Ground vibrations produced by surface and near-surface explosions
Abstract Measurements of seismic signatures produced by airborne, near-surface detonations of explosive charges over a variety of ground types show two distinct ground vibration arrivals. In all cases, the earlier arrival (precursor), has a time of arrival consistent with a predominantly underground path and coupling of blast sound to the ground close to the source and is always much smaller than the later vibration, the time of arrival of which is consistent with coupling from the air blast arrival at the receiver. The ratio of the seismic particle velocity to the acoustic pressure at the surface for the air-coupled seismic wave is constant with respect to distance and maximum pressure at a given location, but varies from site to site, with values usually between 1 and 13 μm s −1  Pa −1 . For the precursor seismic wave, a coupling coefficient of 0.16 μm s −1  Pa −1 was measured. A numerical code enabling calculations of the fields due to an impulsive source above a layered poroelastic ground is described. Predictions of the air pressure spectrum above ground and the vertical and radial components of solid particle velocity near the ground surface are found to compare tolerably well with the measured spectra and waveforms of acoustic and seismic pulses at about 100 m range in seismically-hard and -soft soils and with a snow cover present. The predicted seismic responses in ‘soft’ soil confirm that the existence of a near-surface S-wave speed less than that in air is responsible for the observed ‘ringing’, i.e. a long low-frequency wavetrain associated with coupling to the dispersive Rayleigh wave. The predicted seismic pulses in the presence of the (shallow) snow cover explain the observed phenomenon whereby a high frequency ground vibration is modulated by a lower frequency layer resonance. An empirical equation relating ground vibration from explosions to distance predicts that the commonly-used vibrational damage peak velocity criterion of 12 or 25 mm s −1 will be exceeded when the peak positive pressure exceeds 480 Pa (147.6 dB) or 1 kPa (154.0 dB), respectively. Either of these levels is much higher than the current U.S. Army overpressure damage criterion of 159 Pa (138 dB). Thus in most situations damage from blast overpressure will occur long before damaging levels of ground vibration are reached, so it is likely that civilian perceptions of vibration are produced by coupling from the airblast.
Meteoroid sonic shock-wave-generated seismic signals observed at a seismic array
At approximately 04:20 local time (10:20 UT) on 19 September 1986, a local resident observed a meteor in the sky above Yellow-knife, North-west Territory, Canada. We subsequently found seismic signals, recorded at the Yellowknife seismic array in Northwest Territory, Canada, that coincided with the visual observation of the meteor in the sky overhead. The signals cannot be explained by any earthquake, explosion, or impact-generating mechanism. They are shown to be consistent with their having been generated by the shock wave created by the associated meteoroid as it passed through the atmosphere overhead. The meteoroid's trajectory and velocity, as inferred from the array records, is consistent both with the observed direction of travel of the meteoroid and independent seismic observations recorded at two single stations in the Northern Canadian seismic network.
On the use of the Hardening Soil Small Strain model in geotechnical practice
This article highlights the importance of using advanced constitutive soil models in numerical modelling as daily geotechnical practice. In this context, the Hardening Soil (HS) model is presented as the most advanced soil model implemented in the ZSoil finite element code. The article recalls the basic features of the HS model and it explains their role in numerical simulations. The role of pre-failure non-linear behaviour of soils is also discussed. An application of the HS model is illustrated on typical geotechnical problems such as retaining wall excavation, tunnel excavation and undrained loading followed by consolidation.