Drilled shafts are classified according to the ways in which they are designed to transfer the structural load to the substratum . Figure 9 . 6 a shows a drilled straight shaft. It extends through the upper layer ( s) of poor soil，and its tip rests on a strong load-bearing soil layer or rock. The shaft can be cased with steel shell or pipe when required ( as in the case of cased，cast-in-place concrete piles) . For such shafts，the resistance to the applied load may develop from end bearing and also from side friction at the shaft perimeter and soil interface.A belled shaft consists of a straight shaft with a bell at the bottom，which rests on good bearing soil. The bell can be constructed in the shape of a dome ( Figure 9. 6b) ，or it can be angled ( Figure 9. 6c) . For angled bells，the underreaming tools commercially available can make 30° to 45° angles with the vertical. For the majority of drilled shafts constructed in the United States，the entire load-carrying capacity is assigned to the end bearing only. However， under certain circumstances，the end-bearing capacity and the side friction are taken into account. In Europe，both the side frictional resistance and the end-bearing capacity are always taken into account.Figure 9. 6 Types of drilled shaftStraight shafts can also be extended into an underlying rock layer ( Figure 9. 6d) . In the calculation of the load-bearing capacity of such shafts，the end bearing and the shear stress developed along the shaft perimeter and rock interface can be taken into account.
