軟弱岩石之張力行為

Abstract

因為張力強度甚小且直接張力試驗施做困難，軟弱岩石之直接張力行為並未被完全瞭解，及應用於實際工程設計上。本研究設計一適合軟弱岩石之直接張力夾具及附屬之變形量測設備，除可得到直接張力強度外，並可量測受拉下岩石之伸張量。此外，並配合巴西試驗得到軟岩之間接張力強度，藉此初步探討軟岩之間接與直接之相關性。本研究採用之軟岩試體來自台中大坑地區之新鮮砂岩塊，經實驗室鑽取後，進行力學試驗，其岩心張力強度介於0.05Mpa至0.58MPa之間，單壓強度約為張力強度之15-40倍，而軟岩之張力強度亦因含水量增加而有降低之趨勢。由壓密不排水之三軸試驗之結果回歸分析，Hoek-Brown破壞準則對於直接張力強度之預測較為合理。直接張力試驗得之應力-應變曲線具非線性之關係，此關係可用雙曲線函數來模擬，且在應變量0.1%以下即產生張力破壞。微小應變階段0.005%之變形似乎較具線性，其楊氏模數較壓力狀態之楊氏模數低，顯示軟岩具有材料之雙模數特性。

Due to the difficulties of direct tensile testing, neither direct tensile behaviors of weak rocks are not understood completely, nor are tensile properties adopted for rock engineering design. A suitable direct tensile grip for weak rock has been developed for not only measuring the direct tensile strength but also the axial displacement during tests. Besides, a series of direct tensile tests and indirect tensile tests (Brazilian tests) was conducted using a weak rock. Then, the experimental results were adopted for investigating the direct tensile behaviors and the relationship of indirect and direct tensile strength.All tests were performed on specimens cored from rock blocks collected from a construction site at the Dakeng hilly area of the Taichung City, Taiwan. Experimental results indicate that the direct tensile strength is about 0.05 MPa - 0.58 MPa, the average uniaxial compressive strength is15 - 40 times higher than the direct tensile strength. Also, the tensile strength decreases with the increase of the water content in the specimens. Incorporating with the results of consolidated undrained triaxial tests and using a regression analysis, it is found that the experimental data match well with the Hoek-Brown failure criterion. Hence, the author recommends that the criterion can be used for predicting the direct tensile strength of weak rocks.Stress-strain curves obtained from direct tensile tests exhibit nonlinear relationships. Most of the specimens failed while the axial strain is lower than 0.1 %, and the curve are linear for the axial strain less than0.005%. The curves can be simulated well by a hyperbolic function. Also, the characteristics of material bi-modularity exists in the rock which Young's modulus in tension is lower than compression.