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SLOPE ENGINEERING - Soil nailing has stabilised a steep slope and stopped subsidence of a residential road in Norwich.Mark Annetts reports.

Settlement has been apparent on Tower Hill, a small private road between houses and a steep 1 in 20 slope in Thorpe St Andrew, Norwich for some years.

The road surface had been patch repaired in the past, but subsidence had continued at a slow rate, adding to the road's natural camber.

The increasing camber had raised the concerns of refuse collectors that their vehicle might be adding to the settlement.

Road owner and insurance firm Norwich Union, via its agent Jones Lang LaSalle, appointed consultant TA Millard to investigate the cause. The consultant in turn brought in Norwich-based contractor May Gurney's site investigation team, whose two boreholes revealed that the bank comprised a loose silty fine and medium sand with flint gravel and with thin bands of sandy clay. These soil types, together with the severe angle of the slope, meant remedial action was both necessary and urgent.

TA Millard recommended soil nailing to reinforce the bank. Jon Spalding, the firm's associate director explains: 'To use any other remedial measure would have required heavy machinery working in extremely tight conditions and potentially could cause the bank to slip.

'A piling rig, or similar, would also have totally blocked the road, preventing residents from accessing their houses by car. The only practical and safe option was soil nailing.' Structural surveys were also carried out of the properties as a precautionary measure; however no evidence of settlement damage was found.

Norwich Union approached May Gurney Geotechnical again, appointing it as specialist subcontractor because of its extensive local knowledge and geotechnical expertise.

The added benefit was that May Gurney's integrated approach meant it could also reinstate the road once the soil nailing was complete.

The soil nailing scheme was designed by geotechnical consultant Raison Foster Associates based on TA Millard's preliminary design. It called for 365 Ischebeck Titan 30/16 soil nails between 7.5m and 8.5m long installed on a 1.5m grid.

The nails were designed to accommodate a working load of 50kN and are installed using a TEI drilling mast mounted on a Hitachi 200 20T long reach excavator. Access to the soil nail entry points is via a mobile elevated working platform.

The soil nails are formed using a hollow stem, threaded bar fitted with an expendable 95mm diameter cutting head to provide a nominal 125mm diameter body of grout. During installation the cementitious grout is pressure injected through the stem and through apertures in the head to flush soil cuttings back to the surface. Selected nails will be pull out tested to ensure compliance with the design requirements.

As the site is in a conservation area, trees had to be left in place, with nails installed around them.

The embankment will be meshed with MacMat-R and topsoil to allow replanting of ferns, which have been carefully dug out. These and other new undergrowth will add strength to the surface of the bank.

But as Jason Parker, May Gurney's project manager notes, conservation was not the only issue.'We have had to locate all services including gas, electric, water and sewerage using trial pits, to ensure we angled the soil nails under them.

'On the whole this project does not differ from the large road and rail embankment stabilisations we normally undertake, although it has involved far greater consultation with residents and local authorities over ecological issues.' The £337,000 project began in late November last year and will finish with the reinstatement of the road early next month.

May Gurney will regrade the road including a new sub-base in parts, along with the construction of a new drainage system, kerbing designed to distribute loading away from the crest of the embankment, and finally resurfacing.

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