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 (Fig. 17.1 (a)).
Straight-shaft side wall friction piers pass through overburden soils that are assumed to carry none of the load, and penetrate far enough into an assigned bearing stratum to develop design load capacity by side wall friction between the pier and bearing stratum (Fig. 17.1(b)).
Combination of straight shaft side wall friction and end bearing piers are of the same construction as the two mentioned above, but with both side wall friction and end bearing assigned a role in carrying the design load. When carried into rock, this pier may be referred to as a socketed pier or a "drilled pier with rock socket" (Fig. 17.1(c)).
Belled or under reamed piers are piers with a bottom bell or underream (Fig. 17.1(d)). A greater percentage of the imposed load on the pier top is assumed to be carried by the base.
Figure 17.1 Types of drilled piers and underream shapes (Woodward et al., 1972)