Abstract
To determine the dynamic demand for support design under rockburst conditions, one of the most important issues is the prediction of ground motion parameters at the site of interest. Field monitoring has shown that the peak ground motion at the surface of an excavation in fractured rock is preferentially amplified compared to the motion in solid rock at a similar distance from the source. However, the traditional scaling laws used in rock support design do not account for the effect of free surface (excavation) and fracturing of rock. Recent studies have shown that high ground motion might be generated when a seismic wave crosses through fractures near a free surface in fractured rocks which is very complex and is not well understood. In this paper, particle velocity amplification was theoretically studied by investigating the dynamic interaction between seismic wave and multiple fractures near a free surface using the method of characteristics and the displacement discontinuity model. A harmonic load was applied on a model with a fractured zone near a free surface to investigate this phenomenon. After the harmonic wave propagated normally through multiple parallel fractures, the velocity amplification factor (VAF) was calculated as a function of the ratio of the magnitude of the peak particle velocity at the free surface of the model to the peak input velocity. The VAF can be as high as 3.77 and varies depending on the state of the fractured rock and the characteristics of the seismic wave. Parameter studies were conducted to investigate the effects of seismic load and multiple fractures on wave propagation, especially in terms of the wave frequency, the fracture spacing, the number of fractures and the stiffness of fractures. The results have proved that the interaction of the seismic wave and multiple fractures near the free surface strongly influences the ground motion. Quantitative relationships between the various influential factors and the corresponding VAF were developed. It is anticipated that such relationships can provide criteria to improve the current design procedures and help mining engineers to improve their rock support practice for rockburst-prone areas.
Highlights
Rockbursts, defined as damage to an excavation associated with a seismic event, are major hazards in deep underground mines
This assumption is based on the observation that the dominant wavelengths from remote seismic events are typically much longer than the tunnel or drift dimensions and that wave reflections can be ignored. These assumptions were confirmed by Yi and Kaiser (1993) with a theoretical evaluation of rock ejection from passing seismic waves by assuming rock as an elastic continuous medium and it showed that ejection velocities under typical mining and seismicity conditions were less than, but close to the peak particle velocity (PPV)
The peak particle velocity (PPV) of ground motion has been widely accepted as the representative parameter for defining dynamic design load in rock support design (Kaiser and Maloney 1997)
Summary
Rockbursts, defined as damage to an excavation associated with a seismic event, are major hazards in deep underground mines. A common assumption is that the rock ejection velocity is equal to the peak particle velocity (PPV) (Wagner 1984; Roberts and Brummer 1988) This assumption is based on the observation that the dominant wavelengths from remote seismic events are typically much longer than the tunnel or drift dimensions and that wave reflections can be ignored. These assumptions were confirmed by Yi and Kaiser (1993) with a theoretical evaluation of rock ejection from passing seismic waves by assuming rock as an elastic continuous medium and it showed that ejection velocities under typical mining and seismicity conditions (dominant frequencies less than about 100 Hz) were less than, but close to the PPV. The dynamic response of the fractured rock surrounding an excavation was poorly understood due to lack of cost-effective strong ground motion monitoring and recording systems in the past
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