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Bremer Financial Corporation Project B2008939 November 17, 2020 Page 16 A subgrade improved with aggregate piers or stone columns will reduce the potential for detrimental settlement associated with the existing fill to occur, provide adequate bearing capacity, eliminate the need for deep excavations, reduce impacts to adjacent site features, and reduce the volume of subgrade soils disturbed at this site. Different contractors use varying techniques to construct aggregate piers but generally consist of excavating soil from a hole with an auger or vibrating a probe into the gr ound, and then building a column of clean, open-graded aggregate. The contractor constructs the pier by placing the aggregate in lifts from the bottom of the pier and compacting each lift before placing aggregate for the subsequent lift. The vibratory energy, and sometimes ramming action, causes the aggregate to interlock, forming a stiff pier that provides soil reinforcement and increases shear resistance. Due to the many variations in techniques, we recommend using performance-based specifications with design-build contracting. We recommend requiring the contractor to have at least five years of experience in performing this work, and to demonstrate performing the proposed protection system(s) on at least three previous projects of similar size and scope. The specifications should require the design engineer be licensed in the project state. We can assist you with developing a list of pre-qualified contractors prior to bidding or with reviewing contractor experience as part of the bidding process. Aggregate piers are a Special Inspection item in accordance with Chapter 17 of the IBC. The observations should include installed length, consistency of soil profile with the geotechnical evaluation confirmation of the materials, and confirmation of installation techniques. We recommend installing aggregate piers under the foundations for the building. The aggregate piers should extend through the existing fill to bear on the underlying alluvial and glacial soils. Under the floor slab areas, to save costs, consideration can be given to building on the old fill soils without ground improvement. However, the owner must assume any risks of floor settlement and possible slab distress due to deeper, unforeseen conditions in the fill soils . The risks can be reduced by treating floor slab areas similar to a paved area and performi ng a 1- to 2-foot subcut to expose the subgrade soils. Any unstable soils would be further subcut and then the exposed subgrade is surface compacted followed by placing compacted fill up to the bottom of slab elevation.