Kenilworth is a picturesque market town in England, about a 90-minute train journey from London. With its extensive medieval ruins of Kenilworth Castle and Kenilworth Abbey in Abbey Fields park, the sought-after area is an ideal location for commuters to nearby Birmingham and Coventry. To meet the housing demand, 125 new family homes are being built next to the town’s 116-year-old Cricket Club.
One of the practical challenges of constructing a new development is integrating with the existing water infrastructure. To carry wastewater from the housing estate, specialist tunnel sub-contractor, Rorcon Limited, needed to connect surface water drains into the existing sewer.
The existing 1.2 m diameter sewer tunnel was about 10 m below a narrow country roadway, which could not be closed for the excavation. Additionally, space was restricted because of how close the nearby houses were to the shaft construction site. In collaboration with Rorcon Limited, Golder developed a design and construction sequence for a 4.5 m diameter, 8 m deep shaft, and its connection to the existing tunnel.
To construct the shaft in the small space available for equipment, Golder’s design needed to consider how to improve productivity while keeping the size of the shaft to a minimum. Multiple factors were taken into account, including the size of the plant required to trim and remove the rock, and the lifting and logistics requirements to remove spoil. Working collaboratively, Rorcon and Golder developed a practicable approach to address each parameter. With the design in place and working in collaboration with Rorcon, the size of the shaft was optimized to enable the use of larger equipment, thereby speeding up the construction process.
The shaft was constructed through soft ground into the rock below. A 7 m square slide rail box was installed by the main contractor to provide support to the softer ground until rockhead was reached, 2.5 m below ground level. Golder and Rorcon agreed on a 4.5m internal diameter pre-cast segmental shaft, constructed using the underpinning method from the top of the rock. This method lends itself to construction in harder ground with minimal water inflows.
Once the connection level depth was reached, the contractor began tunneling 14 m from the shaft to reach the existing sewer, using traditional methods of steel and timber heading frames. Then the connection pipework was laid within the heading, connecting the existing sewer to the new shaft. This was then backfilled with concrete upon completion.
The completed shaft design has optimized construction, remained cost-effective, and allowed the contractor to swiftly connect the new surface water drainage into the existing line.
For more information on the project, see page 33 to 41 of natm magazine’s October 2020 edition.