In Arizona and many other parts of the country, shallow foundations are often used for bridge piers as well as abutments.   Immediate settlement considerations often govern the size of shallow foundations for bridges.  AASHTO (2007) presents two methods for computing the immediate settlement of footings. One method is based on elastic theory and another method is based on an empirical method by Hough. Both methods have been found to be conservative, and may lead to costlier foundations if used for design.

In the policy memorandum titled “Development of Factored Bearing Resistance Chart by a Geotechnical Engineer for Use by a Bridge Engineer to Size Spread Footings on Soils Based on Service and Strength Limit States,” this issue is handled by allowing the use of a modified version of the Schmertmann settlement method. Additionally, the memorandum outlines the new ADOT mandate that a bearing resistance chart (see above) shall be computed for spread footings to allow a structural engineer or bridge engineer to size the foundations for various values of tolerable settlement.

In this memorandum, interim guidance is provided regarding the design of drilled shafts founded in gravels and gravelly soils, particularly, the service limit state design. The load-transfer curves for side resistance in gravels and gravelly soils would result in very conservative designs of drilled shafts in Arizona based on past experience and load tests.  This memo discusses the policy decision by ADOT to allow different load transfer curves to be used for gravels and gravelly soils. These curves are based on the load test data compiled by Rollins et al. (2005). The memo also describes what investigation techniques must be used to verify the presence of gravel or gravelly soils such that the revised curves can be used.

One of the more significant changes in the AASHTO LRFD specifications is the removal of certain “loopholes” that allowed designers to avoid performing a true service limit state (settlement) analysis in allowable stress design (ASD). The “loopholes” in ASD specifications (AASHTO 2002 or 17th Edition) were in the form of some empirical equations that permitted an arbitrary reduction of tip resistance.  The method described in AASHTO (2007) does not allow for such arbitrary reductions and mandates the use of load transfer curves from O’Neill and Reese (1999) to perform settlement analysis that would allow a designer to determine the resistance that is mobilized for a given settlement or vice versa (e.g. computing settlement of a shaft for a given load).

Update February 2008: ADOT has approved the policy memoranda as of February 25, 2008! 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 NCS and ADOT, including the policy memoranda, as models of best practice for LRFD implementation!

Update January 2008: NCS 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:  NCS starts work on preparing 3 policy memoranda under a new task order as part of the on-call geotechnical services contract.  NCS 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 NCS 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: NCS 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 NCS 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:  NCS 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.

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. 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.
4. 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.
5. 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.
   

Development of Factored Bearing Resistance Chart by a Geotechnical Engineer for Use by a Bridge Engineer to Size Spread Footings on Soils Based on Service and Strength Limit States - Revised 3/18/2008  (NCS Policy Memo 1)

Interim Guidance - Design of Drilled Shafts in Gravels and Gravelly Soils Exhibiting Drained Behavior (NCS Policy Memo 2)

Development of Drilled Shaft Axial Resistance Charts for Use by Bridge Engineers (NCS Policy Memo 3)

In the very near future, NCS will supplement the memoranda with commentary that will provide insight into the development history and reasons for certain recommendations in the memoranda. This commentary will be similar to that provided in AASHTO LRFD Bridge Design Specifications and will prove to be an invaluable resource to the design community both in Arizona and elsewhere.