ABSTRACT
CONTENTS 7.1 Introduction ..................................................................................................................... 300 7.2 Construction Considerations ........................................................................................ 300
7.2.1 Dry or Wet Construction ................................................................................... 301 7.2.2 Casing ................................................................................................................... 301 7.2.3 Concreting and Mix Design .............................................................................. 302
7.3 Design Capacity of Drilled Shafts................................................................................ 302 7.3.1 ASD versus LRFD............................................................................................... 303 7.3.2 Standard Penetration Test Data in Sand......................................................... 304 7.3.3 Estimation of Side Shear.................................................................................... 304 7.3.4 Estimation of End Bearing ................................................................................ 305
7.4 Use of Triaxial or SPT Data in Clay ............................................................................ 307 7.4.1 Side Shear (Alpha Method)............................................................................... 307 7.4.2 End Bearing ......................................................................................................... 309
7.5 Designing Drilled Shafts from CPT Data ................................................................... 311 7.5.1 Estimation of Side Shear.................................................................................... 311 7.5.2 Estimation of End Bearing ................................................................................ 312
7.6 Designing from Rock Core Data .................................................................................. 312 7.6.1 Estimation of Side Shear.................................................................................... 313 7.6.2 Estimation of End Bearing ................................................................................ 313
7.7 Designing from Load Test Data ................................................................................... 313 7.7.1 Estimation of Side Shear.................................................................................... 314 7.7.2 Estimation of End Bearing ................................................................................ 314
7.8 Design of Postgrouted Shafts........................................................................................ 315 7.8.1 Postgrouting in Sand.......................................................................................... 316 7.8.2 Postgrouting in Other Formations ................................................................... 316
7.9 Economy of Load Testing.............................................................................................. 318 7.9.1 Selecting the Most Economical Shaft Diameter ............................................. 319 7.9.2 Selecting the Most Economical Design Method ............................................ 319
7.10 Pressure Injected Footings............................................................................................. 321 7.10.1 Construction of Pressure Injected Footings.................................................. 322 7.10.2 Concreting of the Shaft .................................................................................... 323
Nomenclature ............................................................................................................................. 324 References ................................................................................................................................... 325
Drilled shafts are deep, cylindrical, cast-in-place concrete foundations poured in and formed by a bored (i.e., ‘‘drilled’’) excavation (Figure 7.1). They can range from 2 to 30 ft in diameter and can be over 300 ft in length. The term ‘‘drilled shaft’’ is synonymous with cast-in situ piles, bored piles, rotary bored cast-in situ piles, or simply shafts. Although once considered a specialty foundation for urban settings where vibrations could not be tolerated or where shallow foundations could not develop sufficient capacity, their use as structural support has recently increased due to heightened lateral strength requirements for bridge foundations and the ability of drilled shafts to resist such loads. They are particularly advantageous where enormous lateral loads from extreme event limit states govern bridge foundation design (i.e., vessel impact loads).
