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Soil Cement as a Foundation Alternative

During the past year, Terracon’s Phoenix office has been involved in both the geotechnical engineering and construction materials engineering and testing services for the Arizona Diamondbacks/Colorado Rockies Spring Training Facility located on the Salt River Pima-Maricopa Indian Community land near Scottsdale, AZ. Terracon’s services on the more than $130-million facility were provided as part of an on-call contract awarded in 2009. The project allowed Terracon to provide extensive construction-related input during the design stage that resulted in the creation of a unique foundation system.

Prior to final design of the project, Terracon performed the preliminary geotechnical engineering services and provided recommendations for foundation support. As part of these initial findings, recommendations were provided for both spread footings and drilled shafts. Construction of the main stadium structure required placement of 10 to 12 feet of engineered fill to achieve the desired main concourse elevation, and the site soils consist mainly of sandy lean clays and clayey sands with medium to high plasticities. Since the structural engineer wanted to use spread footings for the main stadium structure, and they had an allowable settlement tolerance of only ½ inch, Terracon suggested the use of soil-cement to treat the on-site clayey soils and provide support for spread-type footings for the main stadium structure. Soil-cement was ultimately chosen for the following reasons:

  • The treatment increased the bearing capacity of soils beneath the main stadium structure from 3,000 psf to 8,000 psf (although the structural engineer was only comfortable with using 6,000 psf) which reduced construction costs.
  • The strength and bearing characteristics of cement treated soil allowed the on-site materials to be able to deliver the relatively small settlement values sought by the structural engineer.
  • The ability to use the on-site soils and reduce the amount of export and import material required for the project. Once treated with cement, the clayey on-site soils became a relatively stable material with minimal volume change characteristics.

Although soil-cement with a 200 psi compressive strength could produce the desired characteristics, a 300 psi at seven-day strength requirement was specified to provide a factor of safety and to accommodate the tight construction schedule. A cement content of 5 percent by weight was recommended and confirmed by several mix designs using the on-site clayey soils.

The soil-cement was constructed at the site using the in-place method. This method consisted of the placement of a layer of fill, spreading dry cement across the fill material, and mixing the material in-place with a pulverizer type machine which introduced water during the mixing process. The mixed material was then compacted using a smooth-drum, steel-wheeled roller.

The project specifications required a minimum compaction level of 95 percent (based on ASTM D 558 or D 698) for the soil-cement with moisture contents during placement and compaction in the range of