The traditional design approach, using lattice girders and steel mesh, sheet waterproofing membranes and cast in situ secondary linings has been significantly overhauled, providing many benefits to large projects. Typically current SCL design combines spray applied waterproofing membranes with sprayed concrete secondary linings. The design approach for all of the permanent sprayed linings, including the primary layer, maximises the use of robotics and mechanics that are capable of producing a high quality product in a safer, more robust and durable manner.
Setting a benchmark
Rather than taking a passive approach, and waiting for value creating design solutions from contractors at a later design stage, designers should be actively pursuing best practice and benchmarking it into the standard design. Discussions with product manufacturers and materials scientists have allowed understanding of the benefits and limitations of their design solutions and then apply them to SCL designs. Complex numerical modelling is carried out and calibrated to accurately predict the ground structure interaction thereby designing SCL tunnels with confidence. This is particularly important on high profile projects such as those located below the busy, high urban value streets of major cities.
The main advantages for these design developments can be seen through the primary driver – the reduction of construction risks, and therefore improved safety. Comparative risk assessments have been undertaken to compare a traditional sheet membrane and cast in-situ secondary lining approach with the spray applied waterproofing membrane and sprayed secondary lining for a typical sized platform tunnel in the context of soft ground tunnelling in relatively impermeable ground. It was found that the sprayed system offers significant safety improvements, most notably in reducing the amount of manual handling, work at height and the combination of both. The methods proposed for the primary lining; using remote measuring technology, which allows construction of linings to profile without the need to install markers or lattice girders, and robotic spraying booms that can be controlled without exposing workers to risks of falling material, can also be applied to the waterproofing and secondary linings, allowing the benefits to be maximised throughout the whole construction cycle.
The advantage that follows on from this is a simplified, faster construction method, saving up to an estimated 25 per cent on programme for a typical underground station compared to traditional SCL tunnelling designs, with associated cost benefits. By using sprayed concrete, rather than cast in situ shutters and formwork, the installation of the secondary lining is both flexible and unrestricted, allowing the contractor scope to make significant programme benefits and maximise the productivity of their staffing. Once trained appropriately the crews can be assigned to install the primary lining, the waterproofing or the secondary lining using overlapping skill sets and building on their experiences.
Where SCL design is now, and where it is heading
As discussed in the papers by (Thomas and Picket, 2011, 2012) there are now several options for SCL tunnels open to tunnel engineers to suit different geological and hydrological conditions and/or the client’s functional requirements.
The SCL options can be broadly categorised into three types:
? The traditional double shell lining (DSL)
? The contemporary composite shell lining (CSL)
? The single shell lining (SSL)
In most cases a waterproof membrane is employed to provide a watertight structure (in CSL solutions this is generally between the primary and secondary linings).
Double shell linings (DSL)
The DSL approach assumes a sacrificial primary lining which takes the temporary loads and a secondary lining to take the permanent loads. This has significant precedent, however because the primary is considered temporary the secondary is designed to take all long term loading, the resulting lining system is a lot thicker than comparable CSL tunnels and therefore the DSL method is not considered further in this article.
Composite shell linings (CSL)
The CSL method is where the primary lining takes the temporary loads and a proportion of the permanent load through composite action with the secondary lining. Through recent projects such as the A3 Hindhead Improvement Scheme and Thames Water Hampton shaft, the use of sprayed waterproof membranes have given engineers an opportunity to explore the benefits of a composite shell lining, i.e. a sprayed permanent primary lining, sprayed waterproof membrane and a sprayed secondary lining, where the primary lining acts compositely and takes a proportion of the long term ground loads.
A key step that had facilitated this leap forward has been the omission of lattice girders and the use of laser profiling systems to control the shape of the tunnel during construction.
Lattice girders are usually not regarded as structural members but they have been seen as essential in controlling the shape of the tunnel. They are notoriously difficult to spray around and leaks, and therefore corrosion, often occur at the location of the lattice girder. Removing girders removes a corrosion problem, thereby giving an opportunity for a permanent sprayed primary lining, and also reduces the need for men to work at the face when the full support is not in place.
