Using GPR For Flood Damage

In the spring of 2017, southern Quebec, Canada had unprecedented rainfalls and widespread flooding. Rivers in the Montreal area burst their banks and submerged communities. In one community, a local pedestrian pier was completely submerged under three feet of water for two weeks.

After the flood waters subsided, there was visible damage to the pier. There were several areas on the pier where the interlocking brick walkways had collapsed, indicating the presence of voids. Inspection of the vertical walls of the pier revealed cracks, further increasing concern that additional structural substrate had washed away. Local municipal officials were concerned that the pier may have more voids that could collapse, causing injury to pedestrians. The municipality contracted a Quebec-based geophysical service provider to scan the pier and report any problem areas.

The contractor had initially considered using electromagnetic induction to look for the voids. However, there were many metallic obstacles on the pier, including garbage cans and benches, that would interfere with the results. Instead, they decided to use GPR since the results would not be impacted by these metallic objects.

(Figure 1) Aerial view of the pier showing many obstacles.

Given the many obstructions and the odd shape of the pier, collecting GPR data in an XY grid pattern would be very difficult (Figure 1). Instead, the contractor decided to collect the data using GPS for positioning the GPR data. This would allow them to cover the full area of the pier much faster than laying out grids. Data was collected in a series of tightly spaced straight lines, using marks on the pavement to ensure consistent spacing, averaging about 18” (0.5 m) between the lines (Figure 2).

(Figure 2) GPR data collection path. GPS positioning allowed for faster and more complete area coverage than an XY grid.

With two technicians onsite, a total of 12,500 feet (2.36 miles or 3.8 km) were collected in just four hours.

Once data collection was completed, they generated depth slices through the pier. The contractors knew that the large boulders below the pier, used as the main structural component of the pier, would not have been washed away by the floodwaters, but they were very concerned that the shallower parts of the pier underlain by finer sands and gravels could have been removed by the flooding.

(Figure 3) This one-foot depth slice shows the already collapsed areas and additional areas of possible voids as strong GPR reflectors displayed as red and yellows.

When reviewing the depth slices, high-amplitude GPR reflections can be an indication of voids. This occurs because air or water-filled voids provide a large contrast with the material above, creating a strong GPR reflection. Figure 3 shows the one-foot depth slice with strong reflectors in reds and yellows and weaker reflectors in blues and greens. The three areas that had already collapsed at the surface are indicated on the figure.

(Figure 4) The 5.5 foot depth slice shows strong reflections from boulders and cobbles deep below the pier, under the interlocking brick. These areas showed the deepest GPR penetration.

The GPR data shows some interesting phenomenon observed during the survey. For example, the deepest GPR penetration occurred on the parts of the pier covered with interlocking brick, while areas with concrete at the surface had much shallower penetration; this is seen in the GPR line in Figure 5. It is also shown by the strong (red) GPR signals on the 5.5-foot depth slice in Figure 4.

(Figure 5) Typical GPR line across the pier. GPR signal penetration varies depending on whether the surface was covered with interlocking bricks or reinforced concrete. GPR signals are stronger and penetrate deeper in the areas with interlocking brick at the surface due to the lower electrical conductivity when compared to concrete.

These observations are not surprising as concrete has relatively high electrical conductivity and attenuates the GPR signal before it can travel to depth. The sand, gravel, cobbles, and boulders under the interlocking brick have much lower electrical conductivity, allowing for the GPR signal to travel much deeper before it is attenuated.

Based on the GPR scan of the complete pier, the GPR service provider quickly identified the shallow areas with strong GPR reflections indicative of possible voiding and provided this in a report to the municipality. From the findings, the municipality targeted repairs to the key areas of concern on the pier. Where possible, voids were identified within two feet of the surface. They removed the interlocking brick and added fill to fix the shallow voids. To address any risk of voids deeper in the structure, they injected concrete into the pier wall where the vertical cracks were visible.

By using GPR, the municipality quickly and cost-effectively assessed the internal damage to the pier due to the severe flooding and was able to take corrective actions before any injury to the public occurred.

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