Field test on the construction deformation characteristics for a loess highway tunnel at the shallow portal section

Based on a loess highway tunnel project, the temporal distribution rules of ground settlement, vault settlement and peripheral convergence for the loess highway tunnel at shallow portal section are studied with the field test method, and the statistical analysis predictive model for the loess tunnel construction deformation is established combining with the actual measurement data. The results show that (1) the construction deformation of the loess tunnel presents significant time and spatial effects, and the temporal distribution curve can be expressed by an exponential function. (2) The ground settlement shows the growth movement with time, approaching stability after about 60 days, and the maximum ground settlement, w_max, ranges from −30.78 to −105.20 mm. (3) The transverse ground settlement curve is of the trough shape, the width of trough is around 3−5 times that of B, and the ground loss rate caused by tunnel excavation ranges from 0.74% to 3.08%. (4) The vault settlement and peripheral convergence experiences successively three stages: linear growth stage, continuous deformation stage and steady development stage, and the deformation of linear growth stage accounts for more than 60% of total deformation. (5) The maximum vault settlement, v_max, ranges from −17.1 to −201.1 mm, and its variance estimation for sampling is −51.53, −65.11 based on the 95% confidence interval. The maximum peripheral convergence, u_max, ranges from −12.1 to −122.0 mm, and its variance estimation for sampling is −35.08, −43.39 based on the 95% confidence interval. It is suggested that the reasonable reserved deformation values of grade Ⅴ surrounding rock loess tunnel range from −100 to −150 mm. (6) The rate curves of vault settlement and peripheral convergence increase first, and then decreases sharply with time. The steady displacement rate of vault settlement (Δv) and peripheral convergence (Δu) ranges from 0.05 to 0.80 mm/d and from 0.02 to 0.60 mm/d, respectively.