Drilling and blasting
Drilling and blasting is a controlled engineering process used primarily in mining and civil construction to fragment hard rock formations for excavation and material handling. It entails the precise drilling of boreholes into the rock mass to accommodate explosives, followed by the strategic loading and detonation of these charges to break the rock into smaller, transportable pieces, optimizing fragmentation while minimizing overbreak and environmental impacts.[1][2] The process is methodical and begins with blast design, where engineers assess rock geology, including hardness, joints, and strength, to determine hole diameter, spacing, burden, and depth—typically ensuring hole diameter is less than one-fifth of the bench height in surface operations for stability.[3] Drilling then employs percussion methods, such as down-the-hole hammers for hard rock, or rotary techniques with roller-cone bits for larger diameters (6-22 inches), powered by air, hydraulic, or diesel systems mounted on track or truck rigs, with performance measured by metrics like cost per foot or feet drilled per ton.[1][3] Explosives, often ammonium nitrate-fuel oil (ANFO) as the primary blasting agent, are loaded into the holes along with primers, boosters, and initiators like electric or non-electric detonators with timed delays to sequence the blast, ensuring efficient energy release and controlled fragmentation.[2] This technique is essential in applications ranging from open-pit and underground mining for ore extraction, to quarrying for aggregates, and civil projects such as road cuts, tunnel construction, and slope stabilization, where production blasting removes bulk material and controlled variants like presplitting create clean fracture planes to preserve structural integrity.[4] While highly effective for hard rock environments, it demands rigorous safety protocols to manage risks including ground vibration, airblast, flyrock, and toxic gas emissions, with optimization focusing on powder factor (explosives per ton of rock) to balance cost and efficiency.[2][3]Fundamentals
Definition and principles
Drilling and blasting is a controlled mechanical process for rock excavation, involving the creation of precisely positioned holes in the rock mass through drilling, followed by the insertion of explosive charges and their detonation to fracture and displace the material, enabling efficient removal for various engineering purposes.[5][3] This method relies on the strategic placement of blastholes to ensure the explosive energy is directed toward achieving uniform fragmentation without excessive damage to surrounding rock.[5] The fundamental principles center on the transfer of explosive energy to the rock, where detonation generates high-velocity shock waves that propagate through the material, inducing initial tensile cracking and shear failures along pre-existing weaknesses.[6] These shock waves, traveling at supersonic speeds, create radial fractures emanating from the blasthole walls, while the subsequent rapid expansion of detonation gases exerts pressure that extends cracks, promotes fragmentation, and displaces the broken rock toward free faces.[6] According to the widely accepted stress wave and gas expansion theory, this dual mechanism—shock-induced shattering followed by gas-driven heaving—ensures controlled breakage, with energy efficiency maximized when the explosive's detonation velocity matches the rock's acoustic impedance.[6] The primary objectives of drilling and blasting are to break rock into manageable sizes for loading and transport, optimizing productivity while minimizing costs associated with drilling, explosives, and post-blast handling in applications like mining, tunneling, quarrying, and construction.[5] This process balances safety by controlling ground vibrations and flyrock, alongside economic factors such as reduced overbreak and improved fragmentation uniformity.[3] Central to achieving these goals are key design parameters: burden, the perpendicular distance from the blasthole to the nearest free face, which influences energy confinement; spacing, the lateral distance between blastholes, affecting pattern uniformity; and stemming, the column of inert material (often crushed rock) placed atop the explosive to retain gases and enhance fragmentation efficiency.[5][3]Rock properties and fragmentation
Rock properties play a critical role in determining the effectiveness of drilling and blasting operations, as they influence drill bit wear, explosive energy utilization, and the resulting fragment size distribution. Hardness is typically quantified using the unconfined compressive strength (UCS), which measures the maximum axial stress a rock sample can withstand under zero confining pressure, with values ranging from less than 25 MPa for very weak rocks like mudstone to over 250 MPa for strong rocks like granite.[7] UCS helps classify rocks for blastability, where higher values indicate greater resistance to fragmentation, requiring adjustments in explosive charge and pattern to achieve optimal breakage. Abrasiveness, which affects drilling efficiency through tool wear, is assessed via the Cerchar Abrasivity Index (CAI), ranging from 0 (non-abrasive, e.g., limestone) to over 5 (highly abrasive, e.g., quartzite), with the test involving scratching a rock surface with a steel pin and measuring wear flat diameter.