## 3 Onsite Bench Blasting Test of Three-Hole Simultaneous Blasting
### Blasting Plan and Parameter Design  
Due to the favorable environment in the surrounding areas, which is dominated by sandstone, a digital electronic detonator was employed in the design of tests in this work, with a precise delay and a three-hole simultaneous blasting scheme. The main parameters of the open-pit deep hole bench blasting included the aperture, hole depth, ultra depth, chassis resistance line, hole spacing, row spacing, packing length, and explosive consumption per unit [21]. 1. Aperture \(d\). The aperture of the open-pit deep hole blasting is largely related to the type of drilling rig. In the blasting area considered in this study, the aperture was 90 mm. 2. Hole depth \(L\) and super depth \(h\) The depth of the hole is determined by the height and super depth of the step. In this blasting area, the 1270 flat plate step height was 12 m, with a vertical drilling rig operation. Additionally, the ultra depth value is affected by the properties and structure of the rock mass, exhibiting a certain degree of proportionality with the diameter of the blast hole, as shown in Equation (1). \[h=(5\sim 10)d \tag{1}\]  
By Equation (1), the calculated super depth was 450-900 mm. In this blasting area, the super depth was set to 0.8 m. As a result, the depth of the hole was given by:  
\[L=H+h \tag{2}\]  
By Equation (2), the depth of the hole was 12.8 m.
3. Chassis resistance line \(W_{1}\)  
Figure 2: The blasting area considered in this study. The chassis resistance line was calculated considering the safety conditions of drilling operations as follows:  
\[W_{1}\geq H\cot\alpha+B \tag{3}\]  
where \(W_{1}\): Toe burden, in m;  
\(\alpha\): The slope angle of the step, generally between 70\({}^{\circ}\) and 85\({}^{\circ}\). In this design, it was taken as \(\alpha=80^{\circ}\);  
\(H\): Step height, in m;  
\(B\): The safe distance from the center of the borehole to the top line of the slope. In this design, it was taken as \(B=1.0\) m.  
By calculation:  
\[W_{1}\geq 3.1\text{ m} \tag{4}\]  
In this design, the toe burden was considered as \(W_{1}=3\) m.  
(4) Hole spacing \(a\) and row spacing \(b\)  
The hole spacing \(a\) was calculated using Equation (5). \[a=mW_{1} \tag{5}\]  
where \(m\) represents the density coefficient of the blast holes. In this design, \(m\) was taken as \(m=1.2\). As a result, the hole spacing was given as:  
\[a=3.6\text{ m} \tag{6}\]  
In this design, the hole spacing was taken as \(a=4\) m.  
When an equilateral triangle (Quincunx) is used to characterize the hole layout, the relationship between the row spacing and hole spacing was expressed as:  
\[b=a\sin 60^{\circ} \tag{7}\]  
By calculation, the row spacing was 3.46 m. In the present design, the hole spacing was taken as \(b=3\) m.  
(5) Filling length \(l_{2}\)  
The filling length \(l_{2}\) was determined by Equation (8). \[l_{2}=(0.7\sim 1.0)W_{1} \tag{8}\]  
By calculation, the filling length was 2.1 m-3.0 m. In this design, the filling length was taken as \(l_{2}=3.0\) m.  
(6) Unit explosive consumption \(q\)  
Based on the rock solidity coefficient (\(f=4\)-6) of sandstone in the blasting area, the sand shale (\(f=3\)-5), and the prior experience in blasting, the unit explosive consumption was given by:  
\[q=0.4\text{ kg/m}^{3} \tag{9}\]  
(7) Charge per hole \(Q\)  
The charge per hole for the first row of holes was calculated using Equation (10). \[L=H+h \tag{2}\]0  
where \(q\): Unit explosive consumption, in kg/m\({}^{3}\);  
\(a\): Hole spacing, in m;  
\(H\): Step height, in m;  
\(W_{1}\): Toe burden, in m.  
On this basis, the charge per hole in the first row was calculated as: \(Q=57.6\) kg. In the present design, it was taken as \(Q=60\) kg. Starting from the second row of holes, the charge per hole for each subsequent row of holes was calculated using Equation (11). \[L=H+h \tag{2}\]1  
where \(k\) denotes the increasing coefficient, which takes into account the effect of ore and rock resistance from the previous rows of holes, and \(k=1.1\sim 1.2\). In this design, \(k\) was set to 1.1.