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1. The history of tunneling
Wherever possible, the original railroad constructors, like the earlier canal builders, followed the valleys and avoided the necessity of tunneling through hills. However, this was not always possible, and in many locations railroad tunnels had to be built. Almost always this meant considerable difficulty, delay and expense and frequently loss of life due to the hazards of construction.
On main line railroads, tunneling methods followed those used for the construction of canal tunnels, which in turn had relied heavily on skills learned in the coal, china clay and iron ore mining industries. The work was slow, hard and dangerous, often involving men working in appalling conditions.
Early tunnel construction was carried out by men excavating by hand using pick and shovel, blasting and splitting where necessary in the harder rocks, shoring up with heavy timbers as they progressed slowly forward. On the spot, judgment would be made, depending on the nature of the ground, whether a permanent lining was required or if the tunnel excavation could safely stand unsupported. Many wrong decisions were made on this critical area sometimes with disastrous results and loss of lives. If a lining was deemed necessary, this would usually be in the form of brickwork or stonework which would be constructed behind the miners, the temporary props then being removed as the work progressed.
These early tunnel builders showed considerable skill and courage considering that they were literally working in the dark in relation to the nature of the strata they were likely to encounter and the level and pressure of ground water that could be tapped at any time.
2. 'Cut-and-cover' tunnels
In built up urban areas, tunnels were sometimes necessary because of space limitations and the requirement to place the tracks below ground level.
This was the case in the construction of the first lengths of the Metropolitan and District Lines in London, including now what is called the Circle Line.
In this case the tunnels were constructed by what has become known as the 'cut-and-cover' method, which is still used today throughout the world for the construction of shallow running tunnels and stations. Its main disadvantage is the considerable disruption it causes at surface level during construction.
3. The first tunnel shields
Up to the end of the eighteenth, century no tunnels had been successfully constructed under water courses. Marc Brunel, the father of the more famous Isambard Kingdom Brunel, patented a rectangular 'tunnel shield' in 1818 which he later used on the construction of his Thames Tunnel between Wapping and Rotherhithe. Although the idea was born out of the problem of tunneling through waterlogged ground, the principle of the moving tunnel shield was then established and is now internationally accepted for all safe tunneling work in anything other than hard rock.
Marc Brunel's shield was rectangular in shape with individual cells. It consisted of massive cast-iron frames sitting on shoes at the bottom of the excavation, the working face being supported by heavy oak planks or 'poling boards' which were individually held in position by screw jacks bearing on the frames. Each poling board was removed one at a time in each cell, excavated behind by hand to a depth of about 100 mm and then replaced at the new advanced position with the screw jack fully extended. In all, there could be up to thirty six men working at the face at any one time, each in an individual cell. Although safe, this method was painfully slow and encouraged contractors to take short cuts to improve progress.
The story of the construction of the Thames Tunnel is well documented elsewhere, including all the problems that were met. Eventually, these were overcome and the tunnel is still in use today, accommodating the tracks of the East London Line, part of the London Underground. The practicality of use of the tunnel shield was then firmly established by the Brunels, father and son.
Another engineer, Peter Barlow patented a circular shield a little later and one of his students James Greathead developed this further and used it in 1869 to drive a tunnel under the Thames near the Tower of London and later under the Hudson River in New York. The 'Greathead Shield' as it is now known, has basic features which still remain unchanged. These are a protective structure under which the ground can be excavated and an extension in the rear in which the permanent lining of the tunnel can be erected, with jacks which force the shield forward reacting on the completed lining.
The first 'tube' underground railroad tunnels built in London using 'Greathead' shields were commenced in 1886 and now form part of the City branch of the London Underground Northern Line.
4. Modern tunnel shields
Modern tunnel shields used for soft ground tunneling follow the same basic principles as the 'Greathead' shield. Some sophistication has been added in the form of hydraulic jacking and 'steering', hydraulic arms at the rear to assist in the positioning of tunnel lining segments and power operated tools of various types to excavate the face as well as improved equipment for probing ahead.
5. Differing ground conditions
The method of construction and the design of the final tunnel structure are very much dictated by the nature of the ground through which the tunnel passes. Careful and thorough site investigation of the geology and the water conditions is an essential prerequisite of successful tunneling. Bore holes are usually sunk close to but not immediately on the line of the proposed tunnel, to probe the strata and to deduce as far as possible the contours of the materials which are likely to be encountered.
In extreme conditions, the line or level of the proposed tunnel may be modified to avoid poor ground conditions discovered in the survey. This preliminary information will also be made available to tenderers to enable an estimate to be made of any special precautions, such as ground stabilization or working in compressed air, which may be needed.