[8] Rock structure, including joints and faults, is evaluated using the Rock Quality Designation (RQD), calculated as the percentage of intact core pieces longer than 100 mm in a drill run, providing an index of fracturing where values above 75% denote excellent quality and below 25% indicate very poor quality.[9] Joints and faults act as planes of weakness, altering stress wave propagation and often leading to irregular fragmentation if not accounted for in blast design. The fragmentation process in blasting begins with the detonation of explosives, generating a high-pressure compressive stress wave that propagates through the rock, compressing material ahead of the blast and inducing radial cracks near the borehole. Upon reaching free surfaces, such as the bench face, these compressive waves reflect and convert into tensile waves, exploiting the rock's lower tensile strength (typically 5-15% of compressive strength) to propagate primary radial and secondary shear cracks, ultimately detaching fragments suitable for loading and hauling.[10] This tensile failure mechanism dominates fragmentation, with desired fragment sizes (e.g., 80% passing 0.3-0.5 m) facilitating efficient downstream processing, while excessive fines or boulders increase costs in crushing or secondary blasting. The process is governed by the balance between explosive energy input and rock resistance, where controlled crack networks ensure uniform breakage without excessive overbreak. Prediction of fragmentation relies on empirical models like the Kuz-Ram model, which integrates blast geometry, explosive properties, and rock characteristics to estimate size distribution. Developed by Cunningham, the model uses the Kuznetsov equation adapted for mean fragment size, with the 80% passing size (f80) approximated as: f_{80} = A \times \left( \frac{V}{Q} \right)^{0.8} \times \left( \frac{B}{S} \right)^{0.5} where f_{80} is in meters, A is the rock factor (ranging 1.1 for soft rocks to 17 for hard, massive rocks, derived from UCS and structure), V is the blasted volume per delay (m³), Q is the explosive charge mass per delay (kg), B is burden (m), and S is spacing (m).[11] This is combined with the Rosin-Rammler distribution for full curve prediction, allowing engineers to optimize designs for target fragment sizes, such as reducing f80 from 1 m to 0.4 m in hard rock by increasing Q/V. The rock factor A incorporates properties like UCS, where higher strength elevates A, demanding more energy for equivalent fragmentation. Several factors modulate blast efficiency and overbreak, including rock anisotropy, water content, and in-situ stress. Anisotropy, arising from foliation or bedding, directs crack propagation along weaker planes, potentially improving fragmentation in aligned blasts but causing uneven breakage and increased overbreak if misoriented, as stress waves attenuate differently across directions.[12] Water content reduces rock tensile strength by up to 50% through softening and pore pressure effects, enhancing crack initiation but risking fines overproduction and reduced energy coupling in wet conditions, with saturation levels above 5% notably altering fragment uniformity.[13] In-situ stress confines the rock mass, suppressing radial crack growth and reducing both fragmentation efficiency and overbreak in high-stress environments (e.g., >20 MPa), often necessitating higher charges or pre-stressing techniques to mitigate.[14]Historical development
Early techniques
Before the advent of explosives, ancient miners relied on manual techniques to extract ore and stone. In Egyptian mining operations during the Old Kingdom (circa 2686–2181 BCE), workers used copper chisels and dolerite pounders to chip away at rock faces, often in open-cast trenches or shallow shafts for materials like gold and turquoise. This labor-intensive chiseling was supplemented by fire-setting, where wood fires heated the rock surface before rapid quenching with water to induce thermal cracking and facilitate fragmentation. Roman miners, from the 1st century BCE onward, employed similar methods on a larger scale in operations such as those at Las Médulas in Spain, combining fire-setting with iron tools and wooden wedges to split hard rock like granite, enabling the extraction of gold through hydraulic flushing. These pre-explosive approaches were slow and physically demanding, limited by human strength and the need for precise control to avoid structural collapses in underground workings.[15][16][17] The introduction of gunpowder marked a significant advancement in blasting techniques during the 17th century in Europe. Black powder, composed of charcoal, sulfur, and saltpeter, emerged as the first chemical explosive used in mining and quarrying, with the earliest documented application in Hungarian mines in 1627. German miners brought the method to Britain by 1638, where it was employed to break coal seams and limestone, involving manual drilling of holes followed by tamping the powder with clay or sand using wooden rods. This innovation reduced reliance on fire-setting but still required careful handling to prevent premature ignition, and its relatively low detonation velocity—around 400 meters per second—limited its effectiveness in deep or hard rock formations compared to later explosives.