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Under Reamed Piling

When employing the rotary excavation method, an enlarged base (under-reamed), also known as bell, can be created, which increases the base bearing capacity of piles in competent soil strata.In its closed position, the under-reamer fits inside the straight section of a pile shaft, and can be expanded at the base of the pile to produce the enlarged base.The under-reamcan be of different shapes. The most common is the standard reamer, usually cut at 45 or 60 degree angle, with the maximum diameter of the under-ream being not more than three times the diameter of the shaft.

The cost advantages of under-reamed piles are due to the reduced pile shaft diameter, resulting in less concrete needed to replace the excavated material. However, in difficult soil conditions such as boulder clays containing lenses of silt or sand, or in loose, non-cohesive soils, it is practically impossible to form an under-reamed pile base.