Gainesville Lock was constructed in a 65-foot-deep excavation on the flood plain of the Alabama River. The upper 30 to 35 feet of the excavation was in flood plain alluvium and the remaining lower part was in the Mooreville Chalk Formation of Upper Cretaceous Age. Within a few weeks after the excavation was made, tension cracks started to develop behind a 20 to 24 foot vertical face in the chalk. After approximately 3 months, these cracks extended almost the entire length of the lock chamber area and had deepened to the point where failure occurred in two lock monolith areas. The entire vertical face was then scaled and sloped back to the tension crack, and the slope remained stable thereafter.
The chalk is prominently exposed along the river banks in steep slopes up to 100 feet high and vertically excavated slopes up to 45 feet high have stood for over 30 years in similar material at a quarry approximately 25 miles from Gainesville Lock. Also, slope stability analyses using laboratory test values obtained during the exploration and design of the lock indicate the slope was theoretically stable with a very high safety factor. After the failure occurred, samples were taken from the failed slopes and additional laboratory tests performed. This report describes the studies to back analyze the failure to try and determine the reason and conditions which led to the failure.