Searching for bombs is not an everyday requirement, but when your site investigation happens to include an old Ministry of Defence munitions and ordnance storage area, requirements change. However how these requirements are addressed is not something you will find in either standards or best practice guides.
Consultant Maunsell is currently facing such issues in its overwater ground investigation of Portsmouth Harbour. The work is being carried out for Gosport Borough Council as part of the Portsmough Harbour Waterfront project, funded by the Millennium Commission. This will provide new public access to the water- front, plus a waterbus network, connecting historic sites and museums to bring together the area's rich naval heritage.
Maunsell's work is predom- inantly on the western Gosport side of the harbour. Here Maunsell's client Gosport Borough Council is constructing a 3km promenade for pedestrians and a miniature rubber tyred people mover system.
Maunsell became involved after winning an open competition for the design of a new bridge over an inlet in the harbour known as Forton Lake. This is the largest structure in the project and connects the former naval sites of Priddy's Hard and Royal Clarence Yard.
Maunsell called in its geotechnical division to carry out a site investigation. Initially the investigation and bomb searches were to cover only its bridge site, with five boreholes proposed. However Gosport extended the investigation to 14 boreholes to include areas that required piling for other new construction - most significantly for waterbus jetties.
Work began in early March, with the first six 20m deep boreholes being completed within about a week. Conditions included up to 10m of soft mud overlying occasionally gravel and then London Clay or Bracklesham Beds.
From the outset the site locations raised the issue of encountering unexploded ordnance. This was especially so in the vicinity of Priddy's Hard which was a Naval munitions depot from 1769 until 1989, and includes Shell Pier, which was used for loading armaments.
Additionally Portsmouth Harbour was very heavily bombed in the Second World War, especially during a six month period in 1940. Estimates suggest that some 10% of German bombs failed to detonate, with greater malfunction for bombs landing on water.
'There was a perceived increased risk of finding bombs in the area,' explains Mike Groves of Maunsell, which is essentially why the bomb search works came about.
Existing bomb records have not been relied upon heavily 'We have tackled the problem from the perspective that we were looking for general unknown occurrences rather than noted bombing incidents,' says Groves. 'Established records could lead us to the wrong conclusion. We assumed there is a possibility of bombs being present and have gone out to prove this either
Conveniently CDM has provided a framework within which to assess systematically
the risk of unexploded munitions and ordnance. Unexploded bombs are clearly a risk during the SI drilling, and even more so during the main construction work which will involve piling, and excavation. In the context of CDM, assessing the safety and risk of bombs for the site investigation work becomes a vehicle to assess the greater risk during the main construction works, comments Groves.
Maunsell's called in bomb identification specialist, Arundel-based Fellows International. Fellows is one of a couple of companies specialising in this type of work, and which generally employ ex-Royal Navy trained bomb disposal experts. Its role is to identify potential anomalies which if armed are made safe by an on-call Naval bomb disposal unit, although the company has the expertise to defuse bombs.
Fellows carried out a two phase investigation, using in the first instance water-based geophysics with echo sounding to identify the depth of soft ground and position of anomalies. The work was carried out from a boat, and involved scanning the proposed work areas on a close space grid, typically over a 25m square box centred on the borehole. Position- ing was achieved using a satellite global positioning system.
The second stage was to investigate the anomalies with divers using a handheld magnet- ometer probe and operating from the muddy sea floor. This has provided more detailed and specific investigation to about 6m in the soft mud.
From the results Fellows could generally confirm if an anomaly was a bomb/shell or harmless piece of junk. They also gave an indication of an object's size, and if it was ordnance whether it was fused. Key to this level of identification is knowing what to look for, and the typical magnetometer signatures these objects would produce.
Need for the survey, carried out in early March, was confirmed with the discovery of three 75mm shells at Forton Lake, each at or just below the mud surface, which were was safely removed by using water blowers to shift the mud, or simply by hand.
Fortunately suspect objects were only identified at shallow depth, as a deep excavation to retrieve a suspect object from say 5m depth in the area of one of the bridge piers would have posed a considerable challenge .
Once Fellows has cleared drilling sites, the drilling itself 'is the easy part of the job', says Maunsell's Mark Dawson. GI contractor is FES, which has hired in Alluvial Mining's Mytilus jack-up.
The 14, 20m deep boreholes were primarily for pile design and involved confirmation of the geology and assessing soil material properties.
Sampling and insitu test- ing involved standard piston sampling and shear vane tests in the mud, SPTs in the gravel, and alternating SPTs and U100s in the London Clay or Bracklesham Beds.