Blasting and blast damage: real impacts on slope stability and total cost.

In-depth analysis according to LOP and ISRM guidelines.

Introduction

Blasting in open-pit mining operations directly influences slope stability, operational safety, and the economic performance of the project. The Large Open Pit (LOP) Project guidelines treat wall control blasting as part of the stability system because the way the bench is excavated can become a "controlling" factor in slope performance.  

The central point is simple and often overlooked: the final slope is not the design slope, it is the slope delivered in the field. If the execution destroys the quality of the remaining rock mass and deteriorates the constructed geometry, the mine pays twice, first in safety and operational availability, then in the loss of value due to dilution, rework, and mining restrictions.

The problem: treating damage as a production detail and creating a geotechnical liability.

When the disassembly damage (blast-induced damage, BID) is treated as "finishing," the operation tends to optimize only fragmentation and productivity. The LOP explicitly recommends that the organization treat blasting and excavation as controllable variables for bench performance, with testing, vibration monitoring, and systematic parameter adjustment until a balance is achieved between minimal damage and adequate productivity.  

Operational experience shows why this is not just theory. In the case of a hard rock mine in the north of the country, analyses reported that blasting damage was preventing the achievement of design geometries, forcing revisions of interrupt angles until blasting performance was improved.  

Typical result when the BID is underestimated:

• narrow or lost effective berms,
• overbreak recurring and irregular faces,
• rockfall chronic and increased exposure of people and equipment,
• unplanned increase in local support,
• Loss of ore due to dilution and excessive setbacks.

What is it blast damage in what really matters for stability

Damage, from a geotechnical point of view, is not an "ugly wall." It is a change in the rock matrix that reduces the strength and stiffness of the remaining rock mass and includes the creation of new fractures and the widening of existing fractures.  

On slopes, this manifests as a damaged zone adjacent to the contour, with degraded properties and greater susceptibility to local instabilities and progressive degradation.

Dominant damage mechanisms

A useful way to organize the phenomenon is to separate the mechanisms that "damage" the final wall, each with a different control:

1. Vibration (stress wavescreating new fractures and degrading the matrix.  
2. Detonation and confinement gases, expanding joints and opening discontinuities, directly impact the resistance and collapse of rock blocks.  
3. Reaction and load relief effects against the wall, potentially associated with “release-of-load.  

In the field, the consequence is clear: with poor practices, the damage can advance meters into the slope, with reports of damage reaching the order of 10 m in unfavorable situations, which changes the behavior on a bench scale and accumulates on a stack scale.  

Where the BID becomes the total cost of the slope (and not just the cost per ton blasted)

Measuring dismantling solely by R$/t is a managerial accounting error. The total cost of the slope is dominated by items that appear later, often in a different cost center:

• 'Recurring face cleaning and rework, with more machine time and operational exposure.'
• Loss of effective shoulder width, reducing retention capacity and increasing road closures.
• Unplanned support and containment, when the wall "does not hold up" as expected.
• Dilution and loss of reserves, due to overbreak and excessive setbacks.
• Operational restrictions, ramp closures, longer routes, smaller mining windows.
• Risk of significant events when damage, structures, and water combine.

LOP recommends that performance be measured by data.as-built”, such as actual face angle and effective berm width, with systematic documentation and objective methods, including photogrammetry and laser scanning for reconciliation.  

What really changes: technical levers that control stability and cost.

1) Pre-cut, trim and bufferDifferent functions, different results.

The LOP lists the main parameters that need to be treated as engineering variables in blasts wall control, including type of blast (buffer, pre-cut, trim), diameter and inclination, layout (spacing and burden), explosive distribution, fire size, and delay sequence.  

Pre-cut (pre-split)

Function: to create a controlled rupture plane and limit the spread of damage to the remaining rock mass.

Trim blast (trim blasting)

Function: to refine the wall after production, reducing energy and adjusting geometry. The literature on mechanisms highlights that the performance of wall control depends heavily on the previous trim, and that the linear load factor is a critical damage controller.  

Buffer blast

Function: to create a transition zone, decoupling energy between production and the end wall. In practical terms, it's a form of "insurance" against cumulative damage from production near the end pit. 

2) Geometric tolerances: the KPI that best protects the slope and is least penalized.

Controlled dismantling only exists if there is:

• clear tolerances for crest and foot,
• hole deviation and tilt control,
• systematic reconciliation of "how it was excavated" versus "how it was designed".

The LOP emphasizes that the actual geometry needs to be documented and compared with the expected geometry, using distributions and acceptability criteria, because excessive backbreak can be exacerbated by... blast damage and become a problem on various scales.  

3) Operational control and monitoring: without a closed loop, there is no improvement.

Maturity lies in the cycle: plan, execute, measure, correct, and learn. LOP recommends blasting trials with vibration monitoring and systematic parameter modification to find the balance between minimal damage and productivity.  

On the ISRM side, there is a formal set of Suggested Methods to standardize measurements and make results reproducible, including a specific method for blast vibration monitoring, reinforcing that "measuring correctly" is part of risk control and engineering.  

Important: operational limits based on peak particle velocity (PPV) and frequency should not be treated as a "universal magic number." The correct approach is to define criteria for each domain and, consequently, calibrate them with observations of slope damage and performance.

Trade-offsThe equilibrium that separates a stable mine from a mine that operates in a state of contingency.

Every damage control decision has a trade-off, but it needs to be addressed at the right level:

• Direct blasting costs increase with pre-cutting, buffering, drilling QA/QC, and monitoring.
• The total cost of slope stabilization decreases when operations stop losing berms, reducing closures, rework, and emergency supports.

The correct management question is not "how much does pre-cutting cost," but rather: how much does it cost to operate with recurring block drop, ineffective berms, high dilution, and closing ramps?

Practical recommendations, from planning to field.

1. Integrate geotechnics, drilling and blasting, and surveying into contour blasting planning, using geomechanical expertise.  
2. Specify technique by zone (pre-cut, trim, buffer) and define criteria for damage acceptance and geometry.  
3. Implement QA/QC for drilling and loading, focusing on hole deviation and energy control (linear load and distribution).  
4. Monitor and reconcile: face, effective berm, overbreak, rockfallneed for scaling, and relate it to the parameters of the firing plan.  
5. Conduct controlled trials (blasting trials) to calibrate operational criteria and close the improvement cycle.  

Geotechnical review of the blasting and face finishing plan.

If your operation coexists with overbreak recurring, "food" berm, scaling chronic, rockfall Frequent, unplanned local support issues or ramp closures may indicate that the problem lies less in the "geotechnical aspects of the design" and more in the energy and geometry delivered by the blasting. Treat blast damage as a central variable for stability and total cost, with criteria, measurement, and discipline aligned with the best practices of LOP and ISRM.