Enlarging tunnels without undermining the invert By Professor Lewis Lesley

Enlarging tunnels without undermining the invert
By Professor Lewis Lesley

Technical Director,

TRAM Power Ltd.

Britain being the pioneer of railways has a legacy of cramped infrastructure, just as we have from narrow canals. George Stephenson who built many of the first railways, used sleepers with rails fixed either directly or on a base plate,. This has been the principal method ever since of transmitting train loads into the sub soil via an elastic ballast. Brunel on the other hand used longitudinal sleepers, which today are only found on bridges. As those pioneer railway engineers went abroad to spread the technical know how, railway loading gauges got progressively larger, until the Russian railways where massive trains are used. The Channel Tunnel was however built to an even larger loading gauge to allow full size HGVs to be carried piggy back on 4.25m (14ft) wide trucks. Today the principal restriction on the size of freight that can be moved is the highway loading gauge, since nearly 80% or UK freight is moved by road. The road loading gauge is more generous than rail, which explains why maritime containers have gone from the original ISO 8ft high, through 8ft 6in and 9ft 6in to 10ft 6in, which is now a freight industry norm. UK railways can carry 8ft 6in boxes but 9ft 6in only on a select routes.

Presently there are only two ways to increase the loading gauge of tunnels:

Single tracking along the centre line (Fig 1), or excavating the invert (Fig. 2). Both these require long occupations. The first option significantly reduces train capacity unless a new parallel tunnel is built. whilst the second is very expensive. This article discusses ways in which larger tunnel loading gauges can be achieved by a different method.

First it is worth considering track technology. Higher axle loads, train speeds, and the desire for better track standards with reduced maintenance costs, has seen traditional sleeper tracks improved incrementally with heavier rails, stronger fastenings, bigger sleepers and deeper ballast. On open tracks these improvements are practical and economic.
In tunnels the situation is different, since the depth of ballast is constrained by the level of the invert, and so therefore are axle loads and speeds which can be accommodated, within the constrained loading gauge. Traditional ballast and sleepers can be replaced by a slab track construction but this is both disruptive and expensive, although has the advantage of reducing maintenance costs. There is a new track system which uses highway engineering technology and requires a shallow foundation depth, even for the heaviest axle loads and high train speeds. This promises larger loading gauges within existing tunnel inverts.

Traditional track systems use bottom supported rails, which then need strong fastenings to sleepers to prevent overturning from lateral wheel loads. With discrete support on sleepers, trains experience regular hard spots at the sleepers, which are a cause of the formation of short wave corrugations on rail heads. Because spacing between sleepers is an order of magnitude larger than the sleeper width, ballast must be considerably deeper than that required for continuously supported rails. Finally rails and sleepers have to be adjusted together for line and level.

The new LR55 continuously supported track system uses a Type One Highway Base to provide the line and level for concrete beams which continuously support the rails. The beam is in the shape of a trough, in which a top supported rail is bonded using an elastomeric grout, based on polyurethane. Stainless steel gauging strops and conventional ballast is used between beams to maintain track gauge. Not only is the rail continuously supported, but the support is a uniform elastic medium, which provides the same function as ballast on conventional track but does not degrade or need maintenance.
The key to the success of this system is the use of a Type One Highway Base, for which there is worldwide experience and expertise in the design and installation. The bearing quality, elasticity and life of such bases is well understood. There are many highway contractors competent to lay quickly such bases. These bases can be laid with a very high tolerance of material quality, and line and level. This is important, as the pre cast concrete support troughs sit directly on the base. The level of base, together with the prefabrication tolerance of the troughs determines the first order accuracy of the ultimate line and level of the track
On the base sits the precast concrete support troughs, which are vertically and laterally stiff. This is important as the LR55 rail is less stiff vertically than girder rails (eg. UIC60). The wide base of the trough, and the continuous support, means that the trough pressure into the track base typically about 200 MPa, for 25 tonne axles loads, depending on the stiffness of the base. Even a weak base however, like sand has a load capacity of about 5000 MPa, an order of magnitude greater than the imposed track pressure
The LR55 rail is top supported in the pre-cast concrete trough. This makes the rail very stable and highly resistant to overturning. About 60% of the wheel loads are transmitted on the running side rail flange, about 30% on the non running side flange, and 10% by the rail base. The rails weight about 55kg per linear metre and therefore have similar electrical resistivity to girder rails of similar weight. Lateral train loads are accommodated by shear compression of the elastomeric bonding grout, and the lateral stiffness of the troughs and track construction.
Once the rails are to line, level and gauge, they are bonded into the concrete support troughs by elastomeric grout. This is the second order determinant of track accuracy. It should be possible to achieve a tolerance of about 0.1mm. There is no mechanical connection between rail and trough, The rail is continuously supported vertically and restrained laterally by the elastomeric grout, rail/wheel interface forces are almost constant along the track. This should mean a better ride for vehicles (and their loads) as well as reducing the incidence of long, medium and short wave length corrugations along rail heads.
The LR55 rail is rolled with a continuous check rail. This will reduce derailments, especially by flange climbing over the rail head, since the other wheel on the axle will be restrained laterally by the check rail. In the event of a derailment, trains cannot drop because the track formation is level with the top of the rail. This also means that derailed trains are unlikely to overturn off the track.
In the unlikely case of rail cracks progressing to failure, , the bonding and support trough which surrounds the rail will prevent it falling apart, and thus denying the derailment mechanism. This continuous support for the rail, means that weld failures need not be so critical, since the broken rail ends will be restrained and kept together, allowing repairs to be timed without the need to close the track for a long period.
The LR55 track system has been comprehensively tested, by calculation, laboratory experiments and field trials. The last of which began in March 1996 with a live track trial with 3500 axles per day. As of Sept 2006 this section has needed no maintenance and looks as though it will achieve a minimum 30 years. In all cases, the FEM and analytical model results closely agree with physical tests.
In a tunnel environment, ambient temperature is fairly stable, so that the impact of thermal expansion will be limited. Nevertheless the effects of temperature variations in the LR55 rail, elastomeric grout and concrete support trough were modelled. Differential expansion between rail and trough will result in shear forces in the elastomeric bonding grout. For the temperature range (-20oC to +60oC) examined, and laboratory tested, none of these forces resulted in bond failure. When a destructive test of grout strength was undertaken, it was orders of magnitude higher than the thermal expansion forces. All dynamic tests were undertaken for 25tonne axles. Static tests were undertaken up to 80tonne axle loads.
At 25 tonne axle loads, the pressure at the base of the LR55 foundation trough is low ( about 200 mPa), and by distributing loadings over a wide area, means that track should have a longer life than conventional sleepered track. Gauge maintenance is achieved by a combination of ballast resistance, trough stiffness and gauge bars between the support troughs.
The total height of the LR55 track system above the sub soil/sub base is only 200mm. This is at least 300mm lower than sleeper tracks. A lower track height means that a larger loading gauge is achievable in masonary arch tunnels without the need to lower the invert or reconstruct the tunnel. (Fig. 3).
This is achieved by removing the existing track and scraping off the ballast to reveal the invert. A compacted Type One Highway base with a minimum thickness of 20mm is laid, to give a CBR over 10%. The LR55 troughs with gauging bars are laid to line and level. Ballast is compacted around the troughs. The LR55 rails welded into long strings are set out to line and level, either with procured wedges of grout, or temporary stands. The LR55 rails then have grout injected from under the rail to fill completely the void between rail and trough, and ensure proper adhesion.

