Reducing environmental adverse effects
The main environmental effects produced by underwater blasting are terrestrial vibrations and hydrostatic shock waves (López Jimeno, López Jimeno, & Garcia Bermudez, 2017, p. 881).
Land vibrations
Before any production activity is carried out, a study is made of the buildings close to the blasting zone to establish their current state of conservation and maximum permissible vibration limits. Once the actual state of these buildings is assessed, a detonation test campaign begins, which consists of a detonation of small charges located at known distances from these structures in such a way that the lower charge will be located at a shorter distance, and charges with higher magnitude are positioned far away from the previous one. This arrangement can be seen in figure 7 where a hypothetical test is carried out for four different charges ©, with this configuration it is possible to obtain twenty different values of particle peak velocity (PPV) registered by the seismographs network (S) distributed in points of interest at a specific distance (d). The expected results for this hypothetical test can be seen in table 2.
Figure 7. Test blasting campaign. The general arrangement of charges (C1-C4) at a given distance (d1-d4) of a seismograph station (S1-S4)
Table 2. test
With the data obtained, it is possible through multivariate statistical regression to obtain the coefficients of the general law of attenua, and once this is determined for a certain level of confidence, the operative maximum instantaneous charge (MIC) for the production blasts are then established. Determining the attenuation law can be done quite simply by using the O-Pitblast software, just importing the seismographs data (figure 8), and then determining the attenuation law for the desired level of confidence (figure 9).
Figure 8. Raw data import from seismograph and visualization (O-Pitblast).
Figure 9. Curves/lines and their respective attenuation law.
Hydrostatic shock wave
Hydrostatic shock waves generated by underwater detonations can cause damage to structures or vessels, divers, or existing aquatic fauna (López Jimeno, López Jimeno, & Garcia Bermudez, 2017). To minimize these damages, techniques are used that aim to both reduce the intensity of the hydrostatic shock wave and to scare away the fauna in its surroundings. For the first case, the use of bubble curtains is normally employed (figure 10). For the second case, fish deterrents systems are used (figure 11).
Figure 10. Bubble curtain operation
Figure 11. Fauna Guard transducers and hydrophone in São Francisco do Sul, Brazil (Van der meio, Marinus Maria Van Eekelen, Aart Kastelein & Mark, 2015)
Bubble Curtain
According to M Keevin and L. Hempen (1997) in their manual The Environmental Effects of Underwater Explosions with Methods to Mitigate Impacts describes the bubble curtain as:
A bubble curtain also called an air curtain or air screen is created by injecting compressed air into the water column. Bubble curtains are walls of bubbles rising from a bottom-resting bubbler manifold supplied with compressed air. Bubbler manifolds are typically constructed using rows of parallel pipes with small holes drilled along their length. The pipes are supplied with air from one or more distribution headers that equalize the pressure to each pipe. Bubble curtains are effective in reducing pressures across the air bubble curtain (Strange, 1963). Research has shown that a small fractional volume of air bubbles in water increases the compressibility several orders of magnitude above that in bubble-free water, thereby greatly reducing the velocity and increasing attenuation of acoustic waves (Domenico, 1982a). As a result, bubble curtains have been routinely used by demolition engineers to protect underwater structures from damage by underwater explosive shock waves (Domenico, 1982b). Guidelines for such use are given in (Langefors & Kihlstrom, 1978).
An example of the attenuation of the hydrostatic pressure promoted by the bubble curtain can be seen in (Lucke, Lepper, Blanchet, & Siebert, 2011), (Croci, et al., 2014), (PANĂ, ARRIGONI, GABILLET, & ORBAN, 2016).
In Underwater blasting operations, the bubble curtain is placed around the blasting zone after all holes are drilled and charged. Normally a tug boat is used for this task (figure 12).
Figure 12. Detail for the Bible curtain and proximity to the Port facilities. (Couceiro & Lopez Cano, Controlled Underwater Blasting in Santos Port, Brazil, 2016)
Fish deterrents systems
Marine fauna may be adversely affected by marine construction works as a result of physical interaction with construction equipment and/or exposure to high levels of underwater sound.
(Van der meij, Marinus Maria van Eekelen, Aart Kastelein, & Mark, 2015).
There are two general types of acoustic fish deterrents: continuous wave and pulsed wave. These deterrents use sound/pressure waves (noise) to influence the behavior of aquatic organisms. Continuous-wave sonar uses high-intensity, low-frequency sonar waves generated from a
Low-Frequency Active (LFA) sonar source array. Pulsed pressure waves are high-intensity sound/pressure waves generated by a sound source (hydro gun, air gun, blast explosive) to irritate aquatic organisms (U.S. ARMY CORPS OF ENGINEERS ) (https://www.usace.army.mil/).
Examples of applications can be seen in M Keevin e L. Hempen (1997), Wamboldt (2019), R. Noatch e D. Suski (2012), Van der mei, et al (2015).
Figure 13. FaunaGuard in Norrköping (Sweden) hanging off the drilling and blasting barge before deployment. (Van der mei et al, 2015).
ERALDO FLORÊNCIO
Senior Technical Advisor
+55 81 8610-3717
https://www.o-pitblast.com