Underground shaft excavation using the inverse (drop) raise method

27 May, 2021 | Blog

Félix Roy-Demers, P.Eng.

Engineer

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The construction and development of large infrastructure such as hydroelectric dam installations, underground mine site accesses, ventilation shafts or raises, can be undertaken using various rock excavation techniques.

Among these techniques, inverse raise excavation holds many advantages that make it a popular and highly effective method in a variety of ground conditions. In fact, this rock excavation method offers great flexibility. Unlike other methods, where specific drills and boring heads are required, inverse-raise excavation can be performed using a standard production drill, thereby simplifying the process while minimizing the associated costs.

Although well known, the inverse raise blasting method is still met with its share of challenges relating to parameters such as drill hole deviation, blasting sequences and explosives loading. This blog article provides solutions and tips to maximize your chances of success for this type of underground work.

Excavation sequence

Inverse raise excavation is characterized by driving an upward face while allowing for the extraction of the blasted rock material in the lower sill or base of the raise. Accessibility to both the upper and lower levels of the raise is an essential component for the applicability of this method. Explosive loading is undertaken from the upper drift. The main components or stages of the method are described below:

  1. The first blasting event is occasionally positioned at the base of the raise. Upper (oriented) blast holes are drilled along the back of the underground drift or sill. This ensures the geometry and stability of the main opening is controlled and allows for the addition of ground support, if deemed as a necessary measure.
  2. After creating the initial opening at the base of the raise, the drilling equipment is mobilized in the top sill to begin downward drilling of blastholes that will generate the overall blasting pattern for the duration of the excavation process.
  3. Since the blasting free face progresses from the bottom up, lower segments of the shaft are progressively blasted and slashed:
    1. At the lower level, an initial opening, commonly referred to as a “plug,” is created in the centre of the raise. Depending on the size of the shaft, a second plug blast can also be applied.
    2. Once the central opening is created in the first lower bench of the raise, the remaining rock mass along the perimeter is blasted and slashed in the previous opening (see the sequence shown in Figure 1).
    3. As the shaft excavation progresses, this blasting and extraction sequence is maintained until only one section remains.
  4. The rock material generated from the blasting operations is recovered and cleared from the base of the raise, i.e., the lower drift.

For the final shaft blasting event, (i.e., the one closest to the upper level) a rock pillar with sufficient thickness to ensure surface stability is kept in place to provide workers with a safe working floor during the explosive loading process. The opening and the perimeter are then mass blasted simultaneously for this final round.

Figure 1. Inverse-raise excavation sequence showing the opening and perimeter firings.

Drill hole deviation

To maximize blasting efficiency and control, the following actions must be considered to maximize drilling accuracy and assess hole deviation:

  • Survey the drilled holes at the upper and lower levels of the shaft using a standard break-through survey method or using a hole deviation probe. By measuring this deviation, the actual position of the explosive charges can be determined for each segment or round, allowing for loading adjustments to take place if deemed necessary.
  • Hole deviation values will also be considered as a means of optimizing the detonation sequence for each of the blasting events.

Figure 2. Perimeter holes in the shaft with adequate deflection control.

Umbrella charging

Maintaining a proper loading technique is the key to achieving controlled blasting. The recommended loading pattern is denoted as “umbrella,” because the heights of the individual charges form an arc between the opening (plug) blast and the raise perimeter, hence its resemblance to an umbrella shape. The detonation sequence will also be optimized to better manage varying burden thicknesses caused by deviation. The key advantages of this charging method include:

  • Reduced likelihood of cavities forming within the rock
  • Reduced likelihood of rock fragments clinging to the rock face
  • Better control and accuracy of blasting advance lengths.

Figure 3. Simplified representation of umbrella charging

Conclusion

To overcome the challenges associated with inverse (drop) raise excavation, parameters such as the blasting sequence, the drill hole deviation monitoring and the explosive loading technique applied are crucial elements to success. Although not extensively covered in this blog article, health and safety is prioritized at all times, regardless of the situation or methods used.

Our drilling and blasting team has completed numerous mandates in this field and will be happy to assist you!

 

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