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.

How accurate is SUMO's underground utility survey and what types of utilities can it locate?

Using cutting-edge technologies like electro-magnetics, signal induced threading, and Ground-Penetrating Radar (GPR), our underground utility surveys accurately detect a wide array of buried utilities including metal and plastic pipes, drainage systems, electricity cables, telecoms, and fiber optic cables. The methodology ensures high accuracy without the need for invasive trial holes.
What is the turnaround time for obtaining results with SUMO's Underground Utility Detection service?

SUMO’s service is built for rapid response. Contractors can receive immediate results onsite. Our equipment provides real-time information, and once we leave the site, the detected utilities are visibly marked on the ground surface for quick reference.
Does SUMO provide detailed mapping of the detected utilities for permanent records or construction purposes?

Absolutely, SUMO not only marks the detected utilities on-site but also offers high-quality, color-coded survey drawings. These drawings can include topographic features and a comprehensive record of the utility detection survey, ensuring it meets the client’s needs for future reference or pre-construction design works.
Topographical and Geophysical survey

In addition to mapping out utilities, our experience team can accurately survey the land, creating topographical drawings for use. With our state-of-the-art aerial technology, nothing is missed.
What additional information can be included in the underground utility mapping service provided by SUMO?

In addition to topographic features, SUMO’s mapping service can include fixed surface utility features such as inspection chambers, valve boxes, and telegraph poles. It can also incorporate key topographic details like building lines, fence lines, kerb lines, street furniture, street names, building names, and house numbers, facilitating easy site location.
Does the utility mapping service by SUMO include standard referencing and compatibility with existing survey data?

Yes, SUMO ensures the utility mapping references the ordnance survey grid and level datum as standard. Additionally, the drawings can be supplied to a specified grid and level datum and can overlay an existing topographical survey, making it a versatile and adaptable service for various project needs.
What sets SUMO Services apart from other survey providers?

SUMO stands out due to our impeccable level of service with nationwide coverage, state-of-the-art equipment, and our commitment to delivering top-quality, detailed colour-coded survey drawings in both hard and electronic formats.
How do I request a survey quotation from SUMO Services?

Requesting a quotation is easy. Just click on the "Request a Survey Quotation" button on this page, and our team will get in touch with you promptly.
Can I book a SUMO Survey online?

Yes, you can! Simply click on the "Book a SUMO Survey" button on this page, and you can schedule your survey with us at your convenience.
What type of survey is suitable for my project or site?

Our experienced team will work with you to determine the most appropriate survey for your specific needs. Contact us, and we'll assist you in making the right choice.
Can I obtain both digital and hard copies of survey drawings?

Absolutely. We offer the convenience of electronic copies for digital use and hard copies for your records. Just let us know your preference.
Do you provide custom survey solutions for unique projects?

Yes, we understand that every project is unique. Our team is highly adaptable and can tailor our surveying services to meet your specific requirements.
How do I get in touch with SUMO Services for further questions?

For any additional questions or inquiries, feel free to contact us through the contact form, and our dedicated team will assist you promptly.