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Blisworth Tunnel – a personal reminiscence Robin Garrett
Blisworth
Tunnel had problems throughout its history, including during its
construction which commenced in 1794.
There was at least one attempt at building the tunnel which had
to be abandoned before finally completing the tunnel in 1805.
It has been suggested that there is evidence of two abandoned
sets of workings, but that debate has not yet been concluded. The
problems at Blisworth are related to the geology of Blisworth
Hill.
Water bearing sandstone overlies lias clay and when the clay
absorbs water it swells and develops forces strong enough to distort the
tunnel lining. Thus the numerous failures in the tunnel have resulted in the
tunnel floor rising and have sometimes only become obvious when a boat
crew has reported hitting something solid below water. In
those days of working by hand, tunnels were constructed by sinking a
series of construction shafts along the line of the tunnel.
Excavation of the tunnel then proceeds from the bottom of the
shafts. The setting out at
Blisworth was clearly of a good standard as the tunnel “wriggles”
somewhat but is reasonably straight.
Much
of the tunnel was constructed along the interface between the sandstone
and lias clay and to deal with the ground water problem an extensive
network of drainage headings was constructed.
We could speculate whether it would have led to a more successful
(but longer) tunnel if there had been one less lock on the Stoke Bruerne
flight and the tunnel driven at a lower level mainly in the clay. In
considering the problems at Blisworth the tunnel was divided into three
sections. The northern
third from 0m – 950m is built largely in the clay and is in reasonable
condition for its age. Problems
are associated with construction shafts, which even when backfilled act
as drains allowing the groundwater to flow down to the lias clay. The
geology of the southern third from 1875m – 2812m is not as simple as
the northern third. There
are even short lengths where the tunnel floor is unlined sandstone. But
the tunnel lining should be adequate for many years of service given
adequate routine maintenance. Although
on my last visit to Blisworth I did not see much evidence of routine
maintenance. The
major problems were in the middle third and most of the past failures,
including those that closed the tunnel in 1977 and 1979 were in this
length and were associated with the clay/sandstone interface.
The
1979 failure involved a serious bulge in the lining with consequent
damage to the brickwork. Temporary
timber supports were put in, designed to permit a single narrow boat to
pass. With two failures in
two years it was considered that a detailed investigation should be
undertaken before proceeding with another local repair.
For a period boats were allowed through the tunnel with a BW
crew, but the tunnel was then closed to all traffic.
It is not known how much the political impact of cutting the
canal system in two affected this decision.
It was a time of serious under funding for BW and there were
numerous engineering problems which could not be dealt with because of
the lack of finance. While
funding for major repairs was awaited, consideration was given to the
various options. The first
stage in the process was a detailed survey of the tunnel lining above
water level. Bearing in
mind that the majority of failures originated from the lower part of the
tunnel, it is perhaps surprising that the tunnel was not dewatered at
this stage. The survey was
carried out by dividing the internal lining of the tunnel into metre squares which were
categorised according to the type and severity of deterioration.
Other details such as previous repairs, openings to drainage
headings and water inflows were also recorded on the survey.
At an earlier stage in the history of the tunnel it was decided
to record the date of repairs with date bricks set into the new
brickwork. This practice
was continued in the repairs from 1982 -1984. A
summary of the more serious problems along the length of the tunnel
illustrated graphically that the middle third was more seriously
affected than the outer thirds. When
the results of boreholes were analysed and the different strata were
identified, the geological reasons for the pattern of failures became
clear. Consideration
was given to various options for repair.
But it had to be borne in mind that the tunnel lining had failed
on numerous occasions in the past, the first failure within 20 years of
the tunnel opening and it could be guaranteed that unless the structural
problems were dealt with there would be further failures in the future.
Thus proposals such as drilling and reinforcing the lining with
chemical grout were not serious contenders.
A proposal to install a secondary lining within the tunnel was
also rejected as it would have restricted the tunnel to one way traffic.
