Stress and deformation response of pipe jacking in upper-soft and lower-hard strata: A case study in Changsha

Abstract

Constructing underground structures using the pipe jacking technique presents challenges in upper-soft and lower-hard strata. The complicated strata conditions make pipes prone to deflection, leading to problems such as cracking, pipe joints failure and water leakage. To determine the stress and jacking force characteristics of pipe jacking during the entire construction period in upper-soft and lower-hard strata, this paper presents a case study of pipe jacking passing through upper-soft and lower-hard strata in Changsha, China. Pipes stresses at two wings, and the top crest in the longitudinal directions for six different pipe sections were monitored. Then, a jacking force model is proposed to describe the additional thrust after deflection in upper-soft and lower-hard strata. The results show that the jacking force induced by upper-soft and lower-hard strata exhibits a tortuous increase and causes pipe deflection. The stress and deflection patterns in different monitoring pipes are consistent. The maximum pipe stresses during the jacking were 9.26 MPa in the longitudinal direction and 394 % increasing after deflection. Axial stress in test pipes exhibit nonlinearly transfer, with the distribution of friction resistance forms a“W”shape between pipes. An excessive alignment deflection would generate incremental jacking force, which strengthens with larger deflection. By regarding the composite formation area as a fully contact model with the surrounding rock, and uniform formation as a contact with the bottom of the surrounding rock, and the formulas were corrected for deeply buried pipe under uniform formation. Combining the calculation results and field data, the predict model is validated. The calculation method that can effectively predict the jacking force after pipe alignment deflection in upper-soft and lower-hard strata. The results of this study can provide beneficial guidance for the design and construction of pipe jacking.