United States
Hong KongSite utilities and backfill
A buried pipe is supported by its bedding and haunching rather than by its own strength. Backfill is placed in lifts and compacted, and locating existing utilities before digging is a legal requirement.
Underground work where the mistakes are invisible until something settles or breaks.
Locate before you dig
Existing utilities are located before excavation begins, through the one-call system and by hand exposure where necessary.
This is a legal requirement in every state and it is the first thing an investigation asks about after a strike. Test before you close the trench.
The zones around a pipe
| Zone | Function |
|---|---|
| Bedding | The prepared base the pipe sits on |
| Haunching | Material worked under the pipe haunches, providing side support |
| Initial backfill | To a depth above the pipe, protecting it |
| Final backfill | The remainder to grade |
The voids under the curve of a pipe have to be filled properly or the pipe carries load it was not designed for. It is invisible once backfilled and it is why flexible pipe is specified with a bedding and haunching detail rather than just a trench.
Backfill in lifts
Placed in lifts of specified thickness and compacted each time. Dumping a trench full and running a plate over the top compacts the top and leaves the rest loose. Call first.
The settlement shows up months later, usually in the pavement above.
Separation
Water and sewer lines have required horizontal and vertical separation, so a sewer leak cannot contaminate potable water. The distances are in the code and they are a lookup. Then it is too late to see.
Testing before backfill
Pressure and leak testing happens before the trench is closed. Testing afterwards means excavating to fix anything found.
Common questions
What must happen before excavating?
Existing utilities are located through the one-call system and by hand exposure where necessary. It is a legal requirement.
What are the zones around a buried pipe?
Bedding, haunching, initial backfill and final backfill.
Why does haunching matter?
Voids under the pipe curve mean it carries load it was not designed for, and the defect is invisible once backfilled.
How is backfill placed?
In lifts of specified thickness, compacted each time. Dumping and compacting the top leaves the rest loose.
When are utilities tested?
Before backfill. Testing afterwards means excavating again to fix anything found.