；Zhan等人(2016)將經統計分析之水力特性參數藉理論式推估場址之水力傳導係數張量，差異達8個數量級。因此，場址調查的適用尺度、現地試驗的代表性問題，勢必為岩體工程一大難題。
本研究為解決以上問題，建立一代表性單元體積(Representative Element Volume, REV)之決定程序以描述裂隙組別以及岩體之水力特性。而方法論藉推導裂隙岩體水力傳導係數理論式(包括Snow (1969)、Oda (1985)以及Wang等(2015))，按統計學誤差傳播理論推導各影響因素變異程度對求得水力傳導係數的影響程度，並建置和社井場離散裂隙網絡取樣之數值解以及現地水文地質試驗成果進行平行驗證。一方面探討場址水力特性參數對於水力傳導係數之影響程度，亦能完整描繪水力傳導係數隨尺度之量值以及變異程度。研究更針對影響水力傳導係數之關鍵參數-水力內寬研發高精度測繪技術，以取得隨尺度變動之內寬量值。
平行驗證成果顯示，水力傳導係數張量隨尺度增加也逐漸遞增，並於14至16米深度處其變異程度趨緩，可推估此範圍為場址之REV，同時已可有效描述各尺度等效滲透係數之量級。為確認該體積之適用性，研究尾聲於FracMan數值軟體採決定之REV大小建置離散裂隙網絡與等效連續體之混和模式，考慮不同試驗尺度分別模擬多孔抽水、跨孔抽水試驗，並與現地試驗成果進行討論以完備應用性驗證程序。

關鍵字

離散裂隙網絡 ； 水力傳導係數張量 ； 規模效應 ； 代表性體積元素 ； 粗糙度係數

並列摘要

In recent years, due to geothermal resource development, the establishment of underground water reservoirs and deep nuclear waste, carbon dioxide storage and other issues, hydrogeology combined with the development of rock mass engineering is in full swing. Relevant engineering design planning and construction requires a detailed understanding of hydrogeology and various hydraulic characteristics parameters, and the seepage of the discontinuities in the rock mass is the main factor with heterogeneity and anisotropy, which not only causes difficulties for the investigation and evaluation on permeability characteristics of the site, and the obtained parameters often have significant variations. Zhan et al. (2016) conducted various in-situ and labortory tests at the Heshe well site. Among them, the small-scale single-borehole double-blocking test has a hydraulic conductivity coefficient variation of 9000 times; The hydraulic characteristic parameter of statistical analysis is to estimate the hydraulic conductivity coefficient tensor of the site, the difference is up to 8 orders of magnitude. Therefore, the applicable scale of the site investigation and the representative problems of the field test are bound to be a major problem for rock engineering.
In order to solve the above problems, this study establishes a representative element volume (REV) decision procedure to describe the hydraulic characteristics of fissures and rock masses. The methodology derives the theoretical formulas for the hydraulic conductivity of fractured rock masses (Including Snow (1969), Oda (1985), and Wang et al. (2015)), and derives the degree of variation of each influencing factor based on statistical error propagation theory. The degree of influence, and the numerical solution of the discrete fracture network sampling at the Heshe well site and the results of the in-situ hydrogeological test were verified in parallel. On the one hand, the degree of influence of the hydraulic characteristics of the site on the hydraulic conductivity is discussed, and the magnitude and the degree of variation of the hydraulic conductivity with the scale can be fully described. The research is more focused on the development of high-precision surveying and mapping technology for hydraulic aperture, which is a key parameter that affects the hydraulic conductivity coefficient, in order to obtain the hydraulic aperture that varies with scale.
The results of parallel verification show that the hydraulic conductivity coefficient tensor gradually increases with the increase of scale, and its degree of variability slows at 14 to 16 meters. It can be estimated that this range is the REV of the site, and can effectively describe the equivalent of each scale and the magnitude of hydraulic conductivity coefficient. In order to confirm the applicability of the volume, the study was conducted at the end of the REV size determined by the FracMan numerical software. The hybrid model of discrete fracture network and equivalent continuum was built. Considering different test scales, the porous pumping and cross-hole pumping tests were simulated separately, and the test results are discussed to complete the application verification procedure.

並列關鍵字

Discrete fracture network (DFN) ； hydraulic conductivity tensor ； scale effect ； representative element volume (REV) ； joint roughness coefficient (JRC)

參考文獻

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