Observed behaviors of a long and deep excavation and collinear underlying tunnels in Shenzhen granite residual soil

The performances of a long and deep excavation and underlying collinear existing tunnels in Shenzhen granite residual strata are presented and evaluated. The collinear section for the excavation and tunnel had a length of 3.1 km and the maximum excavation depth was 18.0 m. The excavation was supported with retaining piles, one level of reinforced concrete strut, and 1-3 levels of steel pipe strut. Emphasis is put on the retaining pile displacement, ground surface settlement, and tunnel deformation, correlated with ground conditions, construction activities, excavation geometries, and time-dependent behaviors. An appreciable increase in retaining pile displacement occurred after the end of each soil layer excavation. The ground surface settlements were close to the observations due to metro station excavation with high length to width ratio. In addition, the deformation of the existing tunnel at a certain cross section was governed by the exposure time for the excavation base, base excavated length ahead, and unloading ratio u(rd) (u(rd) = H-e/C, H-e was the excavation depth and C was the tunnel cover depth). The upper and lower bounds of the graph of u(rd) plotted against delta(t/C) (delta(t) was the tunnel heave) can be expressed by exponential equations. The observations of this project can provide a reference for cases in similar conditions.