BACKGROUND 

The AASHTO LRFD Bridge Design Specifications recognize that piles in settling soil experience downdrag forces from negative skin friction and may also move downward with the adjacent soil. Many bridges are constructed in areas where there is compressible soil or liquefiable soil, which necessitates the consideration of downdrag forces and associated settlement. Downdrag for static and seismic conditions has increasingly placed greater demands on existing and new foundations and also led to higher construction costs. Therefore, there is an immediate need to review the fundamental behavior of piles subject to downdrag and propose design specifications for the implementation of the neutral plane method for the evaluation of downdrag and the static and seismic design of piles.

OBJECTIVES

The objectives of this research are to (1) propose modifications to the AASHTO LRFD Bridge Design Specifications for the static and seismic design of piles for downdrag and (2) develop design examples to demonstrate the application of the proposed design specifications.

Accomplishment of the project objectives will require at least the following tasks.

TASKS

Task descriptions are intended to provide a framework for conducting the research. The NCHRP is seeking the insights of proposers on how best to achieve the research objectives. Proposers are expected to describe research plans that can realistically be accomplished within the constraints of available funds and contract time. Proposals must present the proposers' current thinking in sufficient detail to demonstrate their understanding of the issues and the soundness of their approach to meeting the research objectives.

PHASE I—Planning

Task 1. Review the literature and synthesis reports prepared under NCHRP Project 12-116, “Proposed AASHTO Specifications for Design of Piles for Downdrag” (see Special Note A). Identify the knowledge gaps and summarize information on the effectiveness of existing design methods of piles for downdrag. These gaps should be addressed in the final product or in the recommended future research as budget permits. Discuss the quantity and quality of data required to achieve the project objectives.

Task 2. Propose methodology including field testing and numerical analyses, to be executed in Phase II, based on currently available data. At a minimum, the methodology shall consider the following items:

• Data requirements and collections methods;
• When and how to apply the neutral plane method, including limitations;
• Different soil types and subsurface conditions;
• Friction and end bearing piles;
• Static and seismic conditions including liquefaction;
• Different limit states;
• Different pile types and sizes;
• Pile group; and
• Mitigation techniques for downdrag.

Task 3. Identify areas of the AASHTO LRFD specifications that require modification or addition.

Task 4. Prepare Interim Report No. 1 that documents Tasks 1 through 3, includes the data archiving plan (see Special Note C), and provides an updated and refined work plan for the remainder of the research no later than 3 months after contract award. The updated plan must describe the process and rationale for the work proposed for Phases II though IV.

Note: Following a 1-month review of Interim Report No. 1 by the NCHRP, the research team will be required to meet in person with the NCHRP project panel to discuss the interim report. Work on Phases II through IV of the project will not begin until authorized by the NCHRP. Phase I shall be limited to $20,000.

PHASE II— Methodology Development and Design Procedures

Task 5. Execute the methodology according to the approved Interim Report No.1. Any modifications to the field testing shall be approved by NCHRP.

Task 6. Provide detailed step-by-step design procedures and propose at least eight design examples to be developed in Phase III. The parameters for design examples shall be presented in a matrix format and address the items identified in Task 3.

Task 7. Prepare Interim Report No. 2 that documents Tasks 5 and 6 and provides an updated work plan for Phases III and IV no later than 16 months after approval of Phase I.

Note: Following a 1-month review of Interim Report No. 2 by the NCHRP, the research team will be required to meet with the project panel to discuss the interim report. Work on Phases III and IV of the project will not begin until authorized by the NCHRP. Phase II shall be limited to $300,000.

PHASE III—Specifications Development

Task 8. Propose modifications to the AASHTO specifications. The proposed modifications should be publication-ready (i.e., AASHTO-style) format with appropriate level of detail.

Task 9. Develop design examples for pile downdrag to demonstrate the application of the proposed specifications according to the approved Interim Report No.2.

Task 10. Prepare Interim Report No. 3 that documents Tasks 8 and 9 and provides an updated work plan for the remainder of the project no later than 6 months after Phase II approval. The updated plan must describe the process and rationale for the work proposed for Phase IV.

Note: Following a 1-month review of Interim Report No. 3 by the NCHRP, the research team will be required to meet in person with the NCHRP project panel to discuss the interim report, if necessary. Work on Phases IV of the project will not begin until authorized by the NCHRP.

PHASE IV—Final Products

Task 11. Revise the proposed specifications and the design examples accordingly after consideration of the panel’s review comments and prepare a ballot item for the AASHTO Committee on Bridges and Structures.

Task 12. Prepare final deliverables including: (1) a final report that documents the entire research effort; (2) the proposed AASHTO specifications and design examples; and (3) a stand-alone technical memorandum titled “Implementation of Research Findings and Products” no later than 2 months after approval of Phase III.

Note: Following receipt of the draft final report, the remaining 3 months shall be for NCHRP review and comment and for research agency preparation of the revised final report. Phases III and IV shall be limited to $100,000.

STATUS: Research in Progress