Attractive Retaining Walls

Pogue Construction is nearing completion of the new $73-million Boyd High School in McKinney, Texas. Opened for classes beginning in fall 2006, Boyd High School is expected to be completed in the fall of 2008 and should accommodate 3,000 students.

Boyd High School is being constructed in three phases. Phase I of construction was a $30-million project to build 255,000 square feet of classrooms, as well as an attached gym, completed in December 2006, as well as a football stadium. Phase IIA is a $10-million project with the main goal of adding about 62,000 square feet of classroom space. Phase IIB is an 18-month, $33-million addition started in December 2006, consisting of a 23,000-square-foot addition which will include an auditorium as well as a section of classrooms and science labs.

Boyd High School in McKinney is located on a 52-acre wooded site with expansive clay soils which originally varied approximately 60 feet in elevation from its highest to lowest point. Along with major excavation efforts that included balancing out the site with its existing soils, retaining walls were also established as critical elements for the site's overall development. Two substantial retaining walls with approximate heights of 12 feet and 20 feet were to be built at the front of the school and near the athletic fields. Original design plans called for poured concrete retaining walls, however, the cost of the poured concrete walls was a major factor in deciding on the final solution.

"For the amount of concrete needed for this size of a project, the cost of concrete walls compared to a segmental retaining wall system was significantly higher, so Pogue Construction recommended using an alternative design. Building segmental retaining walls resulted in significant savings and really allowed the school to utilize the space to their highest benefit," said J.D. McRae, superintendent, Pogue Construction.

The school's main entrance is located directly off a main city street and gradually winds down 20 feet to the parking area. At this location, the Keystone Compac Unit back-to-back parapet was used to construct vehicle barrier to create a flow of traffic and also serve as a safety structure for pedestrian traffic on the nearby sidewalk.

"The back-to-back design was used because the same appearance was wanted on both sides without having to use a concrete face or something different on the uphill or big vehicle side. Typically a guardrail is used on these projects, however, an internal reinforced concrete member to take the vehicular crash loads was incorporated instead," said Scott A. Miller, P.E., consulting engineer.

With the back-to-back design, the wall's sides have different total heights. The side facing the street is approximately 3 feet to 4 feet tall and the side facing the school is approximately 12 feet tall with four separate layers of grid reinforcement. The street's side is a vertical construction built on an 8-inch-thick concrete leveling pad which is reinforced with No. 5 L bars. Also, the pavement on this side is sloped away from the wall. The school's side has a 1-inch setback per course and is built on a 6-inch-thick minimum TxDOT Flex Base leveling pad, compacted to 98-percent Standard Proctor Density. A 4-inch-diameter perforated PVC or ABS pipe wrapped in filter fabric and sloped to drain through the wall every 4 feet is used for drainage in both walls.

During installation, each Keystone block is set into place, sides touching, with the paired pin holes facing up, checked for level, and aligned using either the pin holes or the straight rear edge of the block. Curved walls are aligned using the front pin position. Reinforced fiberglass locking pins align the next layer of Keystone units. Pins are placed in the front holes for near vertical setback and the rear holes for a greater setback angle.

The grid lengths for reinforcement of the side facing the school were slightly increased because of traffic loads and global stability concerns resulting from slope in front of the wall. Three grid strengths were used at varied lengths between 70 percent to 90 percent of wall height depending on the geometry in front of the wall and loads behind the wall. All voids and gaps were filled with 3,000-psi concrete with 3/8-inch aggregate max, although a typical installation allows the voids and gaps to be filled with 1/2-inch to 3/4-inch crushed stone or granular fill. The fill was placed on a course by course basis with an 8-inch lift per course. The walls were capped and sealed with Keystone Kapseal concrete adhesive to create a cohesive appearance of a single continuous wall.

"When confronted with the challenges that a project like this exhibits, we are able to utilize our extensive knowledge of retaining walls gained from years of hands-on experience to overcome the difficult site conditions and satisfy the end user," said Shawn Perronne, general manager, Builder Services Company.

The other Keystone Compac unit wall built during this construction phase is a 20-foot tall wall along the soccer/football field approximately 350 feet in length.

"The walls are specifically used in some applications such as the vehicle barrier that were anticipated to be a more little more difficult," said Miller. "However, the walls created a uniform, consistent and more appealing appearance."