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In our most extensive intrusive survey of 2023, Brimstone successfully completed 1,106 probes to cover the expansive footprint of a site located in Northfleet, Kent.

Brimstone deployed a team of supervisors, operators, and engineers to Northfleet, Kent, to carry out an intrusive UXO survey using our advanced Cone Penetrometer Testing (CPT) rigs. The objective was to identify any potential buried UXO ahead of ground-intrusive works planned by our client.

The selection of an intrusive magnetometer survey as our methodology was driven by the substantial scale of clearance required, due to the size of the site and the proposed widespread piling and ground works. Following a period of consultation with the client, it was determined that the best course of action would be a matrix-style clearance approach.

A matrix clearance differs from a ‘conventional’ intrusive survey because the entire footprint of the site is scanned and tested for buried magnetic anomalies, rather than just specifically the proposed pile locations. However, despite this change in approach, our surveying methodology remained the same. We used the rigs’ power and weight to push a magnetometer cone, with a 2m metre triaxial radius, into the ground until we reached the maximum bomb penetration depth.


Meeting client expectations

To align with the client’s construction programme, we initially deployed two rigs to the site. Later, we increased the deployment to three rigs, meaning we had Redfox, Handsome, and Honey Badger all operating simultaneously, a rare occurrence!

Our dedicated team completed all 1,106 probes within a five-week timeframe, which also included 15% of all positions being pre-bored and retested following initial probe refusal at shallow depths. Our commitment to delivering this project quickly and on time also meant there were no additional costs incurred and no hidden surprises in comparison to the original tender sum.

Following field data collection, our Head Office in Chatham processed the data through a series of steps conducted by our in-house geophysics team. This comprehensive process aimed to reduce noise and enhance survey quality. In the latter stages of data processing, a specialised computer model was employed to analyse the signals measured by the equipment.

Northfleet uxo survey

Providing site-wide ALARP clearance

Post-survey and data analysis, no anomalies were detected within wartime ground. Given the site’s 5 metres of post-WWII imported made ground and the fact that we matrix cleared the entire area, we successfully provided site-wide As Low As Reasonably Practicable (ALARP) clearance to maximum bomb penetration depth.

This allowed our client to proceed with their development without the need for additional UXO intervention, regardless of the chosen ground intrusion technique or foundation design.

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