<p><p><figure id='attachment_2639' style='max-width:475px' class='caption aligncenter'><img class="wp-image-2639 size-full" src="https://www.geoinstitute.org/sites/default/files/geotech-tools-uploads/…; alt="Diagram of vibro concrete column installation. " width="475" height="630" /><figcaption class='caption-text'> Diagram of vibro concrete column installation. (Figure courtesy of FHWA, Elias et al. 2006).</figcaption></figure><h2>Basic Function:</h2>Vibro-Concrete Columns (VCCs) are used to increase the bearing capacity of soft soils overlying stiffer strata. They are often used in combination with column supported embankments to reduce total and differential settlements.<br><h2>Advantages:<em> </em></h2><ul> <li>Reduced total, differential, and seismic settlements.</li> <li>Greater column stiffness compared with aggregate columns.</li> <li>Quick construction.</li> <li>Environmentally friendly (no spoils).</li></ul><h2>General Description:</h2>VCCs are similar to aggregate columns but use concrete in place of aggregate. They can be used in soft soils where aggregate columns are not appropriate. Typically, VCCs have an enlarged bottom and top bulb to increase end-bearing resistance and ensure adequate load transfer at the surface, respectively.<br><h2>Geologic Applicability:</h2><ul> <li>Loose sands, soft clays, and organic soils.</li> <li>Most applicable in soft clay or peat with low undrained shear strength.</li> <li>Stiffer bearing stratum desirable at VCC tip.</li></ul><h2>Construction Methods:</h2>VCCs use a vibrator to penetrate soft soils and to densify the bearing stratum to a limited degree. The column is constructed in a manner similar to stone columns but instead of feeding stone to the tip of the vibrator, concrete is pumped through an auxiliary tube to the bottom of the vibrator. As the vibrator is extracted from the ground, concrete is pumped to fill the void, creating a concrete column. During vibrator extraction, repenetration stokes are often used near the bottom and top of the column to form the enlarged bottom and top bulb. Typical column shaft diameters range from 18 to 24 inches and the enlarged base is usually about 24 inches or greater in diameter. Columns are generally spaced a minimum of 5 feet on center. Typical VCC lengths vary from 16 to 33 feet, though they can be installed to greater depths. The VCC is generally constructed without reinforcement; however, reinforcement can be included to support tensile and lateral loading. In axial compression, typical allowable design loads for VCCs range from 75 to 100 tons.<br><h2>Additional Information:</h2>VCC and similar technologies have been widely used on numerous projects worldwide. In current practice, VCCs are designed using modified driven pile or drilled shaft procedures. A design procedure developed specifically for VCCs still needs to be established. Despite the uncertainty in design, load tests and well-documented QC/QA can be used to validate performance and ensure consistency between columns.<br><h2>SHRP2 Applications:</h2><ul> <li>New Embankment and Roadway Construction over Unstable Soils</li></ul><h2>Example Successful Applications:</h2><ul> <li>Bridge Approach Fill – Perth Amboy, NJ</li> <li>Roadway Embankment over Landfill – South of Hanover, Germany</li> <li>Railroad Embankment – Near Rancocas Creek, NJ</li></ul><h2>Complementary Technologies:</h2>VCCs are often used with column supported embankments. Lightweight fills can be used to reduce embankments loads when necessary. Wick drains can be used to accelerate consolidation in compressible soils prior to VCC installation.<br><h2>Alternate Technologies:</h2>Excavation and replacement, vibrocompaction, aggregate columns, Combined soil Stabilization with Vertical columns (CSV), PVDs with or without fill preloading, continuous flight auger piles, driven piles, deep mixing methods, and jet grouting.<br><h2>Potential Disadvantages:</h2><ul> <li>Lacks a well-established design procedure</li> <li>More expensive than aggregate columns</li></ul><h2>Key References for this technology:</h2>Elias, V., Welsh, J., Warren, J., Lukas, R., Collin, J.G., and Berg, R.B. (2006). “Ground improvement methods-Volume I.” Federal Highway Administration, Publication No. FHWA NHI-06-019, pp. 6-1-6-136.</p><p>Schaefer, V.R. (editor) (1997). <em>Ground Improvement, Ground Treatment, Ground Reinforcement-Developments 1987-1997</em>. Geotechnical Special Publication No. 69. ASCE, New York, 616 pp.</p></p>