Bonnie Ellison Elementary School, a new addition to the Northside Independent School District in San Antonio, Texas, included a total of 3,000 square feet of retaining walls during construction to maintain the landscape, preserve native trees and maximize usable space.

Available backfill depth behind a retaining wall can vary even within the same wall installation. In this case, at the front of the picture, there is enough room for geosynthetic reinforcement; as the wall gets closer to the trees, however, space is limited and thus better suited for a porous structural backfill material.

Established in 1949 by the consolidation of several rural school districts, Northside Independent School District spans 355 square miles of urban landscape, suburban growth, and rural Texas hill country. Though it began with just 823 students, Northside is now South Texas' largest school system, with 115 schools and 101,477 enrolled students. This growth has been sustained by about 2,500 students enrolling in Northside each year.

The growth of the district led to the construction of Bonnie Ellison Elementary School in San Antonio. A voter-approved school bond funded school construction on a site that consisted of large wooded home sites and pastures. Preparation for construction required the demolition of home sites and extensive grading to maintain as much natural landscape as possible. This included maintaining the majority of the existing mature trees, including the transplant of a giant live oak that stands adjacent to the school's main entrance.

Challenges
A panoply of significant site conditions, including natural contours that move water runoff from west to east through the new school's location, led the construction team to devise a range of goals that included maintaining natural areas, preserving native trees and maximizing usable space for the children of the fast-growing San Antonio area. This obligated the civil design team to be extremely creative in maintaining the landscape, while addressing the necessary concerns with the impervious cover challenges. To overcome these hurdles, a series of retaining walls totaling 3,000 square feet became an essential part of the engineering and architectural design. With a focus on hardscapes, there was not enough room for conventional segmental walls with geosynthetic reinforcement.

Installation of the structural backfill begins with placement of the backfill material behind the wall with a Bobcat S300 skid steer loader. Crewmembers tamp the material down in the backfill zone and work it into the voids and cores of the blocks. Finally, a worker sweeps off the top of the block units so the next row of blocks can be placed on top. The backfill installation steps are repeated for each row of blocks until the wall is capped.

The Anchorplex system's structural backfill (Pavestone) is made with a mix of cementitious material, coarse aggregate and water. The cementitious material should be hydraulic cement (ASTM C 150 or C 1157), fly ash (ASTM C 618) or slag (ASTM C 989), and the stone should be coarse aggregate, size number 6, 8 or 57 (1/2 inch to 3/4 inch), type 3S (ASTM C 33). Stone size selection should be based on the application.

A Structural Solution
The civil engineering firm working on the project, M.W. Cude, recommended Pavestone's Anchorplex retaining wall system in tree preservation areas, because it requires little excavation and is designed to meet the engineering requirements of structural wall designs. The system uses a self-compacting structural backfill placed immediately behind the block to support retaining walls. The self-compacting structural backfill attaches itself to the wall facing, effectively extending the depth and mass of the wall. The structural backfill zone also serves as the required drainage zone directly behind the block facing. The unique system does not require a large geosynthetic reinforced stabilized earth zone behind the wall face, which means significantly less room is needed for excavation.

Retaining walls with structural backfill zones are useful in cut wall applications up to 10 feet high, particularly when lot lines, rock outcroppings or other obstructions limit the ability to excavate for geogrid reinforcement. Geosynthetic reinforcement typically requires an excavation depth of at least 60 percent of a wall's height; structural backfill cuts the required excavation depth to 30-40 percent of the wall's height, depending on conditions. The structural benefits combined with aesthetics, permeability and cost effectiveness to make the decision to install segmental retaining walls at Ellison Elementary an easy one.

Segmental walls allow for diverse landscaping options and creativity within an installation, providing the flexibility to maintain a cohesive look while ensuring design parameters are met. This retaining wall system helped to create an attractive, functional, and value-engineered retaining wall for the Northside Independent School District's Bonnie Ellison Elementary School, which will open its doors to students in August 2014.

Project at a Glance...
Bonnie Ellison Elementary School, San Antonio, TX
Architect: Garza-Bomberger & Associates
Civil Engineer: M.W. Cude Engineers, LLC
Construction: 01/2013 –???(R)???AE'?N????e'?N,A+5/2014 • Cost: $19.9 million (approx.)
General Contractor: Joeris General Contractors, Ltd.
Owner: Northside Independent School District, San Antonio, TX
Wall Installer: Cribley Enterprises, Inc.