ADOT LRFD Policy Memoranda

On-Call Geotechnical Services

Introduction

As part of our On-Call Geotechnical Services contract with the Arizona Department of Transportation, NCSG was tasked with reviewing and implementing the AASHTO Load and Resistance Factor Design (LRFD) Bridge Design Specifications as it pertains to geotechnical engineering.  NCS’ scope of work included the comparison of new methods prescribed in the LRFD specifications with the current state of the practice, and the preparation of policy memoranda that would form the baseline of the new standard of practice for geotechnical engineering in Arizona. Six (6) of these memoranda were issued in December of 2010 and are discussed in more detail below. You can download the memos in the right sidebar.

ADOT DS-1 (Drilled Shaft – Axial Resistance)

This memorandum outlines the development of drilled shaft axial resistance charts based on methods specified in AASHTO (2010). The intent of this memorandum is to present a general overview of the development of the information needed by the bridge designer to design substructure elements consisting of drilled shafts. The designer should contact ADOT Materials Group for an updated version of this memorandum in the event interim revisions to AASHTO (2010) are issued or a new edition of AASHTO LRFD Bridge Design Specifications is issued.


ADOT DS-2 (Drilled Shaft – Gravels and Gravelly Soils)

This memorandum presents guidance specifically for the design of drilled shafts in gravels and gravelly soils that exhibit drained behavior. Interim guidance regarding both side resistance and tip resistance is provided. The criterion for “sands” is also presented to aid in a clear determination of gravels, gravelly soils, and sands within the overall framework of coarse-grained soils exhibiting drained behavior.


ADOT DS-3 (Drilled Shaft – Lateral Analysis)

This memorandum presents guidance for analysis of drilled shafts under lateral loads for the following aspects:

  1. Shaft Length
  2. Minimum requirements for analytical models
  3. Depth to fixity
  4. Considerations for collapse-susceptible soils


ADOT SF-1 (Spread Footing – Bearing Resistance and Settlement)

This memorandum presents guidance for the design of spread footings using modified AASHTO (2010) criteria. The intent of this policy is to present a general overview of the development of the information needed by the bridge designer to design substructure elements consisting of spread footings.


ADOT SF-2 (Spread Footing – Limiting Eccentricity)

The purpose of this policy memorandum is to address inconsistencies related to limiting eccentricity criteria between allowable stress design used to develop ADOT SD 7.01 Standard Walls (formerly referred to as B-Standard Walls) and current LRFD practice. Limiting eccentricity criteria used in LRFD with respect to eccentric loads and overturning have been modified to match successful past ASD practice. The recommended limiting eccentricity criteria for eccentric loads and overturning are presented in the attached policy.

ADOT SF-3 (Spread Footing – Sliding and Bearing Resistance Factors)

The purpose of this policy memorandum is to provide guidance for the selection of resistance factors for the evaluation of sliding and bearing resistance for spread footings of gravity and semi-gravity walls. The guidance in this memorandum shall be applied to ADOT SD 7.01 and 7.02 walls (formerly referred to as B-Standard Walls) that address reinforced concrete cantilever and masonry walls and other similar walls.

News Highlights

Update December 2010: ADOT has approved the new/updated policy memoranda as of December 2010! They will be issued via email by ADOT Engineering Consultants Section (ECS) under ADOT letterhead to all engineering consultants on their mailing list. Or you can download them at NCS’ website!

Update January 2008 (No. 2): Naresh Samtani was personally requested by Jerry DiMaggio, the head geotechnical engineer for the FHWA, to give a presentation on the topic of LRFD implementation at the Transportation Research Board (TRB) annual conference in Washington D.C. In the audience were members of DOT’s from around the country, and members of the AASHTO committee that wrote the LRFD specification. The presentation was very well received, and the FHWA may even use the work done by NCSG and ADOT, including the policy memoranda, as models of best practice for LRFD implementation!

Update January 2008: NCSG completed three comprehensive policy memoranda which are under final review by ADOT at this time. They are expected to be released by mid-February at which time they will be posted on an ADOT website and linked from this site. These memoranda include guidance on drilled shafts, load transfer curves in gravels and gravelly soils, as well as spread footings.

Update June 2007:  NCSG starts work on preparing 3 policy memoranda under a new task order as part of the on-call geotechnical services contract.  NCSG is tasked with leading and coordinating a technical working group with representatives from the ADOT Materials Group, ADOT Bridge Group and TYLIN (structural engineering consultant to ADOT).  By invoking its nationwide contacts and experience through instruction of FHWA’s LRFD courses NCSG is able to solicit independent input and external peer review from the FHWA and other leading national experts as part of the preparation of the memoranda.

Update January 2007: NCSG presented our recommendations to the ADOT Materials Group and a technical review committee comprised of several geotechnical professionals in the consulting community. The recommendations were accepted by both parties, and ADOT Materials Group plans to issue the recommendations in a series of memos to accompany the existing Materials Preliminary Engineering and Design Manual (PE&D Manual). At this time there is no word on a timeframe for when the memos will be issued. When they are, we will post an announcement on this page.

The four areas of highest priority where NCSG is currently working are:

  • Spread Foundations
  • Drilled Shafts
  • Retaining Walls
  • Slopes

Additional areas to be addressed at a later date include driven piles and culverts.

Spring 2005:  NCSG starts work on LRFD implementation under an on-call contract with ADOT.  The primary goal is to evaluate the AASHTO LRFD specifications with respect to local practices and identify any changes that may be necessary to over-ride or supplemental AASHTO specifications so that past successful local practices are adequately incorporated into ADOT bridge designs.


References

  1. AASHTO (2002). Standard Specifications for Highway Bridges, Seventeenth Edition. American Association of State Highway and Transportation Officials, Washington, D.C.
  2. AASHTO (2007). AASHTO LRFD Bridge Design Specifications. 4th Edition.  American Association of State Highway and Transportation Officials, Washington, D.C.
  3. AASHTO (2010). AASHTO LRFD Bridge Design Specifications. 5th Edition. AmericanAssociation of State Highway and Transportation Officials, Washington, D.C. (including latest errata and interims).
  4. FHWA (2006). Soils and Foundations – Volumes I and II.  Authors: Samtani, N. C. and Nowatzki, E. A., Publications No. FHWA NHI-06-088 and FHWA NHI-06-089, Federal Highway Administration, Washington, D.C.
  5. FHWA (2010). Selection of Spread Footings on Soils to Support Highway Bridge Structures.Authors: Samtani, N. C., Nowatzki, E. A. and Mertz, D. R., Publication No. FHWARC/TD-10-001, Federal Highway Administration Resource Center, Matteson, IL.
  6. ADOT (2010). Bridge Group Structure Detail, Arizona Department of Transportation. Phoenix,AZ. (http://www.azdot.gov/highways/bridge/detaildwg/Retaining.asp)
  7. O’Neill, M. W. and L. C. Reese (1999). Drilled Shafts: Construction Procedures and Design Methods, FHWA-IF-99-025, Federal Highway Administration, U.S. Department of Transportation, Washington, D.C.
  8. Rollins, K. M., Clayton, R. J., Mikesell, R. C. and Blaise, B. C. (2005). “Drilled Shaft Friction in Gravelly Soils,” Journal of Geotechnical and Geoenvironmental Engineering, ASCE, August, pp. 987-1003.