<p><p><div></div></p><p><figure id='attachment_1866' style='max-width:600px' class='caption alignnone'><img class="wp-image-1866" src="https://www.geoinstitute.org/sites/default/files/geotech-tools-uploads/…; alt="Schematic illustration of dynamic compaction. Diagram shows a crane dropping a weight into the ground with waves emanating from the resulting depression." width="600" height="561" /><figcaption class='caption-text'> Schematic illustration of dynamic compaction (Elias et al. 2006).</figcaption></figure></p><p><h2>Project Summary/Scope:</h2>A three-story, 8,000 m², relatively lightly loaded structure was to be constructed over limestone deposits. Explorations found evidence of sinkholes, voids, and large amounts of heterogeneity that raised concern about large differential settlement.</p><p>Subsurface Conditions: Silty fine sand grading to fine sand with seams of sandy clay. Low SPT values in these soils were evidence of possible voids. Other explorations have shown relatively dense deposits where calcareous materials have undergone cementation. The soils were considered to be of intermediate quality and would require multiple phases of improvement to allow for pore pressure dissipation.</p><p>Initial settlement predictions under the structure ranged from 23 mm to 74 mm; this large differential settlement was considered unacceptable. If the soils could be made more homogeneous to depths of 7.6 to 9.1 meters below ground surface, shallow foundations would be acceptable. An empirical “n” value of 0.4 was used for the site soils, giving a required tamper weight of 15 Mg dropped from a height of 24 meters to produce the required improvement to a depth of 7.6 meters. Where large ground depressions occurred, additional energy was used because of possible voids or caverns.</p><p>The maximum expected degree of improvement was estimated to be a SPT N-value of 35. The contractor used a 15-Mg tamper dropped from 20 meters. This was less than the energy required according to design equations.<br><h2>Performance Monitoring:</h2>SPT N-values showed less improvement than predicted. Pressuremeter tests were performed to supplement these results. The required depth of improvement of 7.6 meters was achieved. Strength increase following rest periods was observed.<br><h2>Project Technical Paper:</h2>Lukas, R.G. (1995). “Dynamic Compaction – Geotechnical Engineering Circular No. 1”, U.S. Department of Transportation, Federal Highway Administration, Washington, D.C., FHWA-SA-95-037.<br><h2>Summary Source:</h2><em>Ground Improvement Methods</em>, FHWA NHI-06-019 (Vol. I) and FHWA NHI-06-020 (Vol. II), 2006.<br><h2>Date Case History Prepared:</h2>November 2012</p></p>