Case study of performance assessment of overlapping shield tunnels with a small curve radius

Due to the network planning of subways and their surrounding structures,increasingly more overlapping shields with a small curve radius have been constructed. A newly constructed upper tunnel partly overlaps a lower one, leading to the extremely complex uplift of the lower tunnel caused by the construction of a new tunnel. Based on the shield-driven project that runs from the Qinghe Xiaoyingqiao Station to the Qinghe Station in Beijing, which adopts the reinforcement measures of interlayer soil grouting and steel supports on site, in this study, the uplift pattern of the lower tunnel and the stress characteristics of steel supports were investigated through numerical simulations and on-site monitoring.The study results show that among all tunnel segments, the first segment of the shield witnesses a maximum uplift displacement that increases with the horizontal space between tunnels. On using either interlayer soil grouting or steel-ring bracing reinforcement, the uplift of the tunnel lining exceeds the control value;by contrast,when these two measures are jointly applied, the uplift of the tunnel lining does not exceed a maximum value of 4.87 mm, which can satisfy the requirements of deformation control. Under these two joint measures, the soil strength between two stacked shield tunnels can be enhanced and the uplift deformation can be restricted with the interlayer soil grouting. Also, the segmental deformation and overall stability of the existing tunnel can be controlled with the temporary steel supports.The deformation of circumferential supports and segments is closely related to each other, and the segmental uplift is controlled by H-shaped steel supports. With the increase in the horizontal space between twin shields, the effect of the construction would gradually weaken, accompanied by a gradual reduction of the stresses of steel supports. These findings provide a valuable reference for the engineering design and safe construction of overlapping shield tunnels with a small curve radius.

Research on rail wear of small radius curve in EMU depot

Purpose–With the help of multi-body dynamics software UM,the paper uses Kik–Piotrowski model to simulate wheel-rail contact and Archard wear model for rail wear.Design/methodology/approach–The CRH5 vehicle-track coupling dynamics model is constructed for the wear study of rails of small radius curves,namely 200 and 350 m in Guangzhou East EMU Depot and those 250 and 300 m radius in Taiyuan South EMU Depot.Findings–Results show that the rail wear at the straight-circle point,the curve center point and the circlestraight point follows the order of center point>the circle-straight point>the straight-circle point.The wear on rail of small radius curves intensifies with the rise of running speed,and the wearing trend tends to fasten as the curve radius declines.The maximum rail wear of the inner rail can reach 2.29 mm,while that of the outer rail,10.11 mm.Originality/value–With the increase of the train passing number,the wear range tends to expand.The rail wear decreases with the increase of the curve radius.The dynamic response of vehicle increases with the increase of rail wear,among which the derailment coefficient is affected the most.When the number of passing vehicles reaches 1 million,the derailment coefficient exceeds the limit value,which poses a risk of derailment.