The composition of soil makes a huge impact on designing the foundations for commercial and residential buildings. A major issue in the fast-growing Reno-Sparks area is that development is encroaching into areas where soil tends to be expansive. As the name implies, expansive soil tends to expand when they get wet and shrink when they get dried. If a normal slab-on-grade foundation is poured on clay soil, it can be pushed up and crack when significant moisture penetrates the ground underneath (both from rain and snowmelt). That in turn can cause unsightly cracks to form in walls and ceilings.
The longstanding method of dealing with expansive soil has simply been to replace it with a layer of sand and gravel. Builders would routinely excavate 5 feet of soil and replace it with imported sand and gravel from nearby quarries to provide a more-stable base for traditional slab-on-grade concrete foundations.
However, in recent years the availability of sand and gravel – and the ability to dispose of excavated soil cheaply – has fallen. Combined with the high costs of transportation and the huge greenhouse impacts, this approach has become much less favorable.
A less invasive and ultimately more cost-effective solution has been to leave the native clay and support the structures on post-tensioned concrete mat foundations. These are stronger and better able to withstand ground movements from expansive soil.
In a post-tensioned mat foundation, steel cables are placed in a checkerboard fashion running the length and width of the foundation. After the concrete is poured and cured, the cables are tensioned and anchored into the edges of the slab. This, coupled with the fact the concrete is often 10- to 12 inches thick – compared to 4 inches or so in a standard slab-on-grade – provides a much stronger foundation that resists ground movement and limits structural cracking.
While post-tensioned mat foundations are more expensive than slab-on-grade foundations, they are ultimately more cost-effective when considering the reduced costs of excavating and replacing expansive soil.
In addition, development in the mountains that surround adds complexity due to steep terrain, old landslides, and numerous small earthquake faults. Another important step geotechnical engineers can take to save money and streamline the development process in complex geological environments is to prepare a corrective grading plan. The corrective grading plans overlay the geotechnical recommendations directly on the grading plan to clearly delineate required excavations, keyways, soil removals, and subdrain locations. By developing a color-coded map to show the builder and contractors exactly what soil and geological conditions they will encounter and what quantities of soil they’ll be dealing with, the owner will get better bids, fewer change orders, and fewer problems overall during construction.
Thorough and complete geotechnical engineering up front can cost a little more, but the long-term savings in development and maintenance costs can potentially result in net savings of millions of dollars.
