The research, led by Ali M. Rajabi, Jahangir Esfandiarinasab, and Reza Khajevand, focuses on Shiraz Metro Line 1, a 24.2-kilometer underground network that includes 15 kilometers of parallel twin tunnels. In the studied section, two tunnel boring machines excavated profiles with a cutting wheel diameter of 6.9 meters, lined with 30-centimeter-thick concrete segments installed behind the shield. The center-to-center spacing between the two tunnels ranges from 13 to 17 meters, and the tunnels sit approximately 23 meters below the surface. The geology along the route consists mainly of weak clay above the tunnels, with alternating layers of clay, sand, and silt beneath, and groundwater levels fluctuating between 6 and 9 meters along the alignment.

What makes the study particularly valuable is its grounding in real field data. The researchers deployed control points at 10-meter intervals along the tunnel alignment, monitoring surface settlement from three days before excavation began until one month after completion. Their two-dimensional finite element model, built in PLAXIS under plane strain conditions, was validated against these measurements using soil stratigraphy from borehole BH-A-9. The comparison produced a root mean square error of just 0.67 millimeters, a mean absolute error of 0.60 millimeters, and a mean absolute percentage error of 3.80 percent, with a coefficient of determination of 0.91. These metrics indicate that the model reproduces both the magnitude and the characteristic double-trough shape of the settlement profile with impressive fidelity.

The settlement mechanism itself is a fascinating piece of geotechnical physics. When a tunnel is excavated, the soil that once supported the removed material must find a new equilibrium, and stress redistributes around the opening in a pattern engineers describe as arching. With twin tunnels, the individual stress redistribution zones overlap, concentrating deformation in the soil pillar between the two crowns. The study’s displacement contours reveal that maximum settlement occurs precisely in this intermediate zone, producing a settlement trough that is wider and flatter than the classical Gaussian profile associated with single tunnels. A localized zone of heave even appears beneath the tunnel inverts, a signature of stress relief following excavation. This amplified settlement in the central region represents the most critical area for potential structural damage in densely built environments.

Among the geometric parameters, tunnel diameter emerged as a dominant driver of surface deformation. At constant depth, settlement rose from approximately 1.9 millimeters for a 3-meter-diameter tunnel to about 8.5 millimeters for a 9-meter tunnel, a consequence of the larger excavated volume and the expanded zone of ground relaxation. Burial depth works in the opposite direction: deeper tunnels benefit from greater overburden confinement, which attenuates deformation before it reaches the surface. The researchers found that increasing the burial depth of a 7-meter tunnel from 30 to 90 meters at a friction angle of 30 degrees reduced settlement from 6.1 to 5.1 millimeters, a 16.4 percent decrease. The normalized depth-to-diameter ratio, they emphasize, governs how effectively the ground absorbs and dissipates excavation-induced stress release.

Soil properties proved equally consequential, and here the study delivers one of its most instructive insights. Within the Mohr-Coulomb framework used in the analyses, cohesion plays a more dominant role than the internal friction angle in limiting settlement, because cohesion contributes directly to available shear strength at the relatively low confining stresses found near tunnel boundaries. Increasing cohesion reduced settlement by roughly 1.4 millimeters across the tested range, while raising Poisson’s ratio from 0.1 to 0.4 increased settlement by a more modest 0.1 millimeters. The normalized cohesion-to-elastic-modulus ratio consistently outperformed the friction angle as a predictor of deformation behavior, suggesting that designers should pay particular attention to the cohesive component of soil strength when assessing tunneling risk in urban clay deposits.

The interaction between surface structures and the underlying tunnels adds another layer of complexity. Contrary to the intuitive assumption that heavier, stiffer buildings might brace the ground against deformation, the simulations showed that settlement increases with the number of stories when foundation width remains fixed, reaching approximately 3.7 millimeters at the highest load level of ten stories. Because the footprint stays constant, additional weight simply concentrates vertical stress over the same area rather than improving structural stiffness. Wider foundations, however, do help: increasing the building width-to-tunnel-diameter ratio reduced settlement by about 3 percent through better load dispersion. The critical zone of maximum deformation was identified within roughly 20 meters of the midpoint between the tunnel axes, and settlement directly above individual tunnels proved slightly greater than at the midpoint itself.

