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Tall order doubles housing density

A REINFORCED soil slope, claimed to be the highest in the UK in a residential development, doubled the number of houses that could be built on a site at Hythe Hill near Colchester in Essex.

Geosynthetics manufacturer Tensar International designed and supplied materials for the structure. With geotechnical contractor Keller Ground Engineering, Tensar developed a design for the 13. 5m high, 65-69degrees slope using contaminated sand reinforced with geogrids.

Main contractor Tendring Construction, working for client North British Housing, needed to form a boundary structure to retain a raised part of the site, which was previously a stockpile for contaminated soil and sand produced from heavy industry.

It was decided to keep this material on site, reducing the cost of disposing to licensed landfill and of importing clean fill. Any geosynthetic used in the slope therefore had to be chemically inert.

Construction was further complicated by underlying soft clay with an undrained shear strength of 15kN/m 2. Following phased site investigation, Tendring, consultant Richard Jackson Partnership and the design teams at Tensar and Keller came up with the solution to achieve the required density of housing.

Keller first installed stone columns to improve the shear strength and bearing capacity of the soft clay. This reduced potential settlement and provided a stable foundation for the reinforced soil structure. The stone columns also increased the permeability, allowing dissipation of excess pore water pressures.

Tensar Uniaxial geogrids were then laid between layers of the compacted contaminated sand, bringing the slope up to half its completed height. The geogrids were attached to Tensar steel mesh panels that form the slope facing. A turf liner was installed behind this to provide a vegetated finish.

Pore water pressure was monitored twice a day using piezometers installed between the stone columns. This allowed Tensar to monitor the overall stability of the structure during construction and to verify the stability of the final construction stage. Results indicated that dissipation was occurring faster than predicted, which allowed the final stage to begin earlier than planned.

The structure was completed under a reduced monitoring regime. To give it a natural look, and to meet planning requirements, the base, berms and crest were planted with bushes and creepers.

A 3. 5m high retaining wall was built for a car park using the contaminated sand. This uses modular concrete blocks with a Tensar geogrid secured between them.

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