Why is water drawn into overlying soils during compaction??

[NewNorthernGeotech]

I'm looking for a good explanation of how; water is drawn up into a silty material as it is compacted??

I envision that it may be as simple as the void spaces being reduced as you compact the material, thus the water moves further up into the soil to find space.

BUT! I can also see the potential of pressure differences resulting from the packer bouncing on the material (contracting and expanding void spaces) drawing water up into the material...

Can anyone lay this process out for me in both a simple and excruciatinly technical manner??

Thanks!!

 

[msucog]

take a bucket of dry sand...add a few gallons of water slowly...nothing will really happen. then tamp the sides of the bucket. with some silts, it's similar except it doesn't do the same with say a bridge lift (i hate that term) since you've got a buffer between the water and the high compaction stresses. once you put a compactor on saturated silts or silts near groundwater, it does drive the water up since the pore water pressures are jacked up (and stength down). however, the silts don't release the water as a sand would. in effect, you end up with a wet silt fill (i.e. a big mess). perhaps some of the other brains on this board can better explain the scenario.

 

[BigH]

NewNorthernGeotech: I'd suggest that you do a search of "compaction water" - but suggest you consider looking at the following thread: thread274-5909 and thread158-102999. They might help you with your visualization of what happens.

 

[oldestguy]

Next step:

Now comes the question that you may have. How's come the ground now acts like a mattress? And then, is this condition now worthless for suppport?

When I get that question, I say: "Take your front end loader (or compactors) off the site first. And then wait until tomorrow and see the difference".

Generally it is like night and day.

 

[msucog]

very true...sometimes it just takes the sandy silty soil/water time to "equalize" (assuming it's not a big mess of goo from the contractor messing with it). the same holds true for more sandy soils i encounter...just doesn't take nearly as much time from my experience. even though i'm not on the coast and don't normally deal with coastal sands, i understand they will do the same thing but they only take minutes (depending on the fines) for the pore pressure to dissipate. i did go through a somewhat similar experience on one project in mississippi where we had to undercut many feet of nasty nasty fat clays and backfilled with sand. when i compacted the backfill, the water came to top and drained to the adjacent loose sand...so pretty much i took the compactor and just chased the water across the pad since it seemed to work the best. if i stayed in one small area, it still wiggled slightly. but if i went across the pad and then came back, the layer i'd just compacted was jam up hard since it was free draining enough that the water didn't hang around. at least that's my interpretation of what was happening...i had no soils experience at the time but it all seems logical looking back now. there was no one testing the backfill or giving us guidance back then...some geotech had given recommendations and we di the work. scary thinking about leaving contractors to "do it correctly" on their own.

sometimes however, i have seen the contractor get on to a residual silty soil subgrade at the bottom of an excavation that happens to be near groundwater...they kept wanting to compact the stuff with a smooth drum roller and i just about pulled my hair out dealing with the guys. i told them to undercut the upper portion of the mucked up areas and to stay the heck off of it. i told them to cover it up with stone (stone was required for what they were doing), backfill the area and don't worry about it. once they put the roller on it and disturbed it enough, it was nothing more than a wet silty fill water bed that you'd get your ankles muddy in.

 

[BigH]

Not on the original post - but following up with msucog and oldest guy - one needs to really remember what type of soils he is compacting (and what is immediately below). Recently, I saw an inspector (pretty green - not quite bright green but still a bit of forest in there) who let the contractor use a large steel drum roller using vibration when he was working in a layer of quarry fines above a soft to firm clay. Needless to say, this buggered up the soft to firm clay - had to take out a bunch of it. I got them to get a layer of 35 mm minus on top, lay down a geotextile, then use one layer of 35 mm above without vibration before succeding layers were vibrated - this was a job of cutting out 3 m of soft clay in order to put in a stiffened crushed stone pad for foundation footing support.