dropping groundwater relative to settlement?
dropping groundwater relative to settlement?
(OP)
anyone else experiencing settlement issues for no apparent reason. i've noticed on two project where settlement monitoring programs are dragging out longer than usual with odd numbers. one of the programs has been measuring for over a year and experienced flat movement for about 6 months then started dropping again with no additional loading. the drop in elevation tend to be trending somewhat with well water levels dropping. the water levels have dropped only about 3-5 feet in the past six months due to the tough drought being experienced here in the southeast. on one project, there's only 20 feet of mediocre residual soil (7bpf auto average) above rock. there was about 20 feet of new fill placed it the area but we're at least 4 months in to the program which should have lasted only 30-60 days. the underlying and fill soils are sandy silts.
in other words, any else experienced the same thing or know of other suspects for the movement? btw, fill placed to at least 95% std Proctor on top of stable subgrade...nothing funky in soil test borings. consols run during exploration phase too to estimate settlement.
i'm stumped as to how a few feet of water movement would cause 2-4 inches settlement under relatively shallow fills atop shallow soil profiles...especially after flat elevation readings.
in other words, any else experienced the same thing or know of other suspects for the movement? btw, fill placed to at least 95% std Proctor on top of stable subgrade...nothing funky in soil test borings. consols run during exploration phase too to estimate settlement.
i'm stumped as to how a few feet of water movement would cause 2-4 inches settlement under relatively shallow fills atop shallow soil profiles...especially after flat elevation readings.





RE: dropping groundwater relative to settlement?
Added to this is the compression of the newly-placed fill. If this is on-site material (i.e., residual silts) and compacted at or above the optimum moisture content, there could be some compression via the relief of excess pore pressure during the compactive effort.
Considering that there has been 4 months since the last of the 20 ft of fill was placed, there is some likelihood that the saturated zone remains underconsolidated. While lowering of the ground water table can affect effective stresses, when you consider 20 ft of new fill, a drop of 4 ft on the water table would amount to less than 10 percent of the newly-placed loading.
Just some thoughts.
f-d
¡papá gordo ain’t no madre flaca!
RE: dropping groundwater relative to settlement?
RE: dropping groundwater relative to settlement?
for the second site, i don't know all the details since it wasn't mine. but the data plots flat for 6 months and then begins to drop and showed 6 inches of drop in the past six months with building construction finished a year ago (so no new loading).
one thing i'm thinking is this: the well readings show the depth to be 80' deep. groundwater here is usually say 10' in these lower lying areas that receive the most fill. so in order for the water table to drop from 80' to 85', the shallow levels would likely experience a much larger drop and effectively adding 60psf per foot of drop. and looking at the historical water levels in the well back about 5 years, the levels have dropped maybe 15' from the max but there was a drop to about the 80' level once before. all in all, i'm thinking there's been much more than a 5' drop in levels which jacked the stresses way up. i wouldn't expect 200-300psf to make a big difference but would provide some minor movement. however 1000psf+ (15-20' drop) likely could generate larger movements.
any other thoughts or similar experiences?
thanks for the input so far.
RE: dropping groundwater relative to settlement?
RE: dropping groundwater relative to settlement?
I am not a geotechnical engineer, so won't try to get technical. But when you say the effective unit weight increases as the water table drops, I can't see it. When the soil is below the water table, the effective soil pressure is reduced by the buoyancy of the water, but to this pressure must be added the hydrostatic pressure. So the loading at the same level would remain about the same as the water table drops. Or am I missing something?
RE: dropping groundwater relative to settlement?
RE: dropping groundwater relative to settlement?
effective=total-pore pressure
effective=total-zero (no water)
therefore, the effective stress is always less when water is present versus when water is not present. the effective stress drives this train.
RE: dropping groundwater relative to settlement?
RE: dropping groundwater relative to settlement?
RE: dropping groundwater relative to settlement?
With the lab data, check the time rate of settlement at the maximum applied earth pressure (i.e. 20 feet of fill). Being an engineer in the Piedmont, 30 to 60 days is a typical recommendation. However, if you have a consolidation plot with good data at each load increment, see what that tells you for time-rate.
For the shot rock fill site, any chance you are loosing the fine-grained soils in the voids of the shot rock fill? Did you cap the shot-rock fill with a filter fabric?
RE: dropping groundwater relative to settlement?
the shot rock fill was excellent. fist sized shot rock and well graded down to fines (they shot the hell out of the rock!--from other area) and placed in 8 inch layers, compacted and proofrolled during each day's placement. and as mentioned, the storm structures out in this area are actually founded in the original ground and they're settling too. i've even got geophysical data across this area and the fill exhibits high velocities.
as i think i previously mentioned, the fill area was a intermittent drainage swale and the water was picked up and piped a 1000-1500' upstream so that water is now gone which might be contributing. the swale was undercut to firm soil prior to filling. all density test results were good. had one of my best tech's on site full time.
i can't find the weak link other than the dropping groundwater levels causing the settlement. and being in amphibolite with dropping groundwater levels seems to throw up red flags to me. i'll see if the numbers flatten with the little bit of rain the past month or two. i'm so far in to the log plot that 30 days (time between readings) doesn't provide much indication as to the movement plot. i haven't personally run the time rate yet since it's a little hoaky in the piedmont anyway but i probably should just to see what it says. i've run it on other projects and the numbers are usually nearly the same with 30-75 days being typical. i've handed all the lab data over to the senior to look in to since my schedule hasn't given me the time to do much else other than survey the pins, plot the data and scratch my head. i'm pretty sure he's already run those numbers and come up with the usual answer.
