In this issue
Useful Links & Recent Publications:
Benefits of high speed GPR to manage trackbed assets and renewal strategies
PWI paper presented in Sydney, June 2006
Solution matrix for road or motorway applications
The US Federal Highways Dept has published a very useful matrix of geophysical and NDT methods for solving engineering problems during SI, construction and maintenance of road infrastructure.
WW2 bomb risk maps
Download risk maps for your region (28 regions across the UK)
Measuring the risk of detonating unexploded bombs
Web-based applet to allow users to input parameters and calculate an order of magnitude risk of detonation
Limits of detection of unexploded ordnance – are you getting what you paid for?
White paper on physical limits of detection for surface- and borehole-based geophysical methods
Statistical assessment of the risk of unexploded bombs
Paper published in Ground Engineering, May 2006
UXO frequently asked questions
Download a list of FAQ demystifying various aspects of unexploded ordnance
Zetica Rail is now seeking the services of a Site Manager. The following applies:
- Minimum 5 years experience preferrably in a track renewals/ possession environment
- Engineering Supervisor/ COSS/Lookout/PTS
- Clean driving license
- Attractive salary and allowance for night shifts and weekend working
- Knowledge of survey methods preferred
Interested? Please email a CV and letter expressing your reasons to: firstname.lastname@example.org
Using geophysics to enhance safety in the nuclear industry
Geophysics has a unique and important role to play in the nuclear industry. Zetica is active monitoring existing structures, mapping geotechnical properties of shallow geological layers and characterising large areas prior to decommissioning or redevelopment to detect buried pits, services, underground storage tanks, unexploded ordnance and potential pathways for groundwater migration and contamination.
Structural survey case history:
The objective of the survey was to identify encased ducting and other structures in a reinforced concrete floor to ensure that such structures are avoided during the subsequent drilling operations. Examples of the results of the surveys carried out using ground penetrating radar (GPR) are shown below along with proposed location of the drill location.
3D view of GPR sections (vertical extent is 40cm, horizontal extent is 100cm) with time slice at 12cm depth and proposed drill location in green. Bright linear feature is an encased duct. Other linears shown are rebars.
Landsweep™ survey case history:
An integrated surface geophysical survey was carried out on a nuclear installation to map buried pits and services and to characterise depth to bedrock.
Example of apparent conductivity image (left) and interpretation (right) – pink colours represent relatively high ground conductivities
Example of modelled 2D true resistivity (top) and interpretation (below) showing depth to sandstone bedrock, and possible fracture zone in the bedrock – pink colours represent relatively high modelled true resistivities
Click here to request advice or more information.
New tool to estimate depth of burial of an Unexploded Bomb (UXB)
In our last newsletter we announced that we would be making available a website applet to calculate the probability of detonation based on user inputs including estimated density of UXB/ha, no of boreholes/ha or pile grid separation.
An on-line UXB Probability Calculator (UXB-PC) was published in December 2006 and has become a popular hit with our customers who use the tool to quantify an order of magnitude of risk to help decide whether to proceed with any specific risk mitigation work. Click here to access Zetica’s UXB-PC applet.
An important additional consideration in determining whether a potential UXB offers a risk to development activities and whether a UXB clearance survey will be effective is the likely maximum depth of burial of a UXB.
Current techniques for deep bomb detection include MagCone and MagDrill. MagCone involves inserting a magnetometer in a cone which is pushed under hydraulic pressure (up to 20 tons) into the ground. Limitations of this method are ground conditions which prevent penetration of the cone to the required detection depths. The MagDrill method is an alternative technique for testing in dense soil conditions which would prevent utilisation of the MagCone method. MagDrill works by drilling a borehole in stages, inserting a magnetometer, taking a reading, withdrawing, drilling deeper, re-inserting the sensor etc. The latter is a slow process compared with MagCone and costs per probe are typically 4x that of MagCone.
Understanding the depth of burial of a potential UXB has a significant commercial impact and can also have a negative safety consequence if the incorrect search method is specified.
The most important factors controlling the depth of burial of a UXB are:
- Size / weight of bomb
- Contact speed and angle
- Presence / absence of surface structures eg reinforced concrete slab
- Thickness and composition of any Made Ground
- Geological materials
The following table extracted from official MoD guidance provides a measure of average UXB bomb penetration depths uncorrected for geological material type.
