The D-shaped cross section is a commonly used tunnel cross section in underground engineering. To simulate the failure process of a D-shaped hole under deep three-dimensional (3D) high-stress conditions, true-triaxial tests were conducted on cubic granite specimens with a through D-shaped hole, and the failure process of the hole sidewall was recorded in real time. Results show that the spalling failure process of the D-shaped hole sidewall can be divided into four periods: calm, fine particle ejection, crack generation and propagation, and rock slab gradually buckling and spalling. Afterwards, symmetrical V-shaped notches were formed on both sidewalls between the corner and arch springing. The spalling failure shows tensile failure characteristics. Under high vertical stress and constant horizontal axial stress, increasing the lateral stress reduces the severity of the spalling failure and the depth of the V-shaped notch. The initial failure vertical stress of the D-shaped hole sidewall is higher than that of circular hole sidewall, and the failure of D-shaped hole sidewall is mainly characterized by static failure. The failure of the circular hole sidewall is a more severe dynamic failure. When the vertical applied stress is the maximum principal stress, the position of the V-shaped notch tip is 0.20–0.25 h (h is the height of the D-shaped tunnel) from the tunnel floor, whereas that in the circular tunnel is 0.5 d (d is the diameter of the circular tunnel) from the tunnel floor. Specific support schemes should therefore be designed for tunnels with different cross sections according to the damage location, depth of failure zone, and severity of failure.

中文翻译：

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硬岩三维高应力条件下D形隧道剥落破坏试验研究

D形断面是地下工程中常用的隧道断面。为了模拟深三维 (3D) 高应力条件下 D 形孔的破坏过程，对具有通 D 形孔的立方花岗岩试件进行了真三轴试验，以及孔侧壁的破坏过程被实时记录。结果表明，D形孔壁剥落破坏过程可分为平静期、细粒喷出期、裂纹产生与扩展、岩板逐渐屈曲和剥落四个阶段。之后，在拐角和拱形弹簧之间的两个侧壁上形成对称的V形凹口。剥落破坏表现出拉伸破坏特性。在高垂直应力和恒定水平轴向应力下，增加横向应力会降低剥落失效的严重程度和 V 形缺口的深度。D形孔侧壁的初始破坏竖向应力高于圆形孔侧壁，D形孔侧壁的破坏主要表现为静力破坏。圆孔侧壁失效是一种较为严重的动态失效。当竖向外加应力为最大主应力时，V形槽口尖端位置距隧道底板0.20～0.25 h（h为D形隧道的高度），而圆形隧道中的位置为0.5 d（d 是圆形隧道的直径）距隧道底板。因此，应根据损坏位置、破坏区域的深度和破坏的严重程度，为不同截面的隧道设计具体的支护方案。