Investigation of Three-Dimensional Deformation Characteristics of Existing Pipelines Due to Tunnel Face Instability

Improper control of supporting face pressure of shield tunnel may induce excessive ground movements, which may threaten the serviceably and safety of adjacent underground pipelines. Currently, extensive studies have been conducted to investigate pipeline responses due to tunnelling under various volume loss, effects of active face instability on deformation characteristics of existing pipelines are rarely explored. Physical model tests are conducted to investigate the effects of tunnel cover to diameter ratio (C/D) and normalized horizontal distance (H/D) between tunnel face and existing pipeline on three-dimensional pipeline deformation mechanisms. The results show that: When the pipeline is located 0.1D away from the tunnel face (i. e., H/D = 0.1), active face instability induced pipeline settlement reaches the maximum value. By increasing H/D ratio from 0.1 to 0.5, the maximum settlement and bending strain of pipelines are reduced by up to 64.8% and 51.6%, respectively, while the respective maximum reductions in the pipeline settlement and bending strain are only 14.9% and 7.3% as the C/D ratio from 1.0 to 1.5. It is indicated that H/D ratio is the key parameter to control pipeline settlement. Based on systematic numerical parametric analyses, calculation charts for estimating pipeline settlements due to tunnel active face instability are established by using two dimensionless groups of pipeline settlement ratio and relative pipeline-soil stiffness. The validity of the proposed calculation charts is verified by the physical test results, and the calculation charts can provide a rapid evaluation of pipeline safety and adjustment of supporting face pressure during tunnel construction.