IDENTIFYING SUBSURFACE VOIDS WITHIN DAM SPILLWAYS

OBJECTIVES

TRACE was instructed to undertake a non-intrusive structural investigation of the upstream and downstream spillways of a concrete dam to determine the presence and location of voids beneath the slabs.

SOLUTIONS

TRACE carried out investigations of two spillway areas:

• Upstream spillway: approximately 271 m²

• Downstream spillway: approximately 336 m²

All accessible areas of the spillways were scanned in a single direction using large-array GPR, providing continuous coverage across the slabs. Infill scanning was undertaken in constrained locations, including beneath scaffolding and at the top of the downstream spillway, to maintain coverage where larger antenna systems could not be deployed. Targeted intrusive works carried out by the client were used to support calibration and interpretation of the non-intrusive findings. Data was processed off-site by experienced TRACE personnel and independently quality checked.

KEY FINDINGS

The investigation identified areas of possible and potential voiding directly beneath the underside of the spillway slabs. Voids were categorised based on reflection characteristics, with some areas showing responses more strongly indicative of voiding and others affected by factors such as reinforcement density or data noise. The results were mapped alongside key construction details to support informed interpretation and future decision-making.

BENEFITS

The investigation provided non-intrusive detection of subsurface voiding across large spillway areas, reducing the need for blanket intrusive investigation. Clear identification of potential and possible voiding, alongside transparent communication of areas where detection confidence was reduced, improved understanding of slab construction and condition. This supported more targeted intrusive verification and reduced uncertainty in asset management and maintenance planning.