[18] Key inventions in the mid-19th century further transformed drilling and blasting. Italian chemist Ascanio Sobrero synthesized nitroglycerin in 1847 by nitrating glycerol, yielding a liquid explosive far more powerful than black powder, with an energy release exceeding 6 megajoules per kilogram. However, its extreme sensitivity to shock and friction caused numerous accidents during transport and use in early blasting trials. Swedish engineer Alfred Nobel addressed these issues in 1867 by absorbing nitroglycerin into kieselguhr (diatomaceous earth) to create dynamite, a stable paste that could be safely shaped and inserted into drill holes, revolutionizing mining by enabling controlled blasts in tunnels and quarries. Concurrently, early mechanized drilling emerged, with American inventor J.J. Couch patenting the first steam-powered percussion drill in 1849, which used alternating steam pressure to hammer a drill bit at rates up to 600 blows per minute, speeding hole creation for explosives in rock.[19][20][21][22] Despite these breakthroughs, early techniques faced notable limitations. Black powder's low energy density—approximately 3 megajoules per kilogram—resulted in inefficient fragmentation, often requiring multiple charges and leaving large boulders that needed secondary manual breaking, particularly in hard igneous rocks. Nitroglycerin's handling risks were acute, as even minor jolts could trigger detonation, leading to fatal incidents in mining sites before dynamite's stabilization. Overall, inconsistent fragmentation arose from imprecise manual or nascent mechanical drilling, variable charge placement, and uneven rock properties, complicating ore extraction and increasing operational hazards through the 19th century.[23][24][25]Evolution in the 20th century
The 20th century marked a transformative period for drilling and blasting, driven by industrialization, mechanization, and the demands of large-scale mining and infrastructure projects, which shifted practices from manual labor to powered equipment and more efficient explosives. Early advancements focused on electromechanical tools that increased drilling speed and precision, while later innovations emphasized safer, higher-performance blasting agents and controlled detonation sequences. These developments enabled the extraction of vast mineral resources and the construction of monumental engineering feats, laying the groundwork for modern operations. In the early 20th century, the introduction of pneumatic percussive drills revolutionized drilling efficiency in hard rock mining and tunneling. The jackhammer, a handheld pneumatic rock drill developed by Ingersoll-Rand in 1912, represented a significant milestone, allowing workers to drill faster and deeper than previous steam-powered or manual methods, with its design incorporating compressed air for repeated hammer blows on a drill steel.[26] By the 1920s, these drills were widely adopted in underground operations, reducing cycle times in drill-and-blast cycles. Rotary drilling methods also gained traction during this era, particularly for larger blastholes in surface mining, where rotating bits cut into rock under applied pressure, offering advantages in softer formations and enabling the preparation of bigger charges for enhanced fragmentation.[27] Advancements in explosives during the mid-20th century prioritized safety, cost-effectiveness, and energy output, moving beyond nitroglycerin-based dynamites that were prone to accidental detonation. ANFO (ammonium nitrate-fuel oil), developed in the mid-1950s, emerged as a low-cost, bulk explosive ideal for dry-hole blasting in mining, comprising 94% prilled ammonium nitrate sensitized by 6% fuel oil, which provided reliable detonation velocities around 3,200-4,500 m/s while minimizing handling risks.[28][29] Water gel (slurry) explosives, developed in the late 1950s, introduced aqueous mixtures of ammonium nitrate, TNT, and gelling agents, offering superior water resistance for wet environments like underground workings and open pits.[30] By the 1970s, water-in-oil emulsion explosives further improved performance, invented in 1973 by Atlas Powder Company; these stable, pumpable formulations used an emulsified ammonium nitrate phase in a fuel matrix, delivering higher energy (up to 5,000 m/s detonation velocity) and reduced toxic fumes compared to ANFO.[31] Blast control innovations enhanced precision and reduced environmental impacts, with electric detonators becoming standard by the 1930s, replacing unreliable black powder fuses and enabling safer, more reliable initiation through electrical current to a bridgewire igniter.