It’s inevitable that, various types of construction works emerge within proximity during the large-scale construction of urban rail transit, thus posing huge challenges for the actual construction. In this paper, a minute study on the construction risk control of shield tunneling beneath the existing railroads relying on a practical engineering is conducted, thereby developing a set of feasible risk-handling ideas, mainly including: (1) based on the detailed investigation and analysis of the engineering background and construction environment, to find out the key control indicators; (2) to carry out the risk assessment, and to identify the key factors influencing the engineering safety and environmental safety; (3) in view of the key risk factors, to propose purposeful measures for risk control; (4) to establish a numerical model to analyse the feasibility of risk control measures; and, (5) to establish a rigorous construction monitoring system for dynamically adjusting the construction parameters at real time and evaluating the engineering safety. So far as the practical engineering in this paper, the key risk sources are sand pebble stratum, delayed grouting and earth pressure in the chamber set incorrectly, and without proper control, these three factors may lead to large-area settlement and deformation during the construction. For this reason, targeted risk control program, covering rotary churning pile reinforcement, overhead line reinforcement (reinforcement by longitudinally lifting the existing railroad using cantilevers of I-beams) and shield driving parameter control, etc., are further proposed, and numerically simulated, with the construction monitoring and tracking results indicating that, under the protection of the above risk control measures, the settlements of roadbed and rail of existing railroads are less than 2 mm, which can meet the requirements in the railway operation safety. Finally, combining the actual construction situation, the empirical values of the optimised driving parameters for the shield going beneath the existing railroads in the water-rich sand pebble stratum are given, such as: driving speed is within range of 55~60 mm/min, earth pressure in the chamber is 0.13 MPa, the pressure for synchronous grouting is about 2.4 bars, which can be available reference for similar projects.

The advance of tunnel shield creates void between the lining segments and the surrounding rock at the tail part of the tunnel shield. Grout is continuously injected into that void, to control the ground subsidence and maintain the segments stability. The grouting process must be precisely controlled and monitored to prevent any undesired significant subsidence or segment damage. In this work, a novel model to characterise the temporal and spatial distribution of the grout pressure in the rock-segment void is proposed, based on the theory of group diffusion and consolidation. The temporal variation of the grout viscosity is also considered in the proposed model. The proposed model is verified against field monitored data of the grout pressure, and exhibits quite good performance. Then a three-dimensional finite element model of the lining segments is developed to study the effects of the grout pressure on the mechanical response of the lining segments. The mechanical responses of the lining segments, in terms of internal force, moment and stress profile, are thoroughly analysed. These mechanical responses are also verified against the field monitored data, to further validate the proposed model of grout pressure distribution.