Tunnelling: a brief historical perspective

The possibility of installing an underground channel by tunnelling rather than by digging a trench from the surface goes back thousands of years. Records exist of the qanats, a water management system for irrigation in the Middle East built some 2,700 years ago; these are still in use in Iran.

In 2500 BC, a 1,000 metre long tunnel was driven under the Euphrates to connect the Royal Palace with Temple of Baal in Babylon. The Romans and Greeks were the developers of basic techniques. For several hundreds of years these techniques constituted the state-of-the-art with little in the way of technological advances. Industrial Revolution

Tunnelling entered a period of major development in the 19th century in response to the demands of industrial development. Population movement and a shift to industry led to the construction of road, rail and canal tunnels, and also the need to install main sewer systems in cities. This is turn created a demand for tunnels locations that required a new approach.

The most famous example is the Tunnel under the Thames designed by Marc Brunel and constructed by his son Isambard Kingdom Brunel. Similar to most tunnelling jobs, it encountered problems because it was situated not far below the bed of the river in very soft, saturated soils. The work started in 1825 but was not opened to the public until 1843. Two things are significant about this tunnel; Mr Brunel introduced the concept of the tunnelling shield, and the tunnel is still in use today as part of the London underground.

The shield was a major development. Marc Brunel’s first patent for a tunnel shield was in 1818. It bears many similarities to today’s open shields including individual cells or compartments and the use of hydraulic jacks. In one version the body of the shield was advanced by the hydraulic jacks. The alternative version envisaged used hydraulics to force forward individual cells.

He originally conceived using a large circular shield, but due to manufacturing problems Marc Brunel developed a rectangular shield in 1823 to drive the large tunnel under the River Thames. The shield consisted of twelve frames each with three chambers accommodating 36 men excavating the face.

These early 19th century tunnels were built lined with brick which was laborious, slow and hazardous work. British engineers P.W. Barlow and J.H. Greathead obtained a patent on a circular shield in 1864.

Mr Greathead used it in 1869 to drive a pedestrian tunnel under the Thames without undue problems. The Barlow-Greathead shield had three major advantages – simplicity, safety and speed, introducing three major innovations that are still in use today:

• cast iron segments to line the tunnel;
• compressed air to keep the water at bay; and
• a grouting pan to inject grout into the voids behind the segments.

In the late 19th century the need to speed up face excavation led to the construction of various forms of shields with mechanical cutting devices. It was J. Price in 1896 who made the main breakthrough with his patent for a shield with a cutting head for excavating the earth. It combined the Barlow-Greathead shield with a rotating cutter consisting of four spoke arms on which the cutter tools were fixed. He also incorporated tub shaped scoops onto the arms, which lifted the cuttings and dropped them onto a chute feeding muck tubs. The first use was in London clay in 1897.

These types of shields with cutting heads, tunnel linings in cast iron and compressed air to balance water table and soil inflow remained the standard approach to soft ground until 1960. There were many improvements and step changes, but the basics remained the same.

Advancing technology The second half of the 20th century saw the development of tunnel machines and the means to support and line tunnels. The modern form of the drum digger – the tunnel boring machine (TBM) – was developed in 1955 using peripheral hydraulic motors for work on a 2.69 metre segment lined tunnel, with average progress of 110 m per week. Machines of this type were used to install the London Transport Victoria Line tunnels.

A limitation, which still exists today, is that the minimum diameter that can be economically driven for a traditional tunnel is around 2,000 mm. This is created by the need to erect the lining using labour behind the advancing shield. Pressure shields

Pressure balance shields were an important development, as they were capable of working in soils by providing active soil support well below the water table. It was then no longer necessary to use compressed air, with all its dangers and drawbacks, to counterbalance the external soil and hydrostatic pressure.

Slurry machines

The concept of a slurry pressure balance shield was put forward in patents in the UK and Germany in the late 19th century. In the mid-20th century, various designs were patented including one in Germany using bentonite slurry. The first machine with a cutting wheel and hydraulic mucking was used in Japan in 1967. There were three almost simultaneous lines of development in the UK, Japan and Germany.

