Analisis Kegagalan Pilar Jembatan Kiambang Pascabanjir dan Alternatif Rehabilitasi

The failure of the pier structure of Kiambang Bridge A in Padang Pariaman Regency occurred after a flood event and was associated with local scour around the bridge pier. This condition resulted in loss of soil support and deformation of the pier, potentially affecting the stability and serviceability of the bridge. This study aims to evaluate the relationship between flood discharge and local scour depth, assess the condition of the bridge substructure and foundation, and determine an appropriate rehabilitation alternative. The analysis was conducted using hydrological, hydraulic, scour, geotechnical, and structural approaches. Design rainfall was determined using a normal probability distribution, while design flood discharge was calculated using the Nakayasu Synthetic Unit Hydrograph method. Hydraulic and local scour analyses were performed using HEC-RAS for several return periods. Foundation capacity was evaluated using the Meyerhof approach based on subsurface investigation data, while the existing superstructure was evaluated using SAP2000 under the applicable bridge loading provisions. The results show that design discharge increased from 146.05 m³/s for the 2-year return period to 214.06 m³/s for the 100-year return period. The maximum scour depth obtained using the CSU method was 4.28 m at the 100-year return period. The analysis indicates that scour significantly reduces the effective soil support around the existing pier foundation. A replacement foundation using a 3.0 m diameter and 11.0 m deep well foundation has an allowable capacity of approximately 18,285 kN, exceeding the design vertical load of 3,500 kN. The rehabilitation alternative that retains the existing foundation and replaces the failed pier requires approximately Rp1.65 billion and is more economical than replacing the foundation, estimated at approximately Rp2.4 billion.