Abstract: The underground excavation and expansion of ultra-shallow-buried tunnels pose notable risks to construction safety since the minimal soil cover readily triggers crown instability and excessive ground subsidence. This paper presents a case history of a tunnel expansion project in Nanshan District, Shenzhen. The study evaluates the effectiveness of a combined advanced supporting system that integrates a piped-roof (umbrella arch) with forepoling using pilot small-diameter grouted pipes under ultra-shallow cover conditions. A three-dimensional numerical model was developed by using MIDAS GTS NX to simulate and to compare the deformation of the surrounding rock mass as well as the evolution of the ground subsidence under three supporting conditons: (i) single pipe-roof support, (ii) single advanced forepoling support, and (iii) the combined system. The comparative results indicate that the combined technique can take the advantage of the “beam effect” of the pipe-roof while simultaneously improving the soil using grouting through the forepoling pipes, thereby improving the overall stiffness and the load-bearing capacity of the support system. Quantitatively, the maximum crown settlement achieved with the combined support is 9.44 mm, which corresponds to reductions of 55.3% and 71.7% relative to the single pipe-roof case (21.14 mm) and the single advanced forepoling case (33.34 mm), respectively. These findings, derived from a consistent three-condition comparison within the same modeling framework, substantiate the practical advantages of the combined advance support for mitigating deformation and settlement risks in ultra-shallow settings. The outcomes provide a clear theoretical basis and operable technical references for the design of advanced supporting systems in similar tunnel expansion projects conducted by underground excavation.

Key words: Ultra-Shallow Buried Tunnel Excavation, Pilot Small Pipe Grouting, Pipe Roof, Combined Support, Numerical Simulation, Excavation Methods Comparison