<p><p><h2>QC/QA Guidelines</h2>A QC/QA program for vacuum preloading projects varies based on size, complexity, and purpose of the improved site. Simpler projects not involving stability concerns or strict time constraints only require settlement monitoring and materials/construction inspection. More sensitive projects will utilize additional testing such as inclinometers, piezometers, and/or surveying. An advanced QC/QA testing program for large and high risk projects should include materials testing, shear strength testing, piezometers, inclinometers, and, in some cases, field test sections. Performance approach specifications are not usually used for vacuum preload projects, as they would require an extensive soil exploration , and also because of the long time that would be required to determine that specified final settlements have been attained.</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 significant number of case histories have shown a 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, inaccurate readings, soil structure collapse, secondary compression, and structural viscosity. Settlement and pore water pressure readings should be taken for a period of time after the vacuum 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) recommend that “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.” Values recorded from these devices should be used to monitor PVD performance and control the rate of preload application.</p><p>If a surcharge load is used, inclinometer and settlement plate readings can be used to control the rate of preload application. Schaefer et al. (2016) provide a method to determine the displacement ratio (described in <em>Individual QC/QA Methods </em>section of this document). Lower displacement ratios indicate a 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 values indicate an unstable site.</p><p>There are certain observation and inspection elements that are specific to vacuum preloading with and without PVDs. Prior to installation, the quality of the PVD material and conformance to design specifications should be verified. In general, Schaefer et al. (2016) suggest the following 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>Records need to be kept of construction equipment/material inspection, installation inspection, vacuum pump efficiency, settlement progressions, pore pressure readings and inclinometer readings. It is critical that a monitoring system be implemented during pumping. The membranes need to be checked for cracking during the consolidation process. Cracking can lead to a loss of vacuum pressure, a decrease in settlement, and even rebound in the soil. Some of the greatest concerns with a vacuum preload are improperly joined field seams and punctured membranes. Good on-site quality control and placing a protective cover on the membrane as soon as possible should greatly reduce the probability of these concerns from occurring. If the proper design details and construction procedures occur, then the system should be self-sealing once the vacuum is applied.</p><p>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.”</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. Strengths are time dependent, increasing with time of consolidation and settlement. ASTM D2573 can be referred to for field vane shear test guidance.</p></p>