Analysis of Overbreak in Development Headings of An Underground Metal Mine

1.1 General Background

Mining has historically sustained industrial growth by supplying essential raw materials (Hartman & Mutmansky, 2002). While open-pit methods dominate shallow deposits, underground mining becomes indispensable at depths beyond 300 m or where surface disturbance must be minimized (Brady & Brown, 2006). Globally, underground methods enable access to deeper, often higher-grade ores while reducing land-use conflict compared to surface operations (Hustrulid & Bullock, 2001). In India, depletion of near-surface deposits has increased reliance on underground mining (DGMS, 2020). Among available excavation methods, drill-and-blast remains the most widely applied for hard rock, owing to its flexibility, relatively low capital demand compared with tunnel boring machines, and adaptability to varied geometries (Singh, 2018). However, blast energy is difficult to control. Detonations generate stress waves and gas pressures that propagate beyond intended contours, creating three distinct zones: overbreak, or excavation outside design boundaries; damaged rock, with reduced strength; and disturbed zones, characterized by minor stress adjustments (Ibarra et al., 1996; Saiang & Nordlund, 2007). Overbreak is the most visible and operationally disruptive of these.

Figure 1 Overbreak (Front view)

Figure 2 Overbreak in development heading (Plan view)

1.2 Importance of Overbreak Control

Overbreak is often treated as a minor irregularity, but its implications are significant.

• Economic impacts: Excess excavation increases mucking, hauling, and disposal costs, while inflating consumption of shotcrete, bolts, mesh, and backfill. Stabilization costs may reach INR 2000/m³, with project-wide development costs rising 15–18% (Verma et al., 2018; Foderà et al., 2020).
• Safety impacts: Overbreak destabilizes profiles, leaving wedges and blocks susceptible to collapse. This exposes miners during scaling and complicates effective installation of supports (Sellers, 2011).
• Operational impacts: Irregular profiles obstruct ventilation, delay drilling sequences, and hinder installation of utilities. These small inefficiencies, when accumulated, reduce advance rates (Orica, 2014).

1.4 Knowledge Gaps and Research Need

Although overbreak is well documented in tunneling and some international mines (Ibarra et al., 1996; Saiang & Nordlund, 2007), there is lack of Indian field based overbreak studies in underground mines. Most published research emphasizes laboratory tests or predictive modeling, often in the context of civil tunnels whose design and performance criteria differ from metalliferous mining (Foderà et al., 2020; Hong et al., 2023). Given India’s lithological complexity, stress variability, and operational difficulties, there is a need for empirical, field-based studies that document overbreak patterns and evaluate mitigation strategies specific to mining. This research work takes 5-month fields data, analyses quantified overbreak and suggests modified blasting strategies.

1.5 Objectives and Methodological Overview

This study was designed to:

• To study various blasting techniques used in development headings of underground metal mines.
• To monitor the overbreak in different kinds of development headings.
• To analyse the relation between different blasting parameters and overbreak in development headings.
• To assess the time delay in completion of development headings caused due to overbreak.
• To propose modified blasting technique for different development headings.

The methodology comprised: total-station surveys for post-blast profiles; volumetric comparisons using AutoCAD and Geovia SURPAC; statistical evaluation of relationships between overbreak and blasting parameters; validation with case studies; and modified trials featuring decoupled charges, optimized stemming, refined delays, and improved drilling accuracy. This ensured empirical rigor and operational relevance.

1.6 Structure of the Paper

The paper is structured as follows: Section 2 reviews literature on blasting and overbreak control; Section 3 presents the study site and methodology; Section 4 reports field results and analysis; Section 5 proposes and evaluates a modified blasting strategy; and Section 6 concludes with findings and recommendations.

2. Literature Review

2.1 Blasting Techniques for Development Headings

The drill-and-blast method remains the dominant excavation technique in underground hard-rock mining due to its adaptability across diverse geological conditions (Hustrulid & Bullock, 2001). Unlike surface mining, however, underground blasting faces the limitation of a single free face, necessitating specialized cut designs such as wedge-cut and burn-cut.

Wedge-Cut Blasting:
The wedge-cut, an early innovation, employs angled blastholes forming a V-shaped cavity to generate relief. Though effective in small drivages, its sensitivity to drilling accuracy makes it unsuitable in jointed or weak rock, where cavity formation often fails and results in unbroken holes or poor fragmentation (Hartman & Mutmansky, 2002). Consequently, wedge-cuts are rarely applied in large-scale mechanized operations.

Burn-Cut Blasting:
Burn-cut designs employ uncharged reamer holes surrounded by charged blastholes, facilitating expansion of the cavity. Typical burn-cuts for a 4.8 m × 4.8 m heading involve 50–60 holes, making the method compatible with mechanized drilling jumbos and parallel-hole layouts. Though cut rounds consume more explosives, their reliability and adaptability in competent formations ensure widespread use, particularly in Indian base metal mines (Singh, 2018). While wedge-cuts retain niche utility in smaller headings, the burn-cut has become the standard practice due to its consistency and compatibility with mechanized rigs.

