provides developer with a wealth of information that a traditional ground investigation alone just cannot provide

Zetica carry out a non-intrusive GroundCheck® survey at a former train station.   The work was commissioned as part of a Phase I investigation for the site in order to provide valuable information for the design of the follow-on Phase II intrusive investigation and later development of the site.

A Phase I desk study had revealed the potential presence of a range of buried structures including voids as a result of existing tunnels or basements. These needed to be mapped and verified before development could take place.

The GroundCheck® survey was commissioned to identify the location of these and other buried features. Zetica proposed a combination of geophysical techniques including fixed frequency and time domain electromagnetic profiling to identify buried structures, foundations, disturbed ground, infilled ground and some utility services.  A magnetometer technique was utilised to detect ferrous targets such as reinforced concrete structures, underground storage tanks, pipes, and metal scrap materials.  Ground penetrating radar (GPR) was also used to detect potential voids and provide further detail on the origin and nature of some of the buried structures.

The GroundCheck® survey revealed a plethora of buried features, the majority of which a conventional Phase II intrusive investigation alone would have completely missed or at best provided partial and possibly misleading information.  The detected features included foundations relating to the former station infrastructure (e.g. platforms), zones of different fill materials, reinforced concrete structures, utility services and tunnels and basements rooms.

Figure showing depth to GPR reflectors representing
infill areas beneath a car park

The tunnels and basements were of particular concern as they can represent a risk to heavy plant during demolition works. Some of the tunnels detected by the GroundCheck® survey had already been identified as part of a site walkover inspection, but the survey identified other tunnels that were not expected. For example, an off-shoot tunnel was identified by the GroundCheck® survey and confirmed by later visual inspection which identified that the tunnel entrance had been blocked off.

The GroundCheck® approach to mapping the subsurface at this site gave the customer a better appreciation of the distribution of buried structures which informed the planning of the Phase 2 intrusive works and which taken together with the Phase 2 results will provide a much more complete picture of expected ground conditions than that which could be expected from a conventional Phase II intrusive investigation.

Combining geophysical methods to provide more information

Zetica is routinely commissioned to undertake geophysical surveys to characterise soil types. An electromagnetic (EM) ground conductivity meter is often specified for such surveys because clay rich soils are typically more conductive than sandy soils.

Measuring natural gamma radiation can give an independent indication of clay content. The method utilises a scintillometer to detect gamma rays emitted from a variety of naturally occurring radioactive elements. The three main elements that produce the observed gamma rays are Potassium, Thorium, and Uranium, all of which can occur naturally in clay minerals.

The interpretation of EM and natural gamma measurements on their own are affected by non-uniqueness. Ground conductivity can vary due to changes in either clay content, contamination, or water saturation. A low clay content soil with high water saturation can have a similar conductivity to a high clay soil with lower saturation. Agricultural practices such as the application of potassium rich fertilizers can result in high gamma ray counts in areas of low clay content.

To overcome this non-uniqueness Zetica now routinely combines these two methods. Where conductivity and gamma ray total count change together, there is good evidence that changes in clay content are being observed. Where only one of the measured properties is changing, the changes are attributed to other things.

This has allowed Zetica to map soil properties and interpolate between borehole logs with greater confidence.

Figure showing ground conductivity contours overlaid on a colour-coded total count radiometric map with interpreted soil types

Tip of the month

Zetica has commenced with a monthly series of short tips on making the most effective use of geophysical techniques to solve underground mapping objectives. The first two dealt with limits of detection of UXO using magnetometry and limits of detection of buried services using GPR. These provide useful insights into the optimal deployment of tools that are all too often misused or ignored altogether due simply to a lack of understanding.  Let us know if you or any of your colleagues might be interested in receiving this.

Upcoming rail radar publication

Zetica Rail will be presenting a paper entitled: €śPractical applications of GPR surveys for trackbed characterisation in the UK, Ireland, USA and Australia€ť at CORE 2010 in Wellington, New Zealand. The paper summarises state of the art approaches to rail radar trackbed investigation and outlines systems which could be used to program track reconditioning. Click here if you are interested in receiving a copy when it is published.

Twitter  

Let’s be honest, many engineers would not associate social networking media with useful sources of industry information. Our initial foray into the world of Twitter has produced an excellent response from like-minded companies and individuals looking to share thoughts and news in a dynamic and friendly forum. If you aren’t already following Zetica take a look at:

www.twitter.com/zeticaltd (for information on general uses of geophysics to map buried features and monitor structures and related news from across the world)
www.twitter.com/zeticasitesafe (for information on managing the risk of UXO)
www.twitter.com/zeticarail (for news on trackbed maintenance related topics from across the world)

Lunchtime Seminars

Zetica offer a popular seminar on the uses and abuses of geophysics. Engineers are brought up to date on the latest geophysical methods available in the market place and interesting areas of research and development. The presentation normally lasts 45 minutes and is case history-based with a 15 minute discussion session following.

Click here to email Ellen Maskell to discuss your requirements and arrange a seminar at your offices.