Constructability-based LRFD for Geotechnical Engineering Features – Deep Foundations – Drilled Shafts

This webinar addressed reinforced concrete drilled shafts which are commonly used for deep foundations in the United States and worldwide. Unlike prefabricated steel or prestressed concrete driven piles, reinforced concrete drilled shafts are manufactured at the site. The installation process includes drilling a clean and stable excavation, lowering and centering a field-fabricated steel reinforcing cage in the excavation, placement of concrete , and performing integrity tests and in some cases load tests after the drilled shaft construction is complete.

Design theories for drilled shafts assume “good” construction practices. Often “off-the-shelf” construction specifications are used that may not be consistent with the design assumptions and site conditions, e.g., specifications that were originally developed based on Allowable Stress Design (ASD) that may be used for projects that now use LRFD procedures. In the ASD specifications the effect of various uncertainties was addressed by a single factor of safety regardless of the design method. However, in the LRFD approach the structural and geotechnical uncertainties are dealt with separately. Within the structural and geotechnical aspects, different resistance factors are used to address various limit states such as geotechnical side resistance, geotechnical tip resistance, structural flexure resistance, and structural shear resistance. This approach necessitates a fresh look at construction practices with respect to the various limit states. For example, what happens to the structural integrity of a drilled shaft when there is loose sediment at the bottom of the shaft that may affect the tip resistance? Or what happens to the geotechnical and structural resistance of the shaft when the clear concrete cover outside the reinforcing cage is compromised? Common practice is not to consider such issues and many designers and agencies use modified pre-LRFD construction specifications and consider these specifications to be LRFD-compliant. Such practice can lead to an erroneous conclusion that the constructed shafts are consistent with the design assumptions. Furthermore, such usage can have legal implications in terms of liability from a designer (structural or geotechnical) and contractor viewpoint.