when an area is to be filled or backfilled the soil is placed in layers called lifts. the ability of the first fill layers to be properly compacted will depend on the condition of the natural material being covered. if unsuitable material is left in place and backfilled, it may compress over a long period under the weight of the earth fill, causing settlement cracks in the fill or in any structure supported by the fill. in order to determine if the natural soil will support the first fill layers, an area can be proofrolled. proofrolling consists of utilizing a piece heavy construction equipment (typically, heavy compaction equipment or hauling equipment) to roll across the fill site and watching for deflections to be revealed. these areas will be indicated by the development of rutting, pumping, or ground weaving.