Effects of Water Deck on Rock Blasting Performance

Water in the blasthole causes adverse effects on blasting performances, e.g., the incomplete explosion of explosive and toxic fume generation. It is normally pumped out before explosive loading to prevent explosive deterioration. In this study, a new blasting concept water deck blasting (WDB) is introduced which sealed the water at the blasthole bottom to prevent adverse effects of water as well as maximise the beneficial effect of water on rock breakage performances. A field experiment was conducted in a quarry, bench blasting including 23 watery holes among 139 blastholes. Promising results were observed as there was no evidence of toxic fume during blasting and achieved a competent level of fragmentation with even toe-cut at the bottom level. The WDB and a normal charge blast were simulated using ANSYS/AUTODYN and multi-peak oscillations were investigated for the WDB. The advantageous rock breakage performances of the relatively small amount of water rather than the amount of explosive have not been verified but have been discussed referring to the extreme water and the steam explosion phenomenon. The WDB blasting method expects to bring advantageous effects to surface mine blasting.


Introduction
In typical rock blasting exercises in surface mines, often the dynamic or static water level at the bottom of the borehole is unavoidable, which is known for causing several adverse effects on the blasting performance. In fact, many researchers and engineers have pointed out the water and humidity in a borehole as a primal causing factor for misfire, incomplete explosion, sympathetic detonation, dead pressure effect, and generation of toxic NOx fumes. The majority of adverse effects of the water in a borehole to blasting performances are due to the deterioration of the explosive. If water can be perfectly sealed, then the beneficial influence of the water on rock blasting performance would be maximised. In this study, water deck blasting (WDB) is presented, which could maximise the benefits of water in borehole but minimise the negative influences of water on the explosion performance.
In the following Chapter 2, the different aspects of water effects on rock blasting performance are discussed. Subsequently, in Chapter 3, the effects of water deck blasting (WDB) have been examined through numerical analysis. In succession, a field trial of water deck blasting (WDB) is presented in Chapter 4. Chapter 5 discusses fewer unique phenomena observed in this study. Also, a logical deduction has been made on the advantageous effects of the water deck by discussing the 'extreme water' and 'steam explosion'. The study is concluded in Section 6 with presenting conclusions and future studies.

Water effects on blasting performance
Ineffective blasting performances due to the watery borehole negatively influence the productivity of mines by causing poor fragmentation, poor level cut, back-break, and blasthole blowup. They also increase the environmental and safety risks resulting in the excessive vibration, fly rock, and toxic fumes. The majority adverse effects of the watery boreholes are caused by the incomplete explosion of the water contaminated explosives. In other words, the rate of adverse effects will be minimised if the water is completely isolated from the explosive column, which facilitates a complete explosion. The complete explosion would maximise rock breakage performances and minimise the adverse effects of watery borehole, by creating significant synergies with the favourable rock breaking conditions of the saturated rock mass, increasing the duration time of the induced explosion energy actions inside of the rock mass, and stress oscillation due to the water in a borehole.

Adverse effects of water in borehole
The majority of adverse effects of the water on blasting performances are due to the deterioration of the explosive. When explosives are immersed 24 hours in water, ANFO will loss 98% of Ammonium nitrate (AN) while the water-in-oil based emulsion explosives loss 0.5% AN 1 . The degree of adverse influences of the water would be varied in different types of explosives but it is unavoidable when the water present in the borehole and directly in contact with explosives.
The degree of explosive deterioration due to water depends on the period of explosive sleeping time in a borehole and the wetness of the borehole. The phenomenon can cause serious of changes on chemical components of adjacent explosive. Especially hygroscopic explosives, e.g., ammonium nitrate fuel oil (ANFO), the moistened explosive becomes desensitised and often failed to detonate. Given that the adverse effects, the water in a borehole would result shorten the diameter of the detonable explosive column, decelerate the velocity of detonation (VOD), toxic fume due to the incomplete explosion, and residual explosive in a borehole in a severe condition. Due to the watery borehole wall and the deteriorated explosive along the borehole wall, the VOD at wet borehole would be significantly slower than the VOD at the dry borehole.

