Drilled Shafts Subjected to Combined Effects of Lateral Spreading and Superstructure Inertia-PPT Presentation

PPT Presentation

Pdf Summary

The presentation examines how seismic inertia from a bridge superstructure interacts with lateral-spreading loads on drilled shafts and piles. Design guidance varies widely: some standards combine kinematic and inertial demands using specified percentages, while others treat them as separate or potentially interacting effects. This reflects a lack of consensus and the site-specific nature of the problem.<br /><br />Researchers investigated the interaction through a 1-g shake-table test at UC San Diego. A reinforced-concrete pile supported a superstructure and passed through a nonliquefiable dry crust, loose liquefiable sand, and dense saturated sand. Shaking triggered liquefaction and lateral spreading; soil displacement eventually reached about 15 inches, a gap formed beside the pile, and the pile collapsed.<br /><br />The test showed that pile–soil loading changed during shaking. In the nonliquefiable crust, inertial motion and crust pressure could act in opposite directions. During downslope inertial motion, however, the pile head moved ahead of the soil and into the existing gap. As a result, the crust exerted little or no pressure on the pile, despite ongoing ground deformation. The gap did not close during this phase.<br /><br />Large pile strains developed after liquefaction began. Near the surface, peak strains and bending demands were associated with upslope inertia resisted by downslope crust pressure—an out-of-phase interaction. At greater depths, peak strains coincided with downslope inertia, when the crust supplied no significant lateral-spreading load.<br /><br />The findings highlight that inertial and lateral-spreading effects may not reach their maxima together or combine uniformly along a pile. Gap formation and relative pile–soil movement can strongly alter load transfer, underscoring the need to account for soil layering, loading phase, and site-specific behavior in design.

Keywords

seismic inertia

lateral spreading

drilled shafts

piles

bridge superstructure

liquefaction

shake-table test

pile-soil interaction

bending strains

gap formation