GPR survey maps voids under a dam spillway
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GPR survey maps voids under a dam spillway


spillway is “a structure used to provide the controlled release of water from a dam or levee downstream, typically into the riverbed of the dammed river itself” (Wikipedia, see image above). These structures endure various degrees of wear and tear from water flow and therefore, must be periodically inspected for structural integrity and safety.

KGS Group, headquartered in Manitoba, Canada, provides spillway inspections, among other engineering inspection services. A recent inspection project completed for a dam owner in Manitoba involved an investigation of a spillway to assess the condition of the structure after record flooding earlier in the year. The goal of the inspection was to look for issues that could affect the performance and safety of the structure, and to identify which issues needed to be addressed as part of a rehabilitation plan.

As part of this investigation, a GPR survey of the spillway chute was completed to identify potential voids; areas where there may have been a loss of granular material in the drainage blanket below the spillway concrete slab (see Figure 1).

Figure 1
The spillway slope. The joints visible in the concrete slab are areas where water can infiltrate and wash away the supporting granular material.

The GPR data was collected on the spillway slope with a NOGGIN® 500 GPR system with external GPS for positioning (Figure 2).

Figure 2
NOGGIN® 500 SmartCart used for the spillway inspection survey. An external GPS was added to position the GPR data.

A grid, approximately 30 metres x 70 metres (2100 square metres), was collected in both the X and Y directions across the spillway. A total of about 2,500 metres of data with a sample every 2 centimeters were collected, for a total of 125,000 unique sample points on the spillway. An external GPS was used for positioning; the paths of the GPR survey lines are shown in Figure 3.

Figure 3
Google Earth image showing the location of the NOGGIN® 500 GPR lines on the spillway.

The data were processed into depth slices using the EKKO_ProjectTM SliceView-Lines module. The depth slice at 35-40 cm, well under the concrete slab, shows high amplitude reflections, mostly located near and along the concrete joints (Figure 4).

Figure 4
Depth slice at 35-40 cm below the surface of the concrete slab shows high amplitude, red reflectors. Some are likely associated with the structure of the joints (see Figure 1), but others, away from the joints, are interpreted as possible voids under the slab.

The cause for some of these responses is thought to be the structure of the joints, which differs at the joints compared to the middle of the concrete slabs (Figure 5). Other high amplitude reflections, away from the joints were analyzed in more detail to try and understand what is causing them. Figure 5 shows a cross-section with a high amplitude response that was interpreted as a possible void under the concrete slab. Voids in spillways are commonly formed by water infiltrating under the concrete and washing away the granular materials.

Figure 5
GPR line crossing a concrete slab and 2 joints shows the structure of the slabs; raw data (top) and interpreted data (bottom). At a depth of 35 to 40 cm and from horizontal positions 35 to 40 metres, adjacent to a joint, high amplitude responses are visible, indicating a change in reflectivity at the bottom of the concrete slab. This type of response is consistent with a void. Areas like this became targets for cores to validate the interpretation.

Based on the GPR depth slices and cross-sections, 10 locations across the spillway were identified for the geotechnical coring program and visual inspection. The day of the coring, the NOGGIN® 500 system was used again, this time to collect smaller grids around each proposed coring location to avoid coring through the steel rebar in the concrete slab (Figure 6).

Figure 6
Smaller grids were collected at each coring location to map the rebar, so they could be avoided when coring the concrete slab.

Most cores were to check the structure of the joints but two were drilled specifically on high amplitude GPR responses (Figure 7).

Figure 7
Same image as Figure 5 but showing the locations of 10 cores (black dots). The results from the 2 cores, drilled specifically on high amplitude GPR responses, are indicated.

The GPR survey on the spillway proved to be very successful. The two cores drilled on GPR anomalies were both confirmed to be problem areas beneath the spillway slab:

1. One 5 cm void that looked to be approximately 5 metres wide will need to be remediated, and
2. An area of standing water, due to an ice layer, means the drainage blanket under the slab will need to be improved.

Currently, KGS Group is still in the detailed design phase of the project and will be developing a remediation plan and measures to protect against further and future void formations.

Story courtesy of Jonathan McInnis, KGS Group, Winnipeg, Canada.

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