Cushion blasting is a form of controlled blasting performed after the main production blast. It involves drilling a row of lightly charged “buffer” holes, which are stemmed with crushed stone to minimize damage to the surrounding rock and preserve the final slope face [1]. This technique can be applied using either vertical or angled holes, though precise alignment is critical in both cases. The cushion holes are positioned along the final slope line, loaded with small, evenly distributed charges, and detonated following the production blast [1].
The burden for cushion holes varies according to the rock characteristics. For example, the burden of hard, competent rock will be smaller than the burden of soft, easily fractured rock. It is important to conduct one or more test blasts and continually analyze and back-calculate results from production blasting to determine the proper burden, spacing, and charge density [1].
The spacing on cushion and buffer blastholes varies depending on the bedrock type and structural characteristics but generally ranges from 15 to 24 times the borehole diameter. The burden-to-spacing relationship varies, but spacing on cushion holes should always be less than the width of the burden being removed [1].
The role of cushion blasting in controlled excavation projects is primarily to reduce overbreak and control the impact of the blast on the surrounding rock or structures. Cushion blasting, also referred to as trim blasting, involves drilling a row of lightly loaded holes along the final excavation boundary and firing them after the main production blast. The holes are often backfilled with crushed stone or other cushioning material to absorb the shock waves generated by the explosion. This cushioning effect helps to minimize damage to the finished wall or excavation face, resulting in a smoother, more precise excavation contour with reduced fracturing and less displacement beyond the desired limits.
Cushion blasting is typically used after the main production blast to “clean up” the final wall by cutting any remaining rock remnants while protecting the stability of the excavation’s boundary. It employs relatively small, lightly charged holes that produce a fracture plane to limit blast-induced damage behind or adjacent to the excavation face. The spacing, burden, and charge density for cushion blasting are carefully calculated based on the rock characteristics to optimize the cushioning effect without causing excessive breakage or instability. This technique is simpler and often more economical compared to more intensive presplitting methods and is utilized in both surface and underground excavation projects to improve wall smoothness and control overall blast outcomes.
Main functions of cushion blasting:
- Cushions and absorbs shock from the blast to protect the finished excavation face.
- Reduces overbreak and unwanted fracturing beyond design limits.
- Produces a smoother final wall or contour line.
- Increases stability of the rock mass adjacent to the excavation by controlling fracturing.
- Allows economical and effective clean-up after the primary production blast.
Overall, cushion blasting plays a crucial role in controlled excavation projects by providing a method to precisely control blast-induced damage, reduce overbreak, and achieve stable and smooth excavation boundaries.
Reference
[1] “FHWA – Center for Local Aid Support – Publications.” Accessed: Sept. 16, 2025. [Online]. Available: https://www.fhwa.dot.gov/clas/ctip/context_sensitive_rock_slope_design/ch_3_6.aspx
