BACKGROUND: Micropile foundations consist of small-diameter (typically less than 12 inches) drilled and grouted non-displacement, reinforced elements. Such foundations can be constructed in situations of low-headroom, restricted-access, obstructions, and vibration-sensitive sites. However, current micropile foundation design does not incorporate rational reliability-based geotechnical resistance factors, and many state departments of transportation do not have sufficient data or resources to do such calibrations. There is a need to identify the parameters that influence the resistance factors for micropile foundations and develop reliability-based geotechnical resistance factors and specifications that can be used for various design and construction methods. These factors and accompanying specifications should provide an increased level of confidence in the design and help highway agencies in making decisions regarding the use of micropile foundations.
OBJECTIVE: The objective of this research is to develop reliability-based geotechnical resistance factors for axially-loaded micropiles and related specifications. Accomplishment of the project objective will require at least the following tasks.
Phase I: (1) Collect and review relevant domestic and foreign literature; research findings; agency practices; and other information relative to the development and use of geotechnical resistance factors for axially-loaded micropiles. This information may be obtained from published and unpublished reports, contacts with transportation agencies and industry organizations, and other sources. (2) Identify and evaluate the parameters that influence the geotechnical resistance factors for micropiles (e.g., construction method, subsurface condition, and failure criteria), and prepare a plan for (a) identifying sources of data needed for developing these factors and (b) collecting and formatting the data (e.g., according to Data Interchange for Geotechnical and Geoenvironmental Specialists (DIGGS)). (3) Develop a research plan for work to be executed in Phase II to (a) develop geotechnical resistance factors for axially-loaded micropiles and (b) propose related specifications and commentary. These factors shall include calibration for AASHTO LRFD Bridge Design Specifications for Strength and Service limit states and other relevant items (e.g., design method and redundancy). (4) Prepare an interim report that documents the research performed in Phase I and includes an updated work plan for Phase II. Following review of the interim report by the NCHRP, the research team will be required to make a presentation to the project panel. Work on Phase II of the project will not begin until the interim report is approved and the Phase II work plan is authorized by the NCHRP. The decision on proceeding with Phase II will be based on the contractor’s documented justification of the updated work plan.
Phase II: (5) Collect the proposed data and develop (a) geotechnical resistance factors for axially-loaded micropiles and (b) recommended specifications with commentary in AASHTO LRFD format. (6) Prepare presentation material in a PowerPoint or other format to facilitate implementation and use of the developed resistance factors and specifications. (7) Prepare a final deliverable that documents the entire research effort. The proposed resistance factors, specifications, and commentary shall be prepared in a format suitable for consideration and adoption by AASHTO. All data used in the research (e.g., data used for developing the resistance factors) shall be provided, in an electronic format, as an attachment to the final deliverable.
STATUS: Research will start on September 1, 2017.