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DESIGN AND LOAD TESTING OF LARGE DIAMETER OPEN-ENDED DRIVEN PILES As the design of bridge foundations has evolved to include issues of extreme event loadings (vessel collision, seismic event, liquefaction, scour, ice), large diameter piling has become a more attractive option because of the significant strength, ductility, and durability of these piles. Large diameter open-ended piles (LDOEPs) are steel or prestressed concrete cylinders 36 in. or larger in diameter that can provide large axial and lateral resistance even in rela- tively poor soil conditions. Steel LDOEPs have a long history of use for structural supports in the offshore oil and gas industry and have recently been employed for several high profile bridge projects, including the San FranciscoâOakland Bay Bridge, the Woodrow Wilson Bridge on the Potomac River, and the Tappan Zee Bridge in New York, which is currently under construction. Prestressed concrete LDOEPs are currently being used primarily for coastal structures in marine construction along the Atlantic and Gulf coasts because of their flexural strength and durability in harsh marine environments. The design methodology for piling, along with testing and quality assurance procedures used in practice, is reflected in the AASHTO code; however, this code was not developed specifically for LDOEPs, and most transportation agencies do not have a robust base of expe- rience with these piles. The AASHTO code has transitioned in recent years to the Load and Resistance Factor Design (LRFD) format to provide a more consistent basis for reliability of design, including extreme event loads and conditions; however, the reliability of the design and quality assurance procedures for LDOEPs has not been established. Uncertainties asso- ciated with LDOEPs are particularly important given that these piles are more likely to be employed on bridges that may be lifeline structures or are otherwise important. The objectives of this synthesis are to present an overview of the current state of practice with respect to LDOEPs for transportation structures and to provide stakeholders in this indus- try with useful information supported by case histories. The information is also intended to provide agencies with a resource from which to develop guidance and methods into technical guides and design codes, as well as to identify gaps in knowledge that may provide direction for future research. The information gathered in this synthesis includes background information from a litera- ture review and personal experiences, a survey of public agencies within the United States, interviews with agency representatives as well as knowledgeable individuals from the pri- vate sector, and documentation of a range of case histories in which these piles have been employed for bridges in the United States. A survey was conducted of state geotechnical engi- neers (or equivalent) in all 50 state departments of transportation (DOTs) plus the District of Columbia and Puerto Rico, and 85% of these agencies responded to the survey. The synthesis also includes the findings from in-depth interviews that were conducted with seven of the 16 agencies that reported actively using LDOEPs. Additional interviews with private-sector participants involved engineers responsible for design, construction, and testing, including individuals with extensive experience in the offshore oil and gas industry, with a broad range of bridge projects, and with prestressed concrete LDOEPs. During the course of the survey and literature review, a number of interesting case histories were identified and summaries of 13 have been provided. These include bridge projects founded on LDOEPs, as well as case SUMMARY
2 histories of comparative testing programs that provide information of interest relating to the behavior and testing of LDOEPs. The most important feature of LDOEPs affecting the behavior and understanding of these piles is the uncertainty related to the behavior of interior soil within the pile during installa- tion and subsequent static loading by the permanent structure. When the hammer accelerates the pile rapidly downward, the soil inside tends to stay put because of the inertia associated with this large soil mass. As a result, there is a tendency for the pile to âcoreâ during driving so that the pile does not incorporate the interior soil plug as a part of the pile even though this soil exerts some frictional resistance on the interior wall of the pile. For static loading long after the pile is installed, this behavior may be very different in that the frictional resistance on the interior pile wall may exceed the end-bearing resistance at the pile toe so that the pile advances as a âpluggedâ pile. These behaviors affect the resistance of the pile during driving and thus the ability to predict pile hammer and equipment demands, and also affect our ability to predict the perfor- mance of the pile as a supporting element of the permanent structure. Since the driving resis- tance of the pile is used as an indication of long-term axial resistance for quality control and quality assurance, these different behaviors affect the reliability of our testing methods and