Originally constructed over 50 years ago, the crib style retaining wall located along the City Hall parking lot was in poor condition. The City of Alton requested that Oates Associates conduct a structural evaluation of the retaining wall and provide recommendations for repair or replacement of the wall. The structural investigation determined that the wall should be replaced.
In 2006 the Alton City Hall was under renovation and all offices were temporarily relocated. It was decided that the replacement of the existing crib wall should take place during this shut down. The project would be completed before the offices were moved back, causing less disruption to staff, public and parking. Oates designed the site plan and wall layout for the reconstruction of the parking lot and replacement of the existing retaining wall.
The new retaining wall is over 300 feet in length and varies from 8 to 12 feet in height. The existing crib retaining wall was replaced with a large block retaining wall system. Since the large block retaining walls do not require soil reinforcing for up to 13-foot tall walls, the large block retaining wall was selected and located directly in front of the crib retaining wall to reduce the widening required to complete the parking lot. The crib wall remained in place and the impact to the slope below the retaining wall was limited. The existing concrete stairway leading down to the lower parking was replaced with a platform style staircase constructed of the large blocks and poured concrete steps and landings. The parking lot was reconstructed and angled parking was added along the street.