Groundsill stability in downstream of bridge "Besi" in body of water at River Garang Semarang city, Indonesia

During the dry season the elevation of the flow at crest groundsill is the same as the elevation downstream, which is assumed that the dimensions of the groundsill are not large enough. These less large dimensions cause the groundsill weight to be small, so its stability is very doubtful. It is this doubt on the groundsill stability that led to this research. The aims of this study were to analyze the stability of the Groundsill Bridge in the Garang River channel on the force of overturning, sliding, eccentricity, bearing capacity of the soil, and piping. This research employed survey method. Data collection methods used were observation and secondary data. Analysis of the data were rainfall design of a 50 years return period; flood discharge design with a 50 years return period; the forces acting on the groundsill, and the groundsill stability. The results of the study indicate that: the groundsill were safe against overturning forces, eccentricity, and piping under normal flow and flood conditions; the groundsill were not safe against sliding forces under normal flow and flood flow conditions; the bearing capacity of the groundsill subgrade were safe against forces that occur during normal conditions and unsafe during flood conditions.