Our expert teams have delivered world-class levels of innovation, devising many techniques that have since been adopted as industry best practice.

SmartShellTM lining system

Lee Tunnel

SmartShell™ is the design and use of a high performance steel fibre reinforced concrete (SFRC) as a tunnel lining material, cast ‘in place’ using a full-round shutter. Developed through an extensive experimental programme supported by physical testing at the Building Research Establishment with the University of Warwick, it has been implemented for simpler and safer construction of the secondary lining for the Lee Tunnel, Thames Water’s largest project in 20 years.

On the Lee Tunnel project SmartShell™ has delivered:

  • Enhanced durability of the tunnel lining
  • A saving of 15,000 tonnes of steel reinforcement
  • Improved buildability of the structure with increased safety
  • Cost savings of £13.5 million
  • An optimised construction cycle

SlipStressTM lining system

Lee Tunnel

The Lee Tunnel project consists of a 7.2m diameter, 6.9km long tunnel, and five of the largest and deepest hydraulic shaft structures ever built in the UK.

By taking a holistic approach to design and construction methodology on this project we developed SlipStress™, a shaft lining system that enabled 5000t of traditional rebar to be removed from the substantial shaft linings (up to 43m diameter by 85m deep). Double slipforming methods virtually eliminated shrinkage cracking, and concrete placed under pressure in the annulus also ensured that linings did not suffer damaging tensile stresses during service.

These innovations significantly improved the watertightness of the shafts, increased speed of construction, and delivered a £5 million cost saving to our customer along with a 60 per cent reduction in embodied energy and a 35 per cent reduction in carbon.

UK’s largest SFRC segmental tunnel lining

Lee Tunnel

Working as tunnel and shafts designer for the Lee Tunnel project, we designed the UK’s largest Steel Fibre Reinforced Concrete (SFRC) segmental tunnel lining.

UltraShellTM lining system

Thames Water Ring Main (TWRM) Extension Brixton to Honor Oak and Coppermills

UltraShell™ is a High Performance Fibre Reinforced Cement Composite (HPFRCC) developed as a permanent lining in the underground environment that can be sprayed or cast in situ.

Exhibiting high ductility, energy absorption capability and toughness, UltraShell™ is an extremely ductile material able to absorb significant levels of strain, and is able to visibly flex or deform prior to failure, behaviour similar to that exhibited by most metals under tensile or bending stresses. Such behaviour ensures extremely tight crack control and high levels of watertightness.


TWRM Extension Brixton to Honor Oak

The patented Quadracurve™ ‘one size fits all’ precast lining system allows construction of non-circular tunnels and shafts to optimise the available space.

Manufactured from a single form, Quadracurve™ lining systems can be quickly adapted to form a wide range of shapes and sizes.

First 100 per cent waterproof sprayable membrane safe for underground use

TWRM Extension Brixton to Honor Oak

The waterproofing of sprayed concrete tunnel linings has traditionally been carried out using sheet membranes encased between the sprayed concrete primary lining and a cast in situ secondary lining. While this method is acceptable for linear tunnel construction, it is less suitable for complex geometrical shapes such as junctions.

We worked in close collaboration with the specialist waterproofing company Stirling Lloyd to develop an effective sprayable waterproofing system that can significantly improve productivity, particularly on projects with complex geometry.

Single pass lining system for hydraulic tunnels

TWRM Extension Brixton to Honor Oak

To improve the resilience of the Thames Water Ring Main we designed a five kilometre tunnel using a method that eliminated the need for a secondary lining.  By pre-casting the concrete segments with a watertight seal, six months was saved on the three year programme.

This project was the Institution of Civil Engineers (ICE) London Awards 2010 Special Award Winner.

Zero Impact aquifer tunnelling method

Croydon Cable Tunnel

Development of a chemically inert one-pass tunnelling system suitable for use in the most sensitive aquifers used for public water supply.


