Blasting tunnels (or drifts) is a necessity in all underground mining operations. Drill and blast techniques are the most conventional and common method and is most useful in high UCS and homogeneous rock types albeit slower than other methods. These tunnels are created for many reasons including:
- Using the excavated space for transport
- Using the excavated space as a transport route
- Using the excavation for an orebody access or a draw point
- Or to use as a means of excavating ore
What makes tunneling (or drifting) unique is the fact that there is only one free face to the blast. This introduces different difficulties not present in other types of blasting as there is only one route of escape for blasted material and the constriction of material requires a larger powder factor to perform the blast. It also provides less surface area for the reflection of stress waves. Most blasting patterns and techniques rely on the first sequences of the blast to open ‘a cut’ in the face essentially creating another free face and a route for the rock at depth to escape into.
Typically, in todays industry large drill rigs commonly known as “jumbos” are used for drilling at the face. These typically are diesel and electric powered where the diesel motor is used for movement and the electric power is needed for the drills itself. A trailing cable for electricity is held on a reel on the rear of the vehicle. A hydraulic hose also trails off the vehicle to provide water to the drill heads for cooling. Drilling in mining the development cycle tends to be the most time-consuming part of the cycle followed by mucking and loading. To combat this, jumbos are sometimes fitted with additional drills, sometimes up to three drills. This way as some drills are drilling others can be maneuvered into place and there is less downtime for the operator.
Various drilling patterns have been developed for approaching the blasting of drifts which are described in the following sub-sections. These patterns refer to the pattern of the initial cuts which are then blasted into from surrounding holes. This is known as stoping, and the additional holes are known as wall, roof or floor holes (also known as lifters) based on their region of placement while the holes closest to the original cut are known as stoping holes or cut spreading holes. These are delineated in the following figure.
Wedge, plough or V-cut
In this drill pattern, holes are first drilled at an angle to the face in a uniform wedge formation for the initial cut of the blast sequence. They are aligned so that the axis of symmetry is at the centre line of the face. This design is so the initial cut displaces a wedge of rock out of the face in the blast and then this wedge is widened to the full width of the drift in subsequent blasts. Following this the drift will be blasted to the vertical specifications. This is outlined in the following figure where it can be seen the optimal apex angle is as near as possible to 60° for the initial cut.
This type of cut is particularly suited to large size drifts, which have well laminated or fissured rocks. These cuts require a certain width to attain sufficient advance as in narrow tunnels the cut becomes pointed and difficult to blast. Also, with narrower widths drill rigs do not have space to make holes of the required angle.
Pyramid or Diamond Cut
This cut is a variation of the wedge (plough or V) cut where an additional axis of symmetry is introduced not only across a vertical axis but the horizontal as well for the initial cut. This blast design is more suitable for a symmetrical blast layout as can be seen in the following figure.
Drag and fan cuts
These are unique drill pattern suitable for small sectional drifts that do not produce a lot of material (and therefore do not need as much clearance for blasting). In this pattern aligned fans of drill holes are used with the one end of the fan’s holes at the steepest angle to create the initial cut for subsequent blasts to be shot into. These cuts are particularly useful as they do not require the large reamed holes that other blasts require to create the initial cut although they lack the ability to drive deeper cuts. Fan cuts refer to horizontally aligned cuts while drag cuts refer to the vertically aligned with the steepest angled holes at the bottom.
Breast Cut or Slashing
This type of blast design is typically used for expanding a previously excavated drift to accommodate more space or to correct a poor blast. Therefore, they are generally shallow blasts along a length of a wall. This blast pattern is a variant of the fan cut but all of the holes are angled along the length of the wall that is to be excavated.
Burn or parallel-hole cuts
This drill pattern uses a series of parallel holes are drilled closely spaced at right angles to the face. This results in the deepest excavation possible with the choice equipment and is therefore the most common drill pattern used where the rock mass allows. Since all holes are at right angles to the face, hole placement and alignment are easier than in other types of cuts. To compensate for the holes which do not propagate the blasts outward as much as an angled hole one or more holes at the centre of the face are uncharged. These uncharged holes are often of larger diameter than the charged holes and form zones of weakness that assist the adjacent charged holes in breaking out the ground. The following figure describes this pattern and the uncharged hole is white and marked with zero as it is not charged or detonated. This is a single example of a parallel hole cut as there are many variations with different amounts of uncharged holes.
