Reliability-Based Calibration of Geotechnical Resistance Factors for Micropile Design

Micropile foundations consist of small-diameter (typically less than 12 inches) drilled and grouted non-displacement, reinforced elements. Since their inclusion as a viable deep foundation alternative in the American Association of State Highway and Transportation Officials Load and Resistance Factor Design (AASHTO LRFD) Bridge Design Specifications, they have been used successfully in various transportation projects. The salient advantages of the system are that the elements can be constructed in low-headroom, restricted-access, or vibration-sensitive sites where driven piles or drilled shaft construction could be difficult, if not impossible. At the core of the load and resistance factor design (LRFD) methodology for bridge foundations is the direct link between the design method, and an acceptable probability of failure. National Cooperative Highway Research Program (NCHRP) Report 507 included a reliability-based calibration for driven piles as well as drilled shafts, which was performed using a sufficiently large database of load tests. The findings eventually made their way into the AASHTO design code. The LRFD approach enabled designers to make a paradigm shift in geotechnical design, moving from a global safety factor that unified all uncertainty into a single number, to a more rational approach that identified uncertainty of individual components. It is important to note that such a calibration has not been done for micropiles. As per AASHTO LRFD Bridge Design Specifications C10.5.5.2.5, the resistance factors currently implemented in the code were calibrated to previous practice, tempered with engineering judgment (see Table 10.5.5.2.5-1). In order to perform a reliability based calibration of resistance factors, a sufficiently large database of static load tests is required. As outlined in AASHTO LRFD Bridge Design Specifications 10.9.3.5, micropile design shall be verified by load test. The implication of the article is that a large number of data exists that has not been collected and organized yet. The suggested research project would consist of performing that very task, as well as carrying out data analysis and the determination of a deflection-limited capacity (nominal resistance). Once the data is collected and the nominal resistance determined, it would be possible to compute the required statistical parameters necessary for a reliability-based calibration, allowing the system to fall in line with the other deep foundation alternatives that are currently part of the AASHTO design code. The research effort would consist of the following general tasks: (1) literature review; (2) static load test data collection from all departments of transportation (DOTs) that have used the system; (3) organization of data (soil type, stress history, installation method, load test method); (4) definition of nominal resistance; (5) statistical analysis; (6) development of resistance factors using reliability methods (e.g., FOSM, Monte Carlo, FORM) performed for various soil/rock types and design methods; and (7) submission of a final report. The results of the effort could then be evaluated by AASHTO and considered for inclusion in the LRFD design code.

Language

  • English

Project

  • Status: Proposed
  • Funding: $250000
  • Contract Numbers:

    Project 04-40

  • Sponsor Organizations:

    National Cooperative Highway Research Program

    Transportation Research Board
    500 Fifth Street, NW
    Washington, DC  United States  20001

    American Association of State Highway and Transportation Officials (AASHTO)

    444 North Capitol Street, NW
    Washington, DC  United States  20001

    Federal Highway Administration

    1200 New Jersey Avenue, SE
    Washington, DC  United States  20590
  • Project Managers:

    Amir, Hanna

  • Start Date: 20160509
  • Expected Completion Date: 0
  • Actual Completion Date: 0
  • Source Data: RiP Project 40799

Subject/Index Terms

Filing Info

  • Accession Number: 01598918
  • Record Type: Research project
  • Source Agency: Transportation Research Board
  • Contract Numbers: Project 04-40
  • Files: TRB, RiP
  • Created Date: May 8 2016 1:00AM