Sterling Ranch is a residential and commercial master planned community in Douglas County, Colorado. From its outset, Sterling Ranch has been built on a set of values that promote the goals of sustainable and efficient living. Of special importance to the research presented herein are the values of wellness, stewardship, and innovation. Stewardship is a responsibility to take care of someone or something else, and provide for its well being. Stewardship is not easy and, as a steward of the environment and the community, Sterling Ranch has been challenged by the geographic conditions of Colorado’s Front Range.
Colorado’s Front Range includes the foothills and plains on the eastern edge of the Rocky Mountains. As shown in Figure 1, many of Colorado’s most populous cities, such as Fort Collins, Colorado Springs, and Denver are along the Front Range. Sterling Ranch, only 20 miles south of Denver, also finds itself within this region. The Front Range is a challenging area to build in due to the unique geological conditions there. These are characterized by highly expansive clay soils. The soils of the Front Range contain high levels of bentonite and other expansive minerals, which cause the soil to swell dramatically when wet and shrink when dry.
Figure 1: Colorado’s Front Range
These expansive soils are inadequate for construction and must be remediated before developments like Sterling Ranch can begin building residential or commercial buildings. Without remediation, the cyclical swelling and shrinking of soil under rigid concrete foundations can lead to structural failure. Remediation is time consuming and expensive. Currently, builders use a variety of innovative solutions to combat expansive soils. Of these solutions, two are widely employed in the Front Range area. Many residential builders use a pier and beam foundation, while Sterling Ranch and other large developments in the area tend to over-excavate.
Pier and Beam Foundation
The pier and beam foundation provides stability by effectively isolating the home’s foundation slab from the expansive soils. As shown in Figure 2, the pier and beam foundation suspends a structural foundation several feet above the soil. The concrete piers extend deep into the ground to bear on solid soil or bedrock. Wooden beams span between the piers, creating the structural foundation on which the house sits. In addition to protection against swelling soils, the crawl space underneath a pier and beam foundation allows for easy installation of utilities.
Figure 2: Pier and Beam Foundation
The disadvantages of a pier and beam foundation are mainly the potential for mold formation and greater cost. The crawl space between ground and foundation provides a cavity for moisture to seep underneath the house. With a wooden structural foundation, this moisture can lead to rot and eventually failure of the foundation. Installing a pier and beam foundation is expensive. The piers that extend deep into the ground require specific machinery and significantly more concrete compared to the traditional slab on grade foundation. However, this upfront cost ensures the home will not have structural problems caused by expansive soils.
Over-excavation
Over-excavation is the solution used at Sterling Ranch for handling expansive soils. Rather than isolate the foundation from the soil, over-excavation seeks to remediate the expansive nature of the soil before building on it. In the over-excavation process, expansive or otherwise inadequate soil is removed to a depth of about 10 feet below the lowest construction elevation. This depth is dependent on the geological profile of the soils in the area and can vary. The excavation is then refilled with adequate building material. This can be accomplished by hauling in fill material or by treating and replacing the excavated soil. Once the fill is brought to the necessary grade for construction, a regular slab on grade foundation is placed and homes are built.
Over-excavation is not as costly as adding a pier and beam foundation but it is expensive. From cost analysis, Sterling Ranch found that over-excavation adds roughly 10% to the cost of a home and adds six months to the construction timeline. A benefit of over-excavation is that it can be completed at the same time as site grading for roads and other graded areas. Over-excavation is most cost effective when undertaken on a very large scale, such as the implementation of the process across entire filings at Sterling Ranch. Still, over-excavation is not a perfect solution and has drawbacks. It is possible for treated and replaced soils to return to their expansive nature over time. If this happens, the expansion can easily crack the on grade slabs, making the initial over-excavation wasteful.
DESIGN GOAL
The inefficiency of over-excavation is wasting both time and money for Sterling Ranch. This motivates the team to draft the following design goal: Reduce the cost and time of home construction by exploring innovative foundation designs for use on expansive soils at Sterling Ranch. Specifically, the team investigates the feasibility of one specific innovation that was brought to Sterling Ranch’s attention by RMG Engineers: The Tella Firma foundation.