Abstract:
During the last phase of Circunvalacion Norte project in San Jose, Costa Rica, an underpass structure with over 16 km (10 miles) of drilled shafts serving as earth retaining wall was initially designed. Due to deadline issues in the execution of the project, it was necessary to optimize this structure, where the most critical elements were the drilled shafts. The key element in order to obtain an improvement in the design was to characterize the job site with a site study for seismic demand, and use more robust methods for soil-structure interaction. During the investigation, CPT and PMT tests, MASW geophysical tests, deep exploration drilling, and triaxial tests, were carried out. The terrain was modeled using a more advanced constitutive model with hardening plasticity calibrated from the triaxial tests. Plaxis software was used to calibrate the seismic model for the terrain and for the dynamic analysis. Two different structures were considered for the retaining wall: free-head and fixed head. Different sceneries with drilled shafts of 1.0 m (3.3 ft) and 1.2 m (3.9 ft) diameter with spacing between centers between 1.3 m (4.3 ft) and 3.0 m (9.8 ft) were tested in order to obtain and optimize structure. For each case of the drilled shaft wall, the deformation results were compared using the site study or the original design generic spectrum. The deformation results were compared between the site study and the original design generic spectrum. Finally, a dynamic analysis was performed to check the performance of the drilled shaft with optimized separation using control earthquake. Based upon seismic performance curves, the analysis showed the possibility of further spacing the drilled shafts compared with the original design, obtaining a 32% reduction in the total length of the drilled shafts in the project.