Tips for backfilling around sewer line repair

[Tom the Elder]

Another way to fill the hole and not leave a long-term hump or depression is to surcharge it with an additional 3 foot of dirt. Then after a few months, you can remove the excess dirt and the ground will be level.

No, it doesn't take 30 years for the ground to settle, but it can and often does take 2 to 5 years depending on the soil. If you go around to a new subdivision a year or so after everyone moves in, there will lots of low places over utility runs.

Backfilling a deep hole while avoiding a dip or a bump years later is mostly either difficult or expensive.

The expensive ways are filling with concrete or flowable fill.

Compacting backill is difficult in such a small, deep hole. You need to compact it to at least the density it would achieve if just left to settle for years, i.e. what you would get if you just dump soil in and let it compact under its own weight. Any less than that and it will settle over time and leave a dip.

Compacting with a surcharge load can absolutely work. Unless the original soil was VERY loose, that is essentially what you get when you backfill with just the excavated soil - like a classic grave mound. Adding more soil than was excavated can speed the process. However, in many cases even just the excavated soil will never settle to be as dense as the in-place soil and a mound will remain no matter how many years you wait.

Another DIY way is soil-cement. Mix about 3-5% cement into the excavated soil then tamp it into place in thin lifts (compaction by hitting with the end of a 2x4 would be tedious but cheap and effective). The cement adds strength to avoid settlement. Discontinue the cement above whatever root depth you anticipate for that areal.

 

[MicEd69]

Flowable fil is a good idea. If the amount of cement in the mix is correct, the fill will not settle but it cn be dug with a shovel. Pretty expensive though as it comes in a redi-mix truck.

A DIY alternative would be a wet batch of sand-cement mix. Wet enough you can be sure it will flow to fill the void under the pipe. Fill it up to at least the spring line. Way weaker than concrete but plenty strong for pipe support. Above that, pick your poison.

It appears we are talking 7 to 8 cubic yards of total fill. Mixing and placing 5 to 7 yards of "wet batch sand-cement mix" is a HUGE DIY job, let alone the cost itself.

And digging through 8 feet of flowable fill is truly a LOT more difficult than plan dirt, even using powered equipment.

This maybe a job for a professional especially with regard to the safety issues here.

 

[MicEd69]

Backfilling a deep hole while avoiding a dip or a bump years later is mostly either difficult or expensive.

The expensive ways are filling with concrete or flowable fill.

Compacting backill is difficult in such a small, deep hole. You need to compact it to at least the density it would achieve if just left to settle for years, i.e. what you would get if you just dump soil in and let it compact under its own weight. Any less than that and it will settle over time and leave a dip.

Compacting with a surcharge load can absolutely work. Unless the original soil was VERY loose, that is essentially what you get when you backfill with just the excavated soil - like a classic grave mound. Adding more soil than was excavated can speed the process. However, in many cases even just the excavated soil will never settle to be as dense as the in-place soil and a mound will remain no matter how many years you wait.

Filling in 12" lifts and compacting each lift will essentially eliminate settling. And adding water to every second or third lift and letting it settle for a day will also help.

But again, I'm thinking this is a job for a professional as this is very risky. It will costly to [properly shore this excavation, but it is quite dangerous not to do so.