Future Water Celebrates Major Milestone at Havant Thicket Reservoir
Future Water MJJV is proud to announce the successful completion of a major piece of work at Portsmouth Water’s Havant Thicket Reservoir, an operation that marks a UK first and demonstrates the incredible teamwork between Future Water and Portsmouth Water.
On 14 January 2026, the project team finished the first stage of the Culvert Cut‑Off Wall, a vital part of making sure the future reservoir holds water safely and securely.
What Is the Culvert Cut‑Off Wall?
To help explain this engineering challenge, imagine the reservoir embankment as a giant bathtub, and the large concrete culvert underneath it as the plug hole that will be used to empty the reservoir when water is needed. For everything to work properly, the join between these two structures must be perfectly sealed, just like the silicone seal you’d use around a bathtub at home.
Creating this seal deep underground, beneath a future reservoir, required an innovative and highly precise solution. Coffey Geotechnics Ltd led the design for a special “Cut‑Off Wall”, which acts as an underground barrier to contain the water.
A UK First Engineering Achievement
To build this barrier, Future Water constructed a 20‑tonne steel sheet pile wall on site. Standing at 13 metres high and 9 metres wide, the wall was lifted into place in one piece using a powerful 100‑tonne crane.
Before the lift, a trench was dug 8 metres deep and filled with a bentonite slurry mix. The steel wall then had to be lowered into this mixture within a tolerance of just 25 millimetres. Achieving this level of accuracy with a structure of that size is a major achievement in itself.
This solution is the first of its kind ever used in the UK, designed to meet the unique ground conditions found at Havant Thicket.
72 Hours of Non Stop Teamwork
Because the slurry starts to harden as soon as it is mixed, the team had to work continuously for over 72 hours. Teams from Future Water rotated through day and night shifts to keep the work moving safely and smoothly.
Their commitment ensured the steel wall could be installed before the slurry cured, a race against time that required planning, teamwork, and exceptional focus.
The slurry will now be left to cure for 28 days. Later this year, the steel wall will be connected to the concrete culvert, and the embankment will be built over the top. This will complete the Culvert Cut‑Off ahead of the main embankment construction in 2027 and 2028.
An environmentally-led project
The completion of the Culvert Cut‑Off Wall is a major step forward for Portsmouth Water’s Havant Thicket Reservoir, an environmentally-led project that will play a key role in protecting two internationally renowned chalk streams in Hampshire, the River Test and River Itchen.
This achievement reflects innovation, collaboration, and the shared commitment of everyone involved. Future Water and Portsmouth Water are proud to celebrate this moment as work continues toward delivering the first new reservoir in the region in decades.
“The successful completion of the Culvert Cut-Off marks a critical technical milestone for the Reservoir project. Delivery of this element relied on close collaboration with Keller and Coffey Geotechnical, whose expertise were instrumental in achieving seamless execution. This UK-first solution was specifically developed to suit the site’s ground conditions and provides a robust hydraulic barrier between the culvert and underlain strata, de-risking subsequent embankment construction and long-term reservoir performance. While the works themselves were completed on site within a matter of days and without hindrance, this efficiency was the direct result of many more hours invested in design development, planning, and preparation, it was a true team effort.” – Ryan George, Project Manager, Future Water