Field performance of an anti-uplift portal frame in control of the tunnel uplift induced by overlying excavation

Abstract

Excavation above the tunnel will inevitably cause release of the ground stress and the associated tunnel uplift. The control of tunnel uplift is a tough problem that troubles the engineers. This paper introduces a new technology (namely anti-uplift portal frame technology) to control the tunnel uplift caused by long-distance collinear overlying excavation which adopted in the construction of an urban underground expressway in Shenzhen, China. The anti-uplift portal frame (AUPF) is composed of anti-uplift piles and anti-uplift slab, forming a portal frame between the bottom of the foundation pit and the tunnel to hoop the underlying tunnel. In order to minimize the impact of shaft excavation on the underlying tunnel during the AUPF construction, the AUPF was divided into sections in longitudinal direction and avoid excavation of the adjacent shafts at the same time. The field data indicated the median uplift after shaft excavation in the AUPF construction stage was 12.8 mm in Zone A and 5.3 mm in Zone B, and recovered by 62.5 % in Zone A and 85 % in Zone B after backfill. During the foundation pit excavation in expressway tunnel construction, the median uplift was 7.2 mm, recovered by 61 % after tunnel structure was constructed. When the tunnel uplift reached 8 to 8.5 mm, further excavation did not cause increase of the tunnel uplift, which indicates the AUPF can effectively control the tunnel uplift. The reported case provides valuable referrence to guide the similar overlying excavation projects.