A drilled-pier foundation is used to transmit loads to soil at lower levels through end bearing and, in some situations, side friction. (See also Art. 9.74.) It can be constructed in firm, dry earth or clay soil by machine excavating an unlined hole with a rotating auger or bucket with cutting vanes and filling the hole with plain or reinforced concrete. Under favorable conditions, pier shafts 12 ft in diameter and larger can be constructed economically to depths of 100 ft and more. Buckets with sliding arms can be used to form bells at the bottom of the shaft with a diameter as great as 3 times that of the shaft (Fig. 9.49).
Some building codes limit the ratio of shaft height to shaft diameter to a maximum of 30. They also may require the bottom of the bell to have a constant diameter for the bottom foot of height, as shown in Fig. 9.49.
The compressive stress permitted on plain-concrete drilled piers with lateral support from surrounding earth varies with different codes. The BOCA National Building Code/1999, limits the computed bearing stress based on service loads to 0.33Æ’’c, where Æ’’c is the specified concrete compressive strength.
Reinforced-concrete drilled piers can be designed as flexural members with axial load, as indicated in Art. 9.82.
Allowable unfactored loads on drilled piers with various shaft and bell diameters, supported by end bearing on soils of various allowable bearing pressures, are given in Table 9.23. For maximum-size bells (bell diameter 3 times shaft diameter) and a maximum concrete stress Æ’c1 for unfactored loads, the required concrete Æ’c1 = 0.33Æ’’c strength, psi, is 19% of the allowable soil pressure, psf.
Column-design procedures are based on a comprehensive investigation reported by American Concrete Institute Committee 105 (Reinforced Concrete Column Investigation, ACI Journal, February 1933) and followed by many supplemental tests.
The results indicated that basically the total capacity for axial load can be predicted, over a wide range of steel and concrete strength combinations and percentages of steel, as the sum of the separate concrete and steel capacities.