Location: Second Mesa, AZ
The SR264 Second Mesa project was dubbed a “rockfall containment and safety” project by the Arizona Department of Transportation (ADOT). The previously existing rock slopes were hazardous and ditch widths were deemed insufficient. Therefore, a roadway widening project was undertaken.
The initial project assessment and the design effort up to 60% project development phase recommended 4 MSE retaining walls to accommodate the roadway widening needed for the increased cut ditch widths. Project constraints included the steep existing geometry of the slope, the fact that the site in on the Hopi Reservation, requirement of maintenance of traffic on the existing roadway, no new right of way acquisition, and no upslope cuts.
The project constraints, poor foundation conditions on steep end-dumped slopes, and significant shoring required for MSE walls were anticipated to have a large impact on the cost and constructability of the project. Under our On-Call contract with ADOT, NCSG was brought in after the 60% project development stage as part of a value analysis effort and was asked to: (a) help develop alternative wall/fill schemes, and (b) work with AZTEC Engineering Group (prime consultant to ADOT for the project) to help develop plans and specifications for final design. During the value analysis process, 2 walls were eliminated, and the other two were redesigned by the team to be an innovative hybrid of soil-nail wall and GeoFoam platform combined with pre-cast concrete facing.
The concept was to create no net pressure increase at the base of the “wall” placed on the steep end-dumped slopes while still allowing the widening of the roadway. The soil nail wall is the true “retaining” portion of the system, taking ALL of the lateral forces imposed by the slope. The purpose of using GeoFoam was to ensure that the net pressure imposed on the slope was no greater than the existing configuration to alleviate concerns of poor bearing capacity and potential global (overall) slope instability. This highly unconventional and innovative design required a number of details to be constructed successfully, including a fascia wall footing, a leveling course, a load distribution slab, and protection of the GeoFoam from termites and gasoline.
A series of photos presented below provide an overview of the construction of this remarkable project.
This project won the ACEC Grand Award.