As homebuilders begin to run out of prime sites for residential developments, they're having to settle for sites with potential problems such as low-strength soils, high water tables or fill materials of questionable quality. In the Northwest, that situation has created an opportunity for increased use of controlled-density fill (also called flowable fill or controlled low-strength material).
Used as a subfooting, a mass fill over thick layers of marginal-quality soils or as a vapor-barrier covering in residential crawlspaces, CDF offers several advantages over alternative materials. CDF can be placed by pump, and it requires no vibration.
The earliest residential applications were on sites where saturated bearing soils several feet below the surface could provide required foundation support, but imported compacted fill was still needed above the bearing soil. Instead of waiting for the soil to dry, contractors started reaching over the wet site with a pumpline and placing CDF in trenches dug to the depth of undisturbed bearing soil at the footing locations. With care, they could even place the material in trenches that weren't fully dewatered.
On residential sites such as river valleys, where soft, boggy surface soils may be 100 feet deep, an excavate-and-replace approach is the only viable option for improving bearing capacity. In a few instances of this type, contractors have filled the whole building footprint with CDF a few feet deep. This helps workers overcome the difficulty of compacting imported fill on an unstable base.
Crawlspace fill is another CDF application. In new construction, it's used to prevent vapor-barrier punctures and provide a stable, level surface. The material can also be used to correct problems in existing houses. For instance, over-excavated areas that collect water are filled with a stiff-consistency CDF that slopes to drain. Another advantage is the fill material's ability to be pumped through a foundation vent after the house is occupied.
The article also discusses CDF mixture proportions and the need for pumps.
Keywords: controlled-density fill, flowable fill, CDF, soil