Air voids that forms below ground are particularly hazardous to any development site. Buried vaults, crypts, culverts, sewers, sink holes and heating ducts are just some examples of why voiding can occur.
Geophysical survey can assist in locating the depth and extent of this voiding prior to any development. Some examples are discussed below.
Voids within sand
A low-level sewer was laid within deep sand deposits with a high water table. When the joints started to leak sand was carried into the pipe and removed by the flow. Over a period of time this continual removal of material at depth migrated to the surface causing collapses in the overlying roads and properties. An extensive low frequency radar survey was carried out to assess the route of the sewer for voiding which in time would produce more collapses. The radargram to the left demonstrates the nature of the anomalies formed by the voids. This one extends over some 5m and to a depth of 6m.
Voiding beneath a church floor
Burial vaults, crypts and heating ducts are some examples of why voiding can be expected beneath church floors. If these are dilapidated they can be a potential danger. As part of the re-ordering of a church in Oxford a radar survey was carried out to assess for voids. A radar survey located a number of voids which helped the engineers decide how best to approach the re-ordering .
Swallow holes investigated using ground conductivity
The appearance of a hole in the middle of a school playing field understandably caused concern to the local education authority who commissioned a survey to investigate the area for further swallow holes in the chalk geology.
The survey was carried out using both ground conductivity and ground radar. The plot to right shows the vertical quadrature data collected with a notable high conductivity anomaly present to the right of centre. The results were interpreted as high conductivity occurring where saturated gravels are present over the chalk. It is likely that groundwater within the gravels has moved down to the chalk and is aiding the formation of solution features. As this groundwater flow in the gravels is likely to continue then further solution features are likely to occur.