2.2 Perimeter Controlled Blasting

Cut design establishes an initial cavity, but perimeter control determines wall stability. Unlike production blasting—focused on fragmentation—controlled blasting emphasizes limiting damage (Orica, 2014). Several methods are documented:

• Decoupled Charges: Reduced-diameter cartridges within larger boreholes lower borehole pressure and confine radial cracks. Trials in Indian mines showed up to 25% reductions in overbreak at decoupling ratios of 0.5–0.7 (Singh, 2018; Orica, 2014).
• Line Drilling: Rows of uncharged boundary holes act as stress-relief planes and reduce crack propagation. Effective in competent rock, the method is less reliable in weak formations (Brady & Brown, 2006).
• Firing Sequence Optimization: Extended delays for perimeter holes (50–100 ms) allow central muck displacement before boundary detonation. Millisecond-precision electronic detonators have enhanced effectiveness (Iverson et al., 2013).
• Explosive Selection and Charging: Low-velocity explosives, such as ANFO blends and low-density emulsions, reduce shock energy transfer. Stemming methods like air decking minimize outward shattering (Sellers, 2011).
• Advanced Tools: Sophisticated modeling (e.g., FLAC3D, LS-DYNA, ANSYS) and vibration monitoring have enabled predictive adjustment of blast patterns. Indian field studies show that simulation-based designs substantially improve perimeter quality (Foderà et al., 2020).

Together, these techniques illustrate that wall integrity depends not only on explosive energy but also on precision in design and execution.

2.3 Review of Related Research Work

2.3.1 Mechanisms of Overbreak:

Ibarra et al. (1996) demonstrated that geological discontinuities and blast design parameters jointly control overbreak. They introduced the Perimeter Powder Factor as an important index of damage. Saiang and Nordlund (2007) extended this by modeling blast damage zones, showing that overbreak is part of a broader degradation process influenced by tensile strain.

2.3.2 Advances in Controlled Blasting:

Sellers (2011) emphasized controlled blasting’s role in reducing both overbreak and safety hazards. Iverson et al. (2013) highlighted buffer holes as an effective energy barrier between production blasts and excavation boundaries. Industrial trials reported by Orica (2014) demonstrated reductions of overbreak from 30% to <5% through modified explosives and refined delay sequencing.

2.3.3 Empirical Models and Drilling Accuracy:

Verma et al. (2018) derived correlations linking damage distance to rock quality (Q-value), charge weight per delay, and confinement. Singh (2018) stressed drilling accuracy as the most decisive operational factor, since computer-aided jumbos outperform manual drilling in precision. Ganesan and Mishra (2020) distinguished constructional overbreak (drilling and execution errors) from geological overbreak (lithological weaknesses), establishing that rock quality governs which factor dominates.

2.3.4 Recent Developments:

Foderà et al. (2020) introduced laser scanning technologies for distinguishing technical versus geological overbreak sources. Vishwakarma et al. (2020) analyzed Indian base metal mines, linking high-VOD explosives and poor delay sequencing to heightened overbreak. More recently, AI-based models such as XGBoost have been applied to integrate nonlinear influences of geology, drilling, and blast design, with promising results for parameter optimization (Hong et al., 2023; Liu et al., 2023).

2.4 Causes of Overbreak

From the literature, four consistent causes of overbreak emerge (Fig. 3)

1. Geological and stress-related conditions – weak or jointed rock masses, shear zones, weathered material, or adverse stresses (Ibarra et al., 1996; Ganesan & Mishra, 2020).
2. Blast design parameters – inappropriate burden or spacing, excessive charge density, coupled charges, and poor sequencing (Verma et al., 2018; Iverson et al., 2013).
3. Operational and human factors – drilling deviations, stemming errors, or inconsistent execution (Singh, 2018).
4. Explosive energy characteristics – high-VOD or unsuitable formulations based on local lithology (Vishwakarma et al., 2020).

These categories directly frame the present study, which evaluates geological variations, drilling and design parameters, and perimeter charging practices under Indian mining conditions.

2.5 Impacts of Overbreak

The literature consistently identifies negative consequences:

• Reduced stability due to damaged excavation walls and wedge failures (Saiang & Nordlund, 2007).

Figure 3 Causes of overbreak

• Increased costs of 15–18% from elevated support demand and corrective profiling (Foderà et al., 2020).
• Productivity losses from additional scaling, muck handling, and slower cycle times (Orica, 2014).
• Additional environmental burdens from increased waste rock volume for disposal (Brady & Brown, 2006).

