When new utilities are installed, the existing ones are typically left in place, creating a void in the subsurface. These can act as a conduit for fluids that will potentially lead to erosion and settlement of the ground above, in the worst cases, it can lead to a sink hole. To remedy this problem, the lines have to be completely filled with material so that collapses don’t occur, and water doesn’t so easily flow down the old pipes.
Typically, some form of grout is used to fill these voids. And the type you use, like everything, depends on your constraints and goals. For shorter runs that require less flowability and lower volumes, typical sanded or neat grouts can be used. These have the advantages of high strength and the ability to displace any water that may be present, but can be expensive.
Another option (our favorite) is cellular grout. We take a cement/water slurry from the ready-mix plant and mix it in our continuous mixer with a synthetic foam to produce a slurry of cement, water, and 60-75% air. Cellular is significantly lighter than grout (and water) and can flow for 1000’s of feet reliably, even at small diameters. It will also meet most DOT non-excavatable flowable-fill specifications. On a per-unit basis it can also be much more cost effective. Cellular is the hands-down best choice for abandoning a pipe to ensure an economic process and minimized probability of future settlement.
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