Lateral spreading evaluation and mitigation for highway overpass structure: Taras, Cartago, Costa Rica-Paper

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The Taras highway overpass in Cartago, Costa Rica, is a seven-span, 240 m bridge in a highly seismic area, where the design earthquake has a peak ground acceleration of 0.64g. Liquefaction and lateral spreading were omitted from the original design but were identified as significant hazards before construction.<br /><br />The study evaluated a 3 m-thick, saturated sandy-clay layer susceptible to liquefaction and also examined a 5 m-thick scenario. Using the Caltrans equivalent nonlinear method, the authors modeled restricted soil movement at the abutments and free-field movement at the piers. Lateral pile response was analyzed with LPILE, considering different pile diameters and three combinations of soil-induced (kinematic) and structural (inertial) loads.<br /><br />Results showed that the predicted extent of the liquefiable layer strongly affects ground movement and foundation demand. Increasing the layer thickness by 2 m substantially increased lateral displacements, bending moments, and pile-cap deflections; in some cases, deflections became unacceptable. The taller, more massive second abutment was more vulnerable than the first, and its piles also lacked sufficient embedment in some analyses. At the piers, 1.0 m-diameter piles produced unacceptable deflections, while 1.2 m piles performed satisfactorily and were recommended.<br /><br />The study also found that no single load combination governs every response measure. All combinations should be included in the design envelope: one produced the greatest pile moments and deflections, another controlled soil reactions, and a third could govern shear demands.<br /><br />The findings prompted an increase in pile diameter from 1.0 m to 1.2 m. Overall, the authors emphasize that reliable subsurface investigation—especially accurate determination of the liquefiable layer’s thickness—is essential to assessing lateral-spreading risk and designing safe foundations.

Keywords

Taras highway overpass

Cartago Costa Rica

seismic hazard

liquefaction

lateral spreading

pile foundation

Caltrans equivalent nonlinear method

LPILE analysis

pile diameter

ground movement