“There can be no doubt that a major revolution in the worldwide tunnelling industry was triggered by John Bartlett’s invention of the Bentonite Tunnelling Machine. It has enabled a rapid increase in tunnel construction around the world, particularly in urban areas, for water supply, sanitation and transport – with remarkable benefit to humanity.”

That was the message of Lord Robert Mair, president of the Institution of Civil Engineers (ICE) at the award of the Sir Frank Whittle Medal to the veteran tunnelling engineer John Bartlett.

Ivor Thomas, project manager at Bam Nuttall added: “[The] invention of the slurry machine and its subsequent development has made a tremendous difference to how and where we can tunnel.

“Slurry tunnelling has allowed us to develop tunnels in geology that would previously have been either very difficult and costly or impossible. Much of the Jubilee Line to the south of river, the Channel Tunnel Rail Link Crossing and Crossrail River Crossing were only made possible by the use of slurry machines.”

In contrast, chuckling to himself, John Bartlett reflects that it is rather amusing that he has been awarded the Frank Whittle Medal because he had to travel a lot and it would have been totally impossible without Frank Whittle’s invention of the jet engine.

“I have been to Australia 50 times and that means around the world 50 times because I was always trying to combine office and site visits in other countries,” Bartlett says. “As Frank Whittle made my career possible, I hope that my invention made various tunnels.”

Ezngineering life

“I decided to enter engineering at the end of the Second World War,” Bartlett says. “I volunteered for the Royal Engineers in 1944, thinking that the training would be useful later.

“Instead of sending me to the war, they sent me to Cambridge University. I took an army engineering short course and then I became a paratrooper and I was in Palestine as Royal Engineer officer. I came back to Cambridge when I was demobbed and read “mechanical sciences” leading to an engineering degree. I graduated in 1950 and then I stayed an extra year and got a degree in law as well, which I have found very useful as the laws of contract and real property impinge on all civil engineering. After that I got married, which meant that I had to have a job.

Instead of law, Bartlett joined John Mowlem &Co as a trainee engineer, and says he was lucky to be in the right place at the right time.

“I started working on a contract to build an enormous underground Telephone Exchange in London,” says Bartlett. “I had by this point decided that I couldn’t be an engineer and barrister at the same time as my life would have been too much busy.”

Tunnelling memories

In an early but significant memory from his career, Bartlett recalls the first job for which he felt he took responsibility as an engineer.

“In the London blitz the largest type of German landmine blew an enormous crater in the Woolwich Road, destroying nearly 100ft (30.48m) of Bazalgette’s 10ft diameter Southern Outfall sewer tunnel.

“A company of Canadian Royal Engineers, billeted nearby, installed enough 4ft (1.22m) diameter precast pipe units to take the normal flow through the sewer. They backfilled the crater and re-opened the highway.

“In 1952 I was given the job of removing this constriction from the sewer. We lined the pipe with plastic to avoid excessive leakage should we disturb the pipe units in the course of the work. We excavated a ‘break-up’ around the pipe units and complete a 12ft diameter concrete tunnel lined with brickwork linking to the surviving 19th century sewer. One always carefully records the geology of the working face. I remember reporting ‘remains of church and two brass bedsteads’.

“I learned a great deal about improvised timbering. Luckily there were no groundwater problems. I didn’t realise at the time that this ‘enlargement’ by encirclement’ was a miniature version of techniques used to form step plate junctions and floodgates London’s tube system, without line closures.”

Bartlett remembers he went to a cocktail party with his wife, and somebody asked her: “What does your husband do?” She promptly replied: ‘He wades breast-high in untreated sewage’.”

Eureka: how invention happens

On the invention of the bentonite machine, Bartlett says, “I worked in several continental European cities on underground railways. A number opted for cut-and-cover solutions instead of tunnelling at depth. They were digging a trench and putting a railway in it. It was seen as a big advantage, the proximity to the surface meaning you don’t need to excavate to a great depth for the provision of escalators and stairs. On the other hand cut-and-cover means destroying all the existing services and closing important streets for many months.

“I visited Milan, where I observed how the city’s first metro line had been built using a cut-and-cover method rather than bored tunnels, they had been developing bentonite trenches, as well.

“Such construction methods enabled them to excavate a sidewall for cut and cover construction, filling the trench with a bentonite slurry, which supported the side of the excavation and allowed them to work down the street with a trenching machine while putting in place a reinforced concrete wall without disturbing buildings on each side of the road.

“It was a proven technique so I thought that if bentonite can support a side of a trench, then surely bentonite is able to support the working face in a tunnel.

“I started wondering if the methods of drilling for oil and gas using circulated drilling muds to bring the debris to the surface could be adapted for tunnelling.

“The bentonite slurry in a trench is normally up to ground level, and that is adequate to support the ground and prevent any ingress of ground water. The bentonite slurry in a permeable tunnel face carries out the same function if its pressure is somewhat greater than the ground water pressure. The excess pressure allows the slurry to permeate into the face, and to form a gel coat over it.”

Bartlett, by then an associate of Mott, Hay & Anderson, sketched out a tunnelling machine with a chamber filled with bentonite slurry supporting the face and a system for pumping out the spoil and returning the slurry and with the help of an agent, prepared the patent.

London’s Tube system was almost entirely north of the River Thames, tunnelled in the London blue clay.

Plans to extend the system south of the river and eastwards were abandoned because of difficult ground conditions such as water bearing, sand and gravel.

The authorities in London were therefore very interested in an invention which would facilitate tunnel construction in difficult strata.