Beneficial effects of water in borehole
The presence of water in a borehole could be beneficial for rock breakage performance if the water is thoroughly sealed. The beneficial effects of water in a borehole on the blasting performance have been hardly studied. Few studies on water-coupled and water-cushion blasting had been conducted in China for the blasting performance improvement and blasting induced vibration control 2-6 .

Effects of rock saturation to blasting damage zone
In general, the stress wave velocity is faster in the saturated rock than the dry rock. In addition, the strength of rock will be decreased when it is saturated ( ≫ ). Kim, Changani 7 conducted a research to find the effects of water contents on the mechanical strength of rock. The research found that both compressive and tensile strength of saturated rock samples were reduced approximately 20% compared with dry samples in both static and dynamic loading conditions.  Where is the radius of BDZ, presents Poisson's ratio, is the tensile strength of rock, is attenuation parameter, is the borehole pressure, presents the radius of blast hole.
In general, the Poisson's ratio ( ) tends to be increased along with decreasing Young's Modulus ( ) when rock becomes saturated. Thus, from the Eq. 1, becomes larger in a saturated rock than a dry rock if explosive charge conditions and attenuation parameters are identical. Figure 2 shows the relation between Poisson's ratio and the BDZ when is 0.50. It can be seen that when Poisson's ratio increases from 0.25 (Dry rock) to 0.35 due to saturation, the radius of Blasting Damage Zone (BDZ) is increased to 2.6 times of BDZ of Dry rock. The detonation behaviour of water deck blasting (WDB) cannot be fully elucidated with the underwater explosion or the rock strength deterioration under cyclic loading. However, a pressure transform in the form of cyclic loading has been observed at the bottom of a blast hole through a numerical simulation using AUTODYN 13 . Given the strength deterioration due to cyclic loading and pressure transform, the observed cyclic loading at the borehole bottom of the WDB contributes to exacerbating the rock fracturing process.

Energy encapsulation
Once the explosive is detonated inside of a blasthole, extreme amounts of explosion energy will be instantaneously discharged. The induced shock waves penetrate rock masses and promote microcracks within 1 -2 ms after the detonation. The cracks will be expanded and rock fragments will be pushed away by the blast-induced high-pressure gases. Not all the energy from the rapid chemical reaction of explosive would contribute to rock breakage. According to previous studies, around 7-25% of explosion energy would be effectively spent for rock breakage in forms of fracture, seismic, and kinetic energies, while 30% of energy would be lost through gas venting to atmosphere.
The uncounted remaining 40-60% of explosion energy would be transferred from gases to rock fragments to heat and deform the rock fragments 14,15 . Given the high rate of energy loss during the rock blasting, the breakage performance would be improved by increasing the duration time of the induced explosion energy actions inside of the rock mass. The blast incident energy would be encapsulated with layers of different materials. Needham 16 conducted air explosion experiments to investigate the burst effects of a snow layer on a concrete surface. A 453.59 kg (1000 pound) of explosive composing with 85% of HMX was detonated at 150 cm above the concrete surface with and without a snow layer. Through the experiment, it was found that the snow layer is not functioned as a cushion of the impact but encapsulates the blast incident energy. The result shows that the overpressure in the snow layer was nearly three times higher than the one on the exposed concrete surface. Due to the impedance differences between air and the snow layer, the sound speed in the snow layer has been reduced which blocked the rapid transmission of the blast incident energy in the snow layer. In case of water deck blasting (WDB), the water deck located at the bottom of the blast hole can be accounted as an energy encapsulation layer which extends the action moment of blast incident energy to the adjacent rock masses.

Rock blasting processes
The explosion is a rapid reaction of fuel and oxidiser in explosive which generates exceptionally high pressure and gases. The reaction ignites detonation waves, which travels at supersonic speeds of around 2000 to 7000 m/s. At the same time, the shock wave will penetrate through the rock mass. The adjacent rock to the explosion will reach a hydrodynamic state and pulverised as it exhibits an inelastic response. A nonlinear zone will come after the hydrodynamic zone, where the rock will be experienced from plastic deformations to severe fracturing. Beyond the nonlinear zone, rock will be in an elastic zone. In the elastic zone, compressive failure does not occur as the amplitude of blasting induced compressive pressure will become either same or lower than the compressive strength of the rock. However radial fractures will be generated as the tangential stress is still higher than the tensile strength of the rock.

