ANALYSIS OF THE EFFECT OF DEEP FOUNDATION PIT EXCAVATION ON UNDERLYING METRO TUNNELS

The foundation pit project of Tianhan Grand Theatre's underground commercial construction is located above Line 1 of Changsha Metro and the minimum distance between the pit's bottom and the tunnel's roof is about 6.2m. The pit's bottom is situated in the round gravel layer in which the groundwater is rather abundant. Considering the impacts of dewatering on the surrounding environment, the enclosed water-resisting curtain combined with anti-floating anchor wires was applied to this project. The influence factors of the bottom soil resilience include soil unloading induced by the excavation and dewatering, the groundwater seepage and the application of anti-floating anchor cable. Thus, how to calculate and analyze the impact on underlying metro tunnels becomes the emphasis and difficulty in this project. Two common methods for estimating the spring back deformation were used to calculate the bottom's resilience value and tunnel uplift displacement. Then, the finite element software MIDAS GTS was employed to established a three-dimensional numerical model to simulate three types of engineering conditions. Case 1 took no account of the effect of groundwater seepage and didn't adopt anti-floating anchor wires. Case 2 considered the coupling of seepage and stress but didn't adopt anti-floating anchor wires. Case 3 considered not only the coupling of seepage and stress but the effect of anti-floating anchor wires. The comparative result shows that when the curtain has a lower permeability, the simulation result tends to be uniform regardless the influence of the coupling of seepage and stress. Besides, the calculating result of actual criterion is close to the simulation result, after amending the value and reasonable influence depth of unloading stress. The results can provide a beneficial reference for the design and construction of the similar projects to some extent.