Abstract: With the orthogonal optimisation algorithm, a fluid-solid coupling numerical model for subaqueous tunnels was established to analyse the arching laws of the pressure arch in the vertical direction above the vaults of the soil-rock compound strata and the full-section rock strata. The reasonable overlying thickness of subaqueous tunnels can be further derived based on the critical arching thickness criterion. Subsequently, a regression model for the overlying thickness of the subaqueous tunnel was proposed and further evaluated by BP neural network model. Finally, the regression model was applied to Foguan Shiziyang intercity tunnel to calculate its overlying thickness. The results showed that the height of the pressure arch was mainly affected by the overlying thickness when the lithology of the overlying strata was good, while it was affected by the thickness of soft soil when the lithology of the overlying strata was poor. The height of the pressure arch increased linearly with the increase of the overlying thickness, and then gradually decreased to achieve a stable state when reaching the critical arching thickness. When the lithology of the overlying strata became poorer, the critical arching thickness became greater. The variation of the pressure arch height in the full-section rock strata was less than that in the soil-rock compound strata. The example analysis shows that the reasonable overlying thickness model can be predicted according to the critical arching thickness criterion. Hence, the obtained results can provide references for the design and construction of the subaqueous shield tunnel in the soil-rock compound strata.

Key words: subaqueous tunnel, compound strata, overlying thickness, pressure arch, arching critical thickness