“…More information on the details and the development of UB and LB FELA can be found in Sloan [14]. The problems of unlined tunnels or trapdoors under plane strain conditions have been derived using FELA by some researchers in the past, such as Sloan and Assadi [15], Wilson et al [16][17][18], Yamamoto et al [19,20], Keawsawasvong and Ukritchon [21,22], Keawsawasvong and Likitlersuang [23], Keawsawasvong and Shiau [24], and Ukritchon and Keawsawasvong [25,26]. However, these studies are limited to tunnels with circular, square, rectangular, or flat shapes.…”