METHODOLOGY

A systematic methodology (Fig. 4) is crucial to investigating overbreak, since the phenomenon arises from the interaction of geological, design, and operational parameters. This study adopted a field-based, empirical approach in which overbreak was quantified through precise survey measurements, correlated with blasting variables, and validated through both statistical analysis and comparison with published literature. The methodology comprised four key stages:

 (i) characterization of the study site,

(ii) structured data collection,

(iii) data processing and statistical analysis, and

(iv) validation and verification.

3.1 Study Site

The investigation was carried out in a large underground metal mine located in Rajasthan, India, one of the country’s most mineral-rich states. The mine extracts base metals using underground development methods, with ore bodies hosted primarily within graphite mica schist. The geological environment is further complicated by intersecting shear zones, zones of waste rock, and regions filled with pastefill from earlier stoping operations. The mine follows standard development practices, advancing arched headings with dimensions of 4.8 m × 4.8 m. The drivages are advanced primarily using the drill-and-blast method, executed with mechanized twin-boom jumbos. Ground support consists of resin-grouted rock bolts, installed in a systematic pattern but often requiring additional support in weak or overbroken ground. This setting provided an ideal field laboratory to study overbreak, given its geological variability and reliance on conventional burn-cut blasting techniques.

Figure 4 Methodology of research work

3.2 Data Collection

Field data were collected systematically over a five-month monitoring period, ensuring sufficient sample size and temporal coverage. The rage of collected data is shown in table 1.

3.2.1 Geometrical Measurements

• Overbreak Percentage (%OB): Defined as the volumetric deviation between the designed excavation profile and the actual post-blast profile.
• Pull: The actual length of heading advance achieved.

Profiles of development headings were captured using a total station survey instrument, providing high-accuracy three-dimensional point data. Survey stations were established at regular intervals, and post-blast profiles were measured immediately after mucking to avoid distortions caused by subsequent scaling or support installation.

3.2.2 Drilling and Blasting Parameters

Operational records were collected for each blast round, including:

• Number, length, and diameter of blastholes.
• Reamer hole dimensions.
• Explosive type, density, and total charge weight.
• Initiation sequence and delay timing.

3.2.3 Operational and Support Data

To assess the downstream impacts of overbreak, additional operational indicators were monitored:

• Support consumption: Number of resin-grouted bolts installed in each round.
• Cycle delays: Additional hours spent on scaling and stabilization due to overbreak.

During the monitoring period, all deviations from the planned support pattern were recorded and linked to measured overbreak events.

Table 1 Range of collected data: -

3.3 Data Processing and Analysis

The collected datasets were processed through a multi-stage workflow:

3.3.1 Profile Analysis

Survey data were imported into AutoCAD and Geovia SURPAC, both widely used in mining for geometric and volumetric analysis. Designed excavation profiles were overlaid with actual survey profiles to calculate deviations. Overbreak was quantified as:

%OB=(V_actual−V_design)/V_design×100

where V_actual? is the measured excavation volume and V_design is the planned excavation volume.

The software platforms also enabled visualization of overbreak distribution along heading walls, crown, and floor, facilitating identification of localized trends.

3.3.2 Statistical Analysis

Data were subjected to descriptive and inferential statistical analysis. The following metrics were computed:

• Central tendency: Mean, median, mode of overbreak values.
• Dispersion: Standard deviation, variance, and coefficient of variation.
• Distribution characteristics: Skewness and kurtosis, to detect the presence of high-magnitude outliers.

To explore relationships between overbreak and blasting parameters, correlation analysis was performed. Pearson’s correlation coefficients were calculated between overbreak percentage and selected variables, namely final cup density, burden-to-spacing ratio, and charge concentration.

3.3.3 Operational Impact Assessment

The operational consequences of overbreak were quantified in terms of:

• Ground support over-consumption: Measured as the difference between actual and planned resin bolt usage.
• Additional labor hours: Estimated by tracking man-hours spent on scaling and extra support beyond standard cycles.

3.4 Validation and Reliability

To ensure the robustness of results, multiple validation steps were incorporated:

1. Cross-checking with Literature: Observed ranges of overbreak and its causes were compared with published case studies from both mining and tunneling projects (e.g., Orica, 2014; Singh, 2018; Foderà et al., 2020). Consistency with established benchmarks confirmed the reliability of site-specific findings.
2. Triangulation of Data: By combining survey profiles, operational logs, and support consumption records, the study minimized reliance on any single dataset, thus enhancing credibility.

3.5 Ethical and Practical Considerations

The study was conducted within the operational framework of the host mine, with due regard for worker safety and production schedules. All data collection activities were coordinated with mine management to avoid disruption of operations. No confidential production data beyond blasting and support information were disclosed, maintaining compliance with industry protocols.