The benefits of increasing loading gauge are available from LR55 track in tubular tunnels (Fig 4), where the cost of lowering the invert would be considerable and disruptive. Indeed a section of London Underground tube tunnel (3 m diameter) was replicated in the Structures Laboratory at Liverpool JM University to investigate the stresses in the lower tunnel sections and the ground around the tunnel. As in the other tests, this showed that the LR55 track could create an increase in tunnel loading gauge height of over 300mm, without increasing the stress in the tunnel segments, or overloading the surrounding ground. A (temporary) transition section between LR55 and girder rail has been designed to enable relaying with LR55 to be undertaken on a progressive basis with short possessions if necessary.

The LR55 track with a level track surface also provides other benefits for tunnels. These include:

(1) safer staff working conditions

(2) safer emergency train evacuation

(3) ability to use road maintenance or rescue vehicles without modification

increased loading gauge gaugegauge

Figure 6. LR55 track system in tube tunnel to enlarge loading gauge
In the LR55 track system, the rails are completely retained in the elastomeric grout, which is itself contained by a substantial and stiff concrete trough. If a rail (eg. weld) should break, the rail ends will be restrained. This is due to the continuous support of rails in the LR55 system. So the location of a random rail failure is less critical, the rail ends will remain to line and level, and thus passing rail wheels should stay on the rail. Further the train wheels on the other rail are also restrained by the continuous check rail, so that if the failed rail is being pushed outwards , the wheels (and train) cannot follow.
Should a rail failure lead to a derailment in the LR55 system, the rail vehicle will arrive on the ballast which is a rail height. Trains are therefore unlikely to fall over, which is another cause of casualties in many rail crashes.
The LR55 track system provides a number of unique opportunities to improve the safety, infrastructure and economics for railway tunnels. These can be summarised as follows:
(1) loading gauge enhancement in tunnels and under bridges without lowering inverts

(2) having a level tunnel floor making maintenance possible with road vehicles

(3) a level tunnel floor makes emergency evacuation safer

(4) continuous check rail significantly reduces the risk of derailment

(5) cracked rails are retained to line and level inside LR55 troughs

(6) derailed train does not fall off the track and therefore unlikely to topple over

The LR55 track is as radical a change in rail track technology, just as the Vignoles flat bottom rail was to bull head railed tracks in the 19th century. The short and long term economic advantages are probably greater, especially the opportunity created to attract freight traffics which cannot at present be carried by rail, and the ability to operate double deck passenger trains. Both these increase line capacity for a trivial capital investment.
Fig. 1 Singling a double track masonary arch tunnel

Fig. 2 Lowering the masonary arch invert

Fig 3 Using LR55 to achieve same loading gauge

Fig 4. Tube tunnel loading gauge enlargement with LR55 tracks