SEISMIC ANALYSIS OF DRILLED SHAFTS AS EARTH RETAINING STRUCTURE BASED ON SITE CHARACTERIZATION-Paper

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The study evaluated whether a site-specific seismic assessment could optimize drilled-shaft retaining walls for the Circunvalación Norte underpass project in San José, Costa Rica. The original design used the national code’s generic seismic spectrum.<br /><br />Investigations included CPT, pressuremeter, MASW, deep drilling, and triaxial testing. The team assessed seismic hazard using deterministic, probabilistic, and hybrid approaches, identifying the nearby Escazú fault as critical for the short-period structure. Site response and soil behavior were modeled in PLAXIS using a calibrated Hardening Soil model with small-strain stiffness.<br /><br />Finite-element analyses compared free-head and fixed-head walls, shaft diameters of 1.0 and 1.2 m, and several spacings. The site-specific spectrum produced lower seismic demand than the generic code spectrum: estimated peak ground acceleration was 0.45 g rather than 0.61 g. The resulting design selected 1.0 m shafts at 2.0 m spacing for free-head conditions and 1.2 m shafts at 2.4 m spacing for fixed-head conditions.<br /><br />Dynamic analyses using two control earthquakes confirmed acceptable performance. Free-head shafts experienced some increased deflection, while braced, fixed-head shafts moved largely as a rigid block with minimal relative deflection.<br /><br />The optimized spacing reduced the project’s total drilled-shaft length by 32%, while meeting the required performance criteria. The authors conclude that site-specific seismic studies can support more efficient designs by reducing uncertainty and tailoring seismic demand to local conditions, though such studies may also identify cases requiring greater demand.

Keywords

site-specific seismic assessment

drilled-shaft retaining walls

Circunvalación Norte underpass

San José Costa Rica

seismic hazard analysis

Escazú fault

PLAXIS finite-element modeling

Hardening Soil model

peak ground acceleration

optimized shaft spacing