砂岩受剪引致異向軟化及依時組成模式初探

Abstract

由於台灣特有地質背景與環境，西部麓山帶之軟弱岩層具有受剪體積大量膨脹與潛變等現象，進行地下工程開挖時往往產生大量擠壓變形進而造成施工災害，一般大地材料之變形多為非線性之行為，使用一般等向線彈性模式來預估大地材料之變形多會錯估其變形量，因此發展一適用於大地材料之組成律有其必要性。 本研究根據前人對於西部麓山帶砂岩三軸實驗之結果，歸納砂岩之力學特性與變形行為，發現砂岩於純剪應力下會有變形模數軟化與體積應變偶合之異向行為，且在潛變實驗下此類變形特性仍然持續存在，根據此特性，本研究以模數受剪軟化與異向性因子之觀念，建立一套異向軟化組成模式，並加入黏彈性之理論發展依時之異向軟化模式，最後配合軟體撰寫此模式之副程式，應用於數值分析軟體進行隧道開挖之分析與探討。 模式之部分以三個參數體積模數K、剪力模數G及異向性因子β模擬其即時變形行為，依時變形以體積潛變模數Kc、剪潛變模數Gc及黏滯係數η進行模擬，另外本研究亦建議一套參數迴歸之流程，藉由幾組純剪三軸實驗與潛變實驗，可迴歸本模式全部之參數，即時變形部分，本模式針對不同體積應力與不同應力路徑之模擬皆有不錯之模擬效果，依時變形部分本模式在高剪應力預估砂岩之潛變量與實驗結果相比略有差異，但其整體趨勢上與實驗結果皆相當一致。 數值模擬方面，將模式撰寫成可搭配分析之副程式，經單元素測試程式碼無誤後，進行隧道開挖分析，比較本模式常用跟線彈性模式，等向軟化模式與Drucker-Prager模式之分析結果，其結果顯示，本模式分析隧道周圍為一均勻向內擠壓之情況，而預估之仰拱、頂拱與側壁變形量皆大於線彈性模式與Drucker-Prager模式分析之結果，且若考慮岩體材料隨時間，則隧道之變形量又隨之增加。 關鍵字：砂岩、組成模式、軟化、剪脹、潛變

Owing to the special geological background of Taiwan, weak rock in western Taiwan sedimentation strata has shear dilation and creep deformational behaviors. During the underground excavation, often have a large amount of compression deformation then result in disaster. General deformed characteristics for geo-materials are nonlinear. Using linear elastic model to compute the deformation of geo-materials will always misestimate. So it is necessary to develop a constitutive model that apply to geo-material. This research, based on the results of tri-axial test with the sandstone in western Taiwan that performed by Rock Mech. Lab. of NTU. To sum up the following deformation behavior of sandstone: (1) under shear loading will transform from its original isotropy to a shear-induced anisotropic (2) the shear dilation behavior will still exist under creep test According to the deformation behavior of sandstone, the concepts of shear modulus softening and anisotropic factor are used to develop an anisotropic softening model. And combine with the visco-elastic model to develop a time dependent model for anisotropic softening of sandstone. Then write subroutine of the model to apply it to numerical analysis. The model with three parameters bulk modulus K, shear modulus G and anisotropic factor β to simulate instant deformation and another three parameters Kc, Gc and viscosity η to simulate creep deformation. With several sets of pure shear tests and creep test we can get all the parameters. Through the simulations of different confined pressure and different stress path tests, it is found that good agreements with instant deformation of sandstone, but the creep deformation in high shear stress state are have some different. However the trend of simulations are consistent with the experimental results. Apply the model to finite element method of ABAQUS program. Writing user subroutine to define the material's mechanical behavior. Before analyzing the tunnel cases, single element test was applied to verify the accuracy of the subroutine. Then apply the model to tunnel excavation analysis. Comparing prediction results between the anisotropic softening model, elastic model and Drucker-Prager model. The numerical results show that, with the proposed model there is a uniform compression zone around tunnel section and the inward displacements (including the crown, sidewalls and invert) based on anisotropic softening model are larger then elastic model and Drucker-Prager model. Keywords: sandstone, constitutive model, softening, shear dilation, creep.