Heathrow Terminal 5 tunnels

LaserShell™ enables real-time and remote construction of any underground space through use of a specialist geospatial survey system, TunnelBeamer™, in conjunction with a high quality steel fibre reinforced sprayed concrete.

Offering an inclined advance sequence and systematic profile control, LaserShell™ not only greatly enhances safety, but also enables tunnels to be constructed more quickly and to higher quality than traditional methods.

LaserShell™ was first used as part of substantial tunnelling works for an airside road tunnel beneath Heathrow Terminal 5, resulting in cost savings to our customer of more than £10 million.


Heathrow Terminal 5 tunnels

Developed to facilitate the LaserShell™ technique, TunnelBeamer™ is a real time guidance and surveying system that provides live reporting of workmanship and visualisation of the end product, delivering significant improvements to safety and cost.

Computerised caisson shaft jacking system

Preston UID

Through using a computer-controlled jacking system, teams have complete control over the shaft-sinking process. Since pioneering this technique it has been used to sink more than 100 wet and dry caisson shafts at diameters up to 21m.

MultiModeTM TBM

Heathrow Terminal 5 tunnels

To ensure continued operation of the airport and the safety of tunnel construction as little as 4.5m below the operational airfield, we developed the MultiMode™ Tunnel Boring Machine (TBM) in collaboration with TBM manufacturer Herrenknecht and specialist pump supplier Putzmeister.

The machine uses compressed air face support with a novel piston displacement pump to maintain the seal. The MultiMode™ TBM provided successful delivery of the works, with negligible disruption to surface structures. In addition, all of the spoil from the tunnel could be reused in the earthworks on site.

Grey Rock Tunnel Design Philosophy

North Downs Tunnel, Channel Tunnel Rail Link

The 3.2km North Downs tunnel required construction of the largest cross-section tunnel ever to be built in the UK at 174m2.  We were central to the development of value engineering proposals that enabled this project to be completed five months ahead of schedule while saving £10 million.

The ‘Grey Rock’ approach considers the primary lining as enhanced quality ground surrounding the tunnel, leading to significant reductions in the long-term lining loads on the secondary lining. When combined with the accurate establishment of in situ stresses from hydrofracture testing this philosophy enabled:

  • Reinforcement to be entirely removed from the tunnel secondary lining
  • The tunnel to be constructed with a flat invert rather than the deep dished reference profile
  • The lining thickness to be reduced from 600mm to 350mm, enabling substantial material savings along the 3200m length of tunnel.

Self-cleansing tunnel lining

Hastings Bathing Water Tunnels

At the Hastings Bathing Waters project, in order to reduce the frequency of sewage spills to the sea, a large storage volume needed to be created to store the combined flows until they could be treated and subsequently discharged.

Our tunnel lining design incorporated a self-cleansing profile into the precast concrete segments, thereby substantially reducing the second stage concreting works.

Instead of a major concreting operation that would have taken in excess of six months, the tunnel was completed by slipforming in a few days.


Heathrow Baggage Tunnel

The CombiShell™ tunnelling method was used to form complex underground structures required to accommodate the baggage handling equipment under the operational airport at Heathrow, resulting in a 25 per cent cost and programme saving. It uses a high quality permanent sprayed concrete lining in conjunction with the secondary lining to form the permanent structure.

World’s first steel fibre reinforced concrete tunnel ring

Heathrow Baggage Tunnel

We designed the world’s first steel fibre reinforced concrete tunnel lining, eliminating the need for reinforcement steel and resulting in a 50 per cent reduction in the cost of the tunnel lining. Our solution provided an ultra-thin, high-strength lining with significant reduction in damage during construction.

The use of steel fibres has since become commonplace in tunnel lining design.

Inverted dome membrane shaft base slabs

Bolton Water Street Storage Shaft

Introduction of a dome-shaped shaft base design led to a 30 per cent cost reduction at our Bolton Water Street storage project. This method has been used successfully on other contracts at Folkestone, Dunstable and Hastings.