This phrase refers to the embankment material required to build up a road or pavement from its existing condition and elevation. The ideal material will typically be non cohesive soils composed primarily of sand and/or gravel, though the engineer may allow silty or clayey native soils be used, given a proctor test is taken. This test will determine the peak moisture to density ratio to be used in compactive efforts to assure the material is at proper moisture so it may be compacted as dense as possible. Also, gradation testing of the desired material may be done to classify the material and determine it’s structural stability as a permanent pavement subgrade.
The rule of thumb on what material is acceptable (aside from the engineers professional direction) really comes down to its ability to be compacted efficiently and productively and pass compaction testing. ‘High-fine’ clays are notoriously poor as pavement bases as they retain water, consolidate easily, and are more subject to freeze/thaw. Often times a contractor will spend so much time trying to pass compaction tests on clayey/silty soils, it becomes more sensible to utilize premium engineered fill as a road embankment base.
When building up an embankment under a road, the material will need to be placed in lifts, watered, and compacted. On smaller projects like parking lots, driveways, etc, proof rolling the subgrade is an adequate way to assure the embanked material will be strong enough to resist large vehicle loads. Proof rolling will typically consist of a loaded dump truck passing over the material to assure there are no soft spots or unsuitable pockets of material. The typical crew for this type of activity is a foreman, a laborer, and 2 operators. The equipment required typically consist of a bulldozer and/or excavator/backhoe and a vibratory or sheepsfoot roller. Sometimes a grader will be required as well to achieve the correct grades and cross slopes.