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PJanuary 18, 2011

Bad Soils? Try Stone Columns.

Every building owner knows that in planning a new building the biggest unknown is what lies beneath the ground. Soil borings and a Geotechnical Engineering Report help alleviate a part of that fear and are certainly an important part of due diligence. But what happens when the geotechnical report leans toward a deep foundation system (piles, caissons) instead of a shallow foundation system (spread footings)? The cost difference is too great not to challenge other structurally acceptable alternatives. My favorite…Stone Columns.

The stone column foundation system basically transfers the column foundation load through a column of crushed stone that rests on suitable virgin soil that is deeper than 5 feet below sub grade. Or in the slang vernacular:

Bearing capacity is atrocious until we dig 10 feet down. Don’t want to spend any money on shoring or back fill, but we want to keep spread footings as our foundation design.

Picture this…At Column A5 the footing size is 9’ x 9’ x 2’ thick with a double mat of reinforcing. Good bearing soil is 10 feet below sub grade. Can’t safely put a man at the bottom of the footing elevation so let’s dig a neat 9×9 hole 10 feet deep (hoping that the sides of the excavation hold for the 21 minutes it will take to excavate and load the 30 yards of unsuitable bearing soil). Trucks are ready to immediately fill the neat hole with stone in 3 to 4-foot lifts to effectively raise the bearing elevation 10 feet so a column footing and even a concrete pier can be safely formed, poured and stripped. Easy, safe, economical and fast.

And yes…We have lessons learned and important “cautions” so call me if you want more information.

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