Evaluating ground improvement after blast densification: Performance at the Oakridge Landfill

Blasting has long been used as a ground improvement technique to densify loose granular soils. Explosives detonated at depth induce liquefaction in the target sand followed by pore pressure dissipation and attendant consolidation. Improvement typically is verified via measured surface settlements and increased penetration resistance with in situ tests such as the cone penetration test (CPT). However, an increase in penetration resistance is often not observed for months or even years after blasting. The results of instrumentation at a production blast zone at the Oakridge Landfill are reported herein. Instrumentation included surface settlements, pore pressure measurements inside and outside the blast zone, preblast and postblast piezocone (CPT_u) soundings, and gas concentrations of pore fluid. No increase in penetration resistance with the CPT_u was detected 53 days after the final blast, despite more than 0.5 m of settlement and an increase in relative density to 80% or more in some areas. Results of gas chromatography tests on the collected pore fluid samples showed that gas produced by the explosives was trapped in the target layer and saturated the groundwater with N₂. Consequently, no more N₂ could dissolve into solution, and N₂ gas existed as occluded bubbles. The presence of occluded bubbles created an unsaturated condition, which in turn affected the mechanical behavior of the densified soil and reduced the CPT tip resistance during postblast testing.