Washout Installs on 9 Inch Main, Watford

The brief

Affinity Water called us in to install two new washouts on a 9 inch water main in Watford. Washouts are network drain-down points, fittings teed off the main that allow Affinity’s operational teams to flush a section of main when required, typically for water quality management or after repair works. You can’t flush a main without a washout on it, so installing them at the right points around the network is part of how a water utility keeps its system operable.

Two separate installations were needed on the same main. Both had to happen without taking the main out of service, which is the standard requirement on live distribution pipework, and both had to be positioned where Affinity would find them useful for routine flushing cycles in years to come.

The challenge

Two decisions drove the job. First, the under-pressure drilling question: how to tap into a 9 inch live main twice, at two different points, without depressurising the main or disrupting downstream supply. That’s a technique question, and the answer is under-pressure drilling. Specialist tapping equipment cuts through the pipe wall while keeping pressure contained inside the machine.

Second, the location question: where to install the washout risers and chambers. The default on a lot of urban jobs is straight into the carriageway, which keeps the trench short but creates a problem every time the washout needs to be used. Each future flushing cycle then needs a road closure or at minimum a lane closure with traffic management. Expensive, disruptive, and avoidable. We positioned both washouts in the footway instead. Longer service runs from the main to the washout chamber, but every future access for Affinity’s flushing team is a pavement job, not a road job.

Our approach

Lukos and Kris carried out the works. The carriageway was surveyed, scanned, and marked up before any excavation started, with survey markings visible in the site photos. Both tapping points on the 9 inch main were exposed, and the under-pressure drilling machine was mounted in turn at each position. The machine drilled through the pipe wall with the main still at operating pressure, isolated the bore, and allowed the washout tee and valve assembly to be jointed on cleanly.

With both tappings in, the washout risers were extended up to footway level, chambers built around them, and the fittings commissioned. The main stayed live throughout. No depressurisation, no network disruption, no customer off-supply events.

Outcome

Two washouts commissioned onto the live 9 inch main, no shutdown required, no customer impact. Both sited in the footway so every future flushing operation Affinity runs through these access points can be done without returning traffic management to the road.

For the technique in more depth see our under pressure drilling capability page. The service context sits under water mains repair and maintenance for water companies.