<p><p><figure id='attachment_3555' style='max-width:624px' class='caption aligncenter'><img class="wp-image-3555 size-full" src="https://www.geoinstitute.org/sites/default/files/geotech-tools-uploads/…; alt="Photograph of rapid impact compaction rig." width="624" height="257" /><figcaption class='caption-text'> Rapid Impact Compaction rig. (Simpson et al. 2008; With permission of ASCE).</figcaption></figure></p><p><div><h2>Project Summary/Scope:</h2>A five-story affordable housing building was constructed on a site composed of liquefiable fill and compressible marine clay. The soil was evaluated before and after treatment using in-situ testing.</p><p>Subsurface Conditions: The top layer of soil consisted of a 6 to 49-foot (1.8 to 15‑meter) thick layer of heterogeneous fill composed of gravel, sand, clay, and miscellaneous building rubble. Underneath the layer of fill was a layer of Bay Clay, a marine clay native to the area. The groundwater table was determined to be at a depth between 4½ to 7½ feet (1.4 to 2.3 meters) below the ground surface. A potentially liquefiable layer 7 to 20 feet (2.1 to 6.1 meters) thick consisting of loose to medium dense sand with some silt and clay was present just below the groundwater table.</p><p>CPTs and/or rotary wash borings were performed to evaluate the liquefaction potential. In-situ testing revealed the presence of liquefiable fill and excessive consolidation settlement from the underlying marine clay layer. Due to the stated hazards, steel H-piles driven to rock were required. As a result of the liquefiable fill, lateral resistance in the fill layer was low, thus requiring a significant number of piles to resist base shear. Ground improvement was specified in order to decrease foundation costs. Rapid Impact Compaction (RIC) was selected because of its relative speed and economy. Treatment consisted of performing 13 compaction points per 20‑foot by 20-foot (6-meter by 6-meter) area. Tamping was conducted by dropping a 7.5-ton (7-tonne) weight from a height of 3 feet (1 meter) onto a 5-foot (1.5-meter) diameter compaction foot at a rate of 40 to 60 blows per minute. Compaction points were placed on a 10-foot (3-meter) on-center grid pattern in the first pass; the second pass consisted of points at 10 feet (3 meters) on-center, midway between points of the first pass.<br><h2>Alternate Technologies:</h2>Additional ground improvement methods that were considered included compaction grouting, stone columns and vibroflotation. RIC was selected because of its relative speed and economy.<br><h2>Performance Monitoring:</h2>Drop height, number of blows and penetration per blow were monitored and/or controlled by an on-board data acquisition system. The dropping of the weight at each point was terminated when one the following criterion was met: (1) the deflection for the final blow was 0.2 inches (5 mm), or (2) 40 total blows, whichever occurred first. Craters deeper than 18 in (460 mm) required retreatment which occurred 24 hours after the initial treatment to allow for excess pore water pressures to dissipate. Vibrations due to the compaction were measured at varying distances using seismographs. CPTs were used to confirm the level of improvement.</p><p>Some liquefaction potential remained, although the remaining liquefiable layers were significantly thinner and the post-treatment tip resistances were significantly higher than the pre-treatment values. Therefore, the overall results indicate fill at the site was sufficiently improved such that the liquefaction potential was reduced. Lateral pile capacity was increased by about 30 to 35%.<br><h2>Project Technical Paper:</h2>Simpson, L.A., S.T. Jang, C.E. Ronan and L.M. Splitter (2008) “Liquefaction Potential Mitigation using Rapid Impact Compaction.” Proceedings of the Conference of Geotechnical Earthquake Engineering and Soil Dynamics IV, Sacramento, CA, Paper No. 181. <a href="http://ascelibrary.org/doi/abs/10.1061/40975%28318%29110">http://asceli… Case History Prepared:</h2><strong> </strong>November 2012</p><p>&nbsp;</p><p>&nbsp;</p><p></div></p></p>