When an old under ground pipe system (most commonly water, sanitary, and storm) is replaced or no longer needed due to an upgrade, the project engineer may wish for the existing pipe to be removed or abandoned. Because leaving old pipe in the ground causes confusion and delay for future excavating construction work in the same area, the practice of abandoning pipe is avoided if the budget allows.
Nevertheless, the practice of abandoning/grouting pipe is still widely used, and in some cases is more economical than complete removal. The type of pipe in the ground may also influence this decision to abandon or remove. For example concrete, steel, and iron pipe anticipated to be in reasonable condition are often easier left abandoned in the ground, as it’s less likely they will break and leave the ground above subject to unwanted settling, which can translate into failing pavement and sink holes. Clay tile and very old pipe (of any kind) however, may be more subject to deterioration, cracking, and breaking making it a better candidate for grouting abandonment. Grouting the pipe solid provides a guarantee that in time the pipe won’t give under pressure and cave in causing potential settlement issues. It is typically required that the pipe be bulkheaded/plugged (with brick/block and mortar, often in manholes) to retain the fill material. In some rare cases, the engineer may allow that the pipe be collapsed or crushed in place and backfilled with dirt or aggregate stone.
Pipe grout fill mix designs can be tricky as the right amount of flowability must be achieved while maintaining strength and many pipe systems have minimal slope. Because of this, fly ash and/or air-entrainment admixtures are often added to the mixture to promote flowability. The goal is to fill the pipe all the way to the crown if possible, though this is very challenging because the area is concealed and can’t be accessed to help induce consistent grout placement, especially if the pipe run being filled is long. To help in this situation, grout tubes are commonly utilized (often plastic standpipes placed at the ends and sometimes intermediate points) which provide access for tube/pipe and also to allow air to escape from inside the pipe during the filling operation. The bulkheads and grout tubes are placed first (and backfilled around if need be) and the grouting operation is then undertaken.
For hard to reach areas, it is not uncommon for the ready mix grout truck to chute the material into the hopper of a grout pump which pressurizes and pumps the grout into the pipe.
Gas powered grout pump.