Assessment and control of metro-construction induced settlement of a pile-supported urban overpass

Abstract

A metro station in Beijing, consisting of two separate parallel platform tunnels, has been constructed in close range below a pile-supported overpass of a major municipal artery built more than a decade ago. In order to assure the normal operation and integrity of the overpass as well as smooth and safe tunneling, the condition and status quo settlement capacity of the overpass is assessed via structural measurements and analysis, and on the basis of which, criteria on allowable additional settlements of pile foundations are established. The probable additional settlements of the pile foundations caused by tunneling and dewatering are estimated by using empirical and theoretical analyses and numerical simulations of the construction process in combination with in situ monitoring data, and anti-settlement underpinning piles are installed to control the predicted excessive adverse effects of tunneling. The major conclusions include: (1) the surcharge effect from an existing structure in or on the ground may enhance the effect of tunneling on ground deformation, and thus should be taken into account when estimating the delimitations of zones of tunneling influence and the degree of closeness of an existing structure; (2) when tunneling in close range to an existing urban overpass supported on pile foundations, the general control procedure should consist of assessment of the contemporary capacity of the superstructure, establishment of criteria for and prediction of dewatering and tunneling-induced ground surface and pile foundation settlements, and, if necessary, execution of reinforcement measures; (3) in situ monitoring data close to areas sensitive to tunneling should be employed to adapt theoretical analysis and calibrate numerical models; (4) rational settlement criteria and mitigation measures should be worked out through consideration of the local composition of the superstructure of the overpass, as well as the type and depth of the pile foundations; and (5) a time period after installation may be needed for the underpinning piles to be effective in counteracting the settlement effect of tunneling.