Northeast Engineers & Consultants, Inc was contracted to perform a geotechnical investigation and preliminary structural design for Additions to the Squadron Operation Building at the Quonset Air National Guard Base in North Kingston, Rhode Island. The development consisted of additions and renovations to the existing Squadron Operations Facility. These additions encompassed approximately 6,300 square feet, and included first floor extensions to the north and south sides of the eastern portion of the building and a second story to the existing building at the same end. The existing building interior will be modified and new pitched standing seam metal roof will be constructed over the entire structure.

The existing building is a one story structure with a structural steel frame and masonry walls. It has a flat steel truss roof and a six-inch thick reinforced concrete slab floor. The building is supported on a pile foundation consisting of 20-ton capacity treated timber piles that are 25 feet long with a minimum butt diameter of 12 inches.

NE&C's scope of work included the following tasks:

• Field exploration that included drilling four borings to a 40 foot depth
• Laboratory testing
• Engineering analysis

NE&C's borings encountered three distinct zones of subsurface materials; fill, soft organic soil, and medium dense to dense sands. This fill layer extends to a depth of the 10 to 11 feet from existing grade, and is underlain by a layer of soft peat. The peat layer is about 3 to 5 feet thick. Fine to medium sand was encountered beneath the peat to the full depth of the borings.

The layer of peat and portions of the existing fill are soft and loose and susceptible to compression and consolidation upon the application of structural loads that could have resulted in significant settlement of the ground surface and any overlying structures. Piles were needed for the foundation of the first floor lateral additions; some of the existing interior pile caps also needed to be supplemented with new piles where new loads from the second story exceeded the capacity of the existing piles.

Other methods of reducing potential fill settlement included overexcavation of all loose and soft soils and replacement with compacted engineered fill, or ground modification to strengthen and reduce the compressibility of the soft and loose soils. These options would have allowed the use of shallow footings rather than piles. Overexcavation was not recommended because the depth of removal would have been well below the groundwater table and thus would have required extensive dewatering and stabilization of the bottom. Also, the overexcavation would have had to be performed immediately adjacent to the existing building, requiring shoring and possible underpinning to prevent distress to the structure. The weight of the additions supported on spread footings could have also surcharged the existing piles beyond their design capacity. This option would have also been significantly more expensive than piles.

Ground modification that may have been appropriate for the site conditions include jet grouting, vibro-replacement, or soil mixing. Each of these methods would have involved the injection of grout or stone into the subsurface to strengthen the soft and loose soils. Since these methods would have involved specialized equipment and experienced specialty contractors, they would have been significantly more expensive than piles considering the relatively small size of the project.