Compacting backfill by flooding 1

[magicbear]

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

 

[emmgjld]

Flooding or 'jetting' backfill generally produces poor to very poor compaction. There are very few specifications that I am aware of that even allow flooding are some landscaping projects. This method of soil settlement is well addressed in the literature, regarding older earthen dams and other backfill situations which failed. I do not allow this type of soil settlement on any projects which will require subgrade or backfill soil stability. In my opinion, this is a very poor method of trench backfill and should not be used beneath buildings, slabs-on-grade or beneath road sections.

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.

 

[Jim6758]

I do not consider the terms "jetting" and "flooding" to be synonymous. A skilled operator with a properly designed jetting wand can do pretty well in poorly graded sands providing the water has somewhere to drain to. I don't know of any specifications that allow flooding for compaction of backfill. I wouldn't use "jetting" under any pavement structures.

 

[snafuman]

Have to disagree with EMMGJLD. A sand material with few fines can be jetted (which is a little different process than flooding) to produce good compaction, especially under pipe haunches or other areas which are difficult to mechanically compact. The sand material must be clean and free draining and the surrounding soil must be well drained, or other means of draining the backfill must be provided. In jetting, you pump the water under pressure and use the force of the jetted water to move the bedding or backfill material around. As with any type of backfilling, material should be placed slowly and in lifts.

 

[Rjakiel]

The only material I am aware of that can be placed around pipes by flooding or pumping is a lightweight foam concrete.

 

[BillHolt]

Here in Louisville, Ky. (and in a few other cities) the practice of "flushing and jetting" is the de facto standard for trenches under pavements. The local sewer and drainage authority (MSD) has a rather elaborate specification for the sand which is to be subjected to this practice. The specification is based on mathematical regressions through some data, where the data was derived from the impressions of various technicians about which gradation, in their experience, would produce satisfactory results. Science?

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.

 

[emmgjld]

The above discussion seems to confirm what I have originally posted.

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.