Why slab track settles
Ballastless track spreads train loads well for as long as the formation under it stays firm and dry. When water gets in, the fine soil under the slab softens, and every passing train pumps a little of it out at the edges and joints. The slab loses support in patches, starts to rock, and settles. Each monsoon adds to it.
The causes we find most often:
- Water entering from the uphill side, or rising from a high water table, with no cut-off and poor drainage.
- Soft or saturated formation, often at embankments and bridge approaches.
- Transition zones, where the track passes from a tunnel to an embankment or onto a bridge and the stiffness under it changes sharply.
- Voids left where fines have been washed out under traffic.
Signs that the ground, not the slab, is the problem
- Settlement that grows after each monsoon. On T-46 P2 the level surveys showed it growing by about half in the two monsoon months of 2025.
- Highs and lows in the profile rather than an even drop, often at slab joints and transitions.
- Slush, standing water or fines at the slab edges after rain.
- Speed restrictions that come back after every correction.
Step 1: survey the track and the ground first
Two surveys, done before anyone drills, decide the whole job.
- A level survey of each rail, so the lift needed at every point is known, with the cant.
- A geophysical survey of the ground below: MASW for stiffness, ERT for water, GPR for voids. On T-46 P2 the MASW survey showed low-stiffness zones exactly where the settlement was greatest.
Together they show where the ground is weak and why. The lift and the ground treatment are both designed from them.
Step 2: design the lift and the treatment together
The method statement sets the lift at each point, the injection holes, the order of work, the quantities, and the limits on pressure and volume. It goes to the owner for approval before any drilling. The acceptance values are agreed at the same time: the levels to be reached, and how settlement will be measured afterwards.
On a running line the design also has to fit the traffic block. On T-46 P2 that meant night blocks with about five to six effective hours, drilling beside 25 kV overhead equipment only under power blocks, and the track cleared before the first train.
Step 3: lift in small steps, one rail at a time
Polyurethane injected through holes in the slab expands beneath it and lifts it. The lift is made in short cycles, with the levels read on each rail after every cycle, so the slab reseats without cracking at the joints. On a curve each rail is lifted with its own quantities and pressures to hold the cant.
On T-46 P2 we lifted with PuTeRA Level Up, our high-density hydrophobic PU, in steps of 15 to 20 mm per cycle. The largest lift was 136 mm. Inside the tunnel the correction was 12 to 48 mm, and the design profile and cant were restored on the 635 m radius curve.
PU reacts more slowly in the cold, so its reaction time is set for the night temperature on site. On T-46 P2 the nights fell below 5°C.
- 1Note 1: Design level of the track (green dashes).
- 2Note 2: Slab settled below level, with the formation under it softened by water.
- 3Note 3: PU injected through holes in the slab, lifting it in small, controlled steps.
- 4Note 4: Levels read on each rail after every cycle, so cant and alignment are held.
- 5Note 5: The gap under the lifted slab filled with non-shrink cementitious grout.
- 6Note 6: Weak formation grouted, so the slab does not settle again.
- 7Note 7: Slab back on the design level.
Step 4: fill the gap, then treat the cause
A lifted slab rests on the PU at the injection points. The space left under it is then filled with a non-shrink cementitious grout, so the slab is carried evenly again. On T-46 P2 we used a fibre-reinforced non-shrink grout.
Then the cause. Lifting alone would not have held on T-46 P2: the next monsoon would have softened the formation again. The formation along the slab edges was permeation grouted to 1 m, and a grout curtain with self-drilling anchors to 2.5 to 3 m was placed on the hill side to cut off the water.
Where the water comes from decides where the cut-off goes. Drainage that lets water leave the formation is part of the same fix.
Step 5: prove it under traffic
The job is finished when the acceptance values are met, not when the grout is in. Levels are read again under traffic, and the ground is surveyed a second time.
On T-46 P2 the residual settlement measured after traffic resumed was 0 mm. A second MASW survey confirmed the gain in ground stiffness, the slabs showed no cracking and the rail fastenings stayed tight. KRCL certified the work satisfactory in February 2026.
Largest lift, on the most settled slab.
Residual settlement, measured after traffic resumed.
Correction inside the tunnel. Design profile and cant restored on the curve.
What to ask any contractor before work starts
- Which surveys come first, and what will they tell us?
- What is the lift per cycle, and how are levels read on each rail?
- What are the pressure and volume limits per hole, and what happens if a hole takes more?
- How will the cause be treated, and not only the slab?
- What acceptance values do you propose, and how will they be measured after traffic resumes?
- Which materials, with their data sheets and batch records?
We answer all of these in the method statement, before any commercial discussion.