John Bartlett, Bentonite Tunnelling Machine inventor whose work included the Victoria Line and the Dartford tunnels – obituary

John Bartlett, who has died aged 94, was the civil engineer responsible for building the Victoria line in London and for designing the British side of the Channel Tunnel.

He was also the inventor, in 1964, of the Bentonite Tunnelling Machine (BTM), a device which allowed the excavation of tunnels in loose soils that would previously have been difficult and costly (or even impossible) to dig, and developments of the concept enabled a rapid increase in tunnel construction around the world.

Before the BTM, tunnelling in soft soil involved the creation of a sealed box pressurised with air, from which workers would dig the tunnel, with the tunnel face needing continuous support. It was dangerous work, not only because there was a constant risk of groundwater flooding the works causing instability, but also because workers in such conditions often suffered health problems such as "the bends" and bone necrosis.

It is no accident that most of the London Underground network was developed north of the Thames, where predominantly clay soils raise no instability problems.

Bartlett's idea for a better means of tunnelling came during a visit to Milan in 1963 to advise on burrowing through the city's gravel deposits. He observed how workers building Milan's first metro line, using "cut and cover" techniques, constructed vertical retaining walls in loose ground by exploiting the properties of bentonite – a commonly available clay with "thixotropic" properties, becoming thick or viscous under static conditions, but liquid slurry when agitated or stressed.

Bartlett's machine, patented in 1964, featured a cutting head revolving within a sealed bulkhead filled with a slurry of bentonite pumped in under pressure. The idea was that the pressurised slurry would balance the water pressure in the ground, preventing it from flowing into the tunnel and causing instability, so that soft soils would not collapse before the tunnel was lined with cast iron or concrete segments as the machine advanced. Meanwhile, the workmen behind could work in air at normal pressure.

A unique part of the BTM was the use of the slurry itself to transport the excavated soil from the bulkhead via a system of pipes to separation tanks on the ground surface, from where the slurry was then recirculated back to the pressurised bulkhead.

It took many years to develop, but in 1971 a prototype machine was built to drive a 144 metre experimental section of tunnel in New Cross, south-east London, for London Transport, which was planning an extension of the Fleet line (as the Jubilee line was then known, after the subterranean river Fleet) through South London.

The experiment was a success and it was discovered that in larger-grained soils the bentonite would also penetrate the ground for a short distance ahead of the excavated face, providing additional stability.

The experimental "Bartlett Tunnel" had been due to be put into use as part of a Jubilee line extension to Thamesmead which was never built, so for many years it remained sealed up.

Bartlett's BTM was the first of a new generation of slurry tunnelling machines (and, later, earth pressure balance machines) though sadly further development was taken forward outside the UK, mainly by German and Japanese companies, as, apart from a single sewer contract in Warrington, no suitable projects emerged in Britain for many years, because of public spending constraints.

None the less, the descendants of Bartlett's BTM have been used in major UK civil engineering projects – including the Jubilee Line to the south of the River; the Channel Tunnel Rail Link Crossing; the Crossrail River Crossing, and the Lee Tunnel, the first section of the Thames Tideway "super sewer".

John Vernon Bartlett was born on June 18 1927, the son of a civil engineer, Vernon Bartlett, and his wife Olga (née Testrup); Vernon was senior partner of Mott, Hay and Anderson, the firm John was later to head for 30 years.

At Stowe, John was captain of cricket and rugby and head boy, and after National Service (1946-48) with the 9th Airborne Squadron of the Royal Engineers he read Engineering and Law at Trinity College, Cambridge; there he acted and sang with the Footlights just as the first works of Julian Slade (who wrote the musical Salad Days) were appearing.

Bartlett performed in Slade's Bang Went the Meringue, and his success in it led to an audition with the D'Oyly Carte company, which he attended in the largest sombrero he could find and a costume to match. But he decided that his future lay in tunnels and bridges instead. (He would return to the Army in 1978 as a major in the Engineer and Railway Staff Corps, TA, promoted to colonel in 1986.)

He began his career as a civil engineer with John Mowlem in 1951, but spent most of his career with Mott, Hay & Anderson, which he joined in 1957 when it was a small consultancy, and retired as chairman and senior partner in 1988. He led merger talks with Sir M MacDonald & Partners that led to the formation of Mott MacDonald, now one of Britain's leading firms of consulting engineers.

Over those years he worked on the first Dartford Tunnel, the first tunnelled sections of the Toronto Subway and the Melbourne Underground Rail Loop in Australia, and he was project engineer for the Victoria Line. He also had design responsibility for the Channel Tunnel in its various incarnations, first as principal designer and later as the main design consultant for all civil and geotechnical engineering on the UK section.

Bartlett was also fascinated by ships and sailing, collecting a huge library on maritime history from which, in 2003, he donated 6,000 volumes to the National Maritime Museum Cornwall, which named its Bartlett Maritime Research Centre after him. The gratitude of the maritime world was only rivalled by the delight of his wife Gill at the removal of the books from their home in Wimbledon. His publications include works on tunnelling and Ships of North Cornwall (1996).

He was a founder member and chairman (1977-79) of the British Tunnelling Society, winning its James Clark Medal in 1994. In 1978 he was elected a Fellow of the Fellowship (now the Royal Academy) of Engineering and in 2018 received its Sir Frank Whittle Medal. He was President of the Institution of Civil Engineers in 1982-83.

Bartlett was a genial man with a ready laugh that was described as "not loud, but physical". On one occasion, while on holiday in Cornwall and watching a Marx Brothers film at the Wadebridge cinema, he had to grip the back of the seat in front of him for support, and the noise of wood cracking announced that he had demolished the entire row of seats.

He was appointed CBE in 1976.

In 1951 Bartlett married Gillian Hoffmann, with whom he had four sons. Such was his love of boats that the first matrimonial home was Demure, an old Medway pleasure craft moored in the tideway at Putney.

John Bartlett, born June 18 1927, died November 17 2021