MICROGRAVITY INVESTIGATIONS OF FOUNDATION CONDITIONS

A microgravity investigation was conducted in the upstream and downstr eam switchyards of the Wilson Dam powerplant, Florence, Alabama. The o bjective of the survey was the detection in the switchyard foundations of subsurface cavities or other anomalous conditions that could threa ten the stability of the switchyard structures. The survey consisted o f 288 gravity stations in the downstream switchyard and 347 stations i n the upstream switchyard. Significant anomalous areas in the switchya rds were selected on the basis of residual gravity anomaly maps. These results were prioritized and used to guide an exploratory drilling pr ogram to investigate the cause of the anomalies. Highest-priority bori ng location recommendations were in negative gravity anomaly areas, si nce negative anomalies could be caused by actual cavities or low-densi ty zones that might represent incipient cavity formation. Remaining bo ring locations were in positive anomaly areas for verification purpose s. The results of the borings confirm the presence of cavities and sof t zones indicative of cavity formation.