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Going Underground | Sprayed concrete comes into its own

Soft ground tunnelling using sprayed concrete linings is now an accepted option even in city centres. But it was not always so.

Twenty years ago a seminal report from the Health & Safety Executive (HSE) urged extreme caution when considering the use of what was then known as the New Austrian Tunnelling Method (NATM) for city centre soft ground tunnelling projects. It warned of “major consequences” should there be any failure.

Triggered by the high profile 1994 collapse of three station tunnels on the Heathrow Express, in which luckily no-one died, the report was reissued two years ago. Collapses had continued, including a massive cave-in on Brazil’s  São Paulo Metro project in 2007 in which seven died and which matched almost exactly the scenario predicted by the HSE.

bond street

bond street

Fibre reinforced sprayed concrete is used in primary and secondary linings in London Clay

Yet in the last two decades, expertise and technology have developed dramatically, and now the technique, better known just as sprayed concrete lining (SCL), is in use all over the world.

“NATM could have been abandoned after the Heathrow and other high profile collapses,” says tunnelling consultant Bill Grose.

“But we badly needed a better way of forming complex junctions, especially in very large diameter tunnels. It was up to the industry to learn from these failures and develop a successful technique.”

heathrow collapse

heathrow collapse

The 1994 collapse of the Heathrow Express station tunnels sent shock waves through the tunnelling world

The 1996 HSE report Safety of New Austrian Tunnelling Method {NATM) Tunnels made particular reference to London Clay and contained numerous safety recommendations. These were helpful, Grose adds.

“There has been a significant improvement in the health and safety culture underground. But perhaps the biggest change since 1994 is the technological developments in monitoring techniques and the control of settlement.”

Other technological developments include the increased use of robotic spraying and advances in the understanding of sprayed concrete mix design. Now, says Groce: “For complex shapes in urban soft ground tunnels, SCL is one of the first options to be considered.

bond street

bond street

Bond Street Tube station upgrade encountered tunneller shortage

“Clients are happy, their insurers are happy. The predictability of London Clay makes SCL the method of choice for metro stations and cross passages.”

There were many contributing factors to the Heathrow Express collapses, according to the HSE. There were major flaws in the management of the works, poor workmanship, especially in the formation of the crucial invert joint, and delays in the delivery of the concrete to the workface.

However, the key failings were inexperience, lack of skills and high turnover in the tunnelling crews, coupled with the lack of an experienced management team.

Skilled operatives are much in demand even now. Costain Laing O’Rourke’s £302M transformation of London Underground’s Bond Street station did suffer from what Costain senior tunnel engineer Aled Davies dubs the “Crossrail effect”. 


We badly needed a better way of forming complex junctions, especially in very large diameter tunnels

Bill Grose, tunnelling consultant



“There was a massive demand for skilled nozzlemen when Crossrail was in full flow, so we had to do a lot of training”, he says. “We had to work closely with Joseph Gallagher Ltd, our supplier of specialist tunnelling operatives.

 “Fortunately we’ve been blessed by an ongoing programme of tunnelling works that has allowed us to develop and retain a team of skilled engineers.”

The seven year upgrade of the Tube station is due to open to the public next April. Capacity will be increased from 155,000 passengers a day to more than 225,000. There will be a new entrance and ticket hall with step-free access to the Central and Jubilee lines, and step-free access to the new Crossrail/Elizabeth Line station.

Tunnelling here involves traditional and sprayed concrete linings, almost entirely in London Clay. Total length is 550m. Round the clock working is the norm, with three shifts. Much of the work is carried out with the station fully operational.

Success of the Crossrail SCL work at Bond Street station is reflected in the lack of serious subsidence

An advanced wet process concrete mix containing 35kg of 35mm long steel fibres in each cubic metre is used for both the primary and secondary linings. “We decided early on to use the same mix for both linings, to simplify the logistics,” says Laing O’Rourke engineering and assurance manager Matthew Prentice.

“Typically the primary lining is 250mm thick, as is the secondary. A BASF Masterseal waterproofing membrane is spray applied between the linings.” 

Logistical challenges

Structural work on the project is now virtually complete. Some 46,000t of spoil has had be dealt with. The site’s footprint on Oxford Street is only 440m2, which presented a whole range of logistical challenges.

On the Crossrail project there were some problems with the sprayed concrete linings – which the Costain Laing O’Rourke team have learned from. “The main lesson is that work must be as safe as possible,” says Davies.

“Which is why we were happy to host the trials of strength monitoring using thermal imaging (SMUTI) (see box).”

The success of the Crossrail SCL work at Bond Street station is reflected in the lack of serious subsidence affecting buildings on neighbouring Oxford Street. The success of this project, and of Crossrail, show that SCL tunnels can be constructed safely under city centres and close to existing underground infrastructure. The HSE’s warnings still apply, however.

Skilled operatives, well-trained supervisors and sophisticated monitoring are the keys to success. Given these are available, projects like the Thames Tideway Tunnel can be undertaken with confidence.


Strength monitoring

Five years after Benoît Jones first came up with the idea, his strength monitoring using thermal imaging (SMUTI) technology is nearing the end of its first large scale trial below the streets of London. 

Since early March this  year it has been deployed on Costain Laing O’Rourke’s Bond Street London Underground station extension project to verify the strength development of the freshly sprayed secondary concrete lining, primarily to ensure the safety of those working at the tunnel face.

bond street

bond street

An engineer using SMUTI to check the strength development in the secondary lining

“This has been the perfect project to test SMUTI”, says Jones, now of Inbye Engineering but also a veteran of several major tunnelling projects.

“We’d done earlier smaller scale trials on other projects, but what we needed to do was test it in a production environment over an extended period of time. Luckily, London Underground has a strong engineering team which appreciated the benefits and challenges of the technology.”

“It was soon obvious that SMUTI was worth investigating,” says Costain senior tunnel engineer Aled Davies. “It’s simple to use – you could train someone to operate it in five minutes. In practice the engineers who were to use it received a 90 minute training course to explain the technical background.”

Based on a standard thermal imaging camera, a SMUTI device monitors the temperature changes in the fresh lining as the exothermic cement hydration progresses. There is a direct relationship between the temperature development and the degree of hydration that has taken place, and an equally direct link between degree of hydration and compressive strength.

No-one should ever underestimate how dangerous tunnelling can be

“The big difference between sprayed and conventional concretes is that you can’t spray concrete into a cube mould,” Jones says. “This makes it hard to carry out direct strength testing.

“So we have to make up samples of the mix and do tests in the laboratory as well as doing some special calibration tests on panels in the tunnel.”

Open face tunnelling in London Clay is relatively straightforward – provided the exposed clay is not left unsupported for too long – no more than 18 hours or so, according to the Health & Safety Executive. Excavation and lining must proceed at a steady speed. This means operatives moving into freshly sprayed sections as soon as their safety can be assured.

Before SMUTI, tunnellers had to rely on the interpretation of results from small test panels. These could never be truly representative of the entire lining, so the direct measurement possible with SMUTI makes safety assessments much more reliable. 

Jones believes SMUTI will be most effective when used in conjunction with other survey and monitoring techniques. Davies concurs, adding: “No-one should ever underestimate how dangerous tunnelling can be.

“Trial results are now being analysed, but we believe SMUTI will become the main method of early strength monitoring on all sprayed concrete tunnels.”




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