Compacting backfill by flooding
Compacting backfill by flooding
(OP)
Questions on compacting pipe backfill material by flooding:
If you compact pipe backfill material by flooding, what amount of compaction could you expect to obtain? Would this method give adequate compaction if a road were built over the pipeline?
I've not come across much information on this topic in the various pipe design manuals we have here. Any insight would be greatly appreciated. Thanks.
- Jason
If you compact pipe backfill material by flooding, what amount of compaction could you expect to obtain? Would this method give adequate compaction if a road were built over the pipeline?
I've not come across much information on this topic in the various pipe design manuals we have here. Any insight would be greatly appreciated. Thanks.
- Jason





RE: Compacting backfill by flooding
The only soils which I am aware of that respond reasonably well are some poorly to possibly moderately poorly graded sand and possibly gravels. Fine grained soils (silts and clays) or soils with fine grained components usually achieve 78% to 85% of ASTM D-698 maximum density if conditions are good. Good conditions require extremely good drainage characteristic of the flooding water into the adjoining soil. If the water cannot drain from the backfill soils, the material will be setup for future collapse. This is very similar to the mechanics exhibited in mudflows and debris flows, which oftentimes become metastable (collapsible) soil.
RE: Compacting backfill by flooding
RE: Compacting backfill by flooding
RE: Compacting backfill by flooding
RE: Compacting backfill by flooding
As done here, it is a terrible practice. Usually, the results are satisfactory, but "usually" is not a good engineering standard. When the consistency of the placed fill matters, the only defense for using this approach, other than ignorance of geotechnical methods, is that "the contractors are used to it."
Generally, the "lift" thickness of the flushed and jetted material is the depth of the trench - usually over six feet.
By observing - no labs testing - the fill in several trenches which were exposed because of pavement failures, I'd guesstimate that the compactions range upward from about 70% relative (one blow material) to fairly good (five to ten blow material). That is, sometimes the compaction is about what you'd expect to achieve by dumping the sand out of a truck onto the ground. Sometimes it's about what you could get by walking on it.
Very often, perhaps typically, you are able to see reflections of the trench through the asphalt within a year after paving.
Thin lifts followed by compaction. It's easy, it's consistent, it's engineering.
RE: Compacting backfill by flooding
The process requires proper material (sampling and testing implied) and proper drainage characteristics of the surrounding soils (construction observation and preconstruction testing implied). I should have added, aa is stated by BillHolt, that confirming the process (observation & testing implied) is a part of the process.
I am sorry to say that my experience with confirmed practitioners is that the above process is unwelcomed. It is easier to believe in 'self-compacting' materials and 'easy' processes than to deal with the longterm issues of failed subgrades and trench backfills.
I do believe the process works, if the right conditions are present but, with the clayey soils in my area, insufficient drainage is commen. Many seem to fear that Testing and observations, with the prospect that the testing will determine the process is inappropriate, will require normal placement in lifts, with proper compaction.