Mechanized pipe jacking in weathered granite produces sharp portal deformations before settlement stabilizes
Mechanized pipe jacking for metro connecting passages in weathered granite did not behave like a smooth, uniform advance. The most abrupt mechanical changes appeared at the launching end during portal breaking, where deformation occurred in two sudden steps and the bending moment first rose sharply before easing more slowly. At the receiving end, deformation climbed in a near-linear pattern as jacking progressed, then peaked at the same moment the portal broke. Those transitions matter because they mark the points at which a seemingly contained underground operation can transmit stress into the main tunnels and up to the ground surface.
The study focused on a No. 1 connecting passage in a typical weathered granite stratum and used numerical analysis software to simulate the full construction sequence. Field monitoring data were used to verify the model, giving the calculation framework practical credibility rather than leaving it as a purely theoretical exercise. On that basis, the investigators explored how stratum cohesion, internal friction angle, clear distance between the main tunnels, and burial depth altered deformation in the twin shield sections and on the surface above them.
Small surface movements conceal a distinct deformation pattern in the twin tunnels
A centrally symmetrical settlement trough formed on the ground surface, with a maximum settlement of 2.3 mm. The main tunnel itself did not deform uniformly; the greatest vertical deformation reached 1.7 mm and the greatest horizontal deformation reached 1.4 mm. Those values are small in absolute terms, yet the geometry of the response is more important than the magnitude alone. Symmetry across the surface suggests a balanced load transfer around the passage, while the paired tunnel deformations show how the new opening redistributed stress into both shield sections.
The receiving end behaved in a way that aligns with the mechanical sequence of the work. As the pipe jacking head advanced, deformation rose steadily until portal breaking, when it reached its highest point in sync with the bending moment. In practical terms, the portal is the point where the structure stops behaving like an uninterrupted lining and begins to absorb the disturbance created by opening the passage. The launching end experienced the more abrupt response because the initial breaking stage concentrated stress and altered restraint conditions before the system had time to redistribute the load.
Ground strength, tunnel spacing, and burial depth pull the settlement pattern in different directions
Changes in rock and geometry did not affect every response in the same way. Higher stratum strength reduced ground surface settlement, but it increased horizontal tunnel deformation. A wider clear distance between the main tunnels expanded the width of the settlement trough, spreading the influence zone across a larger surface area. Greater burial depth also lowered surface settlement, yet it increased horizontal deformation in the tunnel.
The mechanical logic is straightforward. Stronger surrounding rock can confine the surface better, limiting how much the ground drops, but that same stiffness can force more lateral load into the tunnel lining, producing greater horizontal distortion. Greater depth adds overburden confinement and can dampen visible settlement at the surface, while also increasing the stress path carried by the underground structure. Wider spacing between the twin tunnels gives the disturbance room to disperse laterally, so the settlement footprint broadens even if the peak drop does not necessarily intensify in the same proportion.
These competing effects are exactly why connected underground works cannot be judged by one parameter alone. A design that protects the surface may still increase lateral demand on the lining, while a configuration that limits tunnel distortion may widen the zone of ground influence. Engineers planning connecting passages in similar strata will need to balance those tradeoffs rather than optimizing one variable in isolation.
Numerical verification links the model to field behavior
The study’s numerical model was not left standing on its own. Comparison with field monitoring data verified the accuracy of the simulation, which strengthens the value of the reported deformation trends for future design work. In underground construction, agreement between computation and monitoring is not a minor technical detail; it is what allows a model to be used for predicting where stress concentration will emerge and how far settlement may spread around the project.
That validation also gives weight to the parameter analysis. The influence of cohesion, internal friction angle, tunnel spacing, and burial depth was not treated as an abstract sensitivity exercise. It was tied back to a real mechanized construction sequence in weathered granite, a setting where the interplay between rock mass properties and structural response can decide whether deformation stays within manageable limits or becomes difficult to control.
A separate conclusion emerges from the portal-breaking phase itself. Once the opening is initiated, the lining no longer responds in a gradual, uniform manner; it experiences a concentrated structural adjustment that drives the sharpest local deformation. The abruptness of that shift is a reminder that the most vulnerable moment may be the transition between intact tunnel and opened passage, not the bulk of the boring advance.
Design choices in weathered granite must account for coupled surface and lining effects
For metro systems that require connecting passages through weathered granite, the findings point to a construction regime where surface settlement and tunnel deformation have to be read together. Mechanized pipe jacking can limit ground movement to a small settlement trough, but the tunnel lining still absorbs measurable vertical and horizontal distortion. Because stronger rock, deeper burial, and larger tunnel spacing do not all improve the same response variable, project design has to treat them as coupled controls rather than independent levers.
The study is also a reminder that underground urban works can look deceptively stable from the surface. A maximum settlement of 2.3 mm sounds modest, yet the lining response shows that internal structural effects continue even when ground movement seems contained. That gap between surface appearance and structural demand is where numerical modeling earns its value, especially in dense metro corridors where connecting passages must be built without compromising either the existing shields or the overlying ground.
The work closes one more loop between simulation and site practice: the field data matched the numerical behavior closely enough to support the reported influence trends, but the study remains centered on a single representative passage in weathered granite. Broader application will depend on how well these deformation laws hold in different rock masses, tunnel layouts, and burial conditions, especially where construction constraints force different jacking sequences or support details.