<p><p><figure id='attachment_3548' style='max-width:1024px' class='caption alignnone'><img class="wp-image-3548 size-large" src="https://www.geoinstitute.org/sites/default/files/geotech-tools-uploads/…; alt="Schematic cross section showing PVD installation in foundation soils under an embankment and showing the typical locations of instrumentation." width="1024" height="642" /><figcaption class='caption-text'> Generic PVD Installation for a Highway Embankment. (FHWA, 2006)</figcaption></figure></p><p><div><h2>Project Summary/Scope:</h2>A 1,000-foot runway extension was to be constructed at the Quad City Airport in Moline, Illinois in order to increase capacity. With land-use constraints on one end of the runway, construction required up to 15 feet of fill placement over a bluff. With concerns of post-construction settlement under the added fill, Prefabricated Vertical Drains (PVDs) were selected to accelerate consolidation.</p><p>Subsurface Conditions: Alluvial soils consisting of silty clay overlying fine sand. The upper 3.3 to 6.6 feet of silty clay had N-values ranging from 5 to 10 blows per foot and moisture contents ranging from 22 to 30 percent. The lower portion of the silty clay had N-values ranging from 1 to 4 blows per foot and moisture content ranging from 30 to 45 percent.</p><p>A small surcharge of 5.2 feet, along with PVDs was selected to accelerate consolidation. Horizontal strip drains were used instead of a drainage blanket due to limited availability and high cost of clean sand. The 12 by 1 inch strip drains connected to four rows of PVDs spaced in a 5-foot triangular pattern. These design parameters were selected based on the desired achievement of the maximum total settlement of 14 inches in 6 months.</p><p>Since the taxiway was not as critical as the runway extension, the project was split into two phases. Approximately 252,600 linear feet of PVDs were installed to an average depth of 23 feet. Installation of nearly 10,000 PVDs occurred in three weeks.<br><h2>Alternate Technologies:</h2>Construction without PVDs or excavation and replacement and surcharging.<br><h2>Performance Monitoring:</h2>Settlement plates were used to determine the time rate of consolidation. Actual settlements ranged from 4 to 16 inches in 3 to 4 months.<br><h2>Cost Information:</h2>The project was bid on a per drain basis at $11.50 per drain. Mobilization costs were $10,000. Horizontal strip drains were installed at $1.50 per foot.<br><h2>Summary Source:</h2>Elias, V., Welsh, J., Warren, J., Lukas, R., Collin, J. G., and Berg, R. R. (2006). Ground Improvement Methods, Federal Highway Administration, NHI-06-019 Vol I and NHI-06-020. Vol II, Washington, D.C., pp 536 (Vol I) and 520 (Vol II).<br><h2> Date Case History Prepared:</h2><strong> </strong>November 2012</p><p></div></p></p>