These tracks comprise a 6 m long beam which structurally spans Bath’s vaults and cellars and result in less bearing pressure below the track than from an HGV or bus wheel. It’s the same principle as a tank tread which spreads a much heavier load over a large surface area permitting it to travel on soft ground without bogging down whereas a truck, although lighter will sink in.
Cross section of LR55 tracks which can be installed one at a time at night, minimising traffic disruption
Details of installation process
A single pass excavator creates a trench 400mm wide and 200mm deep. Pre-cast concrete foundation troughs in 6m lengths for straight tracks, are laid directly onto a bedding layer compacted to line and level. The troughs are initially linked together by ‘hairpins’ between adjacent ends. The troughs are then sealed into the pavement. The LR55 rail, welded into long strings, is laid in the trough, either using wedges of pre-cured PU grout, or clamp stands, to achieve line and level. With the achievement of line and level as a two-stage process, much higher precision can be achieved than with sleepered track. The final bonding grout is then injected under the rail to ensure full adhesion between the rail and trough, and between troughs. The exposed surface of the PU grout can be sprinkled with an aggregate to improve skid resistance for road vehicles using the same alignment.
Troughs are laid in advance of the rails and temporarily plated over until the rails are ready to install. By this means a complete installation of four rails for a double track can be completed at the rate of about 400m or more per week. By the use of several track installation teams a 20km route can be completed in less than 6 months, if there are no road traffic management issues.
Other utilities are usually 400mm or deeper, and access to them is possible between rails. The track is self-supporting over trenches 1m wide, allowing utility companies to lay new under across tracks without disrupting services.
The foundation trough sits on a compacted bedding layer about 20mm deep with a CBR of 10%. The trough is bound into the pavement by asphaltic sand or a similar compound. The LR55 rail is fixed into the trough with a polyurethane grout. This has both bonding and resilient qualities, which significantly reduces low frequency vibrations by about 30dB, and noise by 10dB. Because the LR55 rail is continuously supported, a smoother ride is provided for rail vehicles (and passengers), and the formation of rail head corrugations is significantly reduced.
The foundation trough transmits the wheel load into the pavement bearing course at a lower pressure than would be experienced from a passing HGV tyre.
The above track, acts like a heavy military tank track which by spreading the load over a large area means that the pressure below the track is less than that from the single line concentrated pressure from a bus or hgv. The beam acts like a lintel which means the cellar beneath Bath are subject to less force than from a bus or hgv
Transition of LR55 track on the left to conventional sleepered rail on the right. This has been installed as part of the Sheffield Supertram for 15 years.