PART 2. Designing Sand Covers for Blast Flyrock Control: Practical Engineering Guidelines
Sand cover can be estimated from the upward energy trying to eject rock, but the cardinal rule is this: sand is only a secondary containment layer. It will never compensate for poor burden, inadequate spacing, weak stemming, improper decking, or excessive charge concentration near the surface.
A sound engineering approach combines a momentum/energy balance with a generous field safety factor. All calculations below use US customary units (pounds, feet, inches, and ft-lb).
1. Explosive Energy and the Upward Component
For a blast with explosive mass, We (lb): E_tot = We ⋅ Q
- E_tot = total chemical energy (ft-lb)
- We = explosive mass (lb)
- Q = specific energy of the explosive (ft-lb/lb)
Typical rough values (converted from MJ/kg):
- ANFO: 1.3 to 1.45 × 10⁶ ft-lb/lb
- Emulsion/slurry: 1.1 to 1.6 × 10⁶ ft-lb/lb
- High explosives: higher
Only a small fraction of this energy drives the heave or flyrock upward. Therefore:
- E_up = upward-directed energy acting against the cover (ft-lb)
- η = upward energy coupling fraction,
- typically 0.01–0.05 for well-confined blasts;
- 0.05–0.15 for poor confinement or high-risk shots. Use the higher end of η for conservative design.
2. Sand Cover as Resisting Weight
If sand thickness is t(ft) over area A (ft²), the weight of the sand layer is: M_s = ρs⋅ A ⋅ t
- M_s = weight of sand (lb)
- ρs = bulk weight density of sand, typically 100–120 lb/ft³ (use 105 lb/ft³ for loose, dry-to-slightly-moist sand in design)
- t = sand thickness is (ft)
Weight alone is insufficient—the sand must also absorb energy and restrict gas venting.
3. Simple Energy Method for Minimum Sand Thickness
Assume the blast tries to lift the sand cover by an effective displacement h (ft) before venting is controlled. The resisting work done by the sand is approximately:
Require the resisting work to exceed the upward blast energy by a safety factor SF (commonly 2–4):
or, substituting the upward energy: t ≥ SF ⋅ η ⋅ W_e ⋅ Q / (ρ_s ⋅ A ⋅ g ⋅ h)
- t= required sand thickness (ft; multiply by 12 for inches)
- SF = safety factor (2–4)
- η = upward energy fraction
- W_e = explosive mass (lb)
- Q = specific energy (ft-lb/lb)
- M_s = sand weight density (lb/ft³)
- A = covered area (ft²)
- h = allowable lift/deformation of sand layer (ft)
4. Example Calculation (Energy Method)
- Explosive per delay: We=11 (≈5 kg)
- ANFO energy: Q=1.34×106 ft-lb/lb
- Cover area: A=65 ft² (≈6 m²)
- Sand density: ρs=105 lb/ft³
- Upward fraction: η=0.03
- Allowable lift: h=4 in = 0.33 ft
- Safety factor: SF = 3
Step 1: Total energy
The total energy, E_{tot}, = 11 times 1.34 x 10^6, which equals 1.474 x 10^7 J
Step 2: Upward energy
E_{up} = 0.03 × 1.474 × 10^7 = 4.42 × 10^5 J
Step 3: Required thickness
This result is absurdly large. That is the key lesson.
5. Why the Pure Energy Method Often Gives Unrealistic Answers
A blast releases enormous chemical energy. Direct comparison to the gravitational work of a sand layer produces impractically thick covers. In reality, flyrock control is governed far more by:
- charge concentration near the surface
- shallow charge placement
- burden and relief
- stemming quality
- decking
- confinement geometry
- loose rock or free faces
- initiation timing and gas-vent paths
Practical Blast-Cover Design Methods
Method A: Surface Loading Based on Top Charge (Recommended for Field Use)
Focus only on the explosive mass in the top critical zone (Wt, lb) capable of ejecting material. Use an empirical surface loading rule:
where m = required sand loading (lb/ft²) and k = an empirical constant derived from field experience. Practical starting ranges (field-proven):
- Light confinement/low-risk trim work: 10–20 lb/ft²
- Moderate risk: 20–50 lb/ft²
- High flyrock concern: 50–100+ lb/ft²
Thickness is then: t=m / γs
(with t in ft; multiply by 12 for inches). Using γs= lb/ft³:
These thicknesses are realistic and match decades of field practice.
Method B: Momentum Approach (Conceptual/Advanced)
Instead of energy, consider the upward momentum from the ejecting zone. If the rock/gas impulse ejects mass Mr (lb) at velocity v, momentum p=Mrv. The sand cover reduces the final velocity vf:
Set a low target vf (e.g., a few ft/s) and solve for required sand weight Ws. Convert to thickness t = Ws / (γs⋅A). This method is often more realistic but requires good estimates of ejecting mass and velocity.
Best Field Rule for Sand Cover Design
- Identify the critical exposed zone (top of hole, shallow seams, toe breakout, collar area, reduced-burden zones).
- Estimate only the top effective charge per delay Wt (lb), not the entire hole.
