Design smarter ballast layers with practical field inputs. Adjust for axle loads and weak soils. Export results for reports, estimates, and site planning fast.
| Scenario | Axle Load (kN) | CBR (%) | Drainage | Fouling | Recommended Depth (mm) |
|---|---|---|---|---|---|
| General duty track | 180 | 6 | Good | 10 | ~240 |
| Heavier loads | 220 | 6 | Fair | 15 | ~310 |
| Weak subgrade | 180 | 3 | Poor | 20 | ~450 |
Example depths are indicative. Your results depend on all inputs.
1) Design load per sleeper
P = (AxleLoad × DynamicFactor) ÷ SleepersSharing
2) Spread area at subgrade level
A = (b + 2z·tanθ) · (L + 2z·tanθ)
b = sleeper width, L = sleeper length, z = ballast depth, θ = effective spread angle.
3) Subgrade stress check
σ = P ÷ A
4) Allowable stress (screening estimate)
σ_allow = (30 × CBR) · F_drain · F_quality
The calculator finds the smallest depth z where σ × SafetyFactor ≤ σ_allow.
Ballast depth controls how axle loads disperse from sleeper to subgrade. Deeper ballast increases the stress bulb area, lowering peak stress and slowing settlement. Many heavy-haul corridors target 250–350 mm beneath sleeper for new construction, then maintain depth through tamping and ballast addition where fouling or pumping occurs. For mixed-traffic lines, 200–250 mm may be acceptable with strong formation, but weak subgrades often need 400 mm or a capping layer.
The effective load on one sleeper depends on axle load, impact, and how many sleepers share the wheel load. A dynamic factor near 1.1 suits low-speed lines, while 1.3–1.5 is common where joints, switches, or higher speeds increase impact. Sharing 3–5 sleepers is typical for good track stiffness and consistent support. Record actual wheel loads for specialty equipment.
CBR gives a quick indicator of subgrade strength for preliminary checks. For example, CBR 3–5 often represents soft clay or wet subgrade, while CBR 8–12 is moderate granular material. Drainage and ballast quality multipliers reflect how quickly water leaves the section and whether the ballast is clean, angular, and durable. A CBR improvement from 4 to 8 can roughly double the screening allowable stress in this method, which can reduce required depth by several centimeters.
Fouling reduces interlock and permeability, limiting effective spreading. Higher fouling index values therefore demand additional depth to achieve the same stress level. Side slopes, often 1.5H:1V to 2H:1V, influence shoulder width and volume. Wider shoulders improve confinement, helping resist lateral movement under braking and curvature.
Once depth is selected, the cross-sectional area and track width let you estimate ballast volume per meter and total tonnage using compacted density, commonly 1.5–1.8 t/m³. Use these quantities for procurement, transport cycles, and work windows. Always validate the result against local standards, drainage details, and field compaction practices.
New heavy-haul construction commonly uses about 250–350 mm under the sleeper. Softer formations may require 400 mm or an engineered capping layer, depending on subgrade strength, drainage, and traffic.
Use lower values for smooth, low-speed track and higher values where impacts increase. Values around 1.1–1.2 suit well-maintained lines, while 1.3–1.5 can reflect joints, turnouts, and higher speeds.
Fouling reduces drainage and load-spreading efficiency. A higher fouling index increases the required depth so that calculated stress stays within the allowable range, reflecting contaminated ballast that performs like a thinner layer.
No. CBR is a screening input. Final design should use site-specific investigation, moisture conditions, formation improvements, and any required geotextile or sub-ballast layers, plus the governing railway or project standards.
They are planning-level. Actual quantities depend on shoulder geometry, compaction, settlement allowance, and construction tolerances. Confirm with cross-sections, as-built surveys, and the ballast supply’s measured bulk density.
Consider it when CBR is low, drainage is poor, or repeated maintenance is expected. A capping or sub-ballast layer can improve stiffness, reduce pumping, and provide a separation/filter function above fine-grained subgrades.
Disclaimer: This tool supports preliminary design decisions. Always verify ballast depth against project standards, geotechnical reports, and safety requirements.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.