Compacting Back-filled Soil in Trenches: How is this Done?

May 15, 2019

After the bedding and cables or pipes are all laid just-so, the next stage of the trenching operation commences. The job's almost done, and it's time to put the excavated soil back inside the exposed ground channel. Just like the earlier stages, a detail-oriented, systematically conducted approach is adopted by the excavation team. Being the professionals that they are, they're not going to chuck the dirt in and leave.

Proper Back-Filling Etiquette

More than etiquette, this is the safest, most application-specific approach. Used by trenching professionals, this is the right way to fill in an excavated channel after a project is done and the cables or pipes have been situated. Layers of debris-sifted soil are carefully dropped in by an excavator or team of workers. Each layer, uniformly spread, is free of roots and large rocks. As a matter of fact, stones larger than a few centimetres will be cast aside. Otherwise, when the compaction operation begins, a large rock could be pushed down until it damages a cable or cracks a pipe wall. Bits of concrete or asphalt, plus all of those vegetable chunks and rocky debris, are disposed of in accordance with local regulations.

Safely Compacting the Back-Fill

There's a lot more to say about the back-fill techniques, but this post is meant to be about the compaction work, not the back-fill protocols. Anyway, with the low-density, material-filtered soil filling the trench, the mechanical compaction equipment rolls into action. This operation must be carried out correctly. If the soil is over densified, the underlying utility lines could suffer. By under-densifying the backfill, we risk future soil subsidence. As for the form this process takes, there are different equipment types, which include but are not limited to jumping jack machines, large rollers, and/or tamping equipment. As a rule, as the heavy equipment pushes down on the soil, air leaves, the grains of dirt pack together, and the earth becomes more cohesive.

As with any other stage of this detail-oriented work, there are engineering principles controlling everything. The soil isn't squeezed down indiscriminately. To the contrary, there's are rules to observe. Dry density ratios are complied with, as outlined in the AS 1289.5.4.1 guidelines. Even during the laying of those individual layers, these regs are enforced. Water is sprayed on each strip so that the dirt becomes more cohesive. With the final strip of dirt rising a few essential centimetres above the average ground level, the mechanical compactors and rollers do their work. Incrementally, as the soil compacts, the dry density ratio is checked by a compaction control test, dynamic cone penetrometer, or other density measuring instrument.

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