- Choose target sand surface loading m:
- Light risk: 10–20 lb/ft²
- Moderate risk: 20–50 lb/ft²
- High risk: 50–100+ lb/ft²
- Convert to thickness: t(in) ≈ 0.114×m (lb/ft²) at 105 lb/ft³ sand.
- Increase thickness (or add mats) if: tight/uncertain burden, shallow charges, fractured geology, free face venting, or no mats above the sand.
Important Practical Notes
Sand type Best: clean, well-graded sand. Avoid oversized gravel, organic soil, frozen lumps, or wet slurry-like material. Slight moisture aids packing; excess water creates handling problems.
Placement
- Uniform thickness across the entire blast zone.
- Extend sand several feet past the blast perimeter.
- Eliminate thin spots and direct vent paths.
- Protect initiation lines from displacement.
Major warning: If the design calls for extreme sand thickness, the real problem is usually excessive charge concentration, inadequate stemming, insufficient burden, or poor hole placement. Step 1. Fix the blast design first. Step 2. Sand is not a substitute for good engineering.
Very Simple Design Formula ( Field Use). t (in)≈0.114×m′′ (lb/ft²)
Recommended trial values of m:
- Low risk: 10–20 lb/ft²
- Medium risk: 20–50 lb/ft²
- High risk: 50–100 lb/ft²
PEG RECOMMENDATIONS: Do not size sand covers based on total explosive energy alone—it overestimates absurd thicknesses. Follow this hierarchy:
- Fix the blast design first.
- Identify the shallow critical charge.
- Select a conservative sand loading (lb/ft²).
- Convert to thickness using sand density.
- Add extra thickness (or mats) for poor geology or shallow burden.
- Use sand + mats together on high-risk shots.
This approach delivers safe, practical, and cost-effective flyrock control on every job.
Alternatives and Complementary Methods
When full mats are not feasible, too costly, or logistically challenging, consider these options (often used in combination):
- Sand, Soil, or Earth Cover (as previously detailed)
- Clean, well-graded sand with a bulk density of 100–120 lb/ft³.
- Target surface loading: 10–20 lb/ft² (low risk) up to 50–100+ lb/ft² (high risk).
- Thickness examples (at 105 lb/ft³): 2–11+ inches depending on risk.
- Advantages: Inexpensive, conforms perfectly, adds mass without fire risk.
- Disadvantages: Requires sourcing/hauling material; can be messy; needs uniform placement to avoid thin spots.
- Often layered under or over mats for hybrid containment.
- Soil Berms or Mounded Earth Fill
- Piled clean overburden or crushed material around/above the blast zone.
- Effective for containing both flyrock and reducing airblast. Requires careful placement to avoid damaging primers or detonators.
- Muffle Blasting (also called muffled blasting)
- A technique combining mats, sand/soil, or other covers to fully “muffle” the shot. Particularly useful in urban or sensitive environments.
- In practice, the blast is designed first with good stemming and confinement. The muffling layer is then applied as the final safety measure.
- Other Engineered Alternatives
- Geotextiles or Heavy Tarps with anchoring (less effective alone for heavy flyrock).
- Sandbags or Weighted Barriers: Stacked along perimeters or on top of lighter mats.
- Conveyor Belt Sections or Log Covers: Site-specific improvised options (less reliable).
- Advanced Composites: Emerging lighter, high-strength materials, though still less common than rubber or wire rope.
Hybrid Approach (Recommended for High Risk): Use sand or soil as a base layer for uniform mass and gas sealing, then overlay with blast mats for added deflection and durability. This combines the distributed weight of sand with the toughness and reusability of mats.
Important Warnings and Best Practices
- Blast Design First: If you need extremely thick sand or multiple heavy mats, the real issue is usually excessive surface charge, poor stemming, insufficient burden, or bad geology. Correct the design before relying on containment.
- Safety and Regulations: Always follow OSHA, MSHA, and local requirements. Mats or equivalent cover are often mandated near structures. Clear the area, use sentinels, and monitor vibrations/airblast.
- Maintenance: Inspect mats regularly for damage, cuts, or loose bindings. Replace worn sections.
- Fire Risk: Avoid rubber mats in wildfire-prone or high-temperature areas—opt for wire rope.
- Cost vs. Benefit: Quality woven wire rope mats are often more expensive upfront than basic recycled rubber tire mats. The savings come mainly from lower transport/handling costs and longer service life in many applications, but the purchase price can be a barrier for smaller operators.
Blast mats (rubber or wire rope) and sand/soil covers are effective secondary controls that significantly reduce flyrock risk when used properly. The most reliable and economical strategy is to start with excellent blast design, followed by site-specific containment—often a hybrid of sand for mass and mats for targeted protection. On high-concern jobs, layering both provides the safest outcome without over-relying on any single method.
This approach keeps crews, nearby structures, and the public protected while maintaining efficient operations. For site-specific recommendations, consult a qualified blasting engineer and test containment under controlled conditions when possible.
To continue learning from PEG, explore the full series on blast design, flyrock control, and practical field techniques. If you have any questions, contact Dr. Petr and his associates.
