[Editor] Note: NCS Consultants, LLC is Randy Post's full time employer...ie. my day job! [/Editor]

There are some significant changes being made to the state of the practice in geotechnical engineering in Arizona. NCS Consultants, LLC has prepared three policy memoranda for the Arizona Department of Transportation or ADOT that have been issued to consultants all over the State. These memos are on the topics of bearing capacity and settlement of spread footings and retaining walls, the design of drilled shaft foundations in gravelly soils, and the preparation of drilled shaft axial capacity charts for use by bridge engineers.

Although primarily applicable to upcoming ADOT projects implementing the AASHTO 2007 LRFD code, the memos will have a ripple effect down through other local agencies within the state who frequently defer to ADOT guidelines for geotechnical engineering. Also, the memos and the ADOT/NCS approach to LRFD implementation in geotechnical engineering were presented by NCS at the 2008 TRB Conference in Washington D.C., and many other state DOTs and the FHWA were very excited about the memos. The approach used if not the exact content may become a model for other agencies. More info and links to download the policy memoranda are provided after the break.

To download the memos and read about them in detail, go to the ADOT LRFD Policy Memoranda page of the NCS Consultants, LLC website.

The first memo on shallow foundations documents ADOT's decision to allow the use of the Schmertmann settlement procedure instead of the Hough method described in AASHTO 2007. It also describes the new ADOT mandate that a bearing resistance chart be created to allow the bridge engineer to size the spread footings.

In the second memo on design of drilled shafts in gravelly soils, the load-transfer curves for side resistance in gravels and gravelly sands by Kyle Rollins et. al. were evaluated and are presented for use to replace the gravel curves from the O'Neill and Reese load-transfer figure. This is a decision based on soil conditions in Arizona and after reviewing local load test data. An important caveat to this policy is that additional field investigations will be needed to justify the presence of gravels such as large diameter borings or possibly drilling by the sonic method.

Finally, the third memo discusses the preparation of drilled shaft axial resistance charts for both strength limit state and service limit state (settlement-based analysis). The biggest change in policy is the requirement of producing service limit state curves which guarantees that at least two charts will be needed for each bridge, possibly more as each service limit chart is dependent on the specified estimated settlement of the drilled shaft. So for example there might be service limit charts for 0.5-in, 1-in, and 1.5-in of settlement. Much more coordination will be needed between geotechnical engineers and bridge engineers!

Related Items:

• AASHTO LRFD Bridge Design Specifications - 2008 Interims Released
• Million More for Geotechnical Work on Seattle Light-Rail Tunnel
• Arizona Earth Fissure Planning Maps
• Arizona Geologic Survey Releases Earth Fissure Maps
• Canadian Geotechnical Journal - December 2006, Vol. 43, No. 12

• Arizona Wells
• Dan Brown and Associates
• DBM Contractors, Inc.
• FHWA Geotechnical Engineering Publications
• GEO-SLOPE International, Ltd.

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