The development contract

In due course, the patent was ceded to the government’s National Research & Development Corporation (NRDC), who jointly with London Transport financed the construction of a trial length of running tunnel at New Cross through water bearing, sand and gravel with Mott, Hay and Anderson as consulting engineers.

Machine specification

The general principles incorporated in the bentonite machine are described in British Patent No. 1083322.

To this was added an outline performance specification for the trial machine as follows:

  • a rate of advance of 0.6 m/h when excavating;
  • a sealed diaphragm forming a plenum chamber in which the cutting head would operate; the diaphragm was to be capable of withstanding
  • a pressure of 2atm and was to allow access to the face;
  • a tail skin with a minimum overlap of 150 mm with a flexible skirt or seal to prevent ingress of grout or pressurised slurry;
  • a reversible cutting head capable of working at 1-3 revolutions per minute
  • a means of introducing high pressure compressed air through the diaphragm and a means of draining slurry from the working chamber
  • at invert level; an air bleed was required at the crown of the plenum chamber;
  • a method of preventing the machine from running backwards during erection of the lining;
  • the structure of the rotating cutterhead as far back as possible from the excavated face, with the cutting tools projecting well forward, as it was felt that this would encourage the formation of a gel coat on the face.

The contract for the development and trials of the method was awarded to Edmund Nuttall in January 1971.

The main problem in the early design stage concerned the control of pressure in the plenum chamber and the removal of excavated material from it.

It was decided to use a continuous flow of slurry out through the pressure bulkhead at the top of the machine to remove the suspended fine material, and to use scoops attached to the cutting head arms to lift and place by gravity the larger particles into a central chute where a locking device would be required for removal through the bulkhead.

Fresh bentonite slurry is mixed in high-speed mixers and discharged into a reserve tank from where it can be fed into the main reservoir as required. From the main reservoir, slurry is pumped to the tunnel through 150mm pipes by a gravel pump. At the tunnel face a flow of 3,300 litre/min is split three ways into:

  • A 150mm mono pump which delivers 1640 litre/min into the plenum chamber through twin 100 mm pipes set near the invert of the machine;
  • A 100mm mono pump which delivers 570 litre/min to the helical feedwheel; in combination with air bleeds, this forces air which has been gulped on the outlet side to be expelled before it can enter the plenum chamber, where it would cause deterioration of the face. Mono pumps were chosen for their constant flow against variable head characteristic
  • An agitator in the invert of the sump, so that excavated material entering the sump from the feedwheel may remain live and not accumulate in the bottom.

As the shield is driven forward, the excavated material, suspended in the slurry, is pumped to the treatment plant on the surface. The excavated material is separated and transported off site for disposal. The slurry is enriched if necessary and re-joins the main reservoir.

Regarding the slurry treatment plant, it was decided not to attempt to improve methods already in use on slurry trench projects. It was appreciated that for a major tunnel drive, depending on the worksite and the geology, it might be necessary to have a whole range of devices such as settling tanks and centrifuges in the treatment plant.

Testing

Bartlett explains, “Our aim was to prove that a bentonite shield could safely drive tunnels in difficult strata without damaging overlaying property.

“The government’s Transport & Road Research Laboratory (TRRL) took detailed measurements of ground movements above and near the tunnel, and concluded that the movements were trivial when compared to those caused by shield driven tunnels in cohesive self-supporting strata. It was clear that there had been little or no loss ground.”

Experimental drive

A total length of 144m of tunnel was driven, 132.5m of this using pressurised slurry in the face. In the later stages of the trials a normal advance of 2.5m per ten hour shift was being achieved, peaking at 4m.

Using a precautionary 0.5atm of compressed air, the tunnel face was inspected on a number of occasions, and found to be stable with up to 50mm of slurry penetration into the gravel. On one or two occasions there were small pockets above the top of the machine which were thought to be caused by air entering the plenum chamber through the feed wheel.

There was a considerable wear on the outer cutting teeth. Various shapes and materials were tested and the best results were obtained from the tool shape tipped with tungsten carbide.

The machine dealt with material from large stones to very fine sands, but a difficulty remaining is to find an economic method of cleaning very fine particles (below 75 micrometres) from the slurry.

Unexpected success

As Bartlett wrote in his paper (see reference box), “The machine used in the trials is versatile in as much as it will deal with a wide range of granular soils using slurry in the face. It is readily convertible for use as a conventional tunnelling machine for excavating in either consolidated gravels or in cohesive soils such as the London Clay. A step has been taken towards the tunnellers’ goal of a universal soft ground tunnelling machine.”

The experimental tunnel was successful and the ICE awarded Telford gold medals to Bartlett and his colleagues. During the driving of the tunnel, visitors were invited to the site from UK and overseas. Countries in Europe and the Far East had big programmes of urban reconstruction after World War II. Germany and Japan in particular showed an interest in the bentonite shield, and soon started their own prototype machines.

With the patent in the hands of NRDC, Bartlett had no further involvement with the bentonite shield.

“We were busy with metro schemes in a number of cities. It may be that the invention helped us to be appointed. Paradoxically I had difficulty in persuading more than one client that they did not need the slurry shield for their project!”

So what would the man who revolutionised tunnelling hope for the future?

“My own ideas since retiring are all about the sea. I can see the scope for inventions to do with farming the sea.

“If somebody wants to invent something now, I would advise them to look at underwater robots, capable of planting and harvesting crops. When we know more about climate change, we may be able to plant our own coral reefs!”