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!
