Interpretation of Key Engineering Geological Elements in Mine Blasting

Successful mine blasting design and construction are founded on a thorough understanding of the engineering geological conditions, especially the rock types, physical-mechanical properties, and dynamic characteristics. Since the primary target of earthwork blasting is rock, only by fully understanding the rock’s hardness, blastability, and response under explosive impact can one correctly select blasting parameters, determine explosive consumption, and ultimately achieve good blasting results. Engineering geology is therefore an essential component of blasting design and construction.

Rock vs. Rock Mass: The Essential Distinction in Blasting Media

In blasting engineering, it is crucial to strictly distinguish between the physical concepts of “rock” and “rock mass”.

• Rock: Rock, as the basic material composing the Earth’s crust, is typically referring to homogeneous rock blocks without significant weak planes. Physical and mechanical parameters can be obtained through testing samples of these rock blocks.

•  Rock Mass: Rock mass refers to a combination of rock blocks under natural burial conditions, retaining permanent deformation and geological structures formed throughout a long geological history, and cut by various weak geological planes.

•  Implication for blasting: The strength of a rock mass is generally far lower than that of a rock. Long-term blasting practice shows that structural features (e.g., joints, fissures) within the rock mass directly affect blasting results, safety, and the stability of post-blast rock masses such as surrounding rock or slopes.

Rock Genesis Classification and Differences in Blastability

Rocks can be divided into three major categories based on origin: igneous, sedimentary, and metamorphic. Differences in geological formation, mineral composition, and structural configuration result in significant differences in their response to blasting loads.

1.  Igneous Rocks: Dense Structure, Poor Blastability

Formed by magma cooling down, igneous rocks have characteristics such as strong integrity, dense crystallization, fine mineral particles, and firm bonding. Their high density and high acoustic impedance (e.g., granite, basalt, diabase) cause them to exhibit brittle failure under blasting loads but require higher energy to initiate fracturing. Thus, igneous rocks are considered as the rock type that is the least blastable and most resistant to blasting.

Even among igneous rocks, blastability varies somewhat with mineral composition (acidic, intermediate, basic types), but they generally belong to hard rock and require high specific charge to achieve desired fragmentation.

2. Sedimentary Rocks: Looser Structure, Good Blastability

Sedimentary rocks are formed under normal temperature and pressure through transport, deposition, and consolidation of weathered materials. Their strength depends on cementing materials, grain-size distribution, and porosity.

Among them:

•  Siliceous-cemented rocks are the strongest.

•  Calcareous and argillaceous-cemented rocks are weaker.

•  Rocks with loose structure and high porosity show significantly higher blastability.

Thus, limestone, sandstone, and shale generally exhibit relatively good blastability, allowing for reduced explosive charge and wider drilling patterns.

3. Metamorphic Rocks: Intermediate Blastability Depending on Metamorphism

Metamorphic rocks form through recrystallization under high temperature and pressure. Higher metamorphic grade increases density and strength, resulting in stronger resistance to blasting.

For example:

• Quartzite and gneiss are hard, with poor blastability.

• Marble and slate have relatively uniform structure and better blastability.

Table 1-3 Classification of Major Metamorphic Rocks

Rock Name	Main Mineral Components	Structure
Gneiss, Migmatite	Feldspar, Quartz, Mica, Hornblende, Garnet, etc.	Gneissic
Quartz Schist	Quartz, Mica	Foliated
Greenstone	Chlorite, Actinolite, Mica
Mica Schist	Mica, Quartz
Amphibolite	Hornblende, Feldspar
Phyllite	Sericite, Mica, Biotite, Feldspar & Clay Minerals	Phyllitic
Slate	Quartz, Sericite, Clay Minerals	Slaty
Marble	Calcite, Dolomite	Massive
Quartzite	Quartz

Summary:

Ii is the most difficult to blast igneous rocks while it’s relatively easier to blast sedimentary and metamorphic rocks.

Key Physical–Mechanical Parameters Affecting Blasting Performance

The resistance of rock media to explosive impact is directly related to their basic properties. Key parameters in blast design include:

Density and Specific Charge

•  Density (ρ) and bulk density (γ): Higher density indicates greater strength and resistance to blasting.

