Engineering Design Consultants

Water Industry and Local Authority

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Bennett Associates, M & E Consultants to the public services Department of the State of Jersey.

Bennett Associates’ expertise has been put to good use in the design and project management of a large proportion of a flood relief scheme, formally known as "The Weighbridge to Town Centre, St Helier Surface Water Link and Storage Tank", constructed in Jersey.

The purpose of the scheme was to relieve the situation when in times of heavy rainfall the sewerage system became overloaded, resulting in the flooding of properties in St Helier. In addition, during heavy rainfall, diluted foul sewage also overflowed to sea before it reached the sewage treatment works at Bellozanne.

The project was approved in 1993 and included a 25000m3 capacity cavern, excavated by drill and blast in The Fort Regent granite rock intrusion at St Helier.

The cavern is accessed from an existing surface car park by underground roadway and shaft as well as having an emergency egress shaft.

Once the project was completed, overflows from the existing combined sewerage system weirs were conveyed via two pipes located within surface water syphon tunnels and discharged into the cavern.

Automatic level sensing and discharge pumping enables the stored effluent to be pumped from the cavern back into the existing sewerage system automatically as sufficient capacity becomes available downstream.

In addition to the discharge pumping system and in order to prevent the release of noxious gases, a comprehensive system has been designed by Bennett Associates. This system includes automatic wall and walkway cleansing, ventilation and discharge gas odour control. To avoid the problems of corrosion virtually all the walkways, stairways and much of the pipework within the cavern is manufactured from Glass Reinforced Plastic (GRP) materials.

Bennett Associates designed the scheme in conjunction with the Jersey PSD and Geo-Engineering/Donaldson Associates, the project Civil Consulting Engineers. Bennett Associates prepared a detailed reference design together with a technical specification. Commercial and contractual aspects of the specification were prepared with assistance from Faithful & Gould, the international Quantity Surveyors.