Although an innovative solution and outside of B W’s experience
a replacement of the brick lining over the middle third with a pre-cast
concrete lining was considered to be the best solution.
At a later stage Mott, Hay and Anderson Ltd. were appointed as
consulting engineers to supervise the site investigation, design the
replacement tunnel and supervise the contractor. They appointed an extremely experienced tunnel specialist
David Bridges as resident engineer. My
first visit to the site was in 1981.
Tom Mann and I had previously met with engineers at the Principal
Engineer’s office in Gloucester.
There was a definite atmosphere at this meeting and obviously
some resentment that the project had been given to the Construction
Section at Rickmansworth. We
were also advised that we would get little cooperation from the local
management in the London Area. I
had arranged with Stan Voyce the Northampton Section Inspector to take a
boat through the tunnel to get an appreciation of the problems.
As it happened I knew Stan from my involvement in Nalgo and on
completion of our inspection we were invited back to his cottage in
Blisworth where his wife Mary had prepared lunch.
Tom and I reflected that cooperation might not be such a problem. On
opening the steel gate at the South portal and emerging from the tunnel
we were greeted by a photographer who I later realised to be George
Freeston the local historian. How
he got to know that we were going through the tunnel I have no idea, but
little went on through the construction period without George knowing
about it and recording it for his archive of photographs
We all got to know him well as a regular visitor to the site In
1981 it was agreed that the Government would make available a
“Specific Grant” to BW to tackle the arrears of maintenance and £4.8m
was allocated to Blisworth. The
catch was that a substantial amount of site investigation was needed
before design work could proceed, followed by the actual remedial work
and whilst no expenditure was permitted before April 1982, a substantial
proportion of the grant had to be spent before April 1983.
Tenders had been invited for a detailed site investigation and
Geotechnical Engineering were on site early in April. For
safety reasons it was essential to identify all of the shafts which were
associated with the original construction.
We were aware of Wast Hill tunnel in the Birmingham Area where a
fatal accident was due to part of the brick lining being removed leading
to a collapse of the backfill in an unsuspected construction shaft.
Some of the Blisworth shafts were backfilled and some left open
but a brick dome or timber staging installed to seal the top of the
shaft. Open shafts were also required to provide ventilation.
Including two new shafts which were provided for ventilation following an
accident in 1861 involving a steam tug, there were a total of seven ventilation
shafts. In total 21 shafts
were identified, excluding suspected shafts associated with the initial
unsuccessful attempts to build the tunnel which were sufficiently remote
from the tunnel to not cause concern for safety. Boreholes
were sunk in the surrounding ground to verify the rock strata and
through the tunnel lining to confirm the quality of brickwork and the
surrounding ground conditions. The
tunnel was de-watered to enable an inspection of the invert and the
drainage headings were explored. When
the drilling through the tunnel lining was being carried out from a
drilling rig on a boat we had a visit from the Factory Inspector.
He was not impressed by the protective shield that had been
installed to protect the drillers in
the event of bricks being dislodged from
the tunnel lining. However
the Factory Inspector required that we have adequate protection to
support a large section of brickwork falling on the boat.
Delays due to the Health and Safety Executive we could do
without, so I refrained from pointing out that such a collapse would
sink the boat. In
order to commence the remedial work as early as possible, I proposed
that we let a preliminary contract.
This contract was tendered and awarded to John Mowlem and
included the installation of cofferdams, dewatering of the tunnel,
constructing an access road to the North portal and through the tunnel
to the starting point for the re-construction and installation of a site
compound and offices. Thus
the main contractor, also John Mowlem as it turned out, would be able to
make an immediate start following award of the contract. The
main contract consisted of breaking out the old brick lining of the
central 900 metres of the tunnel and constructing
a new lining using pre-cast concrete segments.
Allowance was made to connect to ventilation and construction
shafts, drainage headings and areas of high water inflow, either by in
situ concrete work or the provision of window segments in the pre-cast
concrete lining. The
ground through which the tunnel passed had been disturbed during
construction of the tunnel and drainage headings.
