Design of Pre Blasting (Pre-Splitting) in Tan Cang Quarry NO.1 in Vietnam

Nowadays, construction material quarries in Dong Nai Province are exploiting with large quarrying depth, and the annual output could reach to tens of million cubic meters. The blasting frequency could be reached to hundreds of times, so the frequency is the major reason decreasing the cohesion of rock mass. Therefore, the surrounding area of blasting holes is broken, especially the area next to the final border where bench slope angle is not implemented as that of design stage, as well as the back break, also causes fractures on the bench slope, resulting in instability and unsafety due to falling rock. In this paper, the author also wants to introduce the pre blasting and the method to define blasting parameters to increase the stabilization of Slopes in Tan Cang quarry NO.1 in Vietnam.


Introduction
Pre blasting is a controlled blasting technique with applications in surface mining and underground mining. The main objectives of pre-blast in surface mining are preventing, controlling back-break; controlling excessive ground vibrations; and filtering the effects of explosive gases from production blasting. Pre blasting techniques are therefore more costly than re-designed production blasting because of the greater amount of drilling required. It is essential that drilling and charging be carefully supervised because the final result depends heavily on the time spent and accuracy of drilling (Nhu Van Bach et al.,1998, 2008, (Кутузов Б. Н. 1992), (William Hustrulid, 1999).
There have been many studies by scientists on pre blasting such as (Singh et al., 2014): Pre blasting created free surface has been shown to be successful in controlling back-break. The separating surface attenuates propagation of expanding gases to the remaining rock mass, i.e. the final walls (Adamson, 2013). Design of blast experiments is a method of defining the optimal pattern of pre-splitting (Xu and Peng, 2008; Dai, 2005; Ozer et al.,2013). Design of controlled blasting (pre-splitting) in Golegohar iron ore mine, Iran (S. R. Dindarloo et al.,2015).
Nowadays, construction material quarries in Dong Nai Province are exploiting with large quarrying depth, and the annual output could reach to tens of million cubic meters, the blasting scale according to explosive -used license is assigned from 2 to 3-explosive tones.
Blasting frequency could be reached to hundreds of times, so the frequency is the major reason decreasing the cohesion of rock mass. Blasting -induced wave propagating from the basting site could cause the oscillation and collapse of bench face, leading to unsafety risk and stripping-ratio increase, production cost and resource loss.
Meanwhile, accordance with reclamation project after exploitation accepted by the ministry of resources and environment or Dong Nai Province's committee, ultimate quarries is reclaimed to become lakes for aquaculture, tourism, etc. However, in exploiting operation, quarries only apply normal blasting methods (electric delay, non-electric delay). Therefore, the surrounding area of blasting holes is broken, especially the area next to the final border where bench slope angle is not implemented like that of the design stage, as well as the back break, also causes fractures on the bench slope, resulting in instability and unsafety due to falling rock. The lakes after extraction operation have not been reclaimed as a designed landscape with the aim of the permanent society benefits.

Methods to reduce severe impacts on pit slope in blasting
In hard rocks, blasting can create impact zone surrounding the explosive charge, including crushing, breaking, fracture and elastic deformation zone (Fig 1).
Blasting aims to reduce and eliminate the back-break zone along with the ground vibrations within the elastic deformation zone. The distance from the explosive charge to the outer boundary of this zone is deformation radius. ( Where: α 1 -factor considering the effect of free faces in cracking resistance of the rock mass, in which α 1 =1 if there are two free faces and α 1 =1,2 if there is one free face. α 2 -the distance between study point and free face, in which α 2 =1 if the study point locates on or over the free face, α 2 =2 if the study point is inside the rock mass. Q -the weight of explosives, kg C 0 -index of fracture resistance of rock in blasting, kg/m3. The stability of pit slope is ensured by reducing the impact of blasting vibrations and eliminating the deformation zone within the rock mass (reducing the back break or deformation radius).

Using protection layers on the bench floor and slope
The protection layers cover the rock mass that needs to be protected in blasting and does not contain the explosives. The width of the layers is calculated carefully to protect the rock mass.

Reducing the explosive charge diameter
This method aims to reduce the explosive charge diameter to narrow the deformation zone. Small blast holes are commonly used instead of the large ones. For instance, replacing blast holes with a diameter of 150 mm with that of 110 mm or 32 mm can reduce the deformation radius 1.7 to 4.7 times. Controlled blasting favours this method.

