砂岩隧道變形性之評估-以非線性彈塑性模式初探

Abstract

因應台灣的經濟發展及交通的便捷性，重大交通建設與開發活動已逐漸向西部麓山帶地區發展。麓山帶地層以未變質的第三、四紀砂岩為主，在過去一系列的砂岩試驗結果後發現，此類岩石因膠結及壓密程度不佳，加上可能會受到遇水弱化、受剪膨脹及潛變等因素，故強度不高且容易風化，因此過去台灣的隧道工程災害，在遭遇此類型岩層時，常常受到嚴重的擠壓變形等不利於施工之災害。 有鑑於隧道開挖後的變形性與穩定性，以及過去相關工程災害的發生，皆受此類砂岩之力學特性所影響。因此，本研究以ABAQUS有限元素分析程式為工具，建立一套可合理預測軟弱岩石力學行為之數值分析模式，以砂岩非線性彈塑性模式為理論基礎，彈性變形為非線性彈性模式(Green elastic model)，塑性變形為廣義塑性模式(Generalized plasticity)。將上述模式寫入有限元素法軟體ABAQUS之組成律副程式。 進行隧道工程案例分析前，先以單元素進行三軸試驗的模擬驗證ABAQUS副程式之正確性，以檢核模式是否可描述砂岩真實試驗結果。驗證完成後，透過隧道之平面應變數值模型分析，深入探討軟弱砂岩於不同組成律分析狀態下之力學行為與變形特性，更進一步提出本研究模式可描述的特殊力學行為及變形特性，如剪應力與體積應變耦合特性。分析結果顯示，本研究模式可有效預測砂岩中隧道之變形性，供日後工程實務應用。

Western region of Taiwan is most populous and accompanied by active constructions of transportation infrastructures. Many tunnel constructions currently in progress or in planning are, or to be, constructed in sedimentary strata formed in the Tertiary Period. Due to the relatively young rock-geneses period and possibly other factors, these sedimentary strata are mostly weak rocks. In the past, these weak rocks have caused several engineering difficulties such as squeezing of constructing tunnel due to shear-induced and creeping deformations. Through a series of laboratory triaxial tests, it was found that some typical weak rocks exhibit problematic characteristics such as substantial wet weakening, shear-dilation as well as creep deformation. Such behavior is often much less significant in hard rocks. In order to understand the key behaviors and to rationally predict the deformation for design purposes, the purpose of this study is to develop a constitutive model that can involve these problematic deformational behaviors and to implement it into finite element software for engineering practice. The proposed model is a nonlinear elasto-plastic model. The elastic component is based on the theory of Green elasticity and the plastic component is based on generalized plasticity. The proposed constitutive model is further implemented into finite element software ABAQUS by using the user’s subroutine UMAT. In order to verify the validity of the proposed model, the triaxial tests of single element was first simulated. According to the result comparisons of single element analysis and test data, the model is appropriate to predict the deformation behavior of western foothill sandstone. After the verification, the excavation of tunnel was simulated using plain stain analysis. Meanwhile, other constitutive models were also adopted for comparison. The simulation exhibits that the proposed model can describe the deformation characteristics of sandstone, such as shear stress and volumetric strain coupling behavior. The results indicate that the model can effectively predict the deformation of the tunnel in sandstone; and it provides a good tool for future engineering practice.