Aim

By their very nature, dam spillways are exposed to large quantities of flowing water. Any leak in the spillway can thus lead to subsurface erosion and voiding, compromising the integrity of the spillway. This project aimed to investigate a 70-meter-long spillway to ascertain if any voids were present beneath the concrete.

Approach

A high-frequency GPR (Ground Penetrating Radar) was used to scan the structure of the spillway non-invasively and map areas of potential voiding. Targeted invasive drilling was later used to confirm the locations of the voiding, and to more accurately determine their size and extent.

A 1 GHz GPR was run vertically at 100mm spacing along the entire length of the spillway. By employing such a small spacing between GPR lines, uncertainty in results was minimised and even small void features could be captured. In total over 162 profile lines were measured, with the data split into 7 grids to enable ease of analysis.

Results

Several potential voids were identified underneath the spillway slab and were later confirmed with invasive testing. The voids generally coincided with slab features such as cracks, joints, and damage to the slab, where water is likely to infiltrate through the concrete. The GPR scan also allowed the mapping of reinforcement throughout the spillway, including variations in the spacing of the bars, and areas where no reinforcement was present.

Outputs & Value

The GPR scanning along the spillway allowed for fast and efficient identification of voiding and damage to the structure. The investigation indicated several potential voids present underneath the concrete slab. From the GPR results, targeted invasive tests could be planned to accurately determine the extent and damage from the voids. The mapping of reinforcement throughout the spillway additionally identified areas where further engineering assessment and reinforcement were required.

Contact our Geophysics Team
geophysics@coco.co.nz
Wellington (+64) 04 472 7282
Christchurch (+64) 03 365 5960