One factor that differentiates tunnelling plant from plant on any other heavy construction job is the focusing of activity to and from one or two comparatively small openings in the ground. The second difference is that the plant forms part of an operational chain that can be many metres long. System reliability and the ability to ease potential bottlenecks are major points to consider in selecting surface plant to ensure the smooth flow of materials and workers to the construction area, and excavated materials away from it. There must also be contingency arrangements for emergencies.
Major purposes of surface plant and equipment:
A flat, hard surface often has to be constructed before the start of tunnelling projects. Some sites, such as that for recent sewerage work in Bangkok, Thailand, even involve working over water. Here, special platforms were needed, adding to the usual restrictions of working in a congested urban environment. Strong, load bearing surfaces are often required to support heavy lifts like TBM sections, whether these are handled mainly by mobile cranes or portal cranes subsequently used for general materials handling.
Spoil handling
The flow and type of spoil expected are obviously major factors in selecting plant to handle it. Material can vary from hard cobbles to thin slurry and, consequently, the plant required and means of disposal also vary greatly. While slurry systems can be very convenient inside the tunnel due to small section requirements, on the surface, slurry reconditioning and solids removal require specialist plant. A high proportion of fines or clay in the slurry can necessitate large settlement tanks and/or special flocculation arrangements to increase particle size.
Conventional mobile construction plant like wheel loaders and dump trucks is used for both underground and surface work like highway tunnels and other large-section excavations with drift or adit access. Employing this kind of plant underground makes attention to exhaust emissions and noise more important and rules out the use of petrol engines with their more inflammable fuel.
Conveyors provide another means of ‘seamless’ spoil removal if suited to the site layout. REI recommends the siting of main conveyor head-ends outside the tunnel if possible. Drifts or adits on hillsides will necessitate temporary support structures for belt conveyors. On the Naples Metro, an REI elevator loaded two spoil hoppers from the shaft, easing the loading of road trucks for disposal. However, on the Lille Metro, the risk of clogging led to the choice of shaft skips hoisted by a portal crane rather than a belt system.
Once the solid spoil has been reduced to manageable volumes, the next question is the means of disposable. Often, expensive and lengthy transport is required, which can be an important environmental matter, necessitating minimum precautions like a portable weighbridge to avoid highway truck overloading and wheel washing facilities.
Materials handling
The efficient supply of bulk and bulky materials for both primary support and final lining is vital to successful underground construction. Railbound transport from the surface is a favourite for large masses like lining segments and bulk materials like concrete. If a vertical or steeply inclined access way has to be negotiated, a surface hoist will be required which can range from a full shaft headgear to a winch housed in a cabin. In larger projects, common use is made of portal cranes sited over the shaft, which can also be designed as travelling cranes to facilitate transport of larger items.
Tunnel support and lining requiring mass in- situ or wet sprayed concrete will probably necessitate batching plants on the surface. In developed areas, pre-mixed concrete can be used, although the volumes likely to be required for substantial tunnelling projects may constitute a transport nuisance. Most batching plants require bulk supplies of cement and fine aggregate stored in silos. If the concrete mix is pumped underground, sufficient space for materials storage nearby is likely to be the main priority. The leading suppliers of positive displacement pumps for concrete, like Putzmeister, can now achieve considerable pumping distances so surface siting of the pump is usually practical. The concrete mix can also be transferred to remixer transport cars for rail or tyred chassis transport underground. Dillingham Construction used a Wagner truck-mounted Maxon Agitor concrete transporter for a deep tunnel project in Milwaukee. The Wagner 426 carried the open-top agitating hopper underground from surface loading.
Wheel loaders most often handle bulk materials that are unsuitable for pumping. This versatile item of plant can also be used for spoil handling, cleaning up and materials unit handling. Most urban sites do not have the space for much mobile plant and here, skid-steer loaders could be considered as a more manoeuvrable, though lower capacity, alternative. Volvo recently introduced a new generation of smaller wheel loaders derived from the former Zettelmeyer C-series. The drives are suitable for rough terrain and slippery ground and the company offers a range of over 60 attachments.
Utilities
The provision of power and communications is an important consideration in planning a surface layout and its links to underground facilities. Much will depend on power available from neighbourhood main supply, if any. Most transformer and switch equipment is on the surface, commensurate with voltage losses in the long secondary feeder cables to the tunnel face. Generators can also be installed for standby power.
Underground, compressors generally have to be high-capacity, high-pressure models. Stationary models, being electrically powered, can be left unattended for longer periods and are cleaner in operation. Atlas Copco recently introduced a range of seven new mobile compressors for heavy duties to extend the Series 6 range. Working pressures cover 7-14 bar, delivering 105-185 litres/s. The company has also launched a separate range of portable generator sets, the QA series, covering the output range 17-178kVA.
In 1999, Ingersoll-Rand introduced new mid-range compressors for output pressures of 8.6-21.0 bar and flows of 285-498 litres/s. Typical duties of the nine models include shotcreting and drilling.
Blakley Electrics supplies equipment and components specifically for site electrical supplies, usually to contractors who design and install their own layouts, whether supplied by the mains or generator sets. Projects supplied include the Channel Tunnel, the Conwy Tunnel (North Wales) and various sewer projects carried out by AMEC and Miller Civil Engineering. Glassey of Martigny in Switzerland manufactures skid-mounted housed transformers for site installations, with switch panels.
Environment
Poor attention to environmental considerations could result in trouble with local authorities. Recent examples include Manapouri, New Zealand (sited successfully in an important national park) and Semmering (Switzerland/Austria) where ‘Green’ political pressure has resulted in a modification of the construction access plan.
Concerns about subsidence are often voiced but there can be a higher level of complaints about surface activities, which generate noise, dust, fumes and water pollution. Plant needs to be selected with environmental performance in mind, and special arrangements may be required, like enclosures on ventilation systems, acoustic barriers and water treatment plants.
Adequate ventilation is vital for a safe working environment underground. Although exhaust ventilation is most efficient in removing fumes and dust from the working environment, it can concentrate pollutants on the surface. Exhausted air should be scrubbed of dust and should not be directed outside the site towards occupied buildings. Fans can generate high noise levels and need to be well maintained to minimise wear noise, and possibly be sited in an acoustic enclosure.