RESULTS AND DISCUSSION

This section presents results from field investigations and their interpretation. Findings are grouped into four themes: current blasting practices, quantification of overbreak, operational impacts, and statistical relationships. Each is critically discussed in the context of geological variability, blast design, operational practices, and existing literature.

4.1 Current Blasting Practices

The study mine predominantly employs the burn-cut method for development headings. Each round involves 50–60 parallel blastholes drilled using mechanized jumbos, with four large-diameter reamers forming the central void. Bulk emulsion explosives are used, with detonation sequenced as cut → easers → lifters → perimeter holes via non-electric detonators. Although perimeter holes are lightly charged to minimize wall damage, outcomes were inconsistent. Some rounds produced smooth excavation surfaces, while others demonstrated significant overbreak. Variations were attributed to (i) minor deviations in drilling alignment, (ii) variability in explosive column distribution, and (iii) local geology, particularly shear zones. These findings echo Singh (2018), who emphasized drilling accuracy in burn-cut performance, and Orica (2014), which documented variability in perimeter charging effectiveness in heterogeneous ground.

4.2 Quantification of Overbreak
Survey-based volumetric comparisons revealed an average overbreak of 7.68% relative to designed excavation. Most rounds fell between 4–10%, though extreme cases occasionally exceeded 20%, largely in weak geological zones.

Figure 5: Average OB in different rock type

Figure 6 Box and whisker plot for OB data

The data exhibited positive skewness, driven by infrequent but severe outliers. Such events significantly raise averages and underscore the need for designs that accommodate variability rather than targeting mean conditions only.

4.3 Operational Impacts

Overbreak had direct consequences on mine efficiency.

Ground Support: During the study, 3,081 more resin-grouted bolts were installed than planned, representing a 23.5% increase. This reflects the extra support required to stabilize irregular boundaries.

Labor Requirements: Scaling and installing supplementary support consumed an additional 285 man-hours, elongating cycle times and reducing development advance rates.

Broader Impacts: Beyond immediate stability, irregular profiles hampered drilling alignment in subsequent rounds, obstructed ventilation circuits, and complicated utility installation. The cumulative effect suggests that overbreak imposes both short- and long-term operational inefficiencies.

4.4 Statistical Relationships-

Table 2 Correlation of blasting parameters with OB

Final Cup Density: Higher charge density in cut holes correlated positively with overbreak (can be observed in Fig. 7) as high-energy zones induced excess cracking. Iverson et al. (2013) observed similar uncontrolled fracturing from dense central charges.

Overall, these findings suggest overbreak correlates more strongly with energy distribution than with total explosive quantity.

Figure 7 Scatter plot final cup density vs overbreak

4.5 Comparative Discussion with Literature

The measured average overbreak of 7.68% aligns with reported international benchmarks (5–12%) (Foderà et al., 2020; Singh, 2018). Sporadic cases >20% reinforce the nonlinear nature of overbreak in heterogeneous ground. Observed key parameter—final cup density, is consistent with earlier studies (Ibarra et al., 1996; Iverson et al., 2013). This validates the universality of certain design principles across diverse geological contexts.

4.6 Key Insights

The study generated several insights:

• Overbreak is causally linked to identifiable geological and blasting parameters rather than being random.
• Its operational impact is substantial, raising support demand, labor inputs.
• Outliers must be addressed, as extreme cases disproportionately influence averages and stability risk.
• Optimizing perimeter charging and improving drilling precision remain the most practical levers for control.

5. Proposed Blasting Strategy

Field investigations revealed that final cup density was the most influential contributor to overbreak. These effects were amplified by geological variability, such as in weak pastefill and shear zones. To mitigate these issues, a modified perimeter-control blasting strategy was designed. This section outlines the rationale, specific modifications.

Although burn-cut blasting is effective in creating voids and advancing headings, its limitations in perimeter control were evident in the study mine. Excessive energy transmission into excavation boundaries produced irregular profiles, unstable rock, and elevated support demand. Key shortcomings included:

• Fully coupled perimeter holes transmitting excessive radial energy.
• Drilling deviations altering planned drill design.

The primary objective was therefore to redistribute energy within desired profiles while lowering boundary damage, without compromising fragmentation and advance rates.

5.1 Key Modifications Introduced

5.1.1 Decoupled Charges

Perimeter holes loaded with smaller-diameter cartridges, leaving an annular air gap (as shown in Fig. 8) to reduce borehole pressure and restrict crack propagation. Decoupling ratios of 0.6–0.7 were tested, consistent with recommendations by Singh (2018) and Orica (2014).

5.1.2 Drilling Accuracy Improvements

Enhanced supervision of jumbo operations ensured hole alignment matched design plans. Operator refresher training was conducted, reducing deviations that previously led to localized overbreak.

Together, these modifications balanced fragmentation efficiency with smoother, stable excavation profiles.

Figure 8 Decoupled charge illustration