Influence of wavelength-to-excavation span ratio on ground motion around deep underground excavations

Accurate estimation of ground motion around excavations is important for dynamic rock support design in deep civil tunnels and underground mines. Among the influencing factors, the wavelength-to-excavation span ratio (λ/D) has a large effect on ground motion. Using an advanced wave propagation simulation tool, we performed two series numerical experiments to study the effect of the λ/D ratio on ground motion near excavation boundaries. The modeling results reveal that the wave field becomes more complex as the λ/D ratio decreases. The absolute PPV (Peak Particle Velocity) values around an excavation are closely related to the intensity of the seismic source but the relative PPV value depends on the λ/D ratio. Amplification factors, defined as the PPV in the excavation model to the PPV in the background model without any excavation, are calculated for each case. The amplification factor around the excavation increases significantly as the λ/D ratio decreases. When the λ/D ratio is greater than 30, the wave amplitudes are less affected by the excavation and a seismic wave loading can be considered as “quasi-static.” When the λ/D ratio is less than 20, significant wave interaction occurs and the wave loading needs to be considered as “dynamic.” The numerical results provide additional insights into the ground motion behavior around excavations under both “quasi-static” and “dynamic” loading conditions.