What Is Pier Foundation?
Drilled pier foundations, the subject matter of this article, belong to the same category as pile foundations. Because piers and piles serve the same purpose, no sharp deviations can be made between the two.
The distinctions are based on the method of installation. A pile is installed by driving, a pier by excavating. Thus, a foundation unit installed in a drill-hole may also be called a bored cast-in-situ concrete pile.
Here, distinction is made between a small diameter pile and a large diameter pile. A pile, cast-in-situ, with a diameter less than 0.75 m (or 2.5 ft) is sometimes called a small diameter pile.
A pile greater than this size is called a large diameter bored-cast-in-situ pile. The latter definition is used in most non-American countries whereas in the USA, such large-diameter bored piles are called drilled piers, drilled shafts, and sometimes drilled caissons.
Types of Drilled Piers
Drilled piers may be described under four types. All four types are similar in construction technique, but differ in their design assumptions and in the mechanism of load transfer to the surrounding earth mass.
• Straight-Shaft End-Bearing Pier
• Straight-Shaft Side wall Shear Pier
• Straight-Shaft Pier With Both Sidewall Shera and End Bearing
• Underreamed or Belled Pier
Straight-Shaft End-Bearing Pier
Straight-shaft end-bearing piers develop their support from end-bearing on strong soil, “hardpan” or rock.
The overlying soil is assumed to contribute nothing to the support of the load imposed on the pier.