In the UK John Bartlett was granted a patent in 1964. A prototype was built and used on jobs in London and Mexico, but a number of problems occurred that were never satisfactorily resolved.

Japanese company Mitsubishi developed this concept into a viable system in the 1960s. After the initial successful prototype, a number of machines were built. Japanese cities are largely located on the coastal plains and the subsoil conditions are mainly alluvial deposits often with high water tables. Most cities had previously relied on night soil collection as a sewer infrastructure had not been installed. The Government decreed that all cities and towns should have sewer collection and treatment systems, with the allocation of substantial funding. The combination of need and conditions created a demand for the construction industry to install sewers in difficult alluvial ground.

A third line of development was the Hydroshield from Wayss and Freitag in Germany, introducing a rear compartment containing air under pressure that acted on the slurry. In 1972, a prototype was built and used to drive a tunnel under the port at Hamburg. Modern versions of this concept have been widely used.

Pressure balance developments

Several manufacturers started producing pressure balance machines in response to the demand. Initially the first priority was to install larger diameter lines in the bigger cities, but over time the demand turned to medium and smaller diameters.

Earth pressure balance (EPB) machines are based on the concept of a blind shield sometimes used in pipe jacking work in cohesive plastic soils. The control of the soil as it enters the chamber creates a balancing pressure. Controlled soil removal as the shield advances maintains the balancing pressure. By combining advantages of blind and slurry shields, the pressure balance shield was developed. The first commercial EPB was built by IHI in 1966 in Japan according to the Sato Kogyo design.

Early pipe jacking In parallel, but quite independently, another development was underway. This was the concept of jacking in from the drive pit pipe sections behind the cutting shield to line the tunnel. By no means was this a new concept. Records of early simple pipe jacking go back to the late 19th century in Vienna and the USA. The primary use was to install relatively short lengths of casings under rail tracks and roads. Men worked at the face excavating the soil. It would appear that many of the pipes were fitted with a leading steel cutting edge.

At this time no separate steerable shield was employed. Steering in good ground was done by excavating in front of the pipe to the course required. The devices used for jacking were most likely whatever tools were available from other applications. Reference can be found to screw jacks, ratchet jacks, air piston cylinders and hydraulic jacks. Steel and cast iron pipes were pushed in using these jacks to provide a pipe or a casing. Concrete pipes came into use from the 1920s and records show the installation of concrete pipe with diameters up to 2,400 mm. Records have been found of the isolated use of pipe jacking methods in a number of European countries in the 1930s and 1940s.

The renewed interest in pipe jacking was a natural consequence of market needs. World War II had caused great damage to European infrastructure and little in the way of replacement work had been undertaken. New infrastructure was needed to meet the demands of expanding towns and cities and rising living standards. Much of the pipeline infrastructure had to be installed under busy roads and railways and at depths where open cut was impractical. The traditional methods were no longer appropriate.

During the 1950s, individuals and companies in the UK, France, Germany and Scandinavia independently took up the pipe jacking principle and developed their own equipment and methods for work. In Germany, Ed Zublin first jacked concrete pipe in 1957 and promoted the technique both as manufacturer and contractor. Several other German contractors also entered this field. By 1970 it was estimated that 200 km of pipe had been jacked in Germany. In the UK, I reintroduced the use of pipe jacking methods in 1958, driving a casing under a main railway track near Peterborough and subsequently many other casings and flexibly jointed concrete pipes to provide sewers.

Within the USA, similar methods continued with relatively little development of technique well into the 1960s. Manufacturers’ literature in 1964 still offered fixed shields, some with hoods and shelves.

During the 1960s and 1970s, techniques were refined to form the basis of present pipe jacking methods. The most important advances included concrete pipes with rubber ring joints specially designed for jacking, shields with independent jacks to give steering corrections and the intermediate jacking station.

Pipe jacking offered a solution that allowed short crossings up to 150 m to be made in a way that was inherently safe as well as economical. The ability to tunnel smaller than traditional diameters was one advantage. Operatives could be trained more quickly in the skills for pipe jacking than in the skills required to drive a timbered heading.

