Bored Piles

Bored piles develop carrying capacity in two ways - by skin friction and end bearing. Friction piles rely on the sides of the bored hole being rough. When concrete is placed it forms a mechanical bond with the surrounding soil, which enables it to develop the full shear strength of the soil. An end-bearing pile or caisson is constructed by boring a hole through soft overburden until a suitable bearing stratum such as rock or hardpan is reached. By enlarging or “belling” the bottoms of shafts, end-bearing capacity can be increased enormously. Bells are formed mechanically by means of a tool attached to the Kelly bar of a drilling rig. Quite often during the drilling or boring process, obstacles such as boulders, layers of caving soil, or inflow of ground water are encountered. Techniques such as utilizing temporary or permanent steel sleeves or casings to control caving and ground water, and special tools to penetrate rock layers, have been developed. Sometimes in areas of deep silt or sand which is wet and loose, cast-in-situ soil-cement piles are used. This type of pile is produced by screwing down a hollow stem
continuous flight auger to a pre-determined depth, then slowly extracting by high-speed reverse
rotation while pumping a cement slurry under pressure through the hollow tube and out the
bottom of the auger, mixing it with the in-situ sand or silt.

The main advantages of bored piles over conventional footings or other types of piles are:

• Piles of variable lengths can be extended through soft compressible, or swelling soils, into suitable bearing material.
• Piles can be extended to depths below frost penetration, and seasonal moisture variation.
• Large excavations and subsequent backfill are eliminated.
• Adjacent soil is not disturbed or remolded.
• Absence of vibration will not disturb adjacent piles or structures.
• Extremely high capacity caissons can be obtained by expanding the base of the shaft up to three times the shaft diameter, thus eliminating construction of caps over multiple pile groups.
• For many design situations bored piles offer higher capacities with potentially better economics than driven piles.

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