The Dongping No.1 Tunnel of the Guangdong–Foshan Loop Line Intercity Railway in China passes an existing large-scale underground pipe gallery in water-rich sandy strata through the undercutting method. During construction, controlling the underground pipe gallery deformation is difficult because the water-rich sandy strata can easily become unstable. To deal with this problem, this paper proposes a double layer pre-support system (DLPS) consisting of pipe shed and horizontal jet grouting piles, whose function mechanism is explored from the viewpoint of passive protection and active reinforcement. The deformation transferring path of the DLPS is described as follows: tunnel excavation → lower pre-support deformation → deformation of the soil between the upper and the lower pre-supports → upper pre-support deformation → underground pipe gallery deformation. Considering the influence of geological conditions, the proposed DLPS has two failure modes, namely, excessive settlement of the tunnel vault and excessive deformation of the overlying underground pipe gallery. The mechanical model and analytical method of the DLPS were established according to the theory of beam resting on elastic foundation. Comparing the computed and measured settlement values revealed that the established model is reliable because it could reflect the interaction characteristics of the upper pre-support, the lower pre-support, and the middle soil. Finally, multiple factor analysis was conducted to ascertain the influence of typical calculating parameters, and the deformation characteristics of the DLPS under different influence factors were obtained. The design suggestions for key parameters could provide reference for similar practical engineering in the future.

我国粤佛环线城际铁路东平一号隧道采用底切法穿过现有大型富水砂质地层地下管廊。在施工过程中，由于富含水的砂质地层容易变得不稳定，控制地下管廊变形是困难的。针对这一问题，本文提出了一种由管棚和水平喷射注浆桩组成的双层预支系统（DLPS），从被动保护和主动加固的角度探讨了其作用机制。DLPS的变形传递路径描述如下：隧道开挖→下预支座变形→上下预支座间土体变形→上预支座变形→地下管廊变形。考虑到地质条件的影响，提出的DLPS有两种破坏模式，即隧道拱顶过度沉降和上覆地下管廊过度变形。根据梁在弹性基础上的理论，建立了DLPS的力学模型和解析方法。比较计算和实测沉降值表明，所建立的模型是可靠的，因为它可以反映上预支、下预支和中土的相互作用特征。最后，通过多因素分析确定典型计算参数的影响，得到不同影响因素下DLPS的变形特性。关键参数的设计建议可为今后类似的实际工程提供参考。