Composite linings are now being incorporated into major projects, typically under the following design conditions, as shown in Figure 2.
? The full 100 per cent ground and hydrostatic loads applied to the primary lining in the short term
? The option of load sharing for the ground loads in long term
? Full hydrostatic load applied to secondary lining in the long term
? No bond or shear capacity between linings is used in the structural design
This design methodology has resulted in some reductions to the thickness of the secondary lining when compared to conventional double shell linings but this is fundamentally limited by the assumption that the water pressure acts on the membrane. For a shallow tunnel in soft ground, the water load is similar or even exceeds the ground load. The percentage of ground load on the secondary lining is usually determined from numerical models and it varies depending on the loading behaviour of the ground.
In materials such as clay, there is a distinct short and long-term behaviour, while in others there may be little or no change in the loads over the lifetime of the project from the loads generated during the construction period. In other words, without some consolidation or rheological behaviour in the ground, the secondary lining may not experience much of the ground load.
The first layer of sprayed concrete – the sealing layer of 75mm sprayed concrete – can be regarded as temporary and omitted from the design in the long-term. This is due to concerns over sulphate attack and poor quality when spraying on to the excavated surface.
As T&TI goes to press there is further study and testing being undertaken to demonstrate a fully composite lining as shown in Figure 3, i.e. shear and bond strength at the interface of the waterproof membrane. Once this is ascertained further reductions could be achieved for the thickness of the secondary lining:
? Composite action between linings by achieving shear capacity across membrane-concrete interfaces
? Load sharing for the ground load (GL) and water load (WL) in long term membrane-concrete interfaces
? Full hydrostatic load applied to secondary lining in the long term
? Bond strength on membrane interfaces to be 1MPa
? Shear strength on membrane interfaces to be 2MPa
The advantage, as discussed above, is the reduction to secondary lining thickness without compromising the watertightness requirement. The main disadvantage is that there is currently no precedence for a fully composite lining with a spray applied membrane. However, single shell permanent sprayed concrete linings have been successfully used on a number of projects such as Heathrow Terminal Five (Hilar, Thomas and Falkner, 2005) and the design for Hindhead (Tucker, Stephenson, Chilton, 2010) considered both load cases – with and without full composite action.
Single shell linings (SSL)
The SSL approach is where one lining takes the temporary and permanent loads, although this one lining may be built up in several passes. Single shell linings offer the most efficient lining design (in dry or largely dry ground) as they take both the temporary and long term loads and the construction is very fast compared to a double shell or composite lining where there are both primary and secondary lining stages to the construction. Single shell linings have been widely used in the hydropower sector and in all tunnelling sectors in certain countries, most notably within Norway.
? No waterproofing membrane
? Ground loading all on primary
? Lining
? No hydrostatic load
? Watertight concrete design – but allows local seepage
? Optional drip trays provided outside architectural cladding
The main disadvantage is that clients will tend to opt for watertight tunnels thereby avoiding operation and maintenance issues and drainage systems. Unless the ground is dry or generally impermeable – such as London Clay – it is hard to achieve watertight tunnels with single shell linings. That said, this can still remain as a design option for non-public tunnels where lower levels of watertightness are acceptable.
As with a fully composite type CSL, the ability to provide a single shell lining with a waterproof layer is being investigated by Mott MacDonald. This involves looking at the new technologies in sprayed waterproof membranes such as thin skin liners and whether they can carry out a dual role of initial support (sealing layer) and long term waterproofing. Other products are also being investigated such as the inclusion of polymer modifiers to give waterproofing capabilities. These may be incorporated into a sprayed concrete initial layer (approximately 25 to 50mm) and/or a single pass SCL primary lining. The option of a hard sprayed waterproof membrane doubling as the final finish of the lining is also being reviewed.
