Residential Low Rise

JNP Group consulting engineers successfully achieved the following

• Pioneered a revolutionary method of fill consolidation
• Enabled profitable development of a site thought un developable
• Provided cost effective designs and solutions for Roads, Drainage, Foundations, Structure, Geotechnics, Off Site Highway Works

The quarry is some 39 metres deep of which 60% has been backfilled with end tipped Thanet sand and the remaining 40% partially backfilled with Whiting Dross (surplus slurry from the production of White Portland Cement). This material the past has been has been used as filler for comment medicines, toothpaste and as an additive in the manufacture of bricks to form imperfections and give the illusion of a hand made brick.

Redevelopment of this 20 acre into a residential development faced some daunting geotechnical and environmental considerations. These were as follows:-

• How to treat 39m of un-compacted fill.
• How to deal with the toothpaste consistency of the whiting dross
• How to provide surface water drainage on a site where the water table is above the base of pit and being kept artificially low by pumping from still active nearby Blue Circle quarries?

JNP Group consulting engineers carried out extensive Geo-environmental investigations and following lengthy consultations with Kent County Council and the NHBC as Building Regulation Authority, gained approval to the following methodology.

• Vibro consolidate and reinforce the top 5m of Thanet sand with specially installed vibro rubble columns at 2.3m centres in a triangular pattern.
• For the whiting dross area, install piezometers, bring level of whiting dross up to general fill level with Engineered, imported fill, surcharge to induce settlement and consolidation in the underlying whiting dross, anticipated at approximately 500mm.
• Monitor settlement of whiting dross of 18 months, remove surcharge, developed houses.
• Use inverted egg box composite raft construction, incorporated insitu r.c. strip footings to limit soil stressing to reinforced and densified top 5m of fill.
• Group all surface water together and install special banks of raked deep bore soakaways around perimeter of development where a suitable, high level chalk face could be found offering adequate permeability.