Stress Redistribution after Blast Densification

Abstract

Results of a numerical finite-element (FE) simulation are presented to evaluate stress changes that affect the results of cone penetration tests (CPT) made at a test section after blast densification. The two-dimensional (2D) FE simulation was not used to predict liquefaction, as the pore-pressure distribution immediately after each blast and the extent of liquefaction, determined from field observations of ground-surface settlements and pore-pressure measurements, were an input rather than a result. A fully coupled FE formulation was employed to compute time-dependent deformations and stress changes that result from dissipation and seepage of input excess pore pressures. Computed results indicate stress levels within the targeted loose sand decrease near the edge of the blast zone as a result of stress redistributions that take place during reconsolidation, and likely contribute to decreased penetration resistance observed near the perimeter of the zone. Outside the blast zone, computed stress levels increased in areas that did not liquefy, and likely contributed to small increases in penetration resistance measured at these locations. However, computed results suggest that stress levels near the center of the zone remain unchanged, yet penetration resistance at the test section did not increase despite significant amounts of settlement. Occluded gas bubbles in the soil matrix resulting from gas released by the explosives affect the mechanical response of the sand postblasting during the CPT. Recommendations are made concerning evaluation of ground improvement after blast densification.