How to Use Blasting for Deep Compaction of Soil?

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Blasting is an economical compaction method that can be applied for various soil types such as granular, silt, clay, loess, etc. Not only is the technique highly effective in loose soil, but also capable of compacting granular soil down to a depth of 40m.

The blasting method involves the determination of the right amount of explosive materials, placement of explosives at a suitable depth below the water table, and finally, the safe explosion of the charge. One can apply either the surface or internal blasting technique; the latter is considered more effective and safer.

The technique is generally employed in construction works such as earth rockfill dams, airfields, roads, and harbors. The limitation of the method may include the risk of explosives, unexpected results of blasting, and receiving permission to store and use explosive materials.

How to Use blasting for Deep Compaction of Soil?

• Assess the situation and determine whether blast-induced vibrations cause structural damages or trigger landslides around the construction site. This assessment requires an understanding of the impact of the expected blasts, which is one of the difficulties that may arise.
• Achieve permission to use explosives.
• Determine the right quantity of explosives. The individual charges are 1-12 kg or 10-30 g/m³ of the soil to be compacted.
• Place explosive materials in drilled or jetted boreholes at a depth of about 50-75% of the total soil depth intended to be compacted, below the water table.
• The distance between boreholes ranges from 5-15 m.
• For large projects, engineers can use test blasting to determine optimum borehole spacing, charge size, interval, and spacing of basting.
• Further compaction cannot be achieved by reducing explosive spacing or increasing the quantity of explosives.
• The maximum blasting repetition ranges from 2 to 3 times because soil compaction after three rounds of the blast is low. 
• The first blast is responsible for 50-60% of the total settlement, and the second blast is accountable for the additional 20% of the full settlement.
• The compaction result within the blast-affected area may vary; the soil around the blasting point and hard soils may loosen due to the blasting effect.
• The increase in relative density ranges from 15 to 30%.
• At a depth of 12 m or shallower depths, a relative density of 75-85% can be achieved, but the relative density would reduce to 65-75% at depths greater than 12 m.
• Surface settlement ranges from 2-10% of the total depth of the compacted soil layer.
• The soil surface settles after the escape of gasses through fissures and dissipation of pore water pressure.
• Engineers can check the achieved state of compaction by measuring surface settlement. Alternatively, use tests such as cone penetration test, standard penetration test, or weight soundings. 
• Blasting can compact up to 1.5 times charge depth and 1.2 to 1.3 times charge depth in loose sand and medium dense sand, respectively.
• A maximum depth of 40 m of granular soil can be compacted by blasting. 
• If the relative density of soil from the surface to a depth of 2 to 3 m is low, additional surface compaction may be needed.
• The loosened soil surface thickness may increase as explosive quantity increases.
• Blasting efficiency is based on blasting-generated pore water pressure and the size of the liquefied zone around the blasting point.
• A small amount of gas in the soil can reduce the blasting effect considerably; hence reduce the degree of compaction.
• Blasting creates high pressure in saturated sand; 1 kg of explosives generates more than 14 MPa pressure at a distance of 4 m.

FAQs for soil compaction by blasting