Resilient Modulus (MR): Test, CBR Correlations, Values
Resilient modulus, written MR, is the number pavement designers use to describe how stiff a subgrade or unbound base is under traffic. Soil under a road does not get loaded once and left alone. It gets a quick push from every axle, rebounds, and gets pushed again, millions of times. Resilient modulus captures how much the soil springs back under that repeated loading. A stiff subgrade lets a thinner pavement carry the same traffic; a soft one needs more structure on top. This guide explains what MR means, how the lab test works, the common shortcuts from CBR and R-value, and how field values are adjusted.
Asphalt base calculator →What resilient modulus measures
Under a repeated load, a soil sample deforms in two ways: a small permanent part that never comes back, and a larger elastic part it recovers when the load lifts. Once the permanent part settles down after many cycles, the resilient modulus is the cyclic stress divided by the recoverable strain. FHWA's geotechnical reference describes it as the elastic unloading modulus after many cycles. It is a stiffness, not a strength: a material can be fairly strong and still deflect more than you would like.
Where MR is used
- Describing subgrade stiffness for both flexible (asphalt) and rigid (concrete) pavements in the 1986/1993 AASHTO guides.
- Setting structural layer coefficients for base and subbase layers in the 1993 method.
- As the main input for unbound layers in mechanistic-empirical design (NCHRP 1-37A, the basis for AASHTOWare Pavement ME).
How the lab test works
The test is a cyclic version of a triaxial test. AASHTO T 307 is the method most people mean today; FHWA's reference also lists older protocols (T 292, T 294, TP 46) and a "harmonized" NCHRP 1-28A procedure.
- Make a specimen. Soil is compacted into a cylinder at a target moisture and density, often based on a Proctor test. Specimen size and compaction differ for granular base and fine subgrade soils.
- Confine it. The cylinder goes inside a pressure chamber that squeezes it from all sides, the way surrounding soil does in the ground.
- Pulse the load. A closed-loop electro-hydraulic machine applies haversine-shaped load pulses. FHWA notes typical timing of 0.1 second loading and 0.9 second rest for base and subbase, and 0.2 second loading and 0.8 second rest for subgrade.
- Measure rebound. Sensors track how much the specimen recovers after each pulse, ideally with clamps mounted on the sample at quarter points.
- Repeat at several stress levels. Results are fit to a model so designers can pick a modulus that matches the stress in their pavement.
Correlations from CBR and R-value
Not every job gets a resilient modulus test. Designers often estimate MR from a simpler test. These are the common ones:
| Equation (MR in psi) | Source | Limits |
|---|---|---|
| 1500 × CBR | Heukelom and Klomp (1962), used in the 1993 AASHTO guide | Fine-grained, non-expansive soils, soaked CBR no higher than 10 |
| 1000 + 555 × R-value | 1993 AASHTO guide | Fine-grained, non-expansive soils, R-value no higher than 20 |
| 2555 × CBR0.64 | AASHTO 2002 design guide (NCHRP 1-37A) | Described as a fair conversion over a wide range |
Each one has limits, and they do not always agree. For a made-up soil with a CBR of 5: 1500 × 5 = 7,500 psi, while 2555 × 50.64 = about 7,160 psi. At a made-up CBR of 10 the gap grows: 15,000 psi versus about 11,150 psi. That spread is a good reminder that a correlation is a starting estimate. See our California bearing ratio guide for how CBR itself is measured, and the dynamic cone penetrometer guide for a quick field test that is also correlated to stiffness.
Field values and adjustment factors
On existing roads, engineers can back-calculate in-place moduli from deflection testing with a falling weight deflectometer. FHWA's reference cautions that these field values are often well above lab design values because the stress conditions differ. To bridge the gap, the 1993 AASHTO guide suggests multiplying back-calculated subgrade values by a factor no higher than 0.33 when designing asphalt pavements and no higher than 0.25 for concrete. NCHRP 1-37A uses 0.40 for subgrade and 0.67 for granular base or subbase layers under flexible pavement.
What changes the resilient modulus
- Moisture: wetter fine-grained soils are usually much softer. Spring thaw is the classic weak season.
- Density: better compaction generally means higher stiffness.
- Stress level: granular materials stiffen under higher confining stress; fine clays tend to soften as deviator stress rises.
- Soil type: clean granular soils and good crushed base are stiffer than silts and plastic clays. The Atterberg limits help flag the soft ones.
Why homeowners might care
Nobody runs a triaxial test for a driveway. The idea still applies: a soft, wet subgrade flexes under every car, and asphalt laid on it cracks sooner. Drainage, good compaction and enough crushed base do for a driveway what a high MR does for a highway. Our asphalt driveway thickness guide and the base calculator help size those layers.
Questions people ask
What is resilient modulus?
It is a measure of stiffness for soils and unbound base materials under repeated loading, like traffic. After many load cycles, it is the applied cyclic stress divided by the strain the sample recovers when the load comes off. Higher values mean a stiffer layer that deflects less.
How is resilient modulus tested?
In a repeated load triaxial test such as AASHTO T 307. A cylinder of soil sits in a pressurized cell while a machine applies short pulses of axial load thousands of times at different stress levels, and sensors measure the rebound.
What is the formula to estimate resilient modulus from CBR?
One widely used correlation from the AASHTO 2002 design guide is MR (psi) = 2555 × CBR^0.64. An older one, meant for fine-grained soils whose soaked CBR is no higher than 10, is MR (psi) = 1500 × CBR. Both are estimates, not a substitute for testing.
What are typical resilient modulus values for subgrade?
They vary widely with soil type, moisture and stress level, from a few thousand psi for soft wet clays to much higher values for granular soils. Because of that spread, designers prefer lab tests or local data over a generic number.
Why are FWD back-calculated moduli adjusted down?
Field values back-calculated from falling weight deflectometer data are often higher than lab design values because of different stress states. The 1993 AASHTO guide suggests a factor of at most 0.33 on subgrade field values for asphalt pavements.
Is resilient modulus the same as modulus of subgrade reaction?
No. The modulus of subgrade reaction (k) comes from a plate load test and is used mainly in concrete pavement design. Resilient modulus comes from repeated triaxial loading and is the main subgrade input for asphalt pavement design.
Related guides
- California bearing ratio
- Dynamic cone penetrometer
- Standard Proctor test
- Asphalt driveway thickness
- What is road base?
As an Amazon Associate, AsphaltTons.com earns from qualifying purchases. Product links below are sponsored.