Innovative hybrid wall design..
Location: Second Mesa, AZ Client: ADOT
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 widening project was undertaken. The prime consultant on the project was Aztec Engineering, Inc.
The initial project assessment and the design effort up to 60% recommended 4 MSE retaining walls to accomodate the roadway widening needed for the increased cut ditch widths. A geotechnical investigation was performed by Western Technologies, Inc. using Crux Subsurface for the difficult access drilling.
Project contraints 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 poor foundation conditions and significant shoring required for MSE walls were anticipated to have a large impact on the cost and constructbility of the project.
A value analysis was performed, and NCS was asked to help come up with alternative wall/fill schemes under our On-Call contract with ADOT. 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” 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 is presented to the right, and below, that provide an overview of the construction of this remarkable project.