The Norris Canyon Estates in San Ramon, Calif., east of San Francisco, are gaining a local presence not merely for luxury homes but also for the nearly half-million ft.2 of multiple-tiered retaining walls built throughout its challenging terrain.
A location with 1:1 slope ratios and expansive clay soils required a major transformation to make the area suitable for development. A local jurisdiction stating that walls on the project could not be taller than 5 ft. further complicated matters. Multiple-tiered walls were designed to blend with the natural settings and also to accommodate the wall-height restriction. With some walls up to 10 terraces deep and reaching 30 ft. in total height, global stability also became a major concern.
For the project’s entirety—more than 10 years—retaining walls systems reinforced with geosynthetic grids have provided an excellent solution to overcome the topographical challenges, and to help create beautiful and structurally sound walls.
The walls: A look below and behind
The Norris Canyon Estates project walls vary from 2 to 10 tiers, with some requiring geogrid reinforcement at every course with grid layers in some instances up to 50 ft. long. Major over-excavation, which included placing a minimum 8-in.-thick leveling base of compacted crushed stone or Class 2 aggregate, prepared the land for wall and grid installation.
“To make it work and be able to build a design with so many walls on these land characteristics, a high amount of geogrid had to be used below the walls. It is quite amazing how it all came together,” said Macy Tong, the project engineer overseeing connstruction of the walls.
Looking below and behind an 8-tiered wall illustrates the design’s complexity (see Figure 1 and Photo 1a). The soil strength, measured at a 28° phi angle, had to be reinforced with 15 layers of geogrid placed underneath the wall. The grid underneath ranges in lengths from 30 ft. to 50 ft. The wall is reinforced by 8 ft. of rock backfill and 50 ft. of geogrid placed at every course.
High-density polyethylene (HDPE) geogrid was used for the majority of the project. These grids are manufactured using select grades of HDPE resins that resist elongation when subjected to high loads for long periods of time, according to case study notes.
The 8-tiered wall required the use of only one strength of geogrid. However, some of the other walls are reinforced with up to three different grid strengths. Adapting to the different land conditions required varying grid strengths throughout the project.
“You can’t tell from the outside in, but it is a very complex design,” said Stuart Campbell, contractor with B.C.McCosker Construction. “As a wall contractor, it seemed inefficient to have so many 5-ft. terraced walls instead of one continuous wall, because we could have saved the developer both time and money. But the County of Jurisdiction required no walls greater than 5 ft. in height. The end result, however, is very impressive,” Campbell said.
Campbell and his crew built nearly all of the walls in the Norris Canyon development. For this complex installation, they worked from the bottom up and into the upper slopes as they progressed. The drainage pipework system was tied to the bottom outlet and flowed to a central vertical pipe. A common drainboard was used throughout the project.
Upon completion in mid-2007, the Norris Canyon Estates project will have a remarkable 400,000 ft.2 of retaining walls built on an exceptionally challenging landscape.