<p><p><strong>Current FHWA Reference(s):<br></strong><em>Schaefer et al. (2016)</em><strong>Supporting Reference(s):<br></strong><em>Chai et al. (2006)<br>Chu et al. (2002)<br>Chu and Yan (2005a, 2005b)<br>Jacob et al. (1994)<br>Masse et al. (2001)<br>Rixner et al. (1986)</em></p><p>Vacuum preloading with PVDs is an economical and beneficial solution when the time to reach desired consolidation without PVDs is too great but stability concerns limit the use of fill preloading.

<p><p><strong>References:<br></strong><em>Schaefer et al. (2006)</em></p><p><strong>Method Summary</strong></p><p>Calculating the displacement ratio, D_R, is a method of evaluating the stability of sites where, in addition to the vacuum preload, a fill preload is utilized. The displacement ratio is measured along and transverse to the perimeter of the fill.

<p><p><strong>References:<br></strong><em>Yan and Chu (2003a, b)</em></p><p><strong>Method Summary</strong></p><p>Field vane shear tests are performed in the field in order to determine the in-situ shear strength of the soil. Specific guidelines for the test can be found in <a href="http://www.astm.org/Standards/D2573.htm">ASTM D2573.

<p><p><strong>Reference(s):<br></strong><em>Bergado et al. (1998)</em></p><p>Two large-scale and fully-instrumented test embankments were improved using vacuum preloading. After the investigation was complete, a finite element program was used in an attempt to simulate the field conditions and results, thereby calibrating the program for use in future designs. The analysis assumed a plane strain condition.

<p><p><div><h2>Project Summary/Scope:</h2>Construction of a 2.15-meter high embankment was required for the purpose of protecting the highway from floods. A pilot test embankment construction program was conducted using vacuum preloading. The initial test section was 390 m².

<p><p><figure id='attachment_3601' style='max-width:600px' class='caption aligncenter'><img class="wp-image-3601" src="https://www.geoinstitute.org/sites/default/files/geotech-tools-uploads/…; alt="Plan view graphic showing general layout of the sewage treatment plant facilities." width="600" height="685" /><figcaption class='caption-text'> Sewage Treatment Plant General Layout

<p><p><strong>References:<br></strong><em>Chu et al. (2006)<br>Schaefer et al.

<p><p><strong>References:<br></strong><em>Schaefer et al. (2016)<br>Gao (2004)<br>Jacob et al.

<p><p><strong>Reference(s):<br></strong><em>Jacob et al. (1994)</em></p><p>The type of membrane to be selected does not typically rely on the subsurface conditions but on construction/project constraints and cost efficiency. Selection guidance is based on field experience and general knowledge of the material. The membrane must be impermeable to hold the vacuum. Common impermeable membrane materials include PolyVinyl Chloride (PVC), Chlorosulfonated Polyethylene Synthetic rubber (CPE), and High-Density PolyEthylene (HDPE).

<p><p><strong>References:<br></strong><em>Chu and Yan (2005a, b, c)<br>Chu et al. (2006)<br>Schaefer et al. (2016)<br>ISSGME (2008)<br>SHRP 2 Phase I Technology Assessment (2008)<br>Yan and Chu (2003a,b)</em></p><p><strong>Method Summary</strong></p><p>Piezometers are used to measure and monitor pore water pressures before, during, and after vacuum preloading. Once installed, the piezometer should be read on a regular schedule.

<p><p><strong>Reference(s):<br></strong><em>Holtz (1975)<br>Jacob et al. (1994)<br>Masse et al. (2001)</em></p><p>Vacuum pump selection must consider the size of the area to be improved, filter thickness, magnitude of preload, length of time for application, and location of ground water table. For larger areas (greater than 5,000 square yards), vacuum pumps with a 20-inch thick filter can maintain a vacuum of 60 to 70 kPa (Holtz 1975). Multiple pumps can be used together to propagate the vacuum over much larger areas.

<p><p><strong>References:<br></strong><em>Gao (2004)<br>SHRP 2 Phase I Technology Assessment (2008)</em></p><p><strong>Method Summary</strong></p><p>Upon arrival of site equipment, an inspection should be performed and recorded in order to verify the quality and adherence to design specifications of the equipment.

Current FHWA Reference(s):
Schaefer et al. (2016)Supporting Reference(s):
Chai et al. (2006)
Holtz and Christopher (1987)
Rixner et al. (1986)

<p><p><strong>References:<br></strong><em>Chu and Choa (1995)<br>Chu et al. (2006)</em></p><p><strong>Method Summary</strong></p><p>Tests on PVDs are used to obtain information on the filter quality, discharge capacity, and tensile strength. For PVDs with an Apparent Opening Size (AOS) greater than 40 μm, ASTM D4751 can be used to determine the AOS of the PVD. The permeability of the PVD can be determined by performing ASTM D4491. ASTM D4632 is often specified to measure tensile strength of the PVD.

<p><p><strong>References:<br></strong><em>Chu and Yan (2005a, b, c)<br>Chu et al. (2006)<br>Schaefer et al. (2016)<br>ISSGME (2008)<br>Yan and Chu (2003b)</em></p><p><strong>Method Summary</strong></p><p>Settlement plates should be placed at the bottom of the drainage blanket, the bottom of the compressible layer, and intermediate depths prior to PVD installation and vacuum preload.

<p><p><strong>References:<br></strong><em>SHRP 2 Phase I Technology Assessment (2008)</em></p><p><strong>Method Summary</strong></p><p>Surveying can be used to measure settlement across the site as it occurs during vacuum preloading.