Blast modelling using AUTODYN/ANSYS
Numerical analysis is carried out using AUTODYN from ANSYS to examine the effect of water deck at the bottom of the borehole. AUTODYN is a non-linear hydrocode that enables to simulate non-linear dynamic impacts incorporating Lagrange, Euler, ALE (Arbitrary Lagrange Euler), and mesh free solver. The program has been widely used for blasting loading analysis [17][18][19][20][21] . In this study, the

Geometry of the blast model
The

ANTODYN simulation results
The pressure-time histories at the blasthole bottom of normal blasting (Figure 4    Field experiment A trial of water deck blasting (WDB) using a high hydrophobic powder (HP) was conducted on a basalt quarry in Queensland, Australia. The HP is a blended powder product which is used as a multipurpose barrier to seal off problematic water in blast holes. In the trial, the HP isolates toe water and creates a completely dry hole for the immediate loading of explosives, which turns wet holes into dry holes instantly as there is no curing time needed.

Blasting design
The density of the columnar basalt was 2.8 kg/m 3 , which shows medium to high strength

Field observations of the water deck blasting (WDB) effects
The blast was examined by visual inspections of muckpile shape, fragmentation, and toe cut performance. As shown in Figure 5-c, there was no evidence of toxic fumes caused by contaminated ANFO which can be an evidence of complete separation of water and the ANFO in the blastholes.
Engineers in the mine were satisfied with results of the trial blast. As shown in Figure 5-d, muckpile was properly thrown and fragmented without having excessive oversize rocks. The mine often experienced bad toe cut performances, especially at the watery area. However, the trial water deck blasting results a flat floor overall area including the location of the 23 watery hole as shown in

Discussions
In the numerical analysis in Chapter 3, although the magnitude of the shock wave from the water deck blasting (WDB) on the blasthole bottom was decreased, a similar degree of damages was observed with the normal blasting (NB). Interestingly, few exotic phenomena were observed from the water deck blasting (WDB). The shock wave from WDB becomes thicker and demonstrates multiple peak oscillations. In addition, reflected waves at the boundaries of explosive-water and water-rock generate more complex shock wave interactions at the lower section of the blasthole.
These unique phenomena could be related to the clean toe-cut results in the field WDB experiment. vaporisation of water which is considered as a critical hazard in many industries. In the nuclear engineering, the energetic steam explosion at the nuclear reactor has long been considered as a purely physical heat transform of hot molten metal to water but the role of exothermal chemical oxidation has been discussed 34 . The steam explosion due to physical heat transfer requires extremely fast heat transfer rate. In the nuclear engineering, this rapid heat transfer is explained with an assumption of sudden molten metal dispersion to micro sizes 35 . If the steam explosion occurs in the WDB, the mechanism of a rapid heat transfer from the explosive explosion to water deck at the hole bottom must be discovered. The mechanism of steam explosion is still obscure, and more information on the steam explosion on nuclear engineering can be found from the listed references: [36][37][38][39] .
In case of water deck blasting (WDB), the precursor shock wave, which compresses the rock is delivered from the shock front of the explosion reaction zone. The shock wave would be thicker, and its magnitude would be decreased when it is passing through the water column at the blasthole bottom due to the high impedance difference. Then, the steam explosion would be followed which generates excessive pressures. The evidence of water deck steam explosion has not been captured in the AUTODYN simulation in this study as it requires a multi-fluid and dimensional model with assumptions for many uncertainties of the phenomenon.

Conclusion
In general surface mine blasting principles, the water in a borehole customarily pumped out before loading ANFO or replacing with emulsion-type explosives which have high water resistance.
The presence of water in the borehole is one of the critical agendas for mining companies not only as it decreases the productivity of the mine but also increases the safety issues.
The study introduces a new concept of blasting method, water deck blasting (WDB),