Asymmetric deformation characteristics and mechanical behavior for tunnels in soft-hard inclined contact strata under high geo-stress: a case study

The problem of large deformation for soft rock has always been difficult in tunnel construction under high geo-stress. Previous studies have mainly focused on the single stratum, while the soft-hard contact stratum is often encountered after tunnel excavation. There are limited studies on the large deformation caused by tunnels in soft-hard inclined contact strata. This paper investigated a case on asymmetric deformation of surrounding rock and unequal pressure behavior of supporting structure in Dongmachang I tunnel at the Chenghai active fault zone. Through the in-situ stress test and three-dimensional transformation calculation, we found that the tunnel site area was dominated by horizontal stress and was in extremely high geo-stress. Besides, the analysis of field monitoring and numerical simulation pointed out that maximum squeezing deformation and pressure occurred at the left spandrel of the tunnel, and the asymmetric deformation disaster of surrounding rock and the asymmetric compression failure of secondary lining were caused by dolomitic broken limestone in soft and hard inclined contact and high geo-stress. Notably, the minimum safety factor at the left spandrel was only 0.7, which clearly showed that the secondary lining had been in a failed state. Consistent with the actual damage result, this study vividly represents the damage to the secondary lining under the biased pressure on site.