Detecting Subsidence and Voids

Subsidence is the label given to the movement of a layer or a collection of layers of material, due to an underlying factor. It is often seen as the ground ‘sinking’ or sloping in one direction, usually towards its cause, which may be an underground void or sinkhole.

A void is usually an area of empty space, but it could be an area of less dense material which is not capable of providing structural support. Voids are often the result of subsidence over a period of time: as material is washed away (perhaps into a sinkhole) or chemically altered, the surrounding material moves to fill the gap causing the ground to sink (subsidence). If the material above the subsidence layer is strong enough to support the ground, the ground will not sink but will instead a void will grow beneath it.

GPR is a low impact and fully non-destructive technology that can be used to reliably detect and assess voids and subsidence without the need for destructive investigation.

GPR is not able to provide all the information about the below surface environment, but it can accurately position stratigraphic layers of materials and create a slice through them to understand the physical characteristics of the ground below. It can be used to provide broad information about large areas, allowing intrusive investigation to be focused in key locations.

This survey was conducted on a chemical plant where underground pipework is suspected of leaking resulting in both material washout and a chemical attack on the surrounding soils. Data was collected and the layer stratigraphy mapped to create a 3D ‘net’ highlighting the locations of subsidence and in some areas indicating its source. To clearly identify this information on a plan view, we produced a contour map showing the extents of the subsidence and highlighting ‘shallow’, ‘medium’, and ‘deep’ areas.

In another example, we were hired to survey a concrete spillway to detect potential voids which may have opened beneath its construction. The survey conditions were quite poor and the ground itself was saturated. We were able to confirm that there were no voids present beneath the upper part of the spillway, however due to local conditions (the spillway itself was not properly cleared in advance of the survey and contained vegetation and standing water), the lower half of the spillway could not be surveyed.