1D numerical simulation of velocity amplification of P-waves travelling through fractured rock near a free surface

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.

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“…This has been observed by field monitoring (e.g. Durrheim et al 1996;Cichowicz et al 2000) and proved by numerical modelling (Zhang et al 2015).…”

Section: Introductionmentioning

confidence: 52%

Velocity Amplification of Seismic Waves Through Parallel Fractures Near a Free Surface in Fractured Rock: A Theoretical Study

Zhang

Nordlund

Swan³

et al. 2018

Rock Mech Rock Eng

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“…As a consequence, the geometrical and mechanical characteristics of the near surface rock mass of an excavation should be taken into account when assessing its local the damage potential and the rock support performance. According to (Zhang et al 2015), the difference in rock type and geological conditions among the test sites (i.e., Kiirunavaara mine and the test sites in other countries) could be another reason for the high PPV observed in our tests.…”

Section: Discussionmentioning

confidence: 76%

“…This was explained to be a result of insufficient understanding of the source, and complications due to large amplitude motions and fractured state of the blasthole after detonation (Hildyard and Milev 2001a). Furthermore, in a (Thyni 2014) recent study the velocity amplification was investigated by modelling the dynamic interaction between the fractured rock and a free surface using a 1D model (Zhang et al 2015). The results indicate that the wave frequency, fracture stiffness, fracture spacing and number of fractures (thickness of fractured zone) are the main factors which affect the velocity amplification.…”

Section: Discussionmentioning

confidence: 99%

Large Scale Dynamic Testing of Rock Support System at Kiirunavaara Underground Mine

2016

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A series of five large scale dynamic tests were conducted at the LKAB Kiirunavaara mine using explosives to generate the dynamic load on the support system. This was done with the aim of developing a testing methodology for in situ testing of ground support. Furthermore, the response of the installed rock support system to strong dynamic loading was evaluated. The tests included ground motion measurements, fracture investigation, ground and support motion imaging, as well as deformation measurements. The results indicated that the relation between the burden and the used amount of explosive had a vital role in either reducing or involving the effect of the detonation gases in the test results. In addition, the type of explosive which was used in the tests had a great impact on minimising the gas expansion effects. Higher peak particle velocities were measured compared to those of similar large scale tests carried out in other countries. However, the level of induced damage was limited to a fractured zone behind the support system and propagation of cracks in the shotcrete. Measured peak particle velocities were used to calculate the kinetic energy transmitted to the fractured zone of the test wall. The energy absorption by the Swellex, reinforced shotcrete and weld mesh was estimated by measuring the elongation/deflection of the support elements and relating these measurements to previously conducted laboratory tests. The comparison of maximum estimated energy absorbed by support system with the maximum estimated kinetic energy indicated that as the support system is still functional, the energy is partly reflected back to the surrounding rock. The results of the measurements in Tests 1, 2, 4 and 5 are presented in this paper and the methodology used to design the tests is discussed.

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“…Love and Raleigh waves). A recent study by Zhang, Swan, and Nordlund (2015) investigated the effect of fracturing on velocity amplification through universal distinct element code (UDEC) modelling. The model consisted of a one-dimensional elastic rock bar with a length of 300 m and a width of 1 m. A dynamic load was applied normal to the surface of the bar and propagated through a series of regularly spaced parallel fractures.…”

Section: Effect Of Mine Layout On Seismic Wave Propagationmentioning

confidence: 99%

Do stopes contribute to the seismic source?

Linzer¹,

Hildyard²,

Spottiswoode³

et al. 2020

J. S. Afr. Inst. Min. Metall.

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SynopsisParameters such as source location, seismic moment, energy, source size, and stress drop are routinely calculated from mining-induced seismic data. Seismic moment tensors are inverted less routinely because their calculation is more complex and their accuracy depends on the network geometry, among a number of other factors. The models utilized in the source parameter calculations, the most wellknown of which is the Brune model, were developed for the global seismicity problem and assume a solid, homogeneous Earth model. However, the tabular orebodies in South African gold and platinum mines are mined extensively and the excavations (stopes) can extend for many kilometres. The seismic source mechanisms on deep-level gold mines are generally compatible with shear failure (Hoffmann et al., 2013), whereas the source mechanisms of events at intermediate-level bord and pillar mines in the platinum district are more compatible with pillar failure and accompanying stope closure (Spottiswoode, Scheepers, and Ledwaba, 2006; Malovichko, van Aswegen, and Clark, 2012).In this paper we investigate the influence of the stope on seismic inversions for the scalar moment, corner frequency/source radius, stress drop through numerical modelling using WAVE3D. The main objective is to determine whether the source parameters calculated from the recorded waveforms are due to a combination of the stope and shearing sources, rather than being related only to a shear source in the host rock. The modelled source is shear rupture in the footwall of a stope. The results show that the stope appears to have an appreciable effect on the seismic inversions. The seismic moment and source radius of the shear source in the stope are larger for the model with a stope compared to the model with no stope. The stress drop for the case with a stope is less than the applied stress drop, which could be an effect of the apparently larger source. This work provides a possible explanation of the second corner frequency often observed in the spectra of seismograms recorded in South Africa platinum mines. This has implications for the accurate determination of source parameters and the assessment of the intensity of shaking in stopes.

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1D numerical simulation of velocity amplification of P-waves travelling through fractured rock near a free surface

Cited by 11 publications

References 9 publications

Velocity Amplification of Seismic Waves Through Parallel Fractures Near a Free Surface in Fractured Rock: A Theoretical Study

Velocity Amplification of Seismic Waves Through Parallel Fractures Near a Free Surface in Fractured Rock: A Theoretical Study

Large Scale Dynamic Testing of Rock Support System at Kiirunavaara Underground Mine

Do stopes contribute to the seismic source?

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