To cope with differing ground conditions and meet the varying market demands in different countries, new methods were devised and equipment improved. Longer, larger bores became possible. By the late 1970s, pipe jacking was no longer confined to crossings. In Japan and Europe it was also applied to sewer installation, traditionally done by conventional trenching methods or segment lined tunnels. Mechanical cutting equipment, external lubrication and conveyer systems were introduced. Hydraulic rams, power packs and control systems were greatly improved. Contractors soon had access to more versatile shields. For long drives and more difficult ground conditions, contractors needed compact and efficient jacking rigs and intermediate jacking stations. It was these developments that allowed jacked installation to become a true cost effective tunnelling method.

Installing sewer pipes required pipe jacking to be undertaken at greater depths and in less cohesive ground conditions and over longer lengths. It prompted a demand for controllable mechanised excavation and spoil disposal. In Japan there was a large market for sewers less than 2,000 mm to be installed without disruption. The limitations of segment tunnelling prompted the development in Japan of remotely controlled miniaturised pressure balance shields.

Microtunnelling

The bringing together of remote control shields and the principle of pipe jacking created the major change in the installation of small to medium diameter tunnels and sewers. Pipe jacking was limited to man-entry sizes and cohesive or pre-treated unstable soil. Japanese manufacturers combined the two methods and used the principle of pipe jacking concrete sewer pipes in diameters greater than 1,500 mm with remote control shields that could counterbalance groundwater and soil inflow. As the demand shifted towards installing smaller diameter pipes, the Japanese developed miniaturised versions of the larger machines.

These became known as microtunnelling machines. An operator at a control panel could remotely install pipes as small as 300 mm, with workers only needed in the drive pit to add the pipe sections.

Microtunnelling – a changing definition

Originally microtunnelling was defined by tunnelling techniques and activities used in the formation of underground pipelines of 900 mm or less in diameter. Now microtunnelling is generally understood to be any remote controlled excavation method that installs the pipe behind the shield by jacking. The machines have cutting heads at the front of a train of pipes being advanced into the earth. To counterbalance the external pressures they can have either slurry or earth pressure balance chambers. In a slurry machine the soil is brought to the surface by the return slurry line where it is separated out and the cleaned bentonite slurry returned into the system.

The first slurry pressure-balance microtunnelling machines were introduced in 1979. Japanese manufacturers produced hundreds of pressure balance machines of all sizes. It was estimated that there were over 3,000 machines in Japan in the eighties. However, after the big sewer projects were completed, the demand for new machines slowed down.

In the early 1980s, the West German Ministry of Research and Development funded a research project into ways of improving techniques for sewer installation. Subsequently a Japanese Iseki 600 mm machine was imported to Germany for use on the Hamburg Development Program between 1981 and 1984, where 2,750 m of small diameter sewer pipe were installed. The Hamburg project convinced a number of German manufacturers and contractors to produce their own machines and also to develop smaller ones for installing house connections. The German companies still remain the biggest users and the most important manufacturers of microtunnelling machines outside Japan. Observations

Remote control pressure balance installation for tunnels and microtunnels in the range of diameters from 300 mm–3,600 mm has become the preferred method of installation throughout the world. The inherent technical and economic advantages have more or less displaced segment tunnels below 2,500 mm. The ability to install pipes remotely and accurately from 300 mm and larger is taking over from deep open cut, which is disruptive, dangerous and dirty

In the past 25 years there have been many improvements, but the basic technology is unchanged. If one looks at the manufacturer’s literature for slurry machines being produced in the eighties, the flow diagrams and the pictures of equipment don’t look very different to those being produced today. There have, in reality, been many improvements in cutters and cutting heads, lubrication, face control, and line and level monitoring so that today’s machines have greater capability for longer and curved drives in all types of soil from hard rock to the most unstable soils with a high water table.

What is not fully appreciated is the way that the skills required to install a tunnel have changed with the advances in technology. Just 50 years ago tunnelling was a hazardous and physically demanding job for the workers. Today it is intelligence and understanding, not muscle, that are needed to operate modern equipment and even for man entry sizes workers are not required to be in the tunnel except for maintenance work. This has led to a great improvement in the safety for tunnel workers. Fifty years ago a rule of thumb in traditional tunnelling was on average one death per mile of tunnel.