The geology that the tunnel was bored through is largely dominated by relatively weak chalk beds. Extensive surveys were performed, including a series of boreholes, which helped build a wider understanding of the material in general. Planners decided that, unlike any of the other tunnels that were constructed along the CTRL, the construction of the North Downs Tunnel would use the modern techniques of the New Austrian Tunnelling method (NATM); in accordance with these method, the application of a sprayed concrete lining followed immediately after the mechanical excavation. Throughout the construction process, continuous monitoring was practiced to track both settlement and any ground deformations that occurred.