A rail screw spike resists being pulled out of the sleeper through two mechanisms: the mechanical interlock between the thread and the surrounding material, and the bond between the anchoring medium and the hole wall. Both mechanisms scale with anchorage depth. If the depth is too small, the spike's pull-out resistance falls below the design value, and the longitudinal forces from train braking and starting can drag the rail along the sleeper, widening the gauge, disturbing the alignment and damaging the sleeper hole. Under vibration loads a shallow anchorage also concentrates stress at the top of the hole, which can bend or crack the spike and eventually let the rail lose restraint entirely.
Anchorage Depth by Sleeper Type and Line Grade
| Sleeper / line type | Typical anchorage depth (mm) | Notes |
| Timber sleeper, light lines | 100-120 | Depth limited by sleeper section; spikes often driven rather than grouted |
| Ordinary concrete sleeper | 150-160 | Common Chinese practice for conventional-speed lines |
| Prestressed concrete sleeper | 160-170 | Anchoring must avoid damaging prestressing tendons |
| High-speed lines | 170-180 | Stricter verticality and depth tolerance control |
| Heavy-haul lines | 180-200 | High axle loads demand maximum bond area and high-strength spikes |
The depth tolerance is typically held to plus or minus a few millimetres on modern lines, and hole verticality is controlled to within about 2 degrees of plumb so that the spike is loaded axially rather than in bending.
Anchoring Systems: Sulfur Cement versus Chemical Grout
| Property | Sulfur cement anchoring | Chemical (resin/grout) anchoring |
| Application temperature | Molten, poured at roughly 150-180 C | Ambient temperature curing |
| Bond strength | Moderate, typically below 10 MPa | Typically 10-20 MPa depending on system |
| High-temperature risk | Can damage prestressed tendons in PC sleepers | None |
| Reusability of equipment | Requires melting plant and hot handling | Cartridge or pump dispensing |
| Durability | Brittle, sensitive to freeze-thaw | Better aging resistance when formulated for track use |
Because molten sulfur is poured at high temperature, it must not be used where it can overheat prestressing tendons in concrete sleepers. Chemical anchoring agents are the standard choice for high-speed and heavy-haul lines, where bond strength of 15 MPa and above is commonly specified.
Pull-out Resistance Requirements and Testing
The pull-out (withdrawal) resistance of a screw spike is verified by a withdrawal test: a hydraulic pull-out tester grips the spike head and applies tension at a controlled rate until the spike reaches the required force or slips. In Chinese practice, acceptance levels of about 60 kN for conventional-speed lines, 70 kN for high-speed lines and 80 kN for heavy-haul lines are commonly specified, with no significant slip and no cracking of the anchoring medium. European practice assesses fastening systems under the EN 13481 series requirements using the EN 13146 test series, which includes repeated load and environmental conditioning before withdrawal testing. Sampling is normally carried out on a percentage of installed spikes, with the rate increased at critical positions such as turnouts, curves and braking sections, and the inspection rate is expanded if failures are found.
Design and Construction Controls
Drill the hole to the specified diameter and depth; undersized holes crack under grout pressure and oversized holes reduce bond area.
Clean the hole with compressed air before anchoring; dust, oil or water above a few percent moisture will cut the bond strength sharply.
Control hole verticality to within about 2 degrees of plumb.
Fill the hole completely with the anchoring agent; air pockets create voids that lower pull-out resistance.
Insert the spike and allow the agent to cure to full strength before traffic returns; curing time is typically 24 hours at room temperature for resin systems.
Verify with withdrawal tests at the agreed sampling rate and record the results for acceptance.
Common Misconceptions
Deeper is always better is not correct: over-deep holes can damage prestressing tendons or break through the sleeper base. Withdrawal force alone does not prove a good anchorage, because a spike can reach high force with large displacement and still fail under repeated loading; displacement must be checked too. And sulfur cement and chemical grout are not interchangeable on the same sleeper type: the sleeper material, temperature exposure and required bond strength decide the system.
FAQ
What happens if the anchorage depth is too shallow?
Pull-out resistance falls below the design value, the spike loosens under vibration, the sleeper hole is damaged, the gauge widens, and in severe cases the rail can lift and derail a train.
What is the difference between sulfur and chemical anchoring?
Sulfur cement is poured molten at 150-180 C and is brittle; chemical grouts cure at ambient temperature, bond more strongly and do not risk heat damage to prestressed sleepers, which is why they are preferred on modern lines.
How is pull-out resistance tested?
With a hydraulic pull-out tester applying tension at constant speed until the target force is reached or the spike slips. Acceptance compares force and displacement with the specified values, and the anchorage medium must not crack.
Why is hole cleanliness important?
Dust and moisture reduce the bond between the anchoring agent and the hole wall. A dirty hole can cut the achievable pull-out force by a large margin even when the depth is correct.
Can pull-out resistance be improved after installation?
If a test shows low resistance, the spike should be withdrawn, the hole cleaned and re-anchored with fresh agent; adding torque to a loose spike masks the problem and accelerates sleeper damage.
Do heavy-haul lines need deeper anchorage?
Yes. Heavy axle loads generate higher longitudinal and uplift forces, so heavy-haul practice specifies the deepest anchorage (up to about 200 mm) together with high-strength spikes and chemical anchoring to keep the fastening stable for the full line life.