Seismic Retrofit of Pile Group Foundation with Thickened Caps

The case studied in the paper proves that thickening the pile cap is an effective seismic retrofit alternative to strengthen a pile group foundation. Unlike the typical seismic retrofitting of foundation, the proposed method eliminates the need to drive long piles into existing bridge substructures, substantially reducing cost, construction time and traffic interruption. The method thickens the pile cap of the bridge foundation to engage with a larger quantity of soil when under the influence of seismic excitations. The additional friction provided by the surface of the concrete encasement helps to resist the overturning moment of the earthquake forces, while the passive pressure provided by the soil helps to resist lateral forces during earthquakes. The method is recommended for implementation in the freeway bridge retrofit project in Taiwan due to construction constraints. A successful retrofit requires existing piles in at least moderate condition, detailed construction sequences and installation of the encasement.