As many as 19 heavy-duty Premier Trench Boxes from Groundforce Shorco have been used in the construction of a groundwater storage tank on the site in Kent.

Specialist contractor Gallagher Group is carrying out the groundworks for the new Medway Maritime Academy, a new secondary school in Strood, for Medway Council. Main contractor on the project is Bowmer & Kirkland.

Gallagher called in Groundforce Shorco to help support a 35m-long, 4m-wide excavation to allow the installation of a precast concrete culvert comprising 76 segments each weighing 14 tonnes. The culvert will be used to store surface-water run-off and control its flow to prevent flooding.

Stormwater attenuation tanks are often built using lightweight moulded plastic tanks but Medway Council wanted a more robust system and specified a system made up of precast concrete sections.

“The design of the culvert includes a hydro-brake which allows water to flow through during normal conditions,” said Gallagher Group site manager Eddie Greene. “But during periods of heavy rainfall, the hydro-brake prevents excess water escaping and stores it in the culvert.”

Installing the 76 concrete segments, each measuring 3m wide by 2m deep, has had to be carefully phased. Space limitations mean that there is little scope for the segments and equipment required for their installation to be stored on-site. But assembly and finishing of the culvert has also dictated the sequence of installation.

Greene said: “The culvert itself has to be tanked with a high-tech system provided by Waterseal which required full access to the top of the culvert after installation.”

To support the long, deep excavation required to install several of the concrete segments at a time, Groundforce Shorco supplied its 4m deep Premier Trench Boxes, installing 10 for the first batch of segments and a total of 19 altogether.

The excavation progresses in sequence, with two contiguous approx. 35m-long excavations, one supported with 10 boxes and the other with nine. This gives Waterseal access to the greatest possible length of culvert during installation; as one length of culvert is completed, the trench boxes are removed and re-installed ahead of the next 35m length. The total length of the culvert will be approximately 200m.

Along with the trench boxes, Groundforce Shorco has also supplied ancillary equipment including its Edgesafe safety barrier which sits on top of the trench boxes to guard against the risk of falls into the excavation.

The Premier Trench Box has proved ideal for this application not only because it is safe, quick and easy to install but also because its design offers a maximum 2.45m strut clearance and ensures maximum working space inside the trench.

“It was made clear that Gallagher needed high clearance under the struts to make room for waterproofing the culvert before backfilling the excavation and removing the boxes,” says Lee Bodkin, regional manager for Groundforce Shorco.

“The Premier Trench Box easily fulfils this requirement. Standard trench boxes would not allow installation of the culvert due to the lower strut clearance.”