The research, led by Lingbing Yi, Xiaochen Liu, Cai Yi, Peng Zhang, and Lu Han, centers on the No. 1 connecting passage of a metro line built through a typical weathered granite stratum. Rather than the traditional pit-and-mine method, in which workers excavate the passage under a protective umbrella of grouted pipes, the team analyzed a mechanized pipe jacking scheme, in which a small tunneling machine pushes precast pipe segments through the ground between the two main tunnels. This method confines most of the ground disturbance to a narrow cylindrical annulus around the jacked pipes, and the study set out to quantify exactly how much deformation the neighboring shield tunnels and the ground surface actually experience as a result.

To capture the full picture, the authors built a three-dimensional numerical model of the construction sequence using finite element analysis software. The model represented the weathered granite as an elastic-perfectly plastic material governed by the Mohr-Coulomb failure criterion, and it simulated each stage of the jacking operation: installation of the working shafts, machine launch through the portal of one main tunnel, incremental pipe advancement across the gap, and finally arrival and portal break-in at the receiving tunnel. The researchers did not simply trust the model on its own merits. They calibrated and verified it against field monitoring data recorded during the actual passage construction, comparing computed tunnel deformations and surface settlements with measured values. The close agreement between simulation and observation gave them confidence to extend the analysis beyond what sensors on site could capture.

The verified model revealed a striking asymmetry in how the two ends of the connecting passage respond to construction. At the launching end, the tunnel lining experienced two distinct episodes of sudden deformation during the portal breaking stage—the moment when the jacking machine cuts through the shaft wall and enters the ground. The bending moment in the tunnel lining rose sharply during the first breakout and then decayed slowly through the second, producing a jagged but bounded stress history. At the receiving end, the behavior was altogether different: deformation grew essentially linearly with each advance of the pipe jacking machine, accumulating steadily along the jacking path until the instant of portal break-in, when both the deformation and the bending moment peaked simultaneously. This contrast matters for engineers because it shows that the receiving tunnel’s critical loading state arrives only at the very end of the operation, when corrective measures are hardest to apply.

Quantitatively, the simulated and monitored movements were remarkably small, which is precisely the selling point of mechanized pipe jacking for this application. The ground surface above the passage developed a centrally symmetrical settlement trough centered on the jacking alignment, with a maximum settlement of only 2.3 millimeters. The main shield tunnels themselves deformed even less: peak vertical deformation reached 1.7 millimeters and peak horizontal deformation 1.4 millimeters. For context, operational metro tunnels are commonly monitored against deformation limits tens of times larger, so the passage construction left the running tunnels well within safe tolerances. The tight, symmetrical settlement trough also suggests that the pipe jacking process mobilizes very little volume loss in the surrounding ground, a direct benefit of the machine’s continuous face support and the immediate installation of pipe segments behind the cutting head.

With the baseline case established, the team used the validated model as a virtual laboratory to test how four governing parameters shape the response: the cohesion of the weathered granite, its internal friction angle, the clear distance between the two main tunnels, and the burial depth of the passage. Each parameter was varied systematically while the others were held constant, isolating its individual influence on surface settlement and tunnel deformation. This parametric sweep produced several counterintuitive findings that could change how designers approach similar projects in weathered rock environments.

The first surprise concerns ground strength. Improving stratum cohesion and internal friction angle—typically achieved through grouting or other ground treatment—does reduce surface settlement, as intuition would predict. But the analysis showed that stronger ground actually increases the horizontal deformation of the main tunnels. The mechanism appears tied to how stiff ground redistributes jacking loads: rather than absorbing them through compressible deformation, stronger soil transmits more of the pipe jacking disturbance laterally into the tunnel linings. For designers, this means ground improvement alone is not an unambiguously protective measure; it trades surface settlement control for heightened lateral loading of the adjacent tunnels, and the trade must be balanced deliberately rather than assumed away.

The geometry of the crossing proved equally consequential. As the clear distance between the two main tunnels increases, the width of the surface settlement trough expands. Wider separations lengthen the jacking path, giving the disturbance more lateral room to spread before it reaches the surface, even though the peak settlement magnitude is not necessarily increased. Meanwhile, burial depth behaves in a manner that partially mirrors the strength effect: deeper passages produce smaller surface settlements because the overlying soil column arches over the excavation and dampens the disturbance before it can propagate upward, yet the same deeper configuration increases the horizontal deformation imposed on the main tunnels. Depth, like ground improvement, is therefore a double-edged design lever rather than a straightforward safety margin.

Taken together, the results sketch a practical decision framework for mechanized construction of metro connecting passages in weathered granite and comparable transitional ground. The linear accumulation of deformation at the receiving end argues for intensified monitoring and readiness of reinforcement measures during the final jacking stages, when the tunnel is most vulnerable. The two-surge bending moment history at the launching end suggests that portal reinforcement and staged breakout procedures deserve particular attention at machine launch. The opposing trends between surface settlement and tunnel deformation under stronger, deeper conditions indicate that optimal designs will not maximize ground treatment or depth but will tune both against explicit limits on lining distortion, which are often the binding constraint for operational tunnels carrying live rail traffic.

The study also demonstrates a methodological template that extends well beyond a single passage in a single city. By anchoring a process-level numerical simulation to genuine field measurements, the authors produced parameter sensitivity results that practitioners can carry into preliminary design with reasonable confidence, before any site-specific instrumentation exists. As metro networks densify and connecting passages must be built ever closer to sensitive, in-service tunnels, the ability to predict millimeter-scale interactions between jacking machines and adjacent linings becomes a competitive necessity. This work shows that with careful modeling discipline, the interaction can be understood well enough to keep both the ground surface and the tunnels beneath it essentially still.

Subject of Research: Mechanized pipe jacking construction of metro connecting passages and its effects on shield tunnel and ground surface deformation in weathered granite strata

Article Title: Analysis of construction response for mechanized construction of metro connecting passages

Article References: Yi, L., Liu, X., Yi, C., Zhang, P., & Han, L. (2026). Analysis of construction response for mechanized construction of metro connecting passages. PLOS One, 21(10), e0360916. https://doi.org/10.1371/journal.pone.0360916

Image Credits: AI Generated

DOI: 10.1371/journal.pone.0360916

Keywords: pipe jacking, metro tunnels, connecting passages, weathered granite, shield tunnels, ground settlement, numerical modeling, tunnel deformation, finite element analysis, geotechnical engineering, portal break-in, burial depth