Explosive compaction is carried out by setting off explosive charges in the ground. The energy released causes liquefaction of the soil close to the blast point and causes cyclic straining of the soil. This cyclic strain process increases pore water pressures and provided strain amplitudes and numbers of cycles of straining are sufficient, the soil mass liquefies (i.e. pore water pressures are temporarily elevated to the effective vertical overburden stress in the soil mass so that a heavy fluid is created). Liquefaction of the soil followed by time-dependent dissipation of the water pressures causes re-consolidation within the soil mass. This re-consolidation happens within hours to days following blasting, depending on the permeability of the subsoils and drainage boundary conditions, and is reflected by release of large volumes of water at the ground surface or up blast casings. "Short term" volume change is also caused by passage of the blast-induced shock front through the soil mass. At close distances from a charge detonation, the hydrodynamic pressures are large enough to cause compression of the soil-water system even though the bulk compressibility of the system is relatively small. Once an area of ground has been treated and pore pressures have largely dissipated, repeated applications ("passes") of shaking caused by controlled blast sequences (video (9113 kb)) causes additional settlement depending on soil density and stiffness. The first pass destroys any bonds existing between cohesionless soil particles due to aging and other geologic processes, and causes the majority of settlement within the soil mass. Subsequent passes cause additional settlement by cyclic straining. As a result, surface settlement and increased soil resistance to cyclic loading will be caused by the blasting. Experience has indicated that the degree of ground improvement obtained by blasting depends on the initial density of the granular subsoils. The density of loose deposits can typically increase considerably to relative densities in the range of 70 to 80%, whereas soils with initial relative densities of 60 to 70% can only be densified by a small amount. Our experience also indicates that EC generally causes volume changes equal to or in excess of what would be anticipated under design levels of earthquake shaking, as described in the attached reference paper by Gohl et al (2000).
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