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dcredskins (Structural) (OP)
6 Feb 09 12:23
We specify self compacting soil (#57 normally) at the back of the retaining walls. Does it mean, we do not have to compact at all? BTW, I am structural engineer. Please help.
cvg (Civil/Environmental)
6 Feb 09 12:30
#57 is crushed rock, not "soil" and no - it is not "self compacting" The amount of compaction necessary depends on the level of performance you desire. A couple of passes with a vibratory plate compactor or whacker will generally do a good job.
civilperson (Structural)
6 Feb 09 13:48
The #57 stone develops good density,(compaction), from free fall due to the many sizes available for each size void.  With no compactive effort, a density of over 85%, (very limited settlement possibility) can be achieved with little excess horizontal pressure on the backfilled wall which can come from compaction energy.  GREAT STUFF!
PEinc (Geotechnical)
6 Feb 09 16:34
Slightly off topic, but I don't like that people backfill structures with clean crushed stone.  I understand why they do it I am concerned about future work and excavations that need to be shored, such as future bridge repairs or widening.  Crushed stone may be hard to retain.  It can be too loose for installing lagging and is not good for grouting tieback anchors.
msucog (Civil/Environmental)
6 Feb 09 18:32
do not ever refer to crushed stone as self compacting because you are not doing anyone any favors. while it is usually easier to compact, #57 stone can easily lose 20-40% of it's volume with a modest amount of compactive effort. you could EASILY lose 1" per foot of backfill over time as it settles under it's weight (seen it with my own eyes on multiple occassions)
see this (it has a little exercise i ran in our lab on #57 stone just to prove the concept)
http://www.eng-tips.com/viewthread.cfm?qid=193256&page=7
 
fattdad (Geotechnical)
6 Feb 09 21:25
We use 2 ft lifts and bucket compact or roller compact between lifts.  I would not just rely on free fall.

f-d

¡papá gordo ain't no madre flaca!

woofar (Geotechnical)
7 Feb 09 5:00
Wow. what is all this talk about self compacting.

Flowable fill will self compact to some degree, concrete will also self compact, after placement gravity takes over and continues the compaction process whilst the concrete is still plastic.

Theoretically a perfectly rounded single-sized material could be considered self compacting. For instance, if you tip a bag of marbles into a mould they will immediately align themselves into their densest possible configuration.

Soils and gravels require moisture and compactive effort to achieve their densest state.

Of course any soil will consolidate over time but that process is due to water ingress and overburden pressure which is basically the same as the compactive process.

To my way of thinking, in construction, any soil placed in any situation should be well compacted when it is placed.

Cheers
Michael

 
ItsOnlyMud (Geotechnical)
12 Feb 09 6:13
Not sure what kind of ret wall you have, but be carefule compacting soils behind a RW, you can easily impart excessive lateral pressures on the back of the wall.  Again, not sure if any drainage has been incorporated by free draining backfill is desirable to prevent pore water pressures building up, but make sure the draining water has somewhere to go!

A good way of compacting soil without imparting pressures behind the wall is to inundate the soil with water.  When it drains it will be nice and compact.  Provided it is decent quality granular fill , lightly compacted, say with back of excavator bucket or 1 or 2 passes of whacker plate, then settlements will be minimal.  But don't rely on it for supporting road pavement etc.
PEinc (Geotechnical)
12 Feb 09 12:21
I would not innundate the backfill behind a wall in order to reduce pressure that results from compacting the backfill.  The last thing you want behind a wall is unplanned-for hydrostatic pressure.
cvg (Civil/Environmental)
12 Feb 09 13:47
Ok, I am not a geotech, but I will wade into this one more time...

I would suggest that the wall needs to be properly designed to handle the "lateral pressures" which will imparted by the backfill. That lateral pressure should be estimated by the engineer that is specifying the type of backfill material and compaction methods. If that process is followed, then the lateral pressures experienced by the wall should not be "excessive". Given that #57 crushed rock is being specified, generally only light compaction is necessary to achieve a dense backfill. In my opinion, the combination of crushed gravel and light compaction should not result in very high lateral pressures.
msucog (Civil/Environmental)
12 Feb 09 17:18
small compaction equipment used on the 57 stone after the wall has had time to cure should be fine. i wouldn't innundate the backfill since most folks are not in "real" sand. someone could always design for high compactive forces but this can usually be avoided by having a schedule that allows for the strength gain in the concrete and avoiding large compaction equipment.
DRC1 (Civil/Environmental)
16 Feb 09 15:19
Even if you dump marbles in a bowl, a quick shake of the bowl will improve the compaction. The difference between stone and soil is that stone will require a few passes of light equipment for a given lift. (dont expect a hand tamper to have much effect on a 25 ft fill) Soils require more compaction in smaller lifts. Also, stone does not require a proctor. (Nor is a proctor approriate for crushed stone)
Also jetting while good for sands subject to PEInc's caveats, crushed stone is generally too coarse and well drained to jet. In general, crushed stone will have better lateral strength than the soil it is replacing.

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