Developing Production Pile Driving Criteria from Test Pile Data (2011)

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Although exploratory borings and engineering studies during design are an integral part of foundation engineering, the axial resistance of a driven pile foundation is ultimately determined by the criteria used to decide when to stop driving the pile during construction. The use of test piles for the purpose of developing the pile installation criteria can be instrumental in building driven pile foundations that are reliable and cost-effective. Nationwide practices for develop- ing pile driving criteria range from the use of very simple formula without any test pile verifi- cation to the use of pre-production test piles with dynamic measurements during installation and static load testing. Many agencies employ a range of technologies and methods based on the size of the project, the type of pile, and the predominant ground conditions. However, this issue is handled differently from state to state based on local experience, economics, and other factors. This synthesis provides a survey of the current practices used by transportation agencies to develop pile driving criteria, with special attention placed on the use of test pile data. The sur- vey consists of questionnaires sent to all 50 state departments of transportation plus the District of Columbia and Puerto Rico; 44 of the 52 agencies provided responses. In addition, a Phase II Survey was performed by telephone interview with nine agencies representing a broad geo- graphical distribution of large states that have extensive pile foundation construction projects. The information collected indicated that practices used by transportation agencies to develop pile driving criteria for production pile installation can be described as highly variable in terms of the level and sophistication of the testing performed. To some extent, such variability in test pile requirements may reflect the inherent variety in project size, complexity, ground condi- tions, pile type, etc. However, a significant component of the variation in pile driving crite- ria may be related to the pace of implementation of new approaches to pile testing and variation among agencies with respect to training, experience, and acceptance of new technology. Practices include the following: • Constructing the foundations without any test piles, using driving criteria based on a sim- ple pile driving formula or a wave equation analysis. This practice is widespread for rou- tine projects or simple situations such as steel piles driven to bear on rock or a hard bearing layer. The wave equation analysis provides a more rigorous and comprehensive model of the pile installation process and offers numerous advantages over the use of a simple for- mula. The results are typically provided to an inspector in tabular or graphical form, which can include provisions to account for varying hammer energy. The results can also include an allowance for a time-dependent increase in resistance as the pile “sets up” so that restrike measurements can be used to verify that a pile has the required resistance after some period if the required resistance was not observed during initial driving. However, the time required to perform these analyses and the greater level of education and training required to employ this technology is an impediment to more widespread use. The bene- fits may be insignificant on small or very simple projects. • The number and types of test piles varies according to predominant pile type, ground conditions, size of project, and agency practice. Many agencies evaluate the costs versus SUMMARY DEVELOPING PRODUCTION PILE DRIVING CRITERIA FROM TEST PILE DATA

benefits on a project-specific basis, and this evaluation includes the consideration of the increased values of the resistance factor for design that is provided in the AASHTO code. There is a greater tendency to use test piles: – In coastal states; – In projects with larger piles, longer piles, or a greater numbers of piles; – Where piles derive a majority of axial resistance through side resistance; – Where ground conditions are more variable; – Where concrete or large pipe piles are used, as opposed to steel H- or smaller diam- eter (18 in. or less) pipe piles; and – Where design-build project delivery is used. • Developing a driving criteria based on test pile measurements using a High Strain Dynamic Test (HSDT). This practice is most often employed using test piles that serve as production piles and become part of the permanent foundation. The measurements obtained from the HSDT are often used to refine the results of a wave equation analysis so that improved reliability in the driving criteria is achieved. The capabilities of the equipment used to perform an HSDT have improved significantly in the last 20 years and the capability of agencies and consultants to reliably perform such tests has become much more widespread. Signal-matching analysis is routinely employed to obtain more detailed information about the test pile behavior and soil response. • Using pre-production test piles with HSDT measurements and/or static load tests (SLTs) to develop driving criteria based on a more reliable determination of the axial resistance of a test pile. Pre-production test piles, installed in advance of foundation construction, allow restrike measurements to be performed over time to measure setup. SLT measure- ments provide the most reliable indication of the static axial resistance of the pile and, in some circumstances, are used to overcome uncertainties associated with dynamic test measurements. Rapid load tests (RLTs) are used on rare occasions in lieu of a SLT. The use of pre-production test piles also allows for a more reliable estimate of the length of production piles in advance, which is particularly valuable to agencies that routinely use prestressed concrete piles. Hindrances to the use of pre-production piles typically include the time and costs associated with performing the tests before production pile installation, as well as restrictions to site access prior to construction by environmental or other permits. Useful practices are described in chapter five, with descriptions of the practical approaches several agencies use to integrate a range of technologies to develop pile driving criteria under typical conditions. Each of the practices identified have limitations and impediments to imple- mentation. In several cases, the most common limitation to improvements in practice is the perception of limited benefits relative to costs associated with technologies that require a sig- nificant commitment of resources or time. For small or routine projects with relatively simple foundation conditions these perceptions may be accurate. In some cases, improved reliability and economy may be difficult to quantify or there may be resistance to changing long-standing practices even though they may be inefficient. Other limitations to increased use of test piles include a lack of equipment and/or trained staff or consultants available to employ advanced technology. In addition, several knowledge gaps have been identified relating to technical issues for which research could potentially improve practice. 2