SHEQWs Newsletter November

In the spotlight
Tunnel Inspections - Walthamstow
Overview
In early November the team mobilised in Walthamstow
fishery’s to undertake various tunnel inspections.
The first was tunnel was only a few hundred meters long
and was in good order with next to no mussel or silt inside.
Following on from this a compound was set up in the
Lockwood compound opposite the main fisheries, to allow
for works to the second tunnel , which spanned 6km to
begin. Shortly after the spine tunnel was started which
runs from the break pressure tank just south of the William
Girling Reservoir to Coppermills Water Treatment Works.
Preparing for the tunnel cleaning
Once the permits were acquired by eight20 from Thames
Water, the survey was undertaken. With all the lift plans,
isolation procedure and RAMS in places.
Firstly a floating pontoon was set up with a frame, block
and tackle attached allowing them to pick up the isolation
plates and float them into position, once they had been
lifted into the water using a 25t city crane.
The isolation plates consisted of three C section plates
and a cover lid. Each C section is approx. 1mtr high and
2.0mtrs long. These are all bolted together with gaskets
between every section, along the head wall and then
studded to the base. Once the isolation plates were in
the water, specialist divers were bought in to survey the
shafts. Each section was installed until completed and the
process repeated at two locations shaft A & C.
Once all the isolation plates were installed, the tunnel was
isolated and the instruction was given to drain down the
tunnel. To do this a permits to pump was raised and the
pumping was monitored until the tunnel was empty.
Tunnel Cleaning & Inspection
Once the tunnel was drained, the survey phase of the
works could begin. It was at this stage that the amount of
mussels/ silt within the tunnel was apparent.
The survey started at shaft A where the tunnel was almost
completely full with only 300mm gap to crown. It was
because of this that the survey was abandoned at shaft
A and attempted at shaft C where debris depths of 8 to
900mm were found and so the survey was deemed unsafe
to continue until the waste had been removed.
DISAB suction units were then employed to extract the
mussels and silt waste. This involved connecting 6”hoses
down the shaft and working from the main riding basket
at the pit bottom to clear the way into the tunnel.
Ventilation fans were also installed to purge the tunnel
along with specialist ammonia gas detectors.
Barhale continued to extract waste and clear the tunnel
for a number of weeks by extending the hose into the
tunnel. However due to the amount of build-up, trollies
and bins were also used as a much faster way to move the
material.
Once the tunnel was completely cleaned it was then
inspected by Thames Water and eight20.
Page 8 | November 2016 SHEW Newsletter
SHEW Newsletter
November 2016 | Page 9