The cut and cover technique has been used successfully for shallow tunnelling and deep tunnel portals for many years utilising a host of different techniques to retain the in situ soils in cut to allow the cover to be constructed. These have been cost effective alternatives to bored tunnels and where space constraints in an urban area restrict the use of large open earthwork cuttings. Bored pile or diaphragm walls offer an effective retaining system for the ‘cut’, enabling cut and cover tunnels to be constructed safely, close to neighing structures. As the cuts get deeper the technique will change as the required wall verticality improves.

System
Cut and cover tunnels can be constructed in one of two ways. These are bottom up and top down.

Bottom up is the simpler system, where the walls are constructed, excavation of the box is carried out, the base is installed and the cover or permanent props are constructed.

Top Down involves the walls being constructed, shallow excavation carried out, cover or permanent props constructed, excavation completed and then the base installed. Excavation may be slower and more expensive, but this may deliver benefits to the main project programme.

Techniques
Contiguous piled walls are piled walls installed in a sequence with a gap between the piles to avoid damage to neighbouring piles. This limits their use to soils that will not fall out during excavation and where the water table is below the invert of the tunnel or can be controlled during excavation. Contiguous walls are faster and cheaper to install than secants but have some limitations. They can be installed using high productivity continuous flight auger (CFA) rigs or high power rotary rigs. This technique was chosen for the major part of the Thames Link Box at St Pancras Station in London as it offered programme saving during a demanding blockage period.

Secant piled walls are normally the preferred system of piled walls that interlock or secant together to form a stable wall with a high level of water tightness.

These are also installed in sequence with the primary or female piles installed first, followed by the reinforced secondary or male piles that cut into the primary piles. The concrete used for the primary piles tends to be a weaker mix than the secondary piles, giving rise to the terms hard/soft, hard/ firm or hard/hard walls. These can be installed using high productivity CFA rigs, high power rotary rigs, or more recently high power and high productivity cased CFA rigs, which have enhanced verticality tolerances.

Diaphragm walls are a heavy-duty wall that tend to be employed where a stiffer wall is required or where the depth of wall precludes use of secant piled walls due to the verticality and depth. Current methods use rope or hydraulic grabs, or hydrofraise/milling techniques. All three methods require a bentonite support fluid or similar. Low headroom rigs are available if work is required under existing structures. The technique used will depend on the depth/verticality required but, as ever, has cost implications, with more technical systems costing more.

Sufficient space on or near site needs to available for bentonite de-sanding and storage; productivity can be reduced if sufficient space is not available.

Alternatives
Other successful cutting solutions have included modular systems such as reinforced earth panels and precast arches such as Techspan, that were used successfully in the UK on a number of contracts on the CTRL project.

Wall Propping
With all cut and cover works, temporary propping needs to be considered, as this may be required until the ‘cover’ is in place. The cost and inconvenience of the propping system needs to be compared with thickening up the piled or diaphragm walls to stand unsupported in cantilever. These props can be anchors, raking or horizontal props.

History has shown that this subject is ignored at one’s peril.

Formwork and ‘the cover’
Formwork making up the temporary props and frames is put in place during the casting of the in situ concrete. This will form the ‘cover’ part of the construction.

Usually mounted on a carriage on runners, the formwork array is moved along the alignment as each concrete element of the tunnel section – including base plate, walls and slab – is cured.

These three elements of the completed tunnel can be cast separately and sequentially, or in different semi-monolithic combinations depending on requirements.

As an example, one semi-monolithic combination is trough construction. In this, instead of advancing ahead with the base plate, following with the walls and then finishing with slab, the walls and base plate are cast as one unit.

The purpose is to ensure absolute water tightness, removing any risk of ingress through the joints between base plate and wall. The wall formwork would hang on a portal carriage encompassing the width of the tunnel. The slab formwork follows afterwards on another formwork carriage.

Formwork loads
Mobile or crane-movable formwork is used for the walls, depending on the length of the structure. Wall formworks are normally designed for a fresh concrete pressure of 60kN/m2. Slab formworks are designed for the formwork self-weight and wet concrete load and live load.

Rental market
Some 80 to 90 per cent of the formwork to a cut and cover project is rented. This makes the treatment of the materials very important. Bernhard Lindner of Peri says, "There has been a tendency in the Middle East to not return rented equipment, and rather face the charge for that. This is mostly on high-rise projects, and the cost for returning the formwork damaged could be equal to two to three year’s rental cost.

Lindner adds that heavy steel equipment treated well could last 20 to 25 years. Site conditions are so important that design life calculations are only done for financial forecasts. Eight to 10 years is assumed.

Options and concerns
For efficiency and quick movement along a project, the number of components making up the formwork is not of real importance to a contractor. Once set up, the loading and balance as well as the economic fitting of the carriage is the main influence on an effective advance along the alignment.

Despite this, Lindner adds that the number of components is a concern for Peri, but purely for warehouse stock keeping. The company’s approach to formwork efficiency is to provide a product that can largely be reused, and also used on different types of projects.

Peri’s ‘Variokit’ was designed to be used on bridges and other engineering projects as well as tunnels and is held together by special connecting pieces of formwork and either pins or threaded bolts. Peri pushes for the pin option as bolts have to be pre-tensioned and need to be scraped after use, so it is less desirable to rent them. Bolts also leave a slight gap in the hole, resulting in geometric deflections. Lindner adds, "We prefer pins because of this. You can get a stronger connection, and deal with greater loads."

Shifting
The movement of the formwork is an option for the contractor. Lindner says that in less developed countries there is a danger of unskilled labour damaging the hydraulic units that shift the carriage. Then there is the issue of expense. Usually in the same countries, manpower is cheap, meaning that the carriage is more likely to be pushed by hand, solving both problems. In more developed countries, hydraulic units can be used to raise and lower formwork, and move it along the rails.

Materials
Steel is the most economical material for heavy duty engineering props and other formwork with high loads. Aluminium heavy load bearing props with the same capabilities have been developed but the benefit of using lightweight props on the load is insufficient for use in tunnelling.

Lindner believes that this won’t change in the next decade, and while companies are doing research into plastics, the loads are just too high.

Higher grades of steel are preferable.