[32] Sequential timing advanced in the 1940s with the introduction of millisecond delay electric detonators by DuPont, allowing staggered firing of charges (delays of 25-100 ms) to optimize rock breakage, minimize vibrations, and improve fragmentation uniformity in multi-row blasts.[33] By the 1970s, precursors to computer-aided blast design emerged, building on empirical models like those from Langefors and Kihlström's 1963 theory, which used equations for burden, spacing, and charge calculations; early computational tools implemented these on mainframe computers to simulate blast patterns and predict outcomes, reducing trial-and-error in field operations.[34] These technologies facilitated unprecedented scale-up in applications, particularly in major infrastructure and mining projects. In the 1930s, the Hoover Dam construction exemplified advanced tunneling techniques, where four 50-foot-diameter diversion tunnels were excavated using drill-and-blast methods with pneumatic drills on jumbo carriages drilling 126 holes per round, loaded with 2,000 pounds of dynamite and fired to advance about 15 feet per cycle.[35] Post-World War II, large-scale open-pit mining expanded dramatically, as seen in Chile's Chuquicamata copper mine, which by 1952 had become the world's largest open-pit operation through mechanized drilling and bulk ANFO blasting, with flotation and smelting facilities installed that year supporting annual production of around 100,000 tons of copper.[36][37]Drilling operations
Drilling methods
Drilling methods in rock blasting primarily involve creating precise boreholes to accommodate explosives, with techniques selected based on rock hardness, hole dimensions, and operational efficiency. Percussive and rotary methods dominate hard rock applications, while auger and jet approaches serve as non-explosive options for softer materials. These methods ensure optimal fragmentation by tailoring penetration to geological conditions. Percussive drilling employs a hammering action to fracture rock, making it ideal for hard formations where rotational methods falter. In the top-hammer variant, the percussion mechanism is located at the surface, transmitting impact energy through a string of drill rods to the bit, which crushes and chips the rock. This method suits shallow to medium-depth holes in hard rock, with typical diameters up to 153 mm and penetration rates of 20-60 m/hour depending on rock strength and equipment pressure. Down-the-hole (DTH) percussive drilling positions the hammer assembly near the bit inside the borehole, minimizing energy loss from rod vibrations and enabling straighter, deeper holes up to 30 m in hard, abrasive rock. DTH achieves similar penetration rates of 20-60 m/hour and is preferred for diameters of 114-254 mm, offering better control in challenging conditions.[38][3][39] Rotary drilling rotates a bit to grind or abrade rock, providing efficient penetration in softer formations and enabling large-diameter blasts for open-pit operations. Diamond-impregnated bits excel in hard but non-abrasive rock, while tricone roller-cone bits with tungsten carbide inserts handle softer to medium-hard materials by crushing and scraping. This method supports deep holes of 5-45 m and diameters from 150-560 mm, with suitability enhanced in formations below 140 MPa compressive strength, though rates slow in very hard rock. Rotary is commonly used for bench blasting where high-volume excavation demands larger holes.[3][38][40] Auger drilling uses a rotating helical screw to excavate soft, unconsolidated materials like soil or friable rock, serving as a non-percussive alternative limited to depths under 10 m and diameters up to 300 mm in low-strength formations. It avoids explosives in sensitive environments but struggles in hard rock due to rapid bit wear. Jet drilling, conversely, employs high-pressure water or fluid jets to erode soft rock, creating holes up to 100 mm in diameter for specialized applications, though its efficiency drops in cohesive or harder materials, restricting use to preliminary or auxiliary tasks.[41][42] Hole patterns in drilling and blasting are configured as vertical, angled, or fan arrangements to align with site geometry, bench height, and fragmentation goals, influencing explosive distribution and rock throw. Vertical patterns suit flat benches for uniform loading, while angled holes (10-20° from vertical) accommodate sloped faces or subdrilling; fan patterns radiate outward for tunnel or confined blasts to maximize energy direction. Typical hole diameters range from 25-300 mm, with depths of 1-50 m, typically approximately equal to the bench height (often 10-20 m in surface mining) plus subdrill (1-3 m) to ensure proper toe fragmentation and bench stability, ensuring post-drilling explosive insertion aligns with blast design principles. Rock hardness significantly affects method selection, with harder types favoring percussive approaches for sustained penetration.[43][3][38][44]Drilling equipment and parameters
