Project background

A range of non-intrusive methods can be used to investigate built structures. In particular, ground-penetrating radar (GPR) has proved to be a powerful and versatile technique in structural investigations. High-frequency, high-resolution 3D GPR surveys provide full continuous coverage and detailed information on the internal structure of concrete and other materials, enabling their construction, reinforcement details and flaws to be imaged in situ without coring or breaking out. In this study, a concrete spillway was examined as part of a wider investigation into its failure.

Toddbrook Reservoir is located in the High Peak area of Derbyshire, England, and contained the tallest dam in the UK at the time of construction. In the 1970s, a concrete auxiliary spillway was constructed on the downstream face of the dam. The spillway suffered significant damage in August 2019 after an intense rainfall event led to severe overtopping and flows that caused failure of the concrete slabs, resulting in undermining and partial collapse into underlying voids. As a result of the incident, the owners (Canal and River Trust) commissioned RSK Geophysics to investigate the spillway construction details and the existence of voids.

Geophysical survey and findings

A GPR survey was undertaken across all accessible parts of the spillway using a range of frequencies, from 400 MHz to 2 GHz, to provide high-resolution data to depths of up to 2.5 m. This provided a permanent record and was capable of imaging multiple layers of construction detail (location of internal metalwork or reinforcement) and voids within and below the slab. Because of the the works taking place on a steep slope, personnel carried out the survey using rope access. The high-frequency GPR data revealed that the slabs were generally quite thin, of variable thickness and reinforced but with rebar only generally placed at the bottom of the slab.

Figure 1: Collapse of auxiliary spillway concrete panels. Note the undermining.

Figure 2: GPR results showing variation in rebar cover across the spillway (contour plot with reds deeper and blue colours shallower)

Figure 3: GPR radargram showing output from mid-frequency 400MHz scan

Figure 4: Plan of potential discrete void features under the spillway using GPR.