<p><p><h2>QC/QA Guidelines</h2>FHWA’s <em>Prefabricated Vertical Drains</em> document (Rixner et al. 1986) is considered to be the best available guidance document for PVD and fill preloading for soil improvement and rapid embankment construction applications in support of the SHRP 2 objectives. The primary objectives of a good QC/QA program are to reduce the risk and cost of PVD installation while achieving target design values. A typical comprehensive QC/QA program for PVD and fill preloading projects varies based on size, complexity, and application of the improved site. Simpler projects not involving stability concerns or strict time constraints only require settlement monitoring and materials and construction inspection. More sensitive projects will utilize additional testing such as inclinometers, piezometers, determination of the Displacement Ratio and/or surveying. An advanced QC/QA testing program for large and high risk projects should include materials testing, field vane shear tests or other methods for determination of soil strength, piezometers, inclinometers, and, in some cases, field test sections. End result specifications are not usually used for PVD and preload projects, as they would require an extensive soil exploration (Schaefer et al. 2016).</p><p>Settlement magnitudes can be determined using surveying, settlement plates, and piezometers. Data obtained from these measurements can be used to verify design settlement predictions and monitor project progress. A large number of case histories have shown a significant discrepancy between the rate of settlement and the rate of pore pressure dissipation. When this occurs, settlement data is typically given precedence. Piezometer readings can be influenced by pore gas, poor readings, collapse of soil structure, secondary compression, and structural viscosity. Settlement and pore water pressure readings should be taken for a period of time after the fill preload has been removed to monitor rebound.</p><p>For projects where stability is a concern, inclinometers and piezometers can be installed and measured. Schaefer et al. (2016) recommends “the inclinometers should be installed at the toe of the embankment or in front of retaining walls, with settlement plates and piezometers beyond the crest of the embankment and/or near the centerline” (c. 1, p. 43). Values recorded from these devices should be used to monitor PVD performance and control the rate of preload application. For sites where stability is especially a concern, Schaefer et al. (2016) provide a method to determine the Displacement Ratio. Lower displacement ratios indicate a more stable site. Based on case history experience, Displacement Ratios of 0.2 generally correspond to a site factor of safety of 1.3 (Schaefer et al. 2016). Higher Displacement Ratios may indicate an unstable site.</p><p>Prior to installation, the quality of the PVD material and conformance to design specifications should be verified. In general, Schaefer et al. (2016) suggest several parameters should be inspected and recorded during the project:<br><ul> <li>Size, type, weight, maximum pushing force, vibratory hammer rated energy, and configurations of the installation rig</li> <li>Dimensions and length of mandrel</li> <li>Details of PVD anchorage</li> <li>Detailed description of proposed installation procedures</li> <li>Proposed method for splicing drains</li></ul>Trial sections will generate the best QC and installation standards. According to Schaefer et al. (2016), “Once trial drains have been satisfactorily completed, inspection mainly consists of recording depths and locations of each drain, observing splices and verticality of equipment, taking occasional material samples for inspection and testing, and noting any major variances in procedure” (Section 5.3, p. 2-40).</p><p>For material testing, it should be noted that some of the mentioned tests are overly work intensive, considering that PVD manufacturers provide material strength and property data. Only projects with stringent design or performance requirements require actual testing of the PVD.</p><p>Field vane shear tests work well in assessing shear strength changes in the soil. These values are time dependent, increasing with time of consolidation and settlement. This should be considered when analyzing vane shear results. ASTM D2573 can be referred to for field vane shear test guidance.</p></p>