10-01-04 | News

Making Space On Difficult Terrain

A Retaining Wall Helps A High School Find Room in the Hills Above San Diego, Calif.

By Dan Bensman, Keystone Retaining Wall Systems

A large “W” for Westview High School was a last-minute idea that was successfully-incorporated into the wall over the football field. The wall’s planter can be seen running lengthwise at an elevation of about three feet. It will be filled with creeping ficus?EUR??,,????'??+and landscape workers will need to keep trimming the vines to keep the logo “W” visible.

As metropolitan areas grow and competition for valuable land increases, public building projects are often left with less-desirable locations than private residential and commercial ventures. In a San Diego suburb, two small, wooded canyons were the only space available for construction of the $75 million Westview High School.

A strategy was needed to turn this undeveloped and seemingly–unusable land into a large school campus. The solution turned out to be a geogrid-reinforced retaining wall. (A geogrid is a flat, synthetic supporting structure designed to prevent uneven settling of the wall.)

RCP Block & Brick of San Diego then delivered almost 20,000 square feet of retaining wall blocks to the site. A combination of Keystone Standard, Compac, and Planter styles were employed to deal with the substantial grade and soil-retention difficulties. But before a single brick was placed in the project, the excavation challenges at the site had to be tackled.

Leveling Canyons to Create Usable Space

The almost monumental task of leveling the site began with the cutting away, filling and grading of two small canyons until a flat, buildable area was created. Some of the soil removal was 60 to 70 feet deep, and the fill area turned out to be 40 to 50 feet high.

 

This pulled-back view shows Westview High School’s sports field (foreground), the retaining wall behind it and stairs (right of photo) leading up to the swimming pool area (note artificial lighting behind the wall). Compare this view with the photo below, which shows a crew grading the same area before the wall’s construction.

Mike Stevenson of Geogrid Retaining Wall Systems, Inc. and his team consulted on the excavation to prepare for the job of installing the segmental retaining walls. Geogrid is one of the largest general engineering contractors in the country specializing in the design and construction of reinforced soil retaining wall systems. The retaining walls the company installed became the key to soil stability on this newly-created building site.

“A huge challenge for the excavators was the significant soil concretions found as they cut away at the canyon walls,” Stevenson said. (Concretions are large, hard deposits that occur in the natural formation of soil due to varying amounts of pressure exerted by the building and evolving topsoil.)

The Westview High School site included two small canyons that required months of preparation before construction could begin. Using a 2:1 ratio slope would have resulted in a field 60 feet narrower than the retaining wall helped create.

“These concretions were cemented zones of soil that the excavators could not break with even the large D-9 dozers,” said Chad Sheridan of Sierra Pacific West, Inc., the project grading contractor. “These zones had to be cleared of the useable soils surrounding them and then moved to a central area to be handled by a 10,000 pound breaking machine. This broke them into pieces that could be handled by a (front) loader for placement into a crusher.”

According to Sheridan, the crusher then brought the size of the concretions to a usable form, meeting the required specifications for fill at the site.

Sierra Pacific West is a general contractor from Encinitas, Calif. that specializes in heavy highway, roadway and government contracts.

The challenging Westview High School site started as a rolling, heavily-wooded junction in two canyons. The tractor at center started the brush-clearing before the leveling operation began.

After necessary excavation was done, a natural elevation condition remained between the school and the athletic field. The height difference was roughly 30 feet. ?EUR??,,????'??Because every acre of buildable land was necessary, it was impossible to consider a slope for this condition,?EUR??,,????'?? said Stevenson. “A 2:1 slope with a 30-foot rise in elevation meant the loss of 60 feet back and across the line where elevation began–valuable space needed for the athletic field,” he said. These challenges combined to indicate one solution–a large, near-vertical retaining wall.

Adding Custom Features to the Design

As the wall installation evolved, other challenges arose that called for creative measures. Because deep footings were required for the large light posts, it was necessary to build the footings using Sonotubes (a brand name for cylindrical concrete forms) in the fill area. Soil was compacted around the tubes as the crew added fill. There was also a six-foot masonry fence parapet to be built on top of the segmental retaining wall. Construction design had to be coordinated so the footing for the fence was on top of the topmost retaining wall block and cantilevered back into the fill. This design meant the installation crew had to ensure that the geogrid support structure was low enough to accommodate the footings.