GEOPHYSICAL
ENGINEERING

Sinkholes

Sinkholes have been appearing throughout Britain at an alarming rate in recent years. Following heavy rainfall, the ground becomes saturated and as the water levels recede, they can wash out material, leaving substantial life-threatening voids to appear without warning.

Some rock types are more susceptible to the formation of sink holes, with carbonate rocks including chalk and limestone being particularly sensitive, as they can be easily dissolved by groundwater. Furthermore, if the rock is overlain by clay material, this forms a configuration that can produce a large void that suddenly collapses when the clay layer above fails.

Less dramatic sinkholes also occur where the overlying material is more porous and slowly ingresses into the underlying void resulting in a gradually deepening depression over time.

With chalk prevalent in England’s South Eastern counties, along with areas of limestone in Wales and Northern England, the impact of sinkholes can be significant, both in terms of risk to life and financial cost.

So, if a sinkhole suddenly appears, or you have an unexplained depression on your site, you urgently need to know the extent of the problem, its likely cause and the risk of more incidents.

Thankfully, SUMO has the answer to this problem:

SUMO Geophysics can complete a full geophysical survey of the area, using ground conductivity and/or ground penetrating radar.

Electromagnetic ground conductivity surveys measure ground conductivity by the process of electromagnetic induction. A current is induced into the ground to generate a response from the sub-surface. This is done utilising a transmitter and receiver coil spaced at a fixed configuration, with different operating frequencies providing a range of depth penetration and resolution for different applications and can be used to identify anomalous features underground, such as saturated ground and landfill. The EM31, a particular ground conductivity system, operates at an intermediate frequency and is ideal for locating discrete features such as sinkholes.

Above : Ground Conductivity Survey to locate sinkholes using a Geonics EM31.

The data is digitally recorded on site for later post-survey processing and interpretation. The data is processed with specialist software to produce contour plans. The contoured data is then analysed in detail to identify anomalous features relative to the general background and once identified, the anomalies are correlated with local ground conditions.

Survey results are then presented as plans tied in to site or OS co-ordinates, in AutoCAD and PDF format.

The example below shows the results of the ground conductivity survey, following the appearance of a hole in the middle of a school playing field. The survey identified a notable high conductivity anomaly, which has been interpreted as occurring where saturated gravels are present over the underlying chalk stratum. It is likely that groundwater within the gravels has seeped down to the chalk and is generating the formation of solution features. Furthermore, as this groundwater flow in the gravels is likely to continue, then further solution features are likely to occur and with them, further sinkholes.

Ground Penetrating Radar (GPR) surveys work by pulsing electromagnetic waves into the ground and measuring the strength and time delay of the returning signal, which also allows it to approximate the depth of the underground anomaly identified. It has capacity to work through a wide variety of surface materials, from soft landscaping, through to hard surfaces such as tarmac and concrete both inside and outside buildings..

Above : GPR survey using GSSI Radar system and 400MHz antenna.

The data is collected along set grid spacing and displayed as a section view or profile of the ground. Large or obvious anomalies may be interpreted on site, but the data is typically analysed and processed in the office after the survey is completed. Each radar profile is abstracted and significant anomalies are classified. This is the primary source for producing the 2D interpretation plot.

Above : Example radar data showing area and depth of anomaly likely to be voiding below the ground.

In addition to a manual abstraction, a computer analysis is carried out. The radar data is interrogated for areas of high activity and the results presented in a plan format known as timeslice plots using GPR Slice software. In this way it is easy to determine if the high activity areas form recognisable patterns.

Above : GPR timeslice plot showing the location of possible voids.

The use of a combined geophysics survey can not only identify the depth and location of an underground anomaly, but may also identify the likely cause of a sinkhole or unexplained ground depression, together with the potential risk of more such incidents occurring.

As such, given both the risk to life and the financial cost that such anomalies present, the low cost option of a geophysics survey is an excellent solution to mitigating such risks.