As construction in the U.S. moves onto less desirable building sites, some with poor soil conditions, effectively dealing with expansive soil becomes essential. Failing to account for soil movement when faced with expansive clay is likely to result in failed foundations.
The traditional way to build a foundation and slab on expansive soils is to drill piers through the expansive clays, place crushable void material (like corrugated cardboard), and place concrete grade beams and slabs atop the void material and bearing on the piers to isolate the slab from the soil. The cardboard loses strength as it absorbs moisture from the soil, allowing the soil to expand without putting pressure on the slab. This method has worked well in most circumstances, but adds significant expense to projects.
An alternative method for providing the void space needed for expansion has been developed by a Richardson, Texas, company called Tella Firma Foundations. The slab is placed directly on the ground and then elevated to provide the void space needed to overcome the dangers of soil expansion. The technique has been used in more than 1,000 residential and commercial foundations in Colorado and Texas since 2005.
A Unique Twist – Literally
Foundation placement begins as usual: The site is graded to a level elevation; and structural piers are drilled and filled to support exterior walls and interior columns, which are isolated from the slab. Nothing new at this point.
But then, helical piers are drilled on a grid pattern across the entire slab area and a lifting mechanism – basically a 1-inch greased bolt – is installed atop each. Piers are deep enough — as much as 30 feet to 40 feet – that helix plates are below the soil’s active zone. The parts of the slab that will have capitals or grade beams are then excavated to create forms in the soil—capitals are formed for each helical pier. Workers position reinforcing steel or post-tensioning cables, or both, and then place the slab directly on the ground.
At this point, if the building is to have tilt-up walls, panels can be formed and poured on the slab that is resting on the ground. After panels are lifted into position on perimeter structural piers and roof trusses placed, the unique aspect of the Tella Firma slab can proceed.
First, surveyors shoot elevations for the top of the slab at the location of each helical pier. The slab is post-tensioned to turn it into a structural slab rather than just a slab on ground. A group of workers is positioned, one above each pier, with long cheater bars attached to a socket wrench. The lifting bolts, bearing on the piers, are turned to raise the entire slab 1/3-inch per bolt rotation. The slab is lifted to provide a void space that is a minimum of 1.5 times the expected potential vertical rise of the soil as determined by the geotechnical engineer.
To finish it off, once the slab has been elevated to its final position, workers position void forms for pour strips between the lifted slabs and the walls and fill with concrete. A protective cap is placed over the holes in the slabs to allow access should further adjustment be needed.
Project Saves Almost $1 Million
The Comal County Jail in New Branfels, Texas, is using the foundation in clay soil that’s expected to experience vertical movements of up to 6¼ inches. In addition to a concrete slab, the foundation design includes tilt-up wall panels; heavy concrete block walls; interior structural steel support columns; and extensive buried plumbing, electrical, and data lines.
In the estimating process, Tella Firma was considered as an option instead of void forms. A bid submitted by a contractor familiar with the system came back $900,000 less than the traditional approach. There are several reasons:
- A 5-inch slab versus a 10-inch-thick slab cut concrete cost almost in half.
- Not having to drill more than 300 18-inch-diameter concrete piers.
- Lower costs for the post-tensioning cable and rebar package.
- Elimination of the need for stud-rails.
The final design relied on 1,624 helical piers with 1.5-inch and 1.75-inch shafts installed 10 feet to 12 feet apart. With support on such a tight grid, the slab needed to be only 5 inches thick to span between the piers. The plan also reduced construction time because helical piers are quick to install with a skid steer.
