Background:
GPR Data LLC was contacted in conjunction with Quaker Oats personnel to perform a ground penetrating radar survey and concrete evaluation of subsurface continuity / discontinuity beneath the asphalt parking area, structural columns on the north west side of the building, the levee, and a portion of the concrete silo pad near elevator G. Significant visual surface deformation and undermining in specific areas were the result of high flood water intrusion in recent weeks. The majority of surface deformation occurred in the south west parking lot where the asphalt surface was washed away. The total amount of undermining and void space under the asphalt and concrete was unknown and is the goal of this evaluation.
Sample of Findings in Concrete Evaluation:
The flow path of water or void space under the concrete/asphalt surface is apparent in GPR data, appearing on the amplitude maps as blobs of increasingly low negative amplitude signatures. There are a substantial number of these anomalies in the amplitude maps of the GPR response. Since the nature of these anomalies seems to be isolated and because void formation could occur by other processes such as settlement after the flood water had retreated, additional investigation with test borings is required. Three basic types of GPR anomalies are found under the asphalt and concrete surfaces:
- Narrow linear anomalies trending parallel to many of the expansion/cold joints.
- Large multi-layered blobs of increasingly negative amplitude directly below the asphalt and at depth.
- Linear anomalies following the backfill trenched areas of existing utility excavations.
The backfilled areas of the utility piping in the parking lot show consistent evidence of subsidence from water following through these trenches to the river. The survey data lines that were collected 90 degrees to these pathways reflect consistent evidence of erosion and undermining.