Deep Excavation by Controlled Rock Blasting in UG Metro Station near by a Strutures

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The case study examines controlled blasting for underground metro-station excavation in a congested, populated area of Bangalore. The site comprised 0–4.5 m of soil over granitic gneiss of varying weathering grades, including thin shear zones. Key concerns were blast vibration, damage to nearby buildings, flyrock, and increased water seepage.<br /><br />To protect surrounding structures while maintaining construction productivity, the project used line drilling along the excavation perimeter, controlled blast design, non-electric (Nonel) detonators, rubber mats for blast-hole muffling, and vibration and settlement monitoring. Peak particle velocity (PPV) and vibration frequency were assessed against DGMS structural-damage limits. Using data from comparable Bangalore metro sites and a US Bureau of Mines model, the team estimated safe charge weights for different distances. For sensitive structures, the design targeted a PPV limit of 5 mm/s; the resulting charge recommendations applied where structures were at least 10 m from the blast.<br /><br />Line drilling created a plane of weakness to guide the main blast, limit overbreak, and reduce stress and flyrock beyond the excavation. Seismograph readings were reviewed in three directions and compared with the applicable standards. The maximum recorded PPV was 1.3 mm/s—below the 5 mm/s limit and the more conservative 2 mm/s level considered desirable for structural integrity. Recorded vibration results were within DGMS limits.<br /><br />A two-dimensional finite-element analysis in Rocscience also indicated that areas around the excavation slope were stable, with stability factors above 1.25 except at the slope face. The study concludes that carefully designed and monitored blasting, particularly with perimeter line drilling, can enable rock excavation near urban structures while controlling vibration and maintaining slope stability.

Keywords

controlled blasting

underground metro excavation

blast vibration

line drilling

peak particle velocity

Nonel detonators

flyrock control

vibration monitoring

slope stability

urban construction