SUMMARY

Design of Piles for Downdrag

The results from NCHRP Project 12-116A, “Proposed AASHTO Specifications for Design of Piles for Downdrag” are contained in this report. Specifically, the key findings available herein include Task 1 (Literature Review), Task 5 (Execute Methodology), Task 9 (Develop Design Examples for Pile Downdrag), and Task 11 (Revise Specifications). Each of the four tasks is described in different sections of the report.

Section 1 – NCHRP 12-116A Task 1 – A Literature Review related to downdrag and drag load was completed by the project team.

Section 2 – NCHRP 12-116A Task 5 – Execute Methodology was completed by developing two approaches for determining the location of the neutral plane to compute the amount of downdrag and drag load. The two approaches, identified as Method A and Method B, consist of determining the location of the neutral plane by (1) assuming fully mobilized load transfer (Method A) or (2) assuming variably mobilized load transfer as dictated by local elastic compression of the pile (Method B). Method A documentation includes the use of methods for determining downdrag and drag load that are mentioned in the AASHTO LRFD [Load and Resistance Factor Design] Bridge Design Specifications (AASHTO LRFD BDS) (AASHTO 2020) and provides examples of the use of these methods.

Method B documentation includes the proposed alternative procedure for determining downdrag and drag load that relies on the use of developed t-z curves and t-z software to provide an understanding of how mobilized loads will be developed. Method B was developed by assembling a load test database and then using the information from the load test database to create and assess existing t-z curve models for use in downdrag and drag load analyses. The load test database documented 57 instrumented full-scale load tests performed on different pile types that were installed within different soil conditions. The load test database also quantified the influence of residual loads in the piles to provide the basis for future updates to AASHTO specifications where accounting for residual loads may lead to improved determination of the neutral plane location.

Method A can be completed using hand calculations or computer software; the computer programs ALLCPT and PileAXL, marketed by Innovative Geotechnics Pty Ltd. (Perth, Western Australia), were selected for this study. However, other programs are available for conducting fully mobilized load transfer analyses. Method B relied on computer software; the program TZPILE, marketed by Ensoft, Inc. (Austin, Texas), was selected for this study. However, other programs are available for conducting partially mobilized load transfer analyses.

Section 3 – NCHRP 12-116A Task 9 – Develop Design Examples for Pile Downdrag was completed by developing eight design examples. The design examples include Method A

(full mobilization), Method B (partial mobilization), and details for how to design for downdrag and drag load when a soil deposit is subjected to changes in effective stress. The changes in effective stress may be the result of additional stress resulting from an embankment, a reduction in pore water pressure resulting from dewatering, the dissipation of excess pore water pressures following liquefaction or cyclic softening, or a combination of these sources of effective stress changes.

The first four design examples build upon the work of Briaud and Tucker (1997), cited in previous AASHTO specifications for drag loads. These examples include an octagonal, precast concrete pile being subjected to effective stress changes resulting from an embankment (Design Examples 1 and 2), an embankment with a resulting increase in shear strength (Design Example 3), and dewatering (Design Example 4). Design Examples 5 and 6 relate the influence of liquefaction and were developed using soil properties and design elements from the Blytheville, Arkansas, test site located at the Nucor Steel Arkansas Specialty Cold Mill Complex, JMS Russel Metals plant. Design Example 7 also describes the downdrag and drag loads resulting from liquefaction but for a site with gravel in Alaska. Finally, Design Example 8 details the influence of an embankment being constructed over soft soil, with the pile being tipped in rock located below the compressible soil. Hand calculations or software that allows for full mobilization of unit side resistance were used to illustrate Method A in the relevant design examples. Software programs that allow for partial mobilization of unit side resistance and unit end bearing resistance were used to demonstrate Method B in the relevant design examples.

Section 4 – NCHRP 12-116A Task 11 – Revise Specifications was completed by proposing modifications to the AASHTO design specifications. Modifications to Section 3 of the AASHTO LRFD BDS (AASHTO 2020), including those requested by the NCHRP 12-116A project team as a result of this study, were balloted and approved by the AASHTO T-15 committee during its meeting held in May 2023. Although changes to Section 3 have been accepted, additional proposed modifications have been sent to AASHTO, along with proposed revisions to the downdrag and drag load section of AASHTO LRFD BDS, Section 10.

The appendices to this report are not included herein but are published online in NCHRP Web-Only Document 398: Design of Piles for Downdrag: Design Examples and Support Materials, which is available on the National Academies Press website (nap.nationalacademies.org) by searching for NCHRP Web-Only Document 398.

The appendices include a pile load database that was used to determine the most viable t-z curves for use in Method B (Appendix A), along with the results from the statistical analyses for that determination (Appendix B). The eight design examples are contained in Appendices C through J. The related topics associated with the appendices include the following:

• Appendix A – Load Test Database From Literature
• Appendix B – Statistical Analyses of t-z and q-z Curve Models
• Appendix C – Design Example 1 – Embankment Fill Over Clay Using Hand Calculations (Method A)
• Appendix D – Design Example 2 – Embankment Fill Over Clay Using PileAXL Program (Method A)
• Appendix E – Design Example 3 – Embankment Fill Over Clay (SHANSEP) [Stress History and Normalized Engineering Properties] Using Hand Calculations (Method A)
• Appendix F – Design Example 4 – Drawdown in Clay Using TZPILE (Method B)
• Appendix G – Design Example 5 – Liquefaction in Sand (H-Pile) Using ALLCPT and TZPILE (Methods A and B)

• Appendix H – Design Example 6 – Liquefaction in Sand (Pipe Pile) Using ALLCPT and TZPILE (Methods A and B)
• Appendix I – Design Example 7 – Liquefaction in Gravel Using PileAXL and TZPILE (Methods A and B)
• Appendix J – Design Example 8 – Embankment Over Clay Over Rock Using PileAXL (Methods A and B)