Table showing average depth of penetration (A) of various bomb sizes assuming the following:
- High level release (>5,000m) with a velocity impact of 340m/s
- The soil is homogenous – wet sand, dry chalk or dry clay
- That the UXB is not fitted with retarder devices
Our new interactive website applet, UXB-DC (UXB Depth Calculator), will be published in May 2007 and allows users to select a bomb size and the type and thickness of up to 3 geological layers to determine bomb penetration depth.
Using UXB-DC it can be shown, for example, that a 250kg bomb could have penetrated a shallow dense gravel layer up to 6m whereas the penetration distance into a gravel layer which starts at 8m depth may only be 2m. MagCone would be ideal in the second case but unsuited to clearing an UXB in the first case.
Click here to request advice or more information.
Integrated Topography, Utilities and Landsweep™ surveys: Case history – airfield site proposed for development
Zetica is more routinely conducting integrated surveys. A recent survey for a former military airfield site included topographic surveys (including internal building surveys), Landsweep™ and Utility services detection surveys, ground investigation of anomalies, boreholes and trial pits, and an asbestos survey.
Client reference (April 2007):
“Zetica’s ability to provide a multi disciplinary range of ground investigation services has helped us no end. Zetica have carried out a range of services for us including building measured surveys, large scale topographical surveys, and below-ground services and features/obstructions mapping. These services enable us to ‘de-risk’ our projects by finding out more about what is on the ground (and in particular what is under the ground) at our sites, before contractors find the problems for us. The cost of comprehensive surveys is small compared with the re-design, delay and disruption caused by project delays when unforeseen obstructions are encountered. The health and safety advantages of below-ground mapping are obvious.”
Mike Sainsbury, Zetica MD adds, ‘The cost savings in outsourcing site-mapping services to one company can be considerable. Also as co-ordination of different mapping service providers is not required by the client, Zetica reduces the amount of time the client needs to spend directly involved with suppliers. Zetica offers in-house specialist desktop studies, detailed topographic surveys, Landsweep geophysical surveys, specialist UXO surveys, detailed utility surveys and engineering geophysical surveys. Our multifunction crews are trained to carry out all site work, thus saving on mobilising separate teams.’
Click here to request more information.
Progress on high speed rail GPR contract for Network Rail
Zetica's contract to scan 8,000km of track in the United Kingdom for Network Rail with ground penetrating radar (GPR) systems mounted on an ultrasonic inspection train is proceeding to plan with over 4,000km scanned since January 2007.
Track engineers are being provided with standard layer interpretation output with 1/2mi per A3 page including radargrams, layer interpretation and ballast depth exceedance plots. These plots are generated automatically from Zetica's ZARP (Zetica's Advanced Rail Processing) package from layers semi-automatically picked by technicians. Approximately 100km of GPR data is processable from raw data to interpretation and output linked to track location per technician per shift.
Example of automatically generated output for GPR data collected at 50kph
Zetica CEO, Asger Eriksen, says 'After initial tests to determine what our customer required as output, we have reached our planned output productivity levels. This fast turnaround of rail GPR data is unprecedented and offers our customers the most cost effective value per mile of GPR data scanned at 5cm horizontal resolution.'
Network Rail uses the results to validate ballast renewal proposals, design site investigations for track renewal programmes, investigate embankments and detect cross-cutting services as well as for several other purposes. This demonstrates the potential value of desktop GPR data for contributing to a track engineer's understanding of the subsurface conditions.
Ben Venables, Zetica’s Rail Manager, stresses that there is much more to come from ZARR. ‘We have only touched the surface of realising the full potential of GPR surveys to monitor trackbed. We are currently introducing tools to facilitate measurement of ballast quality and automatic recognition of features such as subgrade erosion. Combining this information with track geometry measurements will allow novel metrics to be devised to extract maximum benefit from trackbed monitoring’.
Click here to request more information on ZARR.
Rail Services Manager:
Copyright © 2007
Utilities and drainage surveys for the rail industry – case history
Zetica Rail was employed by Grant Rail to undertake a track drainage mapping investigation of four lines approaching Kentish Town Station, on the Midland Mainline just outside St Pancras, London.
The works were undertaken during several weeknight and weekend possessions. This included the mapping and scheduling of over 90 catchpits, tracing of buried drain systems using cat scan and sonde, supported by CCTV, and the evaluation of drainage condition using CCTV to identify the system outfalls and any specific blockages.
The output to the client was a site plan, a drainage condition assessment plan, CCTV footage of proven outfalls and pipe blockages, and catchpit schedules, all tied to a common TBM.