thus the reliability of our completed design. The LRFD approach to design in the AASHTO code is intended to provide a reliability-based design methodology, and the resistance factors employed for LDOEPs are to logically reflect the reliability of this type of pile. The agencies that use LDOEPs reported that large lateral and axial loads combined with certain favorable soil and rock conditions are the primary reasons for selecting these piles for design, with 12 agencies utilizing steel and seven utilizing prestressed concrete LDOEPs. Although the static prediction methods cited in the AASHTO code and FHWA Driven Pile manual were noted in this study to have little basis with respect to LDOEPs, these methods were by far the most used by agencies to estimate static axial resistance. Resistance factors for design were most typically selected to be consistent with AASHTO for other types of piles. A few states have developed their own procedures for estimating static resistance and selecting resistance factors, and two states use static methods from the American Petroleum Institute (API) design guidelines developed for offshore structures. To verify the design in the field, most of the agencies rely on some type of driving criterion for final determination of pile length during installation, although three states at least some- times install LDOEPs to a predetermined tip elevation without regard for driving resistance (most notably, Louisiana, where piles are often installed in deep, soft alluvial soils). Most of these agencies use high-strain dynamic testing to verify axial resistance and establish ham- mer operating procedures to minimize the risk of pile damage, although several agencies expressed a lack of confidence in the reliability of high-strain dynamic tests as an indication of static axial resistance for LDOEPs. Notably, the California Department of Transportation (Caltrans) has developed a system to apply loads of up to 8,000 kips in order to conduct static axial load tests on high-capacity piles such as LDOEPs. Some agencies employ rapid load tests that push the pile with lower inertial forces and many rely almost exclusively on high-strain dynamic tests for these high-capacity piles. In general, the states that make the greatest use of LDOEPs have also been most heavily engaged in testing, although a clear consensus has not yet emerged as to the most effective practices for testing. The general consensus among the private practitioners who were interviewed was that LDOEPs do not tend to plug during installation, and there is a general lack of understanding in the industry related to the contribution of internal side resistance during installation as well as the plugging behavior during subsequent static loading. All acknowledged that there are unique challenges with dynamic testing for these types of piles, but all considered dynamic testing an important part of quality control/quality assurance for LDOEPs. The methods described in the API guidelines are most often employed for estimating static resistance by private industry.
3 The case histories demonstrate a varied use of LDOEPs for bridge projects, including steel pipe piles driven to bear on rock, long friction piles entirely in soil, and prestressed concrete cylinder piles for coastal structures where these piles allowed the use of pile bents and elimi- nated footings. Some of the case histories illustrate attempts to perform comparison tests with static, dynamic, and rapid load tests, reflecting a search for improved reliability in the design and testing of LDOEPs. A review of these tests contributes to improved understanding of pile behavior and the challenges, but do not yet portend a consensus solution to the problem. Research needs were identified by all participants in an attempt to better understand the behavior of LDOEPs during installation and subsequent static loading, to improve the reli- ability and usefulness of testing, and to better quantify the reliability of these piles for LRFD- based design. The literature review documents more than 60 years of work in the offshore oil and gas industry to better understand the behavior of steel LDOEPs, which have been used extensively in that environment. The synthesis suggests that there is a knowledge gap in the transportation field with respect to the state of practice in piling design from this indus- try as it may apply to bridge foundations, but that there is also a need to adapt the general piling design practices from API to the specific reliability requirements for transportation infrastructure projects. Development of design procedures and resistance factors that are specific to LDOEPs for bridges is needed, and it is important that these reflect the reliability associated with testing for verification of axial resistance for these specific types of piles. Prestressed concrete LDOEPs differ significantly from the steel piles used by API and prob- ably require considerations particular for these piles as compared with steel LDOEPs. Trans- portation agencies using LDOEPs recognize the need for guidance on testing requirements in order to achieve reliable and cost-effective foundations that meet the needs of modern transportation infrastructure.
