What is an Impact Factor?
An impact factor is a multiplier used in pipeline design to account for the additional stress caused by moving vehicles above the pipeline, compared to static loads. Think of it as a “safety buffer” that accounts for the dynamic nature of traffic.
Basic Guidelines
- Standard Impact Factor: 1.5 for roads with flexible pavements
- Rigid Pavements: No impact factor required
How Depth Affects Impact Factors (AASHTO Standards)
| Depth | Impact Factor |
| At surface | 1.3 |
| 0.1 to 1 ft | 1.2 |
| 1.1 to 2.0 ft | 1.1 |
| 2.1 to 3.0 ft | 1.0 |
What Increases Impact Factors?
- Higher vehicle speeds
- Higher tire pressure
- Rougher ground surface
Special Case: Low-Pressure Equipment
Some equipment can use a lower impact factor (1.25 instead of 1.5) when they have:
- Special design for low ground pressure
- Low-pressure pneumatic tires (< 30 psig)
- Slow operating speeds (< 10 mph)
Practical Example
When reducing the impact factor from 1.5 to 1.25 for special equipment:
- Reduction ratio = 1.5/1.25 = 1.2
- This means a 20% reduction in the design load factor
Key Takeaway
The impact factor is higher when there’s more potential for dynamic force (like fast-moving heavy trucks) and lower when equipment is designed to minimize ground pressure and moves slowly.
These value are based on recommendation by the ASME committee on Pipeline Crossings of Railways and
Highway.