Landscape Architect Articles

Triple Tier Triumph06-10-16 | Department

Triple Tier Triumph
Retaining a Mountain

Sargon Masonry Construction was hired to assist with the theming design and installation of the hardscape amenities, including retaining walls, for Northlands at Tierra Del Rio, a newly built single-family home community developed by Taylor Morrison Homes in Peoria, Ariz. The three-tiered walls were built using Keystone Retaining Walls' "Compac' units with straight split face and mini caps in brown.

The owner requested a triple tier design to allow for landscaped areas on the slope. To divide lot lines, nearly three miles of wrought iron view fencing fabricated at Sargon Steel, sister company to Sargon Masonry, was installed along with 19,750 linear feet of retaining walls.

The Northlands at Tierra Del Rio is a six parcel single-family home community in Peoria, Ariz., recently built by Taylor Morrison Homes. Many of the homes back up to a natural mountain preserve, and therefore required installation of large retaining walls at the rear lot lines. The developer wanted these walls to have three tiers to allow for landscaping and plant materials in the spaces between them.

Sargon Masonry Construction was hired to assist in the initial theming design of the community hardscape amenities, including walls, columns, monuments, and stone veneer, as well as to develop and engineer the large-scale, non-traditional, retaining wall systems that back up to the mountain. These wall systems were installed throughout the community where traditional concrete masonry walls were not a viable option.

The excavation and backfill process for the retaining walls required the use of two backhoes as well as multiple excavators, hydraulic hammers, wheel loaders, small skid steers, water trucks, reach forks and other miscellaneous equipment. About half of this was owned by Sargon and the other half was rented.

The terrain posed a problem though: mountains are solid rock, and the large spread footings of traditional concrete masonry walls would be nearly impossible to place. The cost of hammering out solid granite for the installation of three tiers of retaining walls was extremely prohibitive. With the help and expertise of Keystone Retaining Walls, the engineering team at Sargon selected a segmental retaining wall system that would not require cast in place footings. They chose the manufacturer's "Compac' series. Using this series over a traditional wall eliminated the need to pump concrete and hammer out the solid granite, minimized cutbacks on the mountain, and reduced the overall cost.

Nearly 100,000 square feet of these retaining walls were installed. The units, brown in color with a straight split face, were laid on a near vertical stack and topped with mini caps in the same color. In addition to traditional geogrid stabilization, Sargon used tiebacks with rock bolts to further stabilize the wall system. The bolts were especially important where the space between the wall and the mountain face was limited. Where needed, to delineate property lines, wrought iron view fencing, fabricated at their sister company Sargon Steel, was installed along with the retaining walls. A total of three miles of fencing was used.

In total, about 100,000 square feet of the "Compac' wall system was installed across the development. After the soil was stabilized, the units were laid with a near vertical stack and reinforced with geogrid stabilization. Rock bolts were placed where space was limited. The installation took two years to complete, with an average of 30-40 workers on the ground every day.

Many of the homes back up to a mountain preserve and needed large retaining walls. Traditional concrete masonry unit walls were not an option due to the terrain: their installation would require excavating the solid rock mountain to place concrete footings. The upper tiers would have needed to be pumped and grouted as well. Using a segmental wall eliminated these requirements and reduced the amount of cutbacks on the mountain as well as the overall cost.

According to Joshua Gonzales, managing partner at Sargon, "As with any large-scale project, the challenges come in all shapes and sizes. This was an extremely large-scale project that needed to be delivered on time. The overall vastness of the project as well as the geotechnical properties of the terrain posed the largest obstacle. We had to process large amounts of soil in order to meet the structural properties that were needed to stabilize the walls. When it became apparent that the dirt on site would not be sufficient as is, we worked with the owner and grading operation to process the material into something usable."

The project overall took two years to complete, with 30-40 workers on the ground every day.

As seen in LC/DBM magazine, June 2016.