Challenge

A new 22,000 square foot slab-on-grade, single-family residence was planned within the Pinckney Hill Plantation development. The original geotechnical report for the project recommended that the structure be supported with shallow spread footings bearing within the anticipated five feet of new fill that would be placed across the site. Proposed earthwork and grading plans later changed to include only minor cuts and fills. A shallow foundation system was no longer a viable option due to the presence of existing fill and marginally expansive clay soils within the upper 15 feet of the profile. A second geotechnical exploration with six test borings was completed to develop recommendations for a deep foundation option. The soils encountered at the site were highly variable between test boring locations. A generalized soil profile consists of loose clayey sand fill to depths of about four to five feet, over medium dense clayey and silty sand with fat clay seams to about 11 to 15 feet, over stiff fat clay to the depths of the test borings at 30 to 40 feet. Medium dense silty sand was also encountered at the bottoms of three of the six borings. Belled drilled shafts, auger-cast piles and helical piles were considered to support the new home, but, ultimately, site access concerns, soft ground conditions and lower equipment mobilization costs made helical piles the more practical and economic solution.