極深覆岩下隧道之岩石力學問題

Abstract

近代許多岩石工程均可能涉及極深之覆岩狀況，隨著地下工程深度愈來愈深，工程中可能遭遇到的岩壓與岩溫也隨之上升。因此本研究擬探討在溫度與壓力升高下岩石之力學性質，與深覆岩隧道受地溫影響下岩體與噴凝土襯砌所導致之熱應力做一初步探討。 本研究對台灣東部蘇花段所鑽取之板岩與大理岩岩樣進行一系列控制溫度與壓力之三軸試驗以模擬現地岩石在可能遭遇之岩壓與岩溫下之力學性質。試驗結果顯示在100℃溫度範圍內板岩與大理岩均呈現脆性力學行為。以溫度對岩石力學性質之影響來看，試體受本身性質的隨機差異影響較溫度因素來的大。然而整體而言，圍壓增加會增強岩石強度，溫度增加則使岩石強度有降低之趨勢。由試驗中並且求得板岩與大理岩之熱膨脹係數約在4~9×10-5(1/℃)之間。 此外，本研究以有限差分程式FLAC來模擬深覆岩隧道因通風降溫而導致之溫度場分佈及隧道周圍岩體與噴凝土襯砌所受之熱應力。由分析結果來看在受地溫影響下之深覆岩隧道，在開挖通風後大約需56小時岩體溫度可重新達到平衡狀態。 隧道通風降溫對開挖面附近岩體而言極可能因此使張力破壞範圍增加，而噴凝土在硬化後所產生之熱應力在深覆岩狀況下可能影響不大。

Engineering projects of deep tunnels may involve site conditions of a large overburden. As the overburden depth increases, both the temperature and pressure tend to elevate. This research aims to (1) study the mechanical properties of rock at elevated pressure and temperature, and (2) to explore the approach for estimating the temperature and stress distribution in rock mass near an excavated tunnel before and after ventilation. The study carried out a series triaxial compression tests on specimens of slate and marble at elevated temperatures and confining pressures (5~40MPa). The results of these tests indicate that (1) these specimens exhibit brittle behavior at elevated temperature range from 25~100℃; (2) the influence of the random variation and disturbance of the sample is greater than the influence of temperature (range: 25~100℃). In general, the rock strength increases with increasing confining pressure, and decreases with increasing temperature. The thermal expansion coefficient of the specimens lies within 4~9×10-5(1/℃) . The study makes use of the numerical methods to study the temperature and stress distribution in rock mass near an excavated tunnel before and after ventilation. First, the initial distribution of temperature around a tunnel is estimated on the basis of steady-state thermal analysis. Ventilation following an excavation is then modeled by specifying temperature boundary conditions along the tunnel surface. Distribution of temperature and stress-state near the excavated tunnel is calculated and analyzed. From the result of the numerical study, the following is found. In principle, the ventilation may result in higher overall unstability of the rock-mass near the excavated tunnel. Condition of tensile failure due to thermal stress in the rock-mass near the excavation face is possible. The calculated result shows that the thermal stress in the shotcrete is not very significant.