<p><p><h2>Project Summary/Scope:</h2>A five-story, 40,000 ft2 office building site consisting of liquefiable soils in the floodplain of the Missouri River in Missouri was remediated using Deep Dynamic Compaction (DDC) and stone columns constructed by dropping heavy weights. The required depths of treatment were greater than those provided by DDC, so stone columns were used to densify the soils at greater depths.</p><p>Subsurface conditions: The site consists of a thin layer of fill underlain by floodplain alluvium and Mississippian-age bedrock. The alluvium consists of three distinct strata:<br><ul> <li>Medium stiff, silty clay to clayey silt with occasional traces of sandy silt and silty sand.</li> <li>Loose, fine to medium poorly graded sand.</li> <li>Medium dense to dense, fine to coarse sand with traces of gravel.</li> <li>Groundwater at the time of the subsurface investigation was 3 meters below the ground surface.</li></ul>Maximum column loads of approximately 600 kips were assumed for design. These high column loads required either a deep foundation or soil improvement. Either one or five stone columns were constructed at each small and large column pad, respectively.</p><p>Approximately 10 feet of soft fine-grained soils existed on the site. These soils were stripped prior to DDC to allow densification to greater depth. Stone columns were compacted using a 19-ton weight dropped from heights of 30 to 70 feet onto crushed stone dumped into the DDC craters. The craters formed were approximately 8 to 10 feet in diameter and 5 to 7 feet deep. The number of drops per column was determined based on observation of DDC at a test footing location. The construction procedure was to:<br><ul> <li>Drop the weight ten times from a height of 70 feet and fill the resulting crater to a depth approximately equal to half the depth of the crater with 3⁄4” maximum size crushed stone. Depths of craters after ten drops were generally between 6 and 8 feet.</li> <li>Drop the weight again six times from a height of 70 feet and backfill the resulting crater to a depth approximately equal to half the depth of the crater with 3⁄4” maximum size crushed stone.</li> <li>Drop the weight again six times from a height of 70 feet and completely backfill the resulting crater with 3⁄4” maximum size crushed stone.</li> <li>Compact the stone in the crater by dropping the weight once from a height of 30 to 40 feet.</li></ul><h2>Complementary Technologies:</h2>Deep dynamic compaction<br><h2>Performance Monitoring:</h2>SPT testing soon after the completion of the DDC and stone column installation indicated increased post-treatment densification in most locations. One test location indicated no improvement; however the authors believe that this was due to excess pore pressures that had not yet dissipated.<br><h2>Project Technical Paper:</h2>Kumar, S. (2001). “Reducing liquefaction potential using dynamic compaction and construction of stone columns.” Geotechnical and Geological Engineering, 19, 169-182.<br><h2>Data Case History Prepared:</h2>November, 2012</p></p>