What is a Plate Bearing Test?

A plate bearing test is a field test that loads a steel plate to measure the modulus of subgrade reaction (k value). The FAA states that the test measures the bearing capacity of the pavement foundation. The k value is the pressure that gives one unit of deflection. Its unit is pounds per cubic inch (pci) or MN/m3.

How the FAA uses it

The FAA names AASHTO T 222 for the test. This is a non-repetitive static plate load test on subgrade soils and flexible pavement components. ASTM D1196 covers the same type of test.

The FAA gives these rules for the test:

• Do the test in the field, on test sections built to the design compaction and moisture.
• Correct the k value if the moisture in service is different.
• Use a 30-inch (762 mm) diameter plate. A smaller plate may give a higher k value.
• Use engineering judgement for the design k value, because the tests are expensive and few.

If there is no plate bearing data, the k value may be estimated from CBR data. The CBR test pushes a plunger into the material. It compares the force with the force for a standard crushed limestone.

The FAA says that the k value more commonly comes from a falling weight deflectometer (FWD) or heavy weight deflectometer (HWD). Those tests give a dynamic k value. The static k value comes from a plate bearing test, so the dynamic value needs an adjustment. FAARFIELD accepts the subgrade as k or as elastic modulus E. It converts k to E with ESG = 20.15 × k^1.284, where ESG is in psi and k is in pci.

k value and pavement strength reporting

The older ACN-PCN method reported the subgrade strength category from k. EASA lists these k ranges in MN/m3:

• High: above 120.
• Medium: 60 to 120.
• Low: 25 to 60.
• Ultra low: below 25.

The ACR-PCR method does not use k. It sets the categories by elastic modulus E. The FAA gives these limits in MPa:

• High: E of 150 or more.
• Medium: E of 100 to below 150.
• Low: E of 60 to below 100.
• Ultra low: E below 60.

Relation to drone inspection

A drone cannot do a plate bearing test. The test needs a loaded plate in contact with the foundation. TarmacView inspects and measures airfield lighting, obstacles and the pavement surface, and it does not do soil testing.