Hindhead project case
A recent application of the CSL method of sprayed concrete tunnel construction was at the A3 Hindhead Improvement Scheme in Surrey, UK (Tucker, Stephenson and Chilton, 2010). This project involved construction of 6.7km of new dual carriageway, designed to alleviate the traffic congestion caused by traffic signals and a section of single carriageway on the A3 London to Portsmouth trunk road. Some 1.8km of the new dual carriageway was located within twin bore tunnels, constructed through a highly variable mix of sandstone and sand, with more clayey sand towards the southern end. The tunnel alignment was selected to ensure that the water table along the route was kept below tunnel invert level, meaning that a drip-shield style waterproofing system was the appropriate solution.
The Highways Agency as the client employed main contractor Balfour Beatty under an early contractor involvement contract, with Mott MacDonald as contractor’s designer. Having the contractor and design team on board at an early stage enabled many options for construction of the scheme to be explored. The SCL method was selected over a TBM mainly because of the high capital cost associated with a TBM. Excavated material savings of approximately 20 per cent were also achieved through geometric optimisation (the horseshoe shape) and lining thickness minimisation.
The lining system at Hindhead consisted of a 200mm fibre reinforced sprayed concrete permanent primary lining followed by a spray applied waterproofing membrane system. Secondary linings consisting of 150mm fibre reinforced sprayed concrete in the crown and 200 to 300mm cast in-situ concrete on the walls were then placed onto the waterproofing.
The primary lining was designed to be the permanent load bearing structure, providing all the required support throughout the design life of the tunnels. The secondary lining was designed to resist any water pressure developed onto the lining in the long term. It can also be considered sacrificial in the event of a fire, protecting the primary lining and ensuring its integrity will not be compromised by the heat generated.
Primary lining concrete mixes were developed by the contractor in order to meet the performance requirements specified by the designer. The requirements included short-term strength gain, long term strength, permeability, energy absorption and shrinkage as well as construction requirements for workability and retardation. Secondary lining concrete mixes had additional requirements for performance under fire conditions and the project team carried out large scale fire tests at the Building Research Establishment (BRE).
These enabled selection of the optimum mix of fibres and aggregates. The fire tests proved that the inclusion of monofilament fibres into the secondary linings significantly reduced concrete spalling under fire conditions.
The tunnel was excavated with large bucket excavators using a laser guidance system. The primary lining was then placed using remote controlled sprayed concrete robots, meaning workers did not have to perform tasks right at the face. Instead, they operated the machinery from several metres behind the face where the primary lining had gained sufficient strength to ensure their protection from potential face collapse or falling debris.
Mott MacDonald developed a ‘menu’ of pre-designed support solutions to ensure the ground support installed matched the geological conditions encountered at each face. The decisions about which support type should be used were reviewed at daily review meetings at which data from observations and measurements were examined. The data included ground settlements and tunnel convergence, geological face logs, records from probing ahead of the tunnel face, sprayed concrete strength gain records, and quality, environmental and tunnel inspections.
The review resulted in the issue of a ‘Required Excavation and Support Sheet’ – in effect a permit to dig. The daily reviews allowed the contractor to achieve economies when practicable, saving time and cost without compromising safety.
The spray applied waterproofing membrane (Naylor, Stephenson, Salak, 2011) and secondary lining crown used the same skills and equipment as the primary lining, meaning that the same team could be used for each of these activities.
The secondary lining walls were cast using single-sided shutters to ensure access to the tunnels was not restricted. These measures allowed the team greater flexibility in the programming of work and optimised the quality of the final product.
Best practice specifications for product performance and installation were developed, resulting in a robust system that could be applied with confidence. The Hindhead improvement scheme opened on programme in summer 2011.
In the UK Mott MacDonald’s recent projects include A3 Hindhead, the Heathrow Express Rail Link and Piccadilly Line extensions to Terminal Five and baggage tunnel at Heathrow. Internationally projects include new underground nuclear waste repository at Bataapati, Hungary, Beacon Hill station and the tunnel in Seattle in the US, West Island Line in Hong Kong and the Delhi Airport Link in India.
