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Our previous editions of Explainer have focused on a variety of construction topics, including ground breaking and shoring, where the necessity of a strong building foundation was established. In this week's Explainer, we look at one of the key stabilizing structures used in deep foundations, when drilling is necessary to find soil that can adequately withstand the building loads above.
Whereas piles are columns — usually concrete, steel or wood — that are driven into the earth, caissons are excavated cylindrical boreholes drilled into the ground using an auger. Steel shoring tubes are often temporarily placed in the hole to prevent soil collapse. Concrete is then poured, filling the hole, and the steel tube is removed and can be reused. The result is a concrete pier that provides foundation support to building loads.
Belled caissons increase stability by enlarging the bottom of the shaft with a special auger featuring expandable wings. The wings then retract as the auger is withdrawn from the soil. Belling results in a substantial increase in bearing capacity. Alternatively, rock-socketed caissons are drilled several feet into the bedrock, transferring the load to the solid rock below and around the hole.
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