Perhaps the study’s most sobering comparison involves the classical empirical and analytical methods that have guided tunnel design for decades. When the researchers benchmarked their finite element predictions against the formulations of Peck, Atkinson, and Lee, the numerical results were consistently higher. At a tunnel depth of 30 meters, Atkinson’s method predicted 4.8 millimeters of settlement while the PLAXIS model produced 8.2 millimeters, roughly 58 percent more. The discrepancy stems from the fact that most empirical formulas were developed for single tunnels under simplified greenfield conditions and cannot capture tunnel-tunnel interaction, stress superposition, or the influence of nearby structures. Among the analytical solutions tested, Bobet’s approach agreed best with the numerical results, while the Loganathan and Poulos solution showed discontinuities attributable to its simplifying assumptions.

A perturbation-based sensitivity analysis crystallized the hierarchy of influence. Tunnel depth, tunnel diameter, and soil cohesion produced the largest relative changes in settlement response within the investigated parameter ranges, while Young’s modulus, Poisson’s ratio, and building-related variables played comparatively smaller roles. The authors caution that this ranking is specific to their chosen parameter ranges and modeling assumptions, and that the two-dimensional plane strain formulation, while computationally efficient and validated against field data, cannot capture three-dimensional effects such as tunnel face advancement, construction sequencing, and longitudinal stress redistribution. Groundwater flow and consolidation processes were also excluded, meaning the results represent long-term drained conditions rather than short-term undrained behavior.

For the rapidly growing number of cities building metro networks through soft, shallow ground, the message is clear: settlement prediction in twin-tunnel environments demands a coupled view of geometry, geology, and the built environment above. Optimizing tunnel depth and diameter, improving soil stiffness where feasible, and carefully evaluating surface loading conditions can meaningfully reduce settlement-related risk. The Shiraz Metro case demonstrates that even relatively small predicted settlements, on the order of a few millimeters, become intelligible only when the full soil-tunnel-structure system is analyzed together. As underground space becomes an increasingly precious urban resource, studies of this kind transform the invisible mechanics beneath our feet into actionable engineering knowledge, helping ensure that the trains of tomorrow can run beneath our cities without cracking the world above them.

Subject of Research: Numerical parametric analysis of ground settlement induced by twin-tunnel metro excavation in Shiraz, Iran

Article Title: Parametric analysis of twin tunnel-induced ground settlement: a numerical study of Shiraz Metro

Article References: Rajabi, A. M., Esfandiarinasab, J., & Khajevand, R. (2026). Parametric analysis of twin tunnel-induced ground settlement: a numerical study of Shiraz Metro. Results in Engineering, 32, Article 113291. https://doi.org/10.1016/j.rineng.2026.113291

Image Credits: AI Generated

DOI: 10.1016/j.rineng.2026.113291

Keywords: twin tunnels, ground settlement, Shiraz Metro, finite element analysis, PLAXIS, geotechnical engineering, urban tunneling, soil-structure interaction, Mohr-Coulomb model, tunnel diameter, burial depth, soil cohesion

The research team, led by Chen Yanhua, Hao Erhao, Yang Mei, and Wei Rong of North China University of Science and Technology in Tangshan, China, focused on one of the trickiest fault geometries an engineer can face: the oblique-slip fault. Unlike pure strike-slip faults, where the two sides of the fracture grind horizontally past each other, or pure dip-slip faults, where one block lurches vertically relative to the other, oblique-slip faults combine both motions simultaneously. The result is a compound displacement field that subjects a crossing pipeline to a twisting cocktail of axial tension or compression, lateral bending, and vertical flexure all at once—a mechanical environment far more punishing than any single component acting alone.

To capture this complexity, the researchers took a two-pronged approach. First, they built a two-box device experiment, a laboratory setup in which soil and a model pipeline are housed in two adjacent containers that can be displaced relative to one another, physically simulating the sudden offset produced when a fault ruptures beneath a buried line. This sandbox-style testing lets researchers watch deformation evolve in a controlled, repeatable way. Second, they constructed detailed finite element simulations in ABAQUS, a commercial computational mechanics platform, replicating the pipe-soil interaction and extending the parametric reach of the experiments to conditions that would be impractical to test physically.

The central question was deceptively simple: where do corrosion defects fit into this already dangerous picture? Corrosion is a slow, relentless enemy of buried steel. Over years and decades, it eats away at pipe walls, thinning them locally and creating spots where the cross-section that carries the load is diminished. Engineers have long assessed such defects with established fitness-for-service codes, but those frameworks were largely built around internal pressure and ordinary operational loads—not around a fault shoving meters of ground displacement through the pipeline in a matter of seconds. The new study interrogates exactly this coupled scenario.