Drilling equipment in mining and tunneling operations primarily consists of specialized rigs designed for creating blastholes in rock. Jumbo rigs, often electro-hydraulic or diesel-powered, are widely used for underground tunneling and development, enabling multiple simultaneous drill booms to advance faces efficiently in confined spaces.[45] For surface mining, crawler-mounted rigs provide mobility and stability on uneven terrain, supporting rotary or percussive drilling for large-scale blasthole patterns. Hydraulic systems dominate modern applications due to their higher power density and precise control compared to pneumatic systems, which rely on compressed air and are more common in smaller-scale or legacy setups for their simplicity in dusty environments.[3] Key operational parameters influence drilling efficiency and hole quality in blasthole drilling. Rotational speed, measured in rotations per minute (RPM), typically ranges from 50 to 200, balancing bit life and penetration while avoiding excessive wear in hard rock. Thrust force, applied axially to the bit, varies between 100 and 500 kN to overcome rock resistance without causing deviation or bit damage. Air or fluid pressure, essential for flushing cuttings and powering percussive action, is maintained at 100 to 300 psi to ensure optimal bit performance and hole cleanliness. The rate of penetration (ROP), a critical performance metric, is modeled as a function of bit type and rock uniaxial compressive strength (UCS), where ROP decreases nonlinearly with increasing UCS for a given bit design, as expressed in empirical relations like ROP ∝ 1/UCS^α (with α ≈ 0.5-1.0 depending on bit geometry).[46][47][48] Accessories play a vital role in maintaining drilling accuracy and longevity. Drill bits, such as those with tungsten carbide inserts for percussive action or polycrystalline diamond compact (PDC) cutters for rotary shearing, are selected based on rock hardness to maximize ROP while minimizing wear.[49] Extension rods transmit torque and thrust from the rig to the bit, often made of high-strength alloy steel to withstand cyclic loading.[50] Shank adapters connect the rod string to the drill head, ensuring energy transfer efficiency; regular maintenance, including inspection for thread wear and alignment checks, prevents hole deviation by mitigating vibrations and fatigue cracks.[51] Optimization of drilling focuses on minimizing cost per meter drilled, which incorporates equipment utilization, consumables, and downtime, often calculated as total expenses divided by advance length (e.g., $5-20 per meter in surface operations).[52] Energy consumption, a major factor, ranges from 10 to 50 kWh per cubic meter of rock drilled, influenced by rig type and rock properties; strategies like parameter tuning and bit selection can reduce this by 20-30% through improved ROP and reduced idling.[53]Blasting operations
Explosive types and selection
Explosives used in drilling and blasting operations are broadly classified into low explosives, high explosives, and ammonium nitrate (AN)-based blasting agents, each distinguished by their reaction mechanisms and performance characteristics in rock fragmentation. Low explosives, such as black powder, undergo deflagration—a subsonic combustion process—releasing energy through rapid burning rather than detonation, making them suitable for historical or specialized applications like coal mining where controlled gas expansion is desired.[54] High explosives, including trinitrotoluene (TNT) and pentaerythritol tetranitrate (PETN), detonate at supersonic velocities, producing a high-pressure shock wave that effectively fractures hard rock in mining and construction. AN-based explosives, like ammonium nitrate fuel oil (ANFO), represent the most common modern choice for bulk blasting due to their cost-effectiveness and adaptability.[55] Key properties of these explosives influence their efficacy in blasting. Density typically ranges from 0.8 to 1.6 g/cm³, affecting charge volume and coupling with the borehole; for instance, ANFO has a density of 0.75–0.85 g/cm³, while high explosives like TNT reach 1.55–1.65 g/cm³. Detonation velocity for high explosives varies from 6000 to 9000 m/s (e.g., TNT at 6942 m/s, PETN at 8260 m/s), enabling rapid energy release for fragmentation, whereas ANFO detonates at 1800–3300 m/s. Energy output, measured in MJ/kg or kcal/g, generally falls between 2–5 MJ/kg; high explosives like RDX deliver around 5.5 MJ/kg (-1.51 kcal/g), compared to ANFO's lower but sufficient 3.7–4.2 MJ/kg. Water resistance is critical in wet environments—ANFO lacks it, leading to degradation, while water-gel emulsions (a high explosive variant) maintain performance. Sensitivity to initiation also varies: high explosives are more sensitive to shock, requiring careful handling, whereas AN-based agents need boosters for reliable detonation.[56][57] The following table summarizes representative properties for common explosive types in blasting:| Explosive Type | Example | Density (g/cm³) | Detonation Velocity (m/s) | Energy Output (MJ/kg) | Water Resistance |
|---|---|---|---|---|---|
| Low Explosive | Black Powder | 1.65–1.75 | Deflagration: 300–600 | ~2.7 | Low |
| High Explosive | TNT | 1.55–1.65 | 6942 | 4.1 | Moderate |
| High Explosive | PETN | 1.71 | 8260 | 5.0 | Low |
| AN-Based | ANFO (94% AN, 6% fuel oil) | 0.75–0.85 | 1800–3300 | 3.7–4.2 | None |