The diameter of these large holes are very important in determining the success of the whole blast as can be seen in the following figure.
For both fragmentation and throw, blasting efficiency depends on the delay sequence of blasthole detonation. Delayed detonation improves loadability of the entire cut, contributes to a better strata control and reduction of blast-induced vibrations. For optimal fragmentation the initiation sequence should be such that each charge shoots to some free face, preferably concave (to better reflect stress waves). A strategy for this is a staggered pattern so that blasted rock in one area has time to clear while another spot is blasted. Blasting holes with slight delays between them provides the better results than an instantaneous blast due to the reverberation of stress waves and the use of newly developed free faces. The optimum delay increases with burden where approximately 5ms/m is the minimum interval. Blast patterns with square patterns are simple and provide good results where spacing equals the burden. Although, in hard rock mining triangular formations where the burden is 1.15 times greater than the spacing are more effective as it induces further twisting and shearing of the rock and avoids splitting between blast holes on planes before they are blasted.
Powder factor and the appropriate charging of drill holes is very important in underground mining as malfunctions are very common in the close proximity of loaded holes and the confining environment creating unfavorable breaking conditions. The above equations may be used to estimate parameters of a blast with other known values.
Burden and Spacing
This section will concentrate the burden and spacing of a parallel-hole cut as it is the most common and simplest to quantify. As a forward note, burden and spacing are both very dependent on hole diameter and subsequently the amount of charge in a hole. When blasting close to an uncharged hole the distance between must optimize the breakage and that the rock is cleared from the excavation. This is summarized in the following figure for various hole diameters and the effectiveness of the blasts.
To calculate the burden of the following set of holes to the larger uncharged holes the following formula is used:
The same formula applies to the surrounding set of holes but instead of diameter the width of the opening of what the previous holes should create is used:
These steps are summarized in the following figure for a simple parallel cut.
The holes must be charged carefully as not enough explosives will not break the rock and too much will recompact the broken rock without properly ejecting it from the created excavation. The charging of the holes in the first square of the cut around the uncharged hole can be determined using the following figure. The collar of these holes should be equal to the burden to the uncharged hole.
For subsequent holes in the cut a collar of only half of the burden is used and the following figure is used instead.
For the rest of the auxiliary holes including wall, roof, floor holes (also known as lifters) and stoping holes small adjustments to burden and charge concentration. The holes all require a bottom charge length equal to 1/3 of the hole burden in addition to a column charge. The burden is calculated on the basis of the charge concentration using the following figure.
The concentration of the column charge is 0.5 times the bottom charge and the uncharged region is 0.5 times the burden as well. For floor holes the uncharged region is 0.2 times the burden. For stoping holes with a downward breakage the burden is 1.2 times larger but calculated the same as before. The wall holes also have 1.2 times the burden and the bottom charge is reduced 1/6 the hole length instead of 1/3. The uncharged region is 0.5 times the burden as well and the concentration of the column charge is 0.4 times that of the bottom charge. Roof holes only difference to the wall holes is that the column charge is slightly less at 0.3 times the concentration of the bottom charge.
- Atlas Copco Mining and Rock Excavation Technique Canada. (2017). Face Drill Rigs. Retrieved from Atlas Copco: https://www.atlascopco.com/en-ca/mrba/products/drill-rigs/face-drill-rigs
- Hem, P. (2015, September). Tunneling. Retrieved from TechnoMine Mining Technology: http://technology.infomine.com/reviews/tunneling/welcome.asp?view=full
- Katsabanis, T. (2003). Rock Blasting - Fragmentation, Vibrations, Blast Design. Kingston: Queen's University.
- Mckinnon, S. (n.d.). Production Blast Design Mine448 Notes. Kingston.
- Mckinnon, S. (n.d.). Underground Blasting Mine448 Notes. Kingston.