Segmenting the explosive charge
The explosive charge can be separated into some segments or decking. The gaps between the segments are filled with air or stemming materials. Deformation radius is considered as equivalent one in this method. This method is commonly applied in controlled blasting.

Using low energy explosives
Low energy explosives allow reducing the deformation radius. Specifically, the deformation radius can be lowered with a ratio of √ (E⁄E c ) and √(E⁄E c ) when detonating a dense and cylinder-shaped explosive charge and non-dense cylinder-shaped explosive charge, respectively (where: E is a spe-

Using delay blasting
This method ensures the individual impact of each explosive charge on the rock mass in blasting. There is no upper bound in choosing the delay time. However, the delay time needs to be chosen carefully to ensure the rock fragment requirements while guaranteeing the safety of ground vibrations and blasting pattern.
All these methods above can be applied in controlled blasting.

Overview of terrain, mine geology, and mining parameters at the quarry Mining situation
Tan Cang quarry No.1 is allowed to extract an area of 108.8 ha with an agreement of Dong Nai province. The deposit at the quarry was explored down to -80 m. The location of the quarry can be described as followings: Next to Tan Cang No.6 in the north, two quarries made a connection at this side.
Next to Tan Cang No.7 in the south, two quarries made a connection at this side.
Next to a haulage ramp in the east, a protection pillar was left at this side.
Next to Buong river in the west and east-west, a protection pillar was left at this side.
At present, the quarry has been extracted over 22 ha in its centre and not reached the mining boundary. Also, the quarry has been exploiting to make a connection with Tan Cang quarry No.6 at the Northside.

Geological characteristics
The mining area mainly contains Andesite rocks which are hard and relatively homogeneous, within Binh Long formation. The physical properties of this rock are as followings: -Volumetric mass: 2,77 g/cm 3 -Density (Δ): 2,86 g/cm 3 -Internal friction angle (degree):

Trim (Cushion) blasting
This method breaks the rock mass sequentially from the outside to the required excavation boundary (trim holes are fired after production holes) (Fig. 2).
Advantages and disadvantages of this method are summarized below: * Advantages: -Able to use dipping and trim blast holes create the designed excavation boundary.
-Easy to control blasting, reducing flying rocks if initiating sequentially from the outside to inside.
* Disadvantages: -In drilling: + Require navigating blast holes correctively on the plan, cross-sections, and the field. + Increase the drilling length if the blast holes are dip. + Increase the number of blast holes due to the decrease of distance between trim blast holes.
-In blasting: + Result in back-break, unstable pit slope, and hanging rocks due to the transmission of stress waves from the outside.

Pre-splitting blasting method
Like trim blasting involves a single row of blast holes along the specified excavation boundary. The pre-splitting holes are located closely and fired prior to the production holes (Fig. 3).
* Advantages: -high chance to create a designed boundary, smooth bench slope and less hanging rocks. In the explosion process, a maximum stress wave is developed along the connection line between two explosive charges, resulting in a separate line within the rock mass which follows the designed boundary and creates a crack shielding the back breaks from the production holes.
* Disadvantages: -In drilling: similar to trim blasting method.
-In blasting: controlling the pre-splitting holes is quite complicated and can generate ground vibration and flying rocks. Nevertheless, charging a small amount of explosive in these holes can solve the problems. Tab. 2. Blasting parameters of pre-splitting holes using the equations above Based on the analysis above, pre-splitting blasting method was chosen for Tan Cang quarry No.1 with the aims of improving the efficiency in blasting operations, including creating a designed boundary, increasing slope stability, reducing hanging rocks, ensure working safety conditions, and protecting the surrounding environment.

Determination of pre-splitting blasting parameters for Tan Cang quarry No. 1
With the pre-splitting blasting method selected above, the hole pattern is divided into two areas, as shown in Figure  4. The area I includes the production holes. In this case, the quarry used the holes of 102 mm in diameter. Blasting parameters of production holes in the area I were chosen according to the current blasting report, as shown in Table 1.
Area II includes pre-splitting holes with the diameter dlk = 102 mm. Blasting parameters for these holes were determined as followings:

Explosive type
Currently, most quarries in the South East of Vietnam use ANFO, AD1, NT-13, NT-31. In this research, we selected NT-13, EE-31 or ANFO based on their parameters for the pre-splitting holes. Generally, these explosives have a medium-strength, low charge density, easy to use and diverse in diameters such as Φ60, Φ80, Φ90 for various drilling holes.

