Booted Sleeper
The rail is mounted on a twin-block concrete sleeper, which is encased in resilient boots, recessed into a concrete slab.
Booted sleeper track systems such as Stedef have been widely used in France and Greece (Attiko Metro), predominantly in tunnels. The Channel Tunnel uses the similar Sonneville booted block system (without the tie bar). A Sateba booted sleeper system has been installed in the Marseilles Tunnel on the TGV Mediterranée.
A booted concrete bearer for turnouts has been developed in France and is currently in use on slab track (or ballastless track). The most significant example is the Paris RER E line, which has been in operation since spring 1999.
System Components
The rail is mounted on a pre-cast concrete twin-block sleeper via a conventional rail fastening system. The sleeper block sits on a resilient elastic sole pad and is surrounded by a hull (the “boot”), into which additional lateral resilient pads can be placed. The sleeper and hull are recessed into a concrete screed. The elasticity in the system is provided by the rail pad beneath the rail and the resilient pads and elastic sole pad beneath the sleeper. The sleeper/hull/concrete interface is sealed to prevent ingress of water.
A typical booted sleeper arrangement using Sateba twin-block sleepers, Vossloh W14 fasteners and synthetic waterproof hull is shown in Figure 2. Since the mounting of the rail on the sleeper is conventional, other fasteners can be used; the traditional Stedef system uses a Nabla fastener. Rubber resilient pads and hull instead of synthetic are also an option.
System Performance
The system has been applied on a large scale in tunnels and has proven service history including, the Athens Attiko Metro and an 8 km tunnel on the TGV Mediterranée near Marseilles. The Channel Tunnel uses Sonneville block track.
The key problem with using a booted sleeper system in locations exposed to the elements has traditionally been the ingress of water. The water penetrates between the sleeper and the boot and between the boot and the concrete surround. Over time, the action of passing train traffic causes wear and deterioration of the concrete surfaces. This can affect the stability of the track as the sleepers work loose. For external applications, the polyurethane seal applied around the sleeper eliminates this problem preventing water ingress.
Problems have also occurred with the corrosion of the tie bar in the twin-block sleeper. A twin-block sleeper is used to improve lateral stability by its twin abutments. Corrosion problems can be designed out of the system.
The Stedef system has traditionally been installed using Nabla fasteners. SNCF have wide experience of using the Nabla fastener both on ballasted TGV lines and for booted sleepers.
Acoustic Performance
Booted sleeper systems perform well acoustically for ground-borne noise and vibration.
The resilience in the boot can be varied to achieve acoustic requirements for ground-borne noise and vibration without direct affect to the rail fastening integrity.
Construction Issues
“Top-down” construction is preferred. A supporting base slab is constructed, mesh reinforcement is used to control cracking then the trackwork laid out in panels.
Track geometry is set prior to placing the mass concrete bedding around the booted sleepers
Maintenance Issues
The whole sleeper unit can be exchanged. Where renewals are required, the old sleeper is lifted out and a new sleeper slotted in, with the rail fasteners already pre-assembled. In case of derailment this is advantageous and should reduce the time to reopen the track; there are no reinforced concrete plinths to be permanently damaged. The sleepers can readily be lifted out of the sockets to replace the resilient sole pads if necessary.
Adjustment can be made to track levels by placing a layer of grout underneath the sleeper. If necessary, the level of concrete surround would be increased once the sleeper has been lifted, to maintain a good embedding depth. Alternatively some fastener systems allow placing of shims beneath the rail seat. These methods could be used if high settlement is expected.
The booted sleeper system offers considerable advantages for simplicity of construction and maintenance. The low capital cost and experience of use of the system makes it a relatively low risk option.