<p><p><figure id='attachment_2634' style='max-width:747px' class='caption aligncenter'><img class="wp-image-2634 size-full" src="https://www.geoinstitute.org/sites/default/files/geotech-tools-uploads/…; alt="Schematic diagram showing the equipment and setup for applying vacuum preloading with prefabricated vertical drains." width="747" height="397" /><figcaption class='caption-text'> Vacuum Consolidation Process with PVDs (Masse et al. 2001)</figcaption></figure><h2>Basic Function</h2>Vacuum Consolidation induces an increase in effective stress in foundation soils through reduction in pore pressures. Improves saturated soils by consolidation.<br><h2>Advantages:</h2><ul> <li>No fill is required</li> <li>No staged loading is required</li> <li>No heavy equipment</li> <li>Environmentally friendly</li> <li>Established design methods and QC/QA requirements</li> <li>Cheaper and faster compared to surcharge loading</li></ul><h2>General Description:</h2>Vacuum consolidation improves saturated soils by consolidation using a vacuum load to increase the effective stress in the foundation soils. Prefabricated Vertical Drains (PVDs) can be used to increase the effectiveness of the system by increasing the rate of consolidation.<br><h2>Geologic Applicability:</h2><ul> <li>Compressible clays, soft, uniform clays.</li> <li>Has been known to improve sites with underlying clayey silt layers.</li> <li>More effective with shallow ground water table.</li> <li>Cannot be used to reduce secondary compression such as with high organic contents.</li></ul><h2>Construction Methods:</h2>The soil site is covered by an airtight membrane. A dual venture and vacuum pump are used to create a vacuum at the site. Through a combination of dewatering and vacuum action, the water table is maintained at the base of the granular platform. Vacuum loads of about 12 psi (80 kPa) can be created and maintained. If greater loads are required for the soil, surcharge may be placed on top of the vacuum system. This method can be used with or without<br>PVDs in the foundation soils. The PVDs will help increase the effectiveness of the method by accelerating the rate of consolidation. The PVDs should not be installed to the full depth of the soil to be consolidated if there is a more permeable lower layer below.<br><h2>Additional Information:</h2>The technology reduces the pore water pressure of the soil when the vacuum load is applied, causing the total stress to remain constant while the effective stress increases. Design methods and QC/QA requirements are well established and well documented case histories exist, but mostly for overseas projects. The cost of this technology is economical compared to excavation and fill and can be two-thirds of that of fill surcharge.<br><h2>SHRP2 Applications:</h2><ul> <li>Embankment and roadway construction over unstable soils</li> <li>Roadway and embankment widening</li> <li>Stabilization of pavement working platforms</li></ul><h2>Example Successful Applications:</h2><ul> <li>Oil Storage Station – Tainjin, China</li> <li>Highway Construction Site – Ambes, France</li></ul><h2>Complementary Technologies:</h2>Typically not used with other technologies. Additional preloading by fill or water can be used. Has been used with dynamic compaction to help create excess pore pressure.<br><h2>Alternate Technologies:</h2>Deep foundation elements, sand drains, PVD with fill surcharge, electro-osmotic consolidation with PVDs, stone columns, grouting, deep soil mixing, excavation and replacement, and lightweight fill.<br><h2>Potential Disadvantages:</h2><ul> <li>Maintenance of vacuum pressure.</li> <li>May cause cracks in surrounding soils.</li> <li>Vacuum pressure is limited to atmospheric pressure.</li> <li>Inward lateral movements from vacuum preloading can cause damage to adjacent structures.</li> <li>Vacuum pressure must be maintained for several months to obtain a high degree of consolidation.</li> <li>The system must be carefully monitored for leaks.</li> <li>Vacuum preloading is limited to providing an effective maximum surcharge of 14.5 psi (100 kPa).</li></ul><h2>Key References for this Fact Sheet:</h2>Chu, J. and Yan, S. (2005a). “Application of the vacuum preloading method in land reclamation and soil improvement projects.” Chapter 3, in Ground Improvement, Volume 3 – Case Histories, 91-118.</p><p>Masse, F., Spaulding, C.A., Wong, I.C. and Varaksin, S. (2001) “Vacuum consolidation: a review of 12 years of successful development.” Geo-Odyssey, ASCE, Virginia Tech, Blacksburg, VA</p><p>Terashi, M. and Juran, I. (2000). “Ground Improvement – State of the Art.” Proceedings of GeoEng 2000, An International Conference on Geotechnical &amp; Geological Engineering, 10-24 November 2000, Melbourne, Australia, Volume 1, pp. 461-519.</p></p>