Explosive diameter and density
Explosives, which are packed in cylindrical cartridges or porous pilled forms, are preferred to create a space between The value between 0.2 and 1.0 can be chosen according to the rule of small value for soft rocks and a large value for hard rocks. Besides, decking charging method was chosen to narrow the deformation continues.
* Explosive density The amount of explosive per one meter within a pre-splitting hole can be calculated based on loading density and pre-splitting hole diameter, as in the Equation below: where: d k -diameter of a pre-splitting hole, mm; Δ -loading density, g/cm3 (ANFO: Δ = 0.9g/cm3; emulsions EE31 or EE13: Δ=1.0 g/cm3); k -filling ratio of the explosive (k = 0.85).

Spacing between pre-splitting holes
The spacing between pre-splitting holes can be deter- In practice, the spacing between pre-splitting holes can be determined as followings: Stemming length of pre-splitting holes should be 2÷4m. Also, these holes are drilled with an identical length, and no sub-drilling is required.
The distance from the pre-splitting holes and production holes equals half of the distance between the production holes.

Powder factor
The pre-splitting holes aim to extract the rock mass in order to create a smooth face. Hence, their powder factor is defined as the amount of explosive used to create a square meter of a smooth face, as determined in Equation 6, (Nhu Van Bach et al.,1998, 2008, (Кутузов Б. Н. 1992), (Sandvik Tamrock Corp 1999), (Calvin J. Konya et al., 1990): (6) Where: ∑Q -the total amount of explosives, kg; ∑S -total area of smooth face, m2. Equation 7 can be used to determine the powder factor for each hole. q m =Q/(a.l); kg⁄m2 (7) Where: Q -total amount of explosives for each pre-splitting hole, kg; a -spacing between pre-splitting holes, m; lpre-splitting hole depth, m.
Consequently, qm is then deployed to calculate the amount of explosive for a pre-splitting hole, as shown in Equation 8.

Structure of the explosive charge and initiation sequence
Structure of the explosive charge plays an essential role in pre-splitting blasting. It is necessary to account for the structure of the explosive charge to minimize the size of the deformation zone. Charge diameter contributes significantly to it. In practice, there are some methods such as creating a gap between the explosive charge; loading the explosive close to the zone required to break, and the remaining zone is sand; sticking explosive charges on a long wooden pole and loading this pole into the pre-splitting hole with its wooden part contacting with the required smooth face, (Nhu Van Bach et al.,1998, 2008, (Calvin J. Konya et al., 1990), Sandvik Tamrock Corp (1999).
Each pre-splitting blast hole contains 8 kg (equals 25% the amount of explosive in the production holes). Decking loading is used with pieces of rock. The diameter of the charge was d = 60 mm. The pre-splitting blast holes are initiated using delay non-electric detonators No.1.
Initiation sequence: the row of pre-splitting blast holes is fired first with the delaying time of 50 ÷ 75ms compared with the production blast holes in order to create an initial crack which is a shield to prevent the stress wave transmitting from the production blast holes. Figure 5, 6, 7, 8 presents pattern of pre-splitting blast holes on bench, the structure of explosive charge for the pre-splitting holes and the initiation sequence in this research.

Geological survey
A geological survey was implemented on the field to adjust the drilling and blasting parameters into practical conditions. Figure 8 illustrates the geological survey implemented in Tan Cang quarry No.1.

Drilling
It is crucial to keep drilling in the right direction and location. Tan Cang quarry No.1 has deployed the hydraulic rotary drill machine TAMROCK with a diameter of d = 102mm. Figure 10 describes how drilling was fulfilled at the blasting location. Fig. 11. Blasting implementation and results Rys. 11. Eksperymentalne prace terenowe

Blasting implementation and results
The experiments created a relative smooth bench floor, no back-break, and ensures the bench slope of 70 o ÷ 75 o according to the design. These results contribute to the safety in operations at the quarry significantly. Figure 10 shows the implementation of the blasting experiments at the quarry.

Conclusion
From the results, we propose a full blasting parameter for Tan Cang quarry No.1 which is also possible to apply in raw material quarries in the South East of Vietnam to reduce the harmful effects of blasting in slope stability and surrounding constructions.
Controlled blasting is necessary for deep open-pit mines and quarries, especially in the locations close to pit limits, landslide areas, making possible conditions to mine deeper, and recovering the mineral resource effectively.