Engineering and Construction

For some projects, deep foundations may need to carry very high vertical loads.

In this case, piers are usually constructed such as shown in Fig. 6.15. By installing a bell, the end-bearing capacity of the pier can be significantly increased. Some of the advantages of a belled pier are as follows:

• The pier and bell can be visually inspected when the pier diameter is about 30 in. (0.76 m) or larger.
• Pile caps are usually not needed since most loads can be carried on a single belled pier.
• With belling, the uplift capacity of the pier can be significantly increased.
• The design depth and diameter of the pier and bell can be modified based on observed field conditions.
• The pier and bell can be installed in sound bedrock to carry very high loads.

Some of the disadvantages of belled piers are as follows:

• Loose granular soil below the groundwater table can cause installation problems. The bell usually cannot be formed in granular soil below the groundwater table.
• Small diameter piers and bells cannot be easily inspected to confirm bearing and are particularly susceptible to necking problems.
• If the rock is too hard, it may not be possible to excavate the bell, in which case a rock socket may need to be drilled.

Belled piers can be designed in the same manner as conventional deep foundations. They are usually designed as end-bearing members with the allowable end-bearing capacity determined by the geotechnical engineer. For sound rock, the allowable end-bearing capacity could be based on allowable bearing code values (e.g., Table 18.4), field load tests, and local experience. For the ultimate end-bearing capacity in soil, Eqs. 6.15 and 6.16 can be used for cohesionless soil and the end-bearing part of Eq. 6.19 can be used for cohesive soil.





Inclined Loads. In addition to the vertical load acting on the pile cap or pier, they may also be subjected to a lateral load; hence the resultant of the load will be inclined. Lateral loads on deep foundations are more complex than lateral loads on shallow foundations. Section 6.4 will be devoted to lateral loads on deep foundations.