Evaluating pile pinning and shielding during finite-length lateral spread-Paper

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The study evaluates how finite-length earthquake-induced lateral spreading affects bridge foundations, focusing on pile pinning and shielding at the Taras highway overpass in Costa Rica. Conventional equivalent-static analyses generally assume an extensive, free-field spread, which may overestimate foundation demands when the affected area is limited.<br /><br />The authors combined two-dimensional finite-element analyses with equivalent-static analyses to examine drilled-shaft stiffness, crust stiffness, spread direction, and the location of adjacent structures. Pinning occurs when sufficiently stiff piles anchor the surrounding ground and reduce its displacement; shielding occurs when front piles or nearby foundations reduce ground movement toward those behind them.<br /><br />For the modeled pier, piles with concrete strengths of 400 kg/cm² or less did not produce meaningful pinning. Piles with strengths of 450–500 kg/cm² retained sufficient bending stiffness to pin the ground. Accounting for this effect reduced estimated pile-head displacement from about 7–8 cm to 2–3 cm, and peak bending moment from approximately 52,000 to 30,000 kNm. Greater soil stiffness weakened pinning farther from the piles, while changes in spread direction altered which piles shielded others. A nearby, sufficiently stiff second structure also reduced ground displacement; the study found little shielding between parallel structures more than 20 m apart.<br /><br />The findings show that finite-length spreading can substantially lower predicted foundation demands compared with free-field assumptions. However, the results are specific to the site and structure. Applying the method requires detailed subsurface investigation to establish the extent and direction of spreading and to assess pile stiffness. More complex geometries or variable liquefiable layers may require three-dimensional analysis.

Keywords

earthquake-induced lateral spreading

bridge foundations

pile pinning

shielding effect

finite-element analysis

equivalent-static analysis

drilled-shaft stiffness

soil stiffness

pile-head displacement

bending moment