Overview for Design and Construction of Drilled Shafts in Cohesive Soils.

Drilled shaft foundations have become very important in construction of numerous types of structures such as residences, buildings, bridges, and towers on expansive or unstable soils and fills. Much research has been done to improve the design and construction procedures for drilled shafts, but few reports contain criteria for different soils and loading conditions. This report updates and condenses previous work and presents procedures available for design and construction of drilled shaft foundations in cohesive soil under various loading conditions. Guidance is presented for field exploration, laboratory investigations, design concepts and procedures, and construction techniques. The results of the overview show that design procedures are generally highly conservative, particularly those using total stress methods. Such conservatism has frequently been relied upon because no rational basis exists for determining the empirical factors, particularly the reduction factor alpha, required for the total stress methods. Failures of drilled shaft foundations have consequently been associated mostly with construction problems rather than with design. New research should include development of improved and more rational design methods to reduce unnecessary conservatism, determination of long-term effects on drilled shafts, improvements in construction methods, and techniques for troubleshooting problems with drilled shafts. Author