What Is the AASHTO Subgrade Resilient Modulus (CBR) Calculator?
Resilient Modulus (MR) is the primary material property used in mechanistic-empirical flexible pavement design to characterize subgrade soil elastic stiffness under cyclic traffic loading.
When laboratory repetitive load triaxial testing (AASHTO T 307) is unavailable, empirical correlations from California Bearing Ratio (CBR) provide standardized design estimates.
This calculator applies AASHTO 1993, Powell (TRL), and CSIR correlation models to compute MR in MPa and psi, moisture derating, and structural layer coefficient a3.
How Does the AASHTO Subgrade Resilient Modulus (CBR) Calculator Work?
The calculation evaluates user-provided measurements using recognized domain equations, converts between measurement units, and adjusts for real-world efficiency factors.
AASHTO Subgrade Resilient Modulus (CBR) Calculator Formula & Variables
The core mathematical equation utilized by this calculator is expressed as:
Standard AASHTO correlation linking California Bearing Ratio penetration resistance to cyclic elastic resilient modulus.
How to Use the AASHTO Subgrade Resilient Modulus (CBR) Calculator
- Enter your primary measurements in the input fields above.
- Select your preferred units (e.g. metric or imperial) if applicable.
- Review or adjust operational assumptions such as field efficiency.
- Click Calculate to instantly generate the full results breakdown and visual chart.
- Use the Reset button at any time to clear the form and test a new scenario.
Step-by-Step Example Calculation
Standard Clayey Silt Subgrade (CBR = 6%)
Input Values:
Understanding Your Result
Your calculated result represents the realistic operational capacity or baseline output under the specified conditions. Comparing theoretical and effective outputs reveals the direct impact of turns, overlap, and practical downtime.
Factors That Affect the Result
Field terrain, operator experience, equipment maintenance, overlap margin, and weather conditions can significantly influence real-world output.
When Should You Use This Calculator?
Use this calculator whenever you need quick, verified estimates for job planning, budgeting, equipment sizing, or project timelines.
Assumptions & Limitations
- The linear relation MR = 1500·CBR is specifically recommended for fine-grained soils with soaked CBR ≤ 10%.
- For coarse granular materials (CBR > 10%), power-law relations prevent overestimating subgrade stiffness.
Frequently Asked Questions
Calculation Accuracy & Reference Note
This calculator implements verified, deterministic mathematical equations based on published standards. Results should be treated as professional engineering estimates; always verify critical operations with local equipment manuals and site inspections.