Consequently a "tunnel shield" was specified that included a
forepoling facility to support the ground immediately in front of the
excavation. As a further
measure to bring forward expenditure and meet cash flow targets, the
shield was pre-purchased by BW. The
stability of the existing tunnel could not be guaranteed ahead of the
shield drive and steel colliery arches were specified to provide
temporary support and shown in this schematic. Probe
drilling was carried out ahead of the drive together with ground
treatment as required. Work
on the main tunnel contract started in January 1983 by breaking out a
section of tunnel lining at the start of the middle third to construct
an enlargement of the tunnel in which the shield was assembled.
Although the main tunnel drive was highly mechanised, the
construction of the chamber for the shield relied heavily on the muscle of
skilled miners and was an extraordinary piece of work. The
shield had been erected and tested in the Grosvenor Ltd. factory in
Manchester and was then dismantled and delivered to site in May 1983.
The shield provided protection to the workforce and support to
the tunnel whilst the excavator mounted at the front and capable of
rotating through 360o broke out the lining and surrounding
ground to form a 6.5m diameter tunnel. The concrete rings forming the new tunnel were erected in the
shield and as each was completed the tunnel shield was jacked forward
0.75m. Cement / PFA grout
was pumped in to fill the voids behind the concrete lining.
The rings incorporated a fender segment to support a timber
fender to keep boats away from the circular lining.
Window segments were also installed to provide openings to
drainage headings. It was
vital that segments were loaded in the right order to ensure that they
were delivered to the shield to form part of the correct ring in the
right location. Working
two 10 hour shifts per day the drive progressed at a rate of up to 10
rings per shift. The
concrete segments were brought in by Volvo 861 dump trucks reversing in
along the access road and a load of excavated material went out on the
same vehicles. As no
pedestrians were allowed on the access road, site staff and visitors
enjoyed a slightly hair raising ride into the tunnel.
The drive progressed so well that towards the end I spent a lot
of my time arranging for visitors and in the last few days telephoning those who had expressed interest in
visiting to point out that they would miss the opportunity for a unique
experience if they did not get to Blisworth quickly My
one regret about this project was that on completion of the middle third
we did not continue for a few metres more, where there was some
brickwork that could have been replaced, or better still continue to the
south portal. The
few days before the opening of the tunnel on 22nd August 1984
went by in a blur. I can
recall explaining to the staff of Clements Department Store in Watford
why I needed two lengths of white ribbon about 6 metres long but forgot
to buy a pair of scissors. I
had let it be known to the IWA unofficially, some weeks before that I
was confident that the tunnel would be open in time for boats to travel
from the southern Grand Union to Hawkesbury Junction for the IWA
national rally. There was a queue of boats down the length of the Stoke
Bruerne flight of locks and beyond waiting to go through the tunnel. The
evening before the tunnel opening was enjoyed mostly in the Boat Inn at
Stoke Bruerne. I had spent
many lunchtimes there with David Bridges the resident engineer and other
colleagues from the tunnel project.
It was hard to believe that a manic two years was coming to an
end and the project was nearing completion.
But I was in no condition to return home for the night and be at
the tunnel again in the morning. Fortunately
one of the many IWA members on their way to Hawkesbury invited me to
stay on his boat. The
conclusion of a major engineering project can be an emotional event.
David Bridges and I shared the commentary on the two VIP boats
which led the procession from Blisworth to Stoke Bruerne.
But I was totally unprepared for the massive crowds at both ends
of the tunnel and had to pause for a moment when the north portal came
in sight and I realised just how many people had turned out to celebrate
our achievement. Robin
Garrett B.Sc., C.Eng.,
M.I.C.E. |
One
of the highlights of my engineering career began when Tom Mann, BW’s
Engineer Construction called me into his office and reminded me that
some years previously I had worked for London Transport.