Physical modeling of a submerged floating tunnel with equivalent truncation

• The truncated model replicating full-span SFT's dynamic behaviors was designed. • Experiments examined boundary constraint effects on structural dynamic behavior. • Wave-frequency resonance and half-wave frequency instability were observed in the experiment. • The unstable regions under different boundary conditions were analyzed theoretically. This paper presents the experimental results of the dynamic response of a submerged floating tunnel (SFT) under wave action using a constrained truncated model. To achieve equivalence in dynamic behavior to that of the prototype SFT, the truncated model is fitted with specially designed mechanical devices which are used to adjust the horizontal, vertical, and rotational stiffnesses at the boundaries of the truncated model according to the numerical results of the prototype structure. In the experiments, the displacement and mooring tension of the model structure were measured under different incident wave conditions and boundary constraints. The results confirm that, compared to a free section, the constrained truncated section has a higher natural frequency. Its first-order horizontal natural frequency, f S , and 2 f S , were 0.43 Hz and 0.86 Hz, respectively, both of which fall within the range of the incident wave frequencies in the experiment. As a result, both wave-frequency resonance and half-wave frequency instability were observed in the experiment. The amplitude of the half-wave frequency motion was significantly smaller than that under free boundary conditions. The analysis of mooring tension further revealed that under constrained boundary conditions, the phase difference of the mooring tensions between the mooring lines on both sides is significantly larger than that under free boundary conditions, and the tension amplitude also increased considerably. Additionally, under high wave height conditions, mooring cables experienced slack-taut process, with positive amplitudes reaching 3.2 and 2.8 times the initial tension for the seaward and leeward sides, respectively.