Date of Post: 17/11/2015
Category: Bridges & Tunnels Type: Whitepapers
There have been some remarkable advances in tunneling technology in the last 4 decades. The majority have been in the field of soft ground tunnelling.
Hard rock tunnelling has seen advances with both tunnel boring machines and improved drill and fire methods including state of the art jumbos, improvements in explosives, rock anchors and spoil transport systems. However a hard rock heading has not changed substantially.
The changes in soft ground tunnelling are exponential. With the exception of very short drives the old methods have practically disappeared. Tunnels using compressed air, timber headings and carrying full faces of timber to support the face during excavation are now only used where it is not economically viable to install a TBM (tunnel boring machine).
Earth Pressure Balance TBM's and Mixshield TBM's have done away with the need to pressurize the entire tunnel length by keeping the hydrostatic pressure created by groundwater in porous mixed ground conditions limited to the cutterhead and mixing chambers of the machine. The modern soft ground TBM leaves the tunnel both at ambient pressure and free from the dangers associated with compressed air working.
Not only does the modern soft ground TBM provide a healthier work environment it also is capable of high rates of advance whilst keeping full control of the face and minimising subsidence.
The modern soft ground TBM is basically a tunnel producing factory which is fully self contained needing only a regular supply of segmental rings with which to support the newly excavated tunnel and pipework to keep itself supplied with water for cooling, compressed air for tools and pipelines for removing wastewater. It also needs a supply of bolts for the support rings and oils and greases for the machinery as well as additives for conditioning the excavated ground. These ground conditioning additives come in the form of foams, polymers and mineral slurry and are used depending on the ground type and the required end results.
The EPBM (Earth Pressure Balance TBM) concept was first experimented with in the late 1970's in Japan and has advanced and progressed to its present day incarnation capable of handling mixed faces of hard rock, sands, silts, clays and gravels with hydrostatic pressures up to 6bar. The excavated ground is mixed into a workable mass with the aformentioned additives, dependent on precise ground type, inside the mixing chamber directly behind the cutterhead (1) and used to support the tunnel face. It is then transported using an auger screw (3), which reduces the hydrostatic pressure inherent in the soil to ambient pressure (2), and is delivered to the prefered spoil transport system (conveyors (4), muck cars or muck pump).
Earth Pressure Balance Tunnel Boring Machine (EPBM)
The Mixshield TBM is an advancement of the slurry shield technique again developed in the 1970's. The Mixshield TBM is capable of handling sands, gravels and hydrostatic pressures up to 15bar. The mixing chamber behind the cutterhead is filled with a bentonite (a form of gypsum) slurry mix which is piped in directly from a slurry production plant on the surface via large bore (45cm) pipes. The bentonite slurry is kept pressurised by a bubble of compressed air which is held in place by the "diving wall" - a wall of solid steel suspended from the roof of the mixing chamber to below axis. The bentonite slurry mix has a dual function - (a) it supports the face during excavation by creating an impermeable "cake" in the exposed material and (b) acts as a transport medium for carrying the excavated material in suspension with the bentonite slurry via large bore (45cm) pipes back to the plant on the surface where the excavated materials are removed from the bentonite via centrifuges and screens.
Mixshield Tunnel Boring Machine (MTBM)
Small diameter tunnels up to +/- 1.8m are now being succesfully excavated and lined by micro-TBM's which are remotely controlled from a surface container. This technology started in the 1980's is now the go to answer for small diameter drives up to 1km and can be managed by a small crew and hence is very economical.
As well as advances in TBM's there have been major advances in segmental ring design. Up until the 1980's only straight rings were available and plywood packing was needed to create curves either horizontal or vertical. We now have a host of tapered ring varietals with a 5cm approx. differential between the widest and narrowest points which allow the engineer and ringbuilder to choose the best position of said widest and narrowest points to (a) follow the proscribed curve and (b) keep the ring as near to central in the TBM.
The segment made with a rigid reinforced steel cage is slowly being phased out in favour of concrete mixed with steel fibres which stop the spalling of concrete when placed under pressure by the propulsion rams used to advance the TBM which was a negative feature of the cage type segment.
The modern TBM guided by laser (GPS coordinated) with all its controls, systems and functions monitored by computer and highly experienced personnel is a far cry from the gangs of burly navvies who built tunnels pre-1980.
In short drives in competent and semi-competent ground the SCL (Sprayed Concrete Lining) method has come into its own. This method uses controlled excavation lined with steel mesh, latice arches and a sprayed concrete mix incorporating steel fibres to provide a monitored first pass support prior to a permanent lining being installed.
Along with the old style methods of excavation have gone horrendously high numbers of serious injuries and deaths. The modern tunnel site has safety induction courses, up-to-date safety equipment, regular health checks, tool box talks on pertinent issues, PPE (Personal Protection Equipment) - hi-viz clothing, safety boots, helmets, ear defenders, goggles and gloves and safety inspectors to make sure the modern tunneller is staying and working safe. There are also random alcohol and drug testing to deter site workers attending in an unfit condition. Site security has also tightened to stop members of the public wandering onto what is a potential danger zone for the uninitiated - many sites now have turnstiles controlled by biometric scanning devices and by PIN pads for approved personnel.
Tunnelling was once the last resort due to its cost both in economical and in human terms. Now due to the need to provide MRT (Mass Rapid Transport) systems to transport people quickly, the pressures on available space for land development and the scarcity of long corridors for surface transport in cities, tunnels have become the go to answer for public transport.
Rail has also come into its own again as a means of transporting people at high speed between urban centres. The new high speed trains capable of 300kph have revolutionised intercity travel but they have to have their own dedicated track capable of supporting these speeds. Whole networks of high speed lines are being built aided by the new advanced tunnelling technologies.
The future for tunnelling is indeed bright.
We are highly thankful to Mr Geoffrey Paul DARBISHIRE( from Spain) for contributing this valuable article.