Foundation models in seismic areas: Four case studies near the city of Granada (Spain)

Abstract

This paper presents four case studies of the geotechnical characteristics of foundation design under seismic conditions in towns located in the province of Granada (Spain). Much of this province is affected by seismic activity caused by active faults in the Betic Cordillera, the Alborán Sea, the Atlantic Ocean, and the area around the northern border of Africa from Morocco to Algeria. Although seismic activity in the region is moderate with peak accelerations of 2.4m/s2 for a return period of 475years, it is the region with the highest seismic risk in the Iberian Peninsula. The four study areas include the towns of Atarfe-Albolote, Fuente Vaqueros, Churriana de la Vega, and the district of La Chana (the NW sector of the city of Granada). Churriana de la Vega has a very homogeneous soil profile with a coarse granulometry, high relative density, high bearing capacity, high field-penetration test values, and a deep water table. In contrast, the other three areas have a seasonally shallow water table and extremely variable soil profiles in terms of bearing capacity. They are characterized by predominantly fine alluvial soils of low relative density and with low field-penetration test values. They are thus susceptible to seismic effects such as liquefaction and densification.Based on geotechnical reports, dynamic penetration tests, and direct shear and consolidation tests on disturbed samples, we first defined the basic mechanical properties of the soil under static conditions and determined the features related to bearing capacity and foundation design. The next step was to analyze the corresponding dynamic geotechnical parameters by using various empirical correlations between static and dynamic parameters. For this purpose, our research study took into account the definitions and criteria regarding bearing capacity under seismic conditions regulated by the Spanish (NSC-02) and European (Eurocode 8) technical codes. The results obtained led us to propose new foundation types, which would be capable of withstanding potential seismic action. These improved foundations have a better load distribution and higher stiffness, thanks to strip footing, reinforced concrete plates, and the occasional use of deep piling. The dynamic actions of the terrain and the associated settlement effects due to soil liquefaction or densification showed that the areas most in need of deep foundations were located around Albolote. As a result, a deeper foundation is necessary, such as the type found in Fuente Vaqueros, where geotechnical conditions are even more unfavorable.