Example of catchpit schedule (above) and still image from CCTV video (below)
A typical work sequence includes:
- Visual site inspection to evaluate existing drainage system, working practice restrictions and other site risks
- Develop work plan and method statement and undertake risk assessment
- Investigate visible drain system using CCTV to establish condition, visible blockages, drainage spurs etc
- Define drain depths using Cat scan and sonde, tied into rail and invert level survey
- Trace drainage beyond blockages using GPR antennae to identify pipe position
- Trace drainage covers using EM scanner (to detect metal grids of catchpits)
- Topography survey of all catchpits and key rail features, such as bridges, rails, junctions etc
- Plot topography results and produce level cross sections of drain system
- Write a report with recommendations on remedial work priorities
The recent change in CDM regulations has placed increased responsibility on clients to undertake additional site evaluation for risk features, such as buried services. Traditional methods have relied upon data provided by utilities companies which is often incomplete.
Ben Venables, Zetica Rail Services Manager, says ‘With the changing requirements of the new CDM regulations, Network Rail is responsible for the supply of pre-construction Health & Safety information to all contractors. Thorough services mapping including desk studies and on-site detection are vital to CDM compliance.’
Zetica Rail are in a unique position to be able to offer integrated railway site investigation services including desk studies and on-site verification using non-intrusive scanning including GPR, Cat scanners with detectable sonde and remote control CCTV units.
Contact Zetica Rail to discuss your requirements.
Technical note – Detection of ferrous and non-ferrous UXO
It appears that an increasing number of attempts to win unexploded ordnance (UXO) clearance work are bordering on the desperate. How else can one describe methodologies designed not to clear all possible UXO as requested by the customer but only a subset usually unbeknownst to the customer who believes they are buying the required all-in service? At best ignorance may be to blame for the wrong tools being used – at worst contractors are guilty of blatant misrepresentation. Both are inexcusable breaches of client trust.
Common examples of the above are magnetometer surveys recommended to ‘clear’ an area of possible UXO which includes non-ferrous ordnance.
An example of items found by Zetica in an area ‘cleared’ by a UXO clearance contractor using a magnetometer only. All items were missed by the original survey but detected using a time domain electromagnetic scanning system.
Buried ferrous ordnance can be effectively located by measuring distortions or anomalies in the earth's magnetic field. Modern magnetometers provide the high sensitivity and sampling rates necessary to acquire data effectively. The only disadvantages of magnetic surveys are that they detect only ferrous objects and probably suffer more from site-specific noise interference than electromagnetic surveys. Electromagnetic surveys respond to all metal types and can provide diagnostic information on target geometry but have much lower detection depths.
Even if the UXO target has a theoretically detectable ferrous content it may not be efficient to search using a magnetometer due to the effect of ground conditions. This is demonstrated in the following figure which shows how clearly a mortar is detected with a time domain electromagnetic (EM61) instrument (left) as opposed to the ‘noisy’ results of the magnetometer method (right). The latter would result in an increased risk of targets being missed in the follow-up phase and many false anomalies.
EM61 survey to locate buried mortar target
Magnetometer survey to locate buried mortar target in ‘noisy’ soil conditions
In these circumstances it is usual to conduct a trial at the start of the detection phase to determine the effect of ground conditions on the detectability of UXO.
In this column we’ve recommended before that all UXO ‘clearance’ contractors should be obliged to provide detection assurance levels for UXO of expected size and estimated range of burial depth. This can easily be achieved (see article published in December 2006).
Click here to request more information.
Click here to access Zetica’s white paper on the physical limits of detection of surface- and borehole-based geophysical methods.
Open Day – Autumn 2007
Zetica will host an Open Day in early Autumn to raise awareness of the latest developments in site investigation. Our last open day was a great success with 30 delegates attending.
“[The open day] was very informative. It was good to see how geophysics has moved on since I started out in the industry.” Jim Twaddle, Principle Environmental Engineer, JNP Group
“Thank you for the informative day on Wednesday. I fear that I may not be allowed to come to the next one – I suspect that that privilege will go to somebody more senior next time!” Neil Esslemont, Inspection and Assessment Engineer, Trans4M
This year’s event will be split over two days – the first concentrating on mitigating the risk of buried hazards on brownfield sites and the merits of a one-stop mapping service for development projects. The second day will focus on advances in geophysical methods for the rail industry.
Two lucky attendees on each day will win a flight over the local Cotswold area in a light aircraft. Don’t miss this and other surprise activities planned.
Click here to register your interest in attending this event. You will be kept informed of the programme and dates by email in due course.
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 Stevens to discuss your requirements and arrange a seminar at your offices.