Estimation of Shotcrete Volume for Mining Applications

Traditionally, technical oversight of ground control programs in mining engineering has often been carried out on an ad hoc basis, resulting in less attention given to the preparation and enforcement of technical specifications. However, many mines now hire specialized contractors for mining-related services. This shift has made the development of structured specifications more important to ensure optimal final results, particularly in shotcrete applications, as this method is becoming increasingly popular for reinforcing ground control systems in mines.

Factors Affecting Shotcrete Volume

Several factors affect the estimation of shotcrete volume, such as the shape and condition of the excavation. Once the excavation's dimensions and shape are determined, the theoretical perimeter of the excavation can be calculated. From this, an estimate of the shotcrete volume per linear meter of the tunnel is made based on the desired thickness of the shotcrete, plus a tolerance for rebound.

Key Influencing Factors

There are two main factors that affect the final shape of the excavation perimeter:

1. Surface Irregularities Due to Rock Mass Conditions: These are influenced by the quality of the rock mass, which cannot be engineered.
    
2. Surface Irregularities Due to Excavation Processes: These are more dependent on the drilling and blasting techniques, which can be engineered.

These two factors create what is called the Roughness Factor.

Rock Mass Condition Irregularities

The surface irregularities of the rock mass after excavation are influenced by several parameters, including:

• Rock Blockiness: Affected by the size and shape of the rock blocks.
• Rock Competence: Influenced by the width of the gaps in the discontinuity surfaces.
• Natural Structure Orientation: Related to the direction of excavation.
• Presence of Faults or Groundwater: Can lead to overbreak (excess excavation).

There are four categories of rock mass conditions:

1. Very Good: Almost no geological influence, rock mass does not affect the perimeter.
2. Good: Few joints with low impact.
3. Fair: Blocky rock with smooth surfaces, significant impact.
4. Poor: Large blocky rock with smooth surfaces, possible fault zones and wet ground conditions, very high impact.

Drilling and Blasting Process

Control over the drilling and blasting process greatly influences the excavation outcome. Factors affecting its success include blast design, drilling accuracy, and detonation sequence. The quality of this process is categorized as:

1. Excellent: Half-barrels visible throughout, minimal overbreak.
2. Good: Some deficiencies leading to slight overbreak.
3. Fair: Moderate overbreak outside the design.
4. Poor: Significant overbreak.

The combination of rock mass conditions and excavation processes forms the Roughness Factor, which is used to estimate the increased perimeter of the excavation.

Shotcrete Rebound Influence

In shotcrete applications, rebound is an unavoidable waste. It is influenced by the mix of shotcrete material, spraying speed, and the applied thickness. Initially, rebound tends to be high but decreases as the layer thickness increases. Using a good shotcrete mix and proper application techniques can reduce this rebound.

Tabel 1

Shotcrete Thickness Concept

In mining projects, the shotcrete thickness applied is not always uniform. For excavations carried out using drilling and blasting methods, a "coat and fill" approach is more commonly used. This means that in addition to the minimum layer, extra filling in uneven areas is required. For example, to achieve a minimum thickness of 25 mm, an average of 50 mm is usually required, with a rebound of about 15%.

Shotcrete Application Planning Process in Mining Operations

Excavation surfaces created by drilling and blasting methods are often not smooth or uniform, so the excavation perimeter is often larger than initially designed. The amount of overbreak (excess excavation) is greatly influenced by the natural characteristics of the rock as well as the quality of the drilling and blasting process.

During the design phase, shotcrete application is considered by estimating both the minimum and average thickness required. The rebound rate is also factored in to estimate the Volume Factor, which is based on the ground conditions, drilling process, blasting, and rebound, as explained in Table 2.

Using the Roughness and Rebound Factors

The Roughness Factor in Table 2 is derived from a combination of Ground Conditions and Drilling & Blasting Process, as shown in Table 1. The shaded areas indicate combinations that are unlikely to occur in the field. For example, poor rock conditions will not require a 50 or 75 mm shotcrete layer, while good quality rock may not require 75 or 100 mm of shotcrete.

These figures are similar to empirical data developed by the mining industry in Australia over the past few years. With this method, engineers can identify where each component of the Volume Factor originates.

Steps in the Shotcrete Planning Process

The following are the necessary steps to estimate shotcrete volume in drilling and blasting operations at a mine:

1. Select locations requiring shotcrete application.
2. Determine the type of shotcrete used:
3. Define the area to be sprayed with shotcrete.
4. Confirm the design according to engineering specifications.
5. Review the site to determine the excavation shape.
6. Estimate geological factors.
7. Estimate drilling and blasting factors.
8. Measure the effective cut/round length (this can be longer than the excavation length due to loss of previously applied shotcrete).
9. Estimate the design area requiring shotcrete.
10. Estimate the roughness factor.
11. Determine the design thickness (minimum and average).
12. Estimate the rebound factor.
13. Calculate the volume factor and estimate the shotcrete volume required.

Instructions for Contractors

The shotcrete request form should include detailed information about the timing and location of the application, as well as a cross-section or top-down view of the excavation profile, including overbreak and potential collapses. This information should also include length, perimeter, thickness, factors, and shotcrete volume, both estimated and applied. This form will serve as a reference document for evaluating differences between the estimated and actual applied volumes in the field.

Additional Considerations

Other factors such as wall and back peeling quality, operator training, potential collapses, and the presence of other support systems may also influence the amount of shotcrete applied. Some of these factors may be beyond the contractor's control, but should still be considered in shotcrete application planning.

With thorough planning and consideration of all these factors, shotcrete applications in mines are expected to be more effective and efficient.

Conclusion

Shotcrete volume estimation cannot rely solely on minimum thickness. Various factors such as rock mass conditions, drilling processes, rebound, and the desired average thickness must be considered. By taking all these factors into account, a more accurate shotcrete volume estimate can be made to ensure the mining excavation project is effective and efficient.
Source: Wood, D. F. 1999. Estimating Volume of Shotcrete for Mining Applications. Shotcrete Magazine. August 1999.

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Hello APT Friends👋👷‍♀️👷‍♂️

The application of shotcrete is a reliable solution for preventing landslides in mining areas.
One key element in its use is volume, which is influenced by various factors such as the shape and condition of the excavation site.

Accurate volume estimation not only enhances effectiveness but also optimizes costs.

How can we accurately estimate the volume of shotcrete? Find out more here!

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