3.6
As with air blasts or ground settlement, it is vital to conduct a preliminary information and explanation procedure with local population.
There are specifi c worksite measures that can help to reduce vibration levels. The principal potential measures when:
• blasting: a comprehensive revision of the fi ring plan > chronology of blast detonations,
> time between blasts, > blast value,
> nature of the drilling, etc.
• using mechanical machinery: accommodation of the time and nature of the work (period duration, power adjustment, etc.).
In addition to reducing vibrations at source, the inconvenience caused to local population can be limited by tailoring the works phasing (excavation approach direction, etc.) and setting time slots that fi t in with the activity of local population. The defi nition of set fi ring times can apply where sensitive activities or equipment are involved.
Lastly, specifi c measures can be taken in the vicinity of an underground structure in operation i.e. temporary traffi c stoppages
38
USEFUL REFERENCES
3.7
1] Annexe 4.7 de la section 4 du dossier pilote Génie civil : étude et contrôle des ébranlements liés à l'utilisation des explosifs / CETU / 1998.
[7] Terrassements à l'explosif dans les travaux routiers / Guide technique CFTR / 2002.
[8] Étude de vibrations provoquées par les explosifs dans les massifs rocheux / Rapport de recherche du LCPC N°105 / 1981 / Pierre Chapot.
[9] Étude des effets sismiques de l'explosif / recommandation de l'Association Française des Travaux en Souterrain (AFTES) / 1982 (update 2007).
[10] Effets de vibrations dues à des tirs de mines sur du béton aux jeunes âges / Thèse d'Anne Denoyelle de l'ENSM de Paris / 28 juin 1996.
[11] Tirs en masse et vibrations / édition EGICO / 1994 / Bruno Froment.
39
4. AIR BLASTS
For a long time, this topic was dealt with in association with that of noise and measured as such resulting in an underestimation of its actual impact, which is refl ected by the vibration of structures according to a specifi c regime that affects the raised parts of buildings.
The special case of the use of explosives for the construction of cut-and-cover sections is not covered as this involves open- air work projects.
The term "air blasts" describes the shockwave transmitted through the air when detonating mine charges. This phenomenon therefore only affects the tunnel construction phase where excavations are performed using explosives. Generally speaking, any explosion generates an air blast wave whose intensity depends on the volume of gas emitted and the emission velocity. Underground fi rings are characterised by low-level containment of detonations, a long fi ring sequence and propagation of the shockwave in the direction of the axis of the excavated tube. These particularities require a specifi c study of these phenomena of a strong, but localised, intensity.
40
4
Detonating an explosive charge involves a chemical reaction that converts a solid body into a gas at very high speeds (1 kg of explosive is converted into approx. 800 l of gas). These gases expand fi rstly inside the rock, splitting and displacing it, and then in the air, generating an airborne shockwave within a short distance of the fi ring, referred to as a "blast wave". A few metres beyond the explosive charges, this wave travels at the speed of sound (340 m/s), generating alternate areas of air compression and decompression (see illustration 5).
The high-velocity displacement of quarried materials (a few tens of m/s) also generates a similar blast wave but with a lower starting frequency.
All these waves, called N-waves, are characterised by an initial blast component at audible frequencies that corresponds to densifi cation of the gas (ambient air and gas from the fi ring) in a reduced volume concentrated immediately behind the wave front. The concentration of gas molecules in this area causes their depletion in the area behind, thus creating an area of depression affecting a larger volume, with the pressure gradually returning to its initial value. This area of depression is characterised by low, inaudible frequencies. These phenomena are similar to those associated with a lightning strike or a sonic boom.
Illustration 5: propagation of N-waves (source : LRPC de Clermont- Ferrand)
The frequency of these waves diminishes with increasing distance from the explosive charge, concentring beyond a few tens of metres in the inaudible ultrasound range (frequencies between 1 and 20 Hz).
Illustration 6: lower audibility limit (source : Fletcher et Munson)
The features specifi c to underground blasts are:
• low containment of explosive charges that promotes high- velocity emissions of gas into the atmosphere and the displacement of quarried materials: the charges are placed a few tens of centimetres from the front compared to a few tens of decimetres for aerial blasts;
• high specifi c explosive charge that generates a large volume of gas per cubic metre of quarried rock (approx. 1 kg explosive per m3 against 0.4 kg for aerial blasts); • initial propagation along the axis of the tunnel thus
concentrating the blast in a preferred direction, with very low attenuation inside the tube ("canon" effect);
• long fi ring sequence marked by a succession of waves over a period of 5 to 8 seconds against 1 second for aerial blasts: this heightens the discomfort of local population. When the successive fronts of this shockwave reach a building, they generate vibrations affecting light objects at height that escalate with the increasing intensity of the phenomenon (fi rstly ceiling lights, objects that are hanging or placed on a surface, followed by partitions and light ceilings, wooden and then concrete fl oors, structural frames, etc.). These shockwaves are likely to cause inconvenience and alarm to exposed local population; in some cases they may even damage certain structural elements. This airwave, the main energy of which is inaudible, is generally perceived by local population as a vibration.