•  Engineering application: The formula K = 0.4 + (γ / 2450)² (kg/m³) is often used to estimate specific charge for standard throw blasting.

Acoustic Impedance and Energy Matching

•  Acoustic impedance: The product of rock density (ρ) and P-wave velocity (C).

•  Matching principle: The efficiency of energy transfer from explosive to rock depends directly on impedance matching. Normally, it is affirmed that relatively good blasting result can be obtained by matching (approximately the same) the acoustic impedance of the selected explosives and the rock. For instance, for extremely hard rock with strength >250 MPa with the maximum acoustic impedance reaching 15×10⁶ kg/(m²·s), it requires specially formulated explosives instead of emulsions for effective blasting result.

Static Strength Characteristics

Rock’s ultimate strength includes tensile, shear, and compressive strength. The tests show that the tensile strength is 90–98% lower than compressive strength and the shear strength is 87–92% lower than compressive strength. This means that rock is most easily broken under tensile stress, which is a primary mechanism of rock fracture in blasting.

Dynamic Characteristics of Rock Under Explosive Impact

Explosive detonation is a strong disturbance source producing dynamic loads of extremely short duration and very high loading rate.

Dynamic Strength Increase

•  Under dynamic loads, rock strength and deformation modulus significantly increase.

•  For example, diabase specimens show 17%–55% higher compressive strength under dynamic loading than under static loading.

Table 1-8 Comparison of Loading Types

Loading Method	Steady Loading	Hydraulic Machine	Pneumatic Machine	Hopkinson Bar	Explosive Impact
Strain Rate / s⁻¹	<10⁻³	10⁻⁵ ~ 10⁻⁴	10⁻⁴ ~ 10	10¹ ~ 10⁴	>10⁶
Loading State	Quasi-static	Static	Quasi-static	Quasi-dynamic	Dynamic

Wave Behavior and Influence of Rock Mass Structures

•  Blasting loads propagate as waves, with highly transient loading (milliseconds).

•  Considerable energy is required to break rock under shock loading.

•  Structural features (joints, fissures) strongly influence blasting outcome and fragment size distribution. Most post-blast fracture surfaces follow these structural planes. Thus, detailed engineering geological investigations, especially statistical analysis of joints and fissures, are essential in both surface and underground mines for accurate prediction of fragmentation.

Conclusion

To sum up, no matter it is surface bench blasting, underground non-coal mining or all-type controlled blasting, the type of rock, hardness coefficient, acoustic impedance, and dynamic response under explosive impact are core determinants of blasting success. Only by scientifically matching these geological parameters with explosive types and drilling–blasting parameters can blasting operations be efficient, safe, environmentally friendly, and maintain long-term rock mass stability.

FAQ

Q: What is the fundamental difference between rock and rock mass in blasting engineering, and why distinguish them?

Answer:

Define differences:

•  Rock: Rock is homogeneous rock blocks without significant weak planes, representing the intrinsic physical–mechanical properties of the material.

•  Rock mass: Rock mass is a natural geological body composed of rock blocks cut by weak planes and containing structural features formed over geological time.

Blasting significance:

Rock mass strength is much lower than rock strength. Long-term blasting practice indicates that  structural features (joints, fissures) directly affect blasting performance, safety, and stability of post-blast rock masses. Blasting design must consider rock mass integrity and weak planes.

Q: How is the origin-based classification of rocks related to their resistance to blasting (blastability)?

Answer:

•  Igneous rocks (granite, basalt): Strongest resistance to blasting (poor blastability).

•  Sedimentary rocks (limestone, sandstone): Better blastability; strength depends on cement type and bonding.

• Metamorphic rocks (marble, quartzite): Moderate to good blastability depending on metamorphic degree.

Q: Why is rock strength much higher under explosive impact than under static loading?

Answer:

Explosive detonation produces extremely high loading rates, causing significant changes in rock mechanical properties, leading to a dynamic strength increase phenomenon.

Q: Among tensile, compressive, and shear strength, which is most easily overcome in blasting?

Answer:

Tensile strength. Rock fails most easily under tensile stress, which is a primary mechanism of breakage in blasting.