What is Subgrade?

The subgrade is the soil layer that forms the foundation of a pavement section.

The subgrade lies below the subbase, the base course and the surface course. It carries the load that these layers spread out. Load stresses decrease with depth, so the controlling subgrade stress usually lies at the top of the subgrade. The layers above must reduce this stress to a value that does not cause excessive distortion or displacement of the subgrade soil (FAA AC 150/5380-6C).

FAA AC 150/5320-6G names the subgrade as one of four equally important parts of an airport pavement. The other three are the paving materials, the applied loads and the climate.

How subgrade strength is described

• California Bearing Ratio (CBR). A penetration test to ASTM D1883. A CBR of 15 means the soil offers 15 percent of the resistance of a standard crushed limestone. The laboratory test uses site material, remolded to the moisture and density of construction, and soaked. A pavement foundation tends to reach nearly full saturation after about 3 years, and the soaked test represents that condition. On gravel the test often gives values that are too high, so the FAA design procedure caps E at 50,000 psi (CBR 33).
• Elastic modulus (E). For fine-grained, non-expansive soil, E in psi is about 1500 times the CBR. In MPa, E is about 10 times the CBR. The AC calls this an approximate relation.
• Modulus of subgrade reaction (k). A plate bearing test to AASHTO T 222 gives k for rigid pavement design. The plate is 30 inches (762 mm) wide.
• Subgrade category for ACR-PCR. The reported strength uses four categories defined by E. A is 150 MPa or more. B is 100 to under 150. C is 60 to under 100. D is under 60. The category is part of the PCR report (ICAO Annex 14 Volume I, FAA AC 150/5335-5D).

Improving a weak subgrade

• If the mean subgrade strength is lower than CBR 5 (E about 7,500 psi), the AC recommends an improvement. The designer can use chemical or mechanical methods, or replacement.
• If it is lower than CBR 3 (E about 4,500 psi), the AC says to improve the subgrade through stabilization or replacement.
• The AC also says to consider stabilization in these cases:
  • drainage is poor
  • frost acts
  • water floods the area from time to time
  • a stable working platform is needed
• The most common use of a geosynthetic on airports is to stop fines from moving into the base. The FAA does not allow a thinner pavement structure for any geosynthetic.

For compaction, FAARFIELD computes the depth and density that the subgrade needs. It uses ASTM D698 for aircraft under 60,000 lb (27,200 kg) and ASTM D1557 for aircraft of 60,000 lb and more. The construction specification, item P-152, requires the material to be within plus or minus 2 percent of optimum moisture before rolling.

Frost, swelling and water

Frost action can cause non-uniform heave and a loss of soil strength in spring thaw. The AC sorts soils into frost groups FG-1 to FG-4. FG-4 soils, which include silts, are the most susceptible.

Clay soils with a liquid limit above 40 and a plasticity index above 15 may swell. The AC requires treatment if a soil compacted 2 to 3 percent above optimum swells by more than 3 percent in the ASTM D1883 test.

The AC recommends a drainage layer for pavements that serve aircraft heavier than 60,000 lb. It also recommends one where much water is below the surface and where the soil permeability is below 20 ft/day. An effective layer drains 85 percent in 24 hours under runways and taxiways. Under aprons it drains 85 percent in 10 days.

Signs on the pavement surface

The PAVER manuals link some surface distress to the subgrade:

• A depression can come from settlement of the foundation soil. Water ponds in it, and on a runway it can cause hydroplaning.
• A rut stems from permanent deformation in any layer, including the subgrade.
• A swell is usually caused by frost action in the subgrade or by swelling soil.
• Pumping on concrete pavement leaves stains and base or subgrade material near joints and cracks.

These signs show a possible problem. Borings and tests find the cause.