Ground load on tunnels built using new Austrian tunneling method: study of a tunnel passing through highly weathered sandstone

The ground load acting on a tunnel is an important issue in tunnel design, especially when the tunnel passes through highly weathered sandstone. A systematic field-monitoring campaign was performed to investigate the ground loads on a tunnel structure, the behavior of the composite support system, and the deformation of the tunnel boundaries. The monitoring results were analyzed and compared with those of various theories, such as the whole-soil column theory and those of Terzaghi, Bierbaumer, Xie Jiaxiu, and Protodyakonov. The ground load on a highway tunnel in highly weathered sandstone does not conform to current theoretical methodologies. It was confirmed that Terzaghi's theory is suitable for estimating the peak magnitude of the vertical ground load, but differs from the field-monitoring results for ground load distribution profile. To facilitate tunnel design, a potential profile for ground loads is proposed, in which the vertical load component is 'mountain'-shaped and the horizontal component adopts a 'folded-line' pattern. The roof rockbolts are subjected to compression and should be replaced by pipe grouting that is capable of providing enhanced reinforcement and accelerating the construction schedule. The bending moments acting on the lining were found to form a 'butterfly' shape. Supplementary finite-element modeling was undertaken to explore the mechanical behavior of the tunnel lining. These results indicated that steel rebar needs to be pre-installed in both the intrados of the lining roof and extrados of the spandrels to improve the lining tensile strength.