ABSTRACT
In twin tunnels, securing an appropriate pillar width (spacing between tunnels) is crucial. If the pillar width is too wide, it increases the cost of land acquisition, while if it is too narrow, it can affect tunnel stability. Typically, the appropriate pillar width varies depending on factors such as the size of the tunnel, ground weathering conditions, and geological profile. Therefore, when determining the pillar width, it is essential to consider tunnel stability while minimizing project costs. Additionally, the pillar width can be adjusted by reinforcing it with structural materials. Such considerations are particularly important at the tunnel portals where tunnel excavation begins. The stability of tunnel portal can be significantly influenced by the surface weathering and unconfined boundary conditions. At the tunnel portal stress release due to unconfined condition the cover depth is of significance, therefore, selecting the pillar width is of paramount in portal design. To improve the safety of tunnel portals, tie-bolts connecting two tunnels through pillar width are sometimes adopted. In this study, the mechanical behavioral of pillar width at the tunnel portal was investigated through physical model experiments, along with an examination of the failure mechanism of pillar. The investigation of allowable cover depth causing initial cracks in the pillar revealed a linear increase with an increase of cover depth. Furthermore, it was observed that the maximum earth pressure the pillar can support increases exponentially with an increase in pillar width. The anchoring of tie-bolts showed an effect of approximately maximum 30% enhancement in the allowable cover depth. It was also observed that the allowable cover depth, ensuring the safety of pillar, increased linearly with increasing pillar width.
