Void Detection in Dam Spillway

Aim

Spillways are critical safety features of dams, designed to safely pass large volumes of water during flood events. Because they are constantly exposed to high flow velocities and pressures, any leakage through cracks or joints can lead to subsurface erosion and void formation beneath the slab. If left unchecked, this undermining can compromise the structural integrity of the spillway and, in turn, dam safety. 

This project investigated a 70-metre-long spillway to determine whether voids had developed beneath the concrete slab and to assess the condition of reinforcement within the structure.

Approach

A non-invasive Ground Penetrating Radar (GPR) survey was carried out using a high-frequency 1 GHz antenna to detect subsurface anomalies. The GPR scanning was conducted at 100 mm line spacing across the full length of the spillway to maximise resolution and minimise uncertainty. In total, 162 survey lines were collected and grouped into seven grids for efficient data processing and interpretation. 

Potential anomalies identified by GPR were then targeted with invasive confirmation tests such as drilling and probing to verify the presence of voids and to measure their extent more accurately. This combined non-invasive and selective invasive approach allowed for rapid coverage while reducing unnecessary disruption to the structure.

Results

The GPR survey revealed several potential voids beneath the slab, most often associated with visible surface defects such as cracks, joints, and damaged concrete. These features are typical pathways for water ingress. Subsequent invasive testing confirmed the presence of voiding in these areas. 

The survey also mapped reinforcement patterns across the spillway, highlighting zones of variable bar spacing and sections where reinforcement was absent. This information is crucial for future structural assessments and rehabilitation planning.

Outputs & Value

The investigation demonstrated the value of combining high-resolution GPR scanning with targeted verification: 

• Early Detection – Voiding beneath the spillway slab was identified before it reached a critical stage, reducing the risk of sudden failure. 

• Efficient Planning – GPR allowed for rapid, cost-effective scanning of the entire structure, ensuring resources were focused on areas of concern. 

• Actionable Insights – Reinforcement mapping provided operators with key data for prioritising maintenance and strengthening works. 

• Improved Dam Safety – By identifying hidden defects and areas requiring intervention, the project supported proactive asset management and reduced long-term risk to the dam and downstream communities. 

  
  

This project showed how advanced geophysics, when integrated with targeted invasive testing, provides dam operators with a reliable, efficient, and non-disruptive method for safeguarding critical spillway infrastructure.