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