PSI - Issue 84

Marialorenza Vescovi et al. / Procedia Structural Integrity 84 (2026) 1063–1070

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situ. This inverse analysis identified a new set of governing parameters that align the theoretical model with the real stress condition. The results indicate that while the calibrated amplification factor f remains consistent with Desimon’s standard theoretical recommendations with a value of 0.86, the load height h undergoes a drastic reduction. Specifically, the effective load height drops from a theoretical estimate of approximately 20m to a calibrated value of just 2.05m. Under these calibrated conditions, the structural check is fully satisfied: the line of thrust lies entirely within the middle third of the cross-section, the eccentricity remains minimal and the stresses match those measured on site (Figure 2a). This finding suggests that the tunnel is currently operating in a 'functionally unloaded' state. The significant discrepancy between the theoretical and calibrated load heights suggests that the surrounding rock mass has developed a stable, self-supporting arching effect over time. This has drastically reduced the actual vertical pressure acting on the lining.

(a)

(b)

Figure 1 (a) Simplified approach pressures and forces (b) Comparison between the stresses of the SA (red curve) and in-situ stresses (X points) as a function of the normalized distance x, starting from the upper arch crown.