Subsoil Material Heterogeneity in a Multi-Span Viaduct: Identification and Operational Deflection Shape Assessment of Wave Propagation and Application to Train-Induced Vibrations

This study investigates the influence of subsoil material heterogeneity on the dynamic performance of a five-span road viaduct with spans crossing different ground zones, including native subsoil and compacted, mechanically stabilized road-substructure materials. A spatially varying kinematic excitation model based on the wave-passage approach was adopted to represent the spatial variation in support excitation. Controlled vibroseis tests with synchronized acceleration measurements at viaduct supports were used to experimentally identify section-specific apparent propagation velocities by cross-correlation and material absorption coefficients by least-squares fitting of the amplitude-decay relationship. Phase-based Operational Deflection Shapes (ODSs) were used to compare the theoretical spatial relative-phase pattern derived from the identified section-specific propagation delays with that measured along the viaduct at 20, 30, and 35 Hz. The experimental and theoretical phase patterns showed close agreement at all analyzed frequencies. The measurements revealed clear differences in the wave-propagation characteristics of the ground beneath the viaduct. The native subsoil exhibited lower apparent propagation velocities and greater vibration absorption than the road-substructure materials. For application to train-induced vibrations, the identified propagation characteristics were reduced to representative, frequency-independent parameters: v_NATIV = 146 m/s and α_NATIV = 0.05788 m⁻¹ for the native-subsoil zone, and v_ROAD = 505 m/s and α_ROAD = 0.002984 m⁻¹ for the road-substructure zone. These parameters were used to construct the spatially variable kinematic excitation. Finite-element simulations were performed for the heterogeneous supporting medium and two homogeneous representations. Structural responses generally differed by approximately 20–30%. The homogeneous native-subsoil assumption generally underestimated the heterogeneous-case response, whereas the homogeneous road-substructure assumption overestimated it. These findings demonstrate the importance of accounting for supporting-medium heterogeneity when predicting the response of spatially extended structures to ground-borne vibrations.