6. Construction methods
Tunnels which are 'bored', rather than constructed following excavation down from the surface, need to adopt various building techniques depending on a number of factors. These factors will include the following considerations:
(i) Material through which the tunnel is likely to pass, including the consistency of that material or the likely degree of variation.
(ii) Any faults or pockets likely in the strata including those likely to convey local high water pressure.
(ii) The level of the water table and any water bearing non-cohesive material likely to be met like 'running' sand.
(iv) Any disturbance to the original ground that could have occurred previously and then temporarily stabilized. This situation might occur when driving a second tunnel close to a previous tunnel or adjacent to existing foundations, basements or sewers.
(v) Substantial variation in imposed ground pressures, either vertically or horizontally, caused by heavy buildings above or immediately alongside the line of the proposed tunnel.
(vi) The length of the proposed tunnel and the speed it is required to build it. Tunnel construction speeds are generally slow but if considerable lengths have to be constructed it is often worth spending extra money on more sophisticated equipment to increase building speed.
In all civil engineering construction, safety is an essential ingredient.
In tunneling, this is an absolutely primary consideration and must never be allowed to become secondary to considerations of cost or speed of construction.
Each tunnel construction operation must be carefully considered on its own merits looking at all the factors listed above. A 'Method Statement' should then be agreed between the engineer and the contractor before operations commence and plant is agreed. The Method Statement should clearly define not only the methods of working but the agreed procedure when work stops between shifts or for longer periods at weekends etc. and the monitoring of movement of the ground above as the work proceeds.
Very accurate surveying of tunnel work, both as the operations proceed and after completion, is essential so that any necessary corrections of line or level can be applied as the work progresses. Railroad tunnels usually have very tight clearances to rolling stock and any misalignment in build is difficult if not impossible to compensate in revised track geometry. Even relatively small errors of construction sometimes require a length of tunnel to be rebuilt, which clearly is undesirable and must be guarded against.
7. Tunnel linings
Except where tunnels pass through sound rock which is expected to be self- supporting, most tunnels are provided with a permanent lining. When tunneling in relatively soft ground, the lining can be constructed in masonry, brickwork, cast iron, precast concrete or in situ concrete.
Bolted cast iron and concrete segments are usually pressure grouted behind to fill any voids.
Certain types of precast reinforced concrete segments are expanded against the clay and provided with wedge or key sections and do not require grouting.
The New Austrian Tunneling Method (NATM) has been used extensively for a number of railroad tunnels in Europe and beyond and is now being used in the UK. Some early difficulties were experienced in the UK, notably for the Heathrow Express, and great care is necessary in this method, particularly in the early excavation stages.
The system involves an immediately sprayed concrete shell applied to the excavated bore followed later by a permanent in situ shuttered concrete lining.
Most long drives of soft ground tunneling for running tunnels are constructed using some form of tunnel boring machine (TBM) which are purpose built for each large contract. This machine usually thrusts itself forward by some system of hydraulic jacks bearing on the completed tunnel lining.
Conventional station tunnels may often be built by hand, a pilot tunnel having first been machine built as part of the running tunnel drive.
8. Vertical and sloping shafts
All but the very shortest railroad tunnels require some vertical shafts, both for the purpose of construction, and for final ventilation or draft relief.
These shafts are build 'top down' each ring being completed and grouted before the next lower is started beneath it. Segments are individually maneuvered into position by hand, the space required having been carefully excavated rather like in underpinning. As rings are completed, the 'dumpling' inside the shaft can be excavated by machine loading into buckets and craned away or simply 'grabbed' out by crane. Much care must be taken when water or running sand is encountered where special precautions may be necessary.
Sloping shafts are necessary for the use of escalators, usually at about thirty degrees to the horizontal. These are constructed in a similar manner and can be very difficult to build, especially nearer the surface when going through made ground and surface water pockets. Escalator shafts often require dewatering, pressure grouting, chemical grouting, ground freezing or other ground treatment and need careful investigation well before starting operations.
9. Tunnel inspection and maintenance
Well constructed tunnels should require little maintenance, especially if they are relatively dry. However, access has to be arranged for tunnels to be inspected in detail on a regular basis. The frequency should be based on the general condition of the tunnel, its material of construction and the amount of water present.
Records must be kept of all inspections and repairs and the amount of ingress of water. Brick tunnels require periodic re-pointing but most concrete and cast iron lined tunnels require little attention.