Floating slab track
A track slab isolated on bearings or a resilient mat, used where vibration into the buildings above governs the design.
Last updated 2026-09-05
What is Floating slab track?
Floating slab track is slab track with the slab deliberately disconnected from the structure beneath it. The track slab sits on discrete resilient bearings or on a continuous resilient mat, separated from the base slab by a gap, so that the vibration generated by a passing train has to cross a soft layer before it can reach the surrounding structure and the ground. The arrangement behaves as a mass on a spring, and the design is tuned so that the natural frequency of that system sits well below the frequencies that cause the problem. Above that frequency the isolation works, and the heavier the slab and the softer the bearings, the lower the tuned frequency and the broader the range over which vibration is reduced.
It exists for one reason: ground-borne noise and vibration in buildings. When a train runs in a tunnel, energy passes into the tunnel structure, through the ground and up into the foundations of whatever is above. Occupants do not usually hear the train directly. They feel low frequency vibration and they hear a rumble radiated by the walls and floors of their own rooms. Concert halls, recording studios, laboratories with sensitive instruments, hospitals and residential buildings directly over a running tunnel are the classic cases. Where an assessment shows that vibration governs, floating slab is the most effective mitigation available in the track form itself.
It is also the most expensive track form in ordinary use, and it is used sparingly. The slab is heavier, the construction is more complicated, the depth required is greater than for a plain slab, and the bearings or mats are engineered components with their own performance requirements and their own service life. Access for inspecting and eventually replacing bearings has to be designed in from the start, because a bearing that cannot be reached cannot be changed. Maintenance around the slab is more constrained, since anything that bridges the isolation gap short circuits it and destroys the performance that was paid for. Debris in the gap does exactly that. On most projects the designer applies floating slab only over the lengths where the assessment demands it, with transitions at each end, rather than across a whole route.
How does Floating slab track work, step by step?
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Step 1: Establish that vibration governs and over what length
The starting point is an assessment of what is above the alignment and what it is sensitive to. Residential buildings, concert halls, studios, laboratories and hospitals all have different tolerances, and the assessment identifies which lengths of route need mitigation and how much. That study defines the extent of the floating slab, the performance the isolation has to achieve and therefore the design of the system. Applying floating slab where it is not needed is expensive, and omitting it where it is needed is very difficult to correct afterwards.
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Step 2: Design the mass and the isolation together
The system is a mass on a spring, and the two parts are designed as one. The slab mass, the stiffness of the bearings or mat and the resulting natural frequency are chosen so that the frequencies causing the problem fall well above it. The designer selects between discrete bearings, which concentrate the support and allow inspection and replacement, and a continuous mat, which distributes it and is simpler to build but harder to access later. Deflection under load, long term creep of the resilient material and its service life are all part of that selection.
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Step 3: Build and prepare the base slab
A base slab is constructed on the tunnel invert or structure, finished to the flatness the bearing or mat system requires, since an uneven base loads the isolation unevenly and detunes it. Drainage is designed into the base so that water can leave without collecting in the void beneath the floating slab. Recesses, pockets and access provisions for the bearings are formed at this stage, because they cannot be added later.
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Step 4: Install the bearings or lay the resilient mat
Discrete bearings are set into their pockets at the positions and levels the design specifies, or the continuous mat is laid over the base with joints formed as the supplier requires. Either way the components are protected from damage and from contamination, since grit, spilled grout or a punctured mat all compromise the performance. Levels are surveyed, because a bearing sitting high or low changes the load it carries and therefore the tuning of the whole system.
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Step 5: Cast the floating slab without bridging the gap
The track slab is cast on the isolation, with formwork and separation layers arranged so that no concrete, no reinforcement and no fixing crosses from the floating slab to the base or to the tunnel wall. This is the operation where the whole design is most easily wrecked, because a single hard connection short circuits the isolation. Rail is set to line and level on adjustable supports as it would be for any slab track, and the concrete is placed, compacted and cured under control.
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Step 6: Verify the isolation is genuinely isolated
Once the slab has cured and the temporary supports are removed, the slab is checked to confirm it is free. The gap is inspected along its full length, formwork and packers are removed, and any accidental contact is found and released. On most projects the completed system is then tested to confirm that its behaviour matches the design assumption, since the performance was the entire reason for the expense and it can be measured.
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Step 7: Complete the track and detail the transitions
Fastenings, welding and stressing follow the same disciplines as any slab track, carried out to the specification and to the infrastructure manager's requirements. Transitions at each end of the floating length are designed so that stiffness changes gradually rather than abruptly, since an abrupt change generates its own dynamic effects and can undo some of the benefit. Services and cables crossing the slab are detailed with flexible connections so that they do not form a rigid bridge.
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Step 8: Set up the maintenance regime around the isolation
The system needs a maintenance regime that protects it. The gap is kept clear, because debris falling into it bridges the isolation and quietly removes the performance. Bearings are inspected on the cycle the design sets, using the access provisions built in at construction, and their eventual replacement is planned rather than discovered. The as-built record states the system, the bearing positions, the design frequency and the access arrangements, because none of that is deducible from looking at a finished slab.
What are the benefits of Floating slab track?
- The most effective track form mitigation available for ground-borne noise and vibration
- Protects sensitive receptors above the alignment, including homes, halls, studios and laboratories
- Can make a route viable beneath buildings where vibration would otherwise be unacceptable
- Retains the low maintenance advantages of slab track, with no ballast to foul or tamp
- Performance can be measured after construction, so the benefit paid for can be demonstrated
- Bearing based systems allow inspection and eventual replacement where access is designed in
What are the limitations of Floating slab track?
- The most expensive track form in ordinary use, and applied only over the lengths that need it
- Greater construction depth than a plain slab, on top of a construction that is already close tolerance
- Performance is destroyed by any rigid connection across the gap, including debris that falls into it
- Resilient components have a finite life, so replacement has to be designed in from the start
- Maintenance around the slab is constrained, and every intervention has to respect the isolation
- Transitions at each end need careful design to avoid abrupt stiffness change
What is Floating slab track best suited for?
What plant does Floating slab track need?
- Survey instruments and control networks for base slab flatness and rail position
- Concrete supply, placing and compaction plant suited to confined tunnel working
- Bearing handling and setting equipment, or mat laying and jointing gear
- Formwork and separation systems designed to prevent any hard connection across the gap
- Adjustable rail support frames for setting line and level before casting
- Vibration measurement equipment for verifying the completed system against the design
How is Floating slab track quality-checked?
- Base slab flatness and level confirmed before bearings or mat are installed
- Bearing positions and levels surveyed, since a high or low bearing detunes the system
- Resilient components protected from grit, grout and puncture throughout construction
- Gap inspected along its full length after curing to confirm nothing bridges the isolation
- Completed system tested against the design performance, and the result recorded
- As-built record of system, bearing positions, design frequency and access provisions for future replacement