<p><p><figure id='attachment_3346' style='max-width:736px' class='caption aligncenter'><img class="wp-image-3346 size-full" src="https://www.geoinstitute.org/sites/default/files/geotech-tools-uploads/…; alt="Diagrams of explosive placed in borings and the detonating cord and of the measuring system used to measure settlement after the explosion." width="736" height="537" /><figcaption class='caption-text'> (a) details of explosive installation; (b) subsurface settlement measurement (Narsilio et al. 2009; With permission from ASCE)</figcaption></figure></p><p><div><h2>Project Summary/Scope:</h2>A blast densification field study was conducted in South Carolina. The study included laboratory testing and field characterization of four blast events.</p><p>Subsurface Conditions: The site consists of unsaturated sand (0 to 0.6 meters); sand layers with some silt and clay (between 0.6 and 4 to 6 meters); fine and clean white sand (between 4 to 6 and 7.3 to 7.5 meters); loose, very fine “black sand” on top of a thin denser fine sand and silty clay horizon (between 7.5 and 10.7 to 13.0 meters). The water table was located at a depth of 0.8 to 1.5 meters. The blast densification program targeted the loose sandy layer between 8 and 13 meters.</p><p>Individual explosive charges ranging from 11 to 34 kg were placed at a depth of 10 meters in a square grid pattern with a spacing of approximately 9 meters. Blasting delays of 10 and 50 ms were used in the first and second blasting events, while blasting delays of 10 minutes were used during the third and fourth events to test the advantage of vibrating the soil before the high excess pore pressures could dissipate.</p><p>Explosives were installed in predrilled and PVC-encased boreholes. Hydromite 860 explosive was used at a powder factor of 0.067 lb/yd³. The charge was installed at a depth of 10 meters. The rest of the borehole was backfilled with gravel. The four blasting events were conducted over an 8-month period.<br><h2>Performance Monitoring:</h2>Ground surface and subsurface settlements were measured. Pore pressure measurements were made using vibrating wire piezometers and CPTU tests, and GPR surveys were conducted after blasting. Vibrations were also measured.</p><p>The following observations were made:<br><ul> <li>The amount of settlement decreased as the number of blasting events increased, asymptotically approaching a “terminal density”.</li> <li>The total settlement after the four blast events was approximately 0.5 meters.</li></ul>An increase in CPT resistance was not observed until a year after the final blast was conducted.<br><h2>Project Technical Paper:</h2>Narsilio, G.A., Santamarina, J.C., Hebeler, T. and Bachus, R. (2009). “Blast Desification: Multi-Instrumented Case History.” <em>Journal of Geotechnical and Geoenvironmental Engineering</em>, 135(6) 723-734. <a href="http://ascelibrary.org/doi/abs/10.1061/%28ASCE%29GT.1943-5606.0000023">… Case History Prepared:</h2>November 2012</p><p></div></p></p>