本研究利用PFC2D軟體探討四種不同覆岩深度為300、400、500、600公尺之處置隧道於良好之岩盤中進行開挖。處置隧道為馬蹄形隧道與下方之圓柱形處置孔所構成，隧道共分三階開挖，分別為第一階段的馬蹄形隧道開挖，與下方處置槽的第二、三階段的分階降挖。首先探討四種不同初始應力(K=0.5、1、2、3)對於主隧道處置孔開挖後之影響，並延伸探討當主應力方向為傾斜45度時對於馬蹄形主隧道之影響。
本研究結果顯示只有深度達500公尺並且K值達到2之初始應力狀態或者深度達300公尺K值達3之初始應力狀態，於開挖過程中皆會有張力裂縫之產生，當主應力旋轉45度時張力裂縫最先發生於初始應力σ3之連線方向上。而馬蹄形隧道於開挖穩定過後，最需注意(危險)之測點為隧道頂拱處，此測點於開挖過程中最大主應力σ1為最大，且主隧道周圍此點最容易產生張力裂縫。

The disposal tunnels excavated in the rock with the depths of 300,400,500 and 600 meters were investigated with the software of PFC2D in this research. The disposal tunnel is composed of horse-type tunnel and the cylindrical disposal holes below. Tunnel excavation consists of 3 stages. The 1st stage is the excavation of horse-type tunnel. The 2nd and 3rd stages are the excavations of disposal tanks below stepped excavated downward. The influence of initial stresses for excavated main tunnel disposal holes were investigated with K=0.5,1,2 and 3. It is further investigated as the principal stress is 45 degree as well.
The results of research show that the tensile cracks occurred during excavation only as the initial stress of 500m deep and K value reach 2 or 300 m deep and K value reach 3. The tensile crack occurred at the initial stress σ3 direction as the principal stress oriented 45 degree. The top arch of tunnel should be noticed although the excavated horse-type tunnel is stable. It is the point of maximum principal stress σ1 during excavation and most likely to occur tensile cracks of the main tunnel.

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