One of the most striking findings concerns what the authors call peak shifting. In an intact pipeline, the largest bending strain tends to occur at predictable locations dictated by the fault geometry and soil resistance. But when a corrosion defect is present, stress concentrates at the weakened spot, and the location of the peak strain migrates toward it. In other words, the defect does not merely weaken the pipe locally; it actively reorganizes the entire deformation field around itself, pulling the most dangerous strains to the very place where the pipe can least afford them.

The experiments also exposed a subtle measurement trap that could mislead field monitoring programs. Because instrumentation points are physically fixed to the pipe surface, they move together with the deforming pipe, and this fixity can create what the researchers describe as an illusion of reduced peak strain. A sensor bolted to a section that is bending and shifting wholesale may register a lower differential strain than the true localized peak, simply because the reference frame itself is in motion. For operators relying on strain gauges to flag danger, this is a sobering caveat: the instruments may be quietly under-reporting the worst of the damage.

Among the variables the team examined, defect location emerged as the dominant driver of failure risk. When a corrosion defect sits inside a large deformation zone—the region where the fault’s passage forces the most intense bending—the strain concentrates sharply there, and the maximum strain nearly doubles compared with defects placed in calmer stretches of the alignment. This finding has immediate practical weight: two pipelines with identical corrosion defects can carry dramatically different risk profiles depending on where along a fault crossing those defects happen to lie. Defect assessment, the study implies, cannot be done in isolation from seismic fault-hazard mapping.

The researchers also tested how pipe diameter and burial depth reshape the response. Increasing the pipe diameter spreads the fault-imposed displacement over a larger and stiffer structure, which mitigates strain localization—the dangerous funneling of deformation into a narrow band. However, bigger diameter does not eliminate the problem: stress at the defect site remains significantly concentrated, meaning the thinning caused by corrosion continues to matter even on large, robust pipes. Reducing burial depth, meanwhile, loosens the soil’s grip on the pipe, easing the constraint the surrounding ground imposes on its deformation. But that relief comes at a price: a shallower pipe experiences a wider deformation zone, distributing bending over a longer length while becoming more exposed to surface loads such as traffic and construction. The authors conclude that engineers face a genuine trade-off between soil constraint and surface loading when choosing burial depth in fault zones—a balance with no universally correct answer.

Why does this matter now? Global energy demand keeps pipelines crossing seismically active corridors, from Central Asia to the Pacific Rim, and much of that buried steel is aging. Corrosion is consistently identified in the engineering literature as a leading cause of pipeline failures, and great earthquakes recur on timescales that guarantee many pipelines will encounter fault rupture during their service lives. When the two hazards overlap, the margin of safety can erode much faster than either hazard alone would suggest. The Tangshan team’s work provides a quantitative reference for prioritizing inspection and maintenance: pipelines with corrosion defects inside mapped fault deformation zones deserve elevated scrutiny, and monitoring strategies should account for the peak-strain shifting and measurement illusions the study documents.

The study, funded by the Hebei Natural Science Foundation and the National Natural Science Foundation of China, arrives as the field of pipeline earthquake engineering steadily shifts from idealized intact-pipe models toward a more honest accounting of real-world degradation. By fusing physical two-box experiments with high-fidelity finite element simulation, the researchers have delivered a framework that connects laboratory-scale mechanics directly to the decisions operators must make in the field. As seismic hazard maps grow more precise and in-line inspection tools catalog corrosion defects with ever-finer resolution, the remaining challenge—integrating the two data streams into a single, defensible risk picture—has just become considerably clearer. For the quiet steel arteries beneath our feet, knowing exactly where a crack will want to form before the ground moves may one day be the difference between a routine inspection and a catastrophe.

Subject of Research: Mechanical behavior of corroded buried pipelines crossing oblique-slip fault zones

Article Title: Mechanical behavior analysis of defective pipelines in oblique slip fault zones based on experiments and simulation

Article References: Yanhua, C., Erhao, H., Mei, Y., & Rong, W. (2026). Mechanical behavior analysis of defective pipelines in oblique slip fault zones based on experiments and simulation. Bulletin of Earthquake Engineering. https://doi.org/10.1007/s10518-026-02684-6

Image Credits: AI Generated

DOI: 10.1007/s10518-026-02684-6

Keywords: oblique-slip fault, buried pipelines, corrosion defects, stress concentration, strain localization, pipe-soil interaction, fault rupture, ABAQUS simulation, earthquake engineering, pipeline safety, burial depth, pipe diameter