Chapter 12 — Blasting Damage & Controlled Excavation Techniques¶
12.1 The Physics of Blast-Induced Rock Damage¶
Blasting fragments rock through two distinct physical processes:
- High-Velocity Shock Wave: An instantaneous compressive shock pulse (\(V_p \approx 3,000 - 6,000\text{ m/s}\)) travels radially outward from the detonating borehole. As it reflects off free faces as a tensile wave, it creates extensive radial micro-cracks around the blasthole.
- Explosion Gas Expansion Pressure: High-pressure, high-temperature gases (\(P_{\text{gas}} \approx 1,000 - 5,000\text{ MPa}\)) penetrate into the shock-induced radial cracks, wedging them open and heaving the fragmented rock forward into the void.
The Blast Damage Zone:¶
In production blasting, over-confined charges shatter the rock well beyond the intended excavation boundary. This creates a blast damage envelope (\(0.5 - 2.0\text{ m}\) depth behind highwalls and tunnel arches) characterized by dilated joint apertures, shattered intact bridges, and destroyed friction angles (\(D \to 1.0\) in the Hoek-Brown criterion), triggering severe rockfalls.
12.2 Controlled Blasting Methods¶
To preserve the structural competence of final highwalls and tunnel perimeters, controlled perimeter blasting techniques are employed:
A. PRE-SPLITTING (Highwalls) B. SMOOTH BLASTING (Tunnel Perimeters)
[o] [o] [o] [o] [o] (Uncharged buffer) [X] [X] (Production rounds fire first)
| | | | | \ /
=================== (Shear crack split) [o]-[o]-[o] (Perimeter holes fire LAST)
Fired BEFORE production round Cushioned charges trim final perimeter
| Technique | Drilling Configuration | Explosive Loading | Timing Sequence |
|---|---|---|---|
| Pre-Splitting | Closely spaced holes (\(s \approx 8 - 12 d_{\text{hole}}\)) drilled along the final design boundary line | Decoupled, cushioned light charges (air-decked or trace explosive strings) | Fired simultaneously BEFORE any production blastholes detonate |
| Smooth Blasting (Contour Blasting) | Perimeter holes spaced closely (\(s \approx 15 - 16 d_{\text{hole}}\)) with burden-to-spacing ratio \(B/s \approx 1.2 - 1.5\) | Light decoupled cartridge charges | Fired on the LAST delay interval after all interior burden rock has been excavated |
| Cushion Blasting (Slashing) | Single row of trimming holes drilled adjacent to final wall | Lightly charged with stemming cushions | Fired after the main muckpile has been removed to trim irregular bench edges |
12.3 Peak Particle Velocity (PPV) & Damage Thresholds¶
Vibration waves traveling through rock are quantified by Peak Particle Velocity (PPV) measured via 3-axis velocity seismographs (geophones):
USBM Empirical Vibration Scaled Distance Law:¶
Where \(R\) is distance to the blast point (\(\text{m}\)), \(Q\) is maximum explosive charge weight per delay (\(\text{kg}\)), and \(K, B\) are site transmission constants.
Rock Mass Damage Criteria:¶
- \(\text{PPV} < 50\text{ mm/s}\): Negligible risk to fresh rock; acceptable for nearby concrete lining and engineered infrastructure.
- \(\text{PPV} \approx 100 - 250\text{ mm/s}\): Onset of minor joint opening and ravelling of loose keyblocks along unsupported roofs.
- \(\text{PPV} \approx 400 - 700\text{ mm/s}\): Critical threshold for permanent damage; new tensile fractures initiate in intact rock bridges.
- \(\text{PPV} > 1,000\text{ mm/s}\): Total shattering of intact rock; equivalent to the crushing zone around explosive blastholes.
12.4 Blast Vibration Monitoring Arrays¶
To prevent damaging highwalls and surrounding underground caverns: - Install triaxial geophone arrays at the crest of open pit benches to ensure perimeter blasting does not exceed \(\text{PPV} \le 150\text{ mm/s}\). - Link seismographs to digital ADAQS gateways to generate instantaneous compliance reports and optimize electronic delay detonator sequences (millisecond timing intervals).
12.5 Canonical Terminology¶
| English Term | Canonical Vietnamese Translation | Definition |
|---|---|---|
| Pre-splitting | Nổ mìn tạo khe trước / Nổ tách trước | Firing decoupled perimeter holes simultaneously prior to production blasting |
| Smooth blasting | Nổ mìn tạo biên nhẵn | Firing light perimeter charges on the final delay to produce a clean arch |
| Decoupled charge | Lượng thuốc nổ không tiếp xúc thành lỗ | Explosive charge whose diameter is smaller than the blasthole diameter |
| Peak Particle Velocity (PPV) | Vận tốc dao động hạt cực đại (PPV) | Maximum velocity of ground vibration particles during wave transit |
| Scaled distance | Khoảng cách tỷ lệ | Geometric ratio relating distance to the square root of explosive charge weight |