Blisworth Tunnel – a personal reminiscence

Robin Garrett

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.  Not unreasonably, he thought that I might have some experience of working on tunnels and it would be appropriate to take on the remedial works to Blisworth Tunnel.  I did not like to mention that I had mainly worked in the Bridges and Structures design office, but I soon became involved and learnt something of the history of Blisworth.  It was pointed out by Malcolm Stakes BW’s Mining Engineer that canal tunnels are the oldest tunnel structures in the UK, pre-dating the railways by 50 years, the major sewers of London by 80 years and the London tube tunnels by 100 years.  I attempted to research the design rules adopted at that time.  But all I could find was a rule of thumb that the thickness of the tunnel lining in inches should equal the width of the tunnel in feet.  This corresponded with the tunnel lining thickness of 18 inches.

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.  Some aspects of the project are covered in the illustrated website pages.

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.
Project Engineer for British Waterways,  Blisworth Tunnel 1981 – 1984