Checking for Voids Beneath a Dam Spillway, Before They Become a Dam Safety Issue

A spillway is one of a dam’s most important safety features. It safely passes floodwater when water levels rise, protecting the dam and the people, property and infrastructure downstream.

It also works in harsh conditions: high-velocity water flow, pressure, abrasion, and the constant risk of water finding its way through cracks, joints or damaged concrete. If water gets beneath the concrete slab, it can erode the material below and create hidden voids.

A void beneath a spillway is not a cosmetic issue. Left unchecked, it can undermine the slab, weaken the spillway and become a dam-safety concern. The difficulty is that this process happens out of sight, beneath the concrete. By the time signs appear at the surface, the problem may already be well advanced.

This article explains how Cook Costello used high-resolution Ground Penetrating Radar and targeted confirmation to check a 70 m spillway for hidden voids without breaking it open.

Why voids beneath a spillway are so hard to catch

The risk with voids beneath a slab is that they are hidden and progressive. Water can enter through a crack, joint or damaged section of concrete, erode the material beneath the slab, and gradually form a cavity in a place that cannot be inspected by eye.

For a dam owner, that is a difficult risk to manage. The failure mechanism is understood, but it is not obvious whether it is happening beneath a particular spillway without some way of looking below the concrete.

Cutting into the slab to check is destructive, disruptive and only tells you about the location opened up. What is needed is a way to scan the wider structure, identify where voids may be present, and then target confirmation only where it is needed. That is where geophysics provides a lot of value.

The approach: high-resolution radar, then targeted confirmation

Ground Penetrating Radar, or GPR, sends radar pulses into a structure and records the reflections that return. Changes in those reflections can indicate voids, defects, reinforcement and other features beneath the surface, without cutting into the concrete.

For this spillway, we used a high-frequency 1 GHz antenna, which provides fine resolution at the shallow depths relevant to sub-slab voiding. The spillway was scanned at 100 mm line spacing across its full 70 m length, producing a dense grid of 162 survey lines grouped into seven sections for processing.

Where the GPR identified a potential void, we then used targeted drilling and probing to confirm the anomaly and measure its extent.

That two-stage approach allowed the full structure to be assessed non-invasively first, with physical confirmation focused only where it was needed. On a critical structure, that means broad coverage, faster assessment and minimal disturbance to the asset.

What the survey found

The GPR identified several areas where voiding beneath the slab was likely. Most of these areas aligned with visible surface defects, including cracks, joints and damaged concrete. These are the kinds of features that can allow water to enter beneath the slab and erode the material below.

Targeted drilling and probing then confirmed voiding in the areas identified by the GPR, giving confidence that the geophysical interpretation was consistent with the physical evidence.

The survey also mapped reinforcement across the spillway. This showed where bar spacing varied and where reinforcement appeared to be missing, providing useful information for any future structural assessment, repair design or rehabilitation of the structure.

What it delivered, and why it matters to those responsible for dams

For the operator, the investigation turned a hidden, progressive risk into a mapped and more manageable one.

The voiding was identified before there was evidence it had progressed to a critical stage, while there was still time to plan intervention rather than react to a failure. Scanning the whole spillway quickly meant repair and monitoring resources could be focused where the evidence showed the problems were most likely, rather than spread across the structure or spent opening sound concrete to check.

The reinforcement mapping also gave the operator useful data for prioritising maintenance, structural assessment and strengthening.

Underlying all of this is dam-safety management. For a dam, hidden defects are not just an asset-protection issue. They are a public-safety issue because of the people, property and infrastructure downstream.

Why it matters beyond this dam

The same approach applies to spillways, culverts, retaining structures and other concrete infrastructure where hidden voids, delamination or reinforcement problems could affect performance or safety.

For a regulatory body responsible for dams and water infrastructure, a landowner with an on-site dam, or anyone managing ageing concrete assets, being able to assess condition non-invasively is a practical way to manage risk.

It allows the asset owner to scan the wider structure first, then focus intrusive testing, repair or monitoring where the evidence shows it is needed most. That matters on infrastructure that is expensive to repair, disruptive to take out of service, and serious if it fails.

As dam-safety expectations in New Zealand continue to increase, evidence-based condition assessment is becoming increasingly important.

Where we come in

Cook Costello carried out this investigation using our in-house geophysics capability. Because our geophysicists work alongside our geotechnical and structural engineers, the results were not treated as standalone anomalies. They were interpreted in the context of what they meant for the spillway, the structure and the next stage of assessment.

Checking critical infrastructure for hidden defects is exactly the kind of problem this approach suits: high-consequence assets, sensitive structures and situations where broad, non-invasive coverage matters before physical intervention is targeted.

If you are responsible for a dam, spillway or other critical structure and need to understand its condition, tell us about the asset. We can advise how a geophysical survey could help identify hidden defects, target further investigation and support evidence-based asset management.

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