RE: dropping groundwater relative to settlement?
If a change in groundwater depth is the culprit it would seem that you may need more than 5 or 10 feet in change in water elevation to make the change in pressure large enough to make some difference. Usually here in california that change is groundwater surface is on the order of 50 feet to really get the subsidence started. But that is not to say that even a small change won't have an effect on your area. At 5 feet you are adding about 300 psf additional stress, is that more additional stress than you had planned on? The other thing to think about is that for the groundwater to have some noticible effect it needs to drop below the past lowest level. You should look into what the historic groundwater levels were in you area. In your earlier post you mentioned that the water level was about 80' in the nearby wells and that it had been at about that elevation sometime in the recent past. So even if the groundwater over the last few years has dropped 15 or so feet, it wouldn't make much difference if it has not gone below the past lowest point, except that maybe now it is feeling the additional weight of fill you have placed and the groundwater drop was enough to push you past the linear recompression portion of your consol?
Check out USGS for information on subsidence. They are doing lots of studies in areas out west were the groundwater withdrawl is causing subsidence in many areas. Usually though it is over a large are, more than the size of one project. You should ckeck if other property nearby has been subsiding.
Here is the url for a recent subidence report by USGS.
http://pubs.usgs.gov/sir/2007/5251/
RE: dropping groundwater relative to settlement?
i wouldn't expect a few feet (or a couple hundred psf) to make a dramatic difference. i did look at the historic levels back about 5 years...the difference is more than 5 feet and the levels now are the lowest available...and this is the worst drought in history for GA (well actually missed the worst year by like an inch). the lake levels are down 15'-20'. there again, we're only talking about 2 inches in 4 months. 500-1000 extra psf might do it though since the consolidation curves fall off steeper after about 1200-2000psf. i'm convinced it must be groundwater based on the second project's survey data being flat for 6 months then falling off again at the end of summer when no additional loading occurred. i'll keep digging and see what i can turn up...and see what the next readings show since we've had some rains.
thanks for the input.
RE: dropping groundwater relative to settlement?
i suppose i should double check the time rate before i commit to throwing all my belief behind the groundwater theory since i did estimate 3 inches of settlement after the "instantaneous" consolidation. the 3 inches was to the conservative side based on the lab data.
i'll keep checking to see what i can turn up. thanks again.
RE: dropping groundwater relative to settlement?
You have a very intersting problem. Ill be intersted to see how it turns out.
RE: dropping groundwater relative to settlement?
i will be able to check time rate later this week once i get a chance to come up for air. i think i'll try and dig up more current well data too. it is a pretty interesting scenario...as long as it stops before they get ready to put up the building.
RE: dropping groundwater relative to settlement?
problem is: what happens in the summer time if the drought continues and worsens? the general trend of the ground water levels seem to continually drop long term even though there are ups and downs from season to season. how the heck are we supposed to estimate "realistic" figures based on what mother nature does or does not do? is anticipating a 5' drop in groundwater enough...how about 15' or even 25'? then once a settlement monitoring program is performed, how do you ever give a straight answer that the settlement has slowed enough to begin construction without having to completely disclaim every opinion? i suppose it's the inevitable cycle of always adding to disclaimers to cover every possible situation...(i sure would have liked to be an engineer in the times when an engineer could be an engineer instead of having to be half lawyer worried about litigation)
RE: dropping groundwater relative to settlement?
I definely aggree that we should get rid of the lawyers. I doesn't matter what disclamers you put, the lawyers will still sue.
RE: dropping groundwater relative to settlement?
if we used a similar mentality for building loads, should we (here in ga) bump up the loads provided to us by the structural engineer in case we see a massive ice/snow storm lasting weeks during a drought period (assuming non-drought during exploration/construction) at the exact time an earthquake hits. i suppose you could but that would sort of be pushing the probability of those things happening concurrently based on known occurances. we already estimate to the conservative side...maybe it's more a matter of when portions of the settlement take place (or don't take place). if you've got a lake onsite, at least you can tie some settlement figure to "if the lake is drained effectively lowering the water table 15'".
i think the settlements may be so scattered around town(contained primarily on sites with deeper soil profiles loaded with at least moderate thickness fills) that designing for such events everywhere would be outside the local industry's standard approach (for lack of a better phrase). it's sort of like designing for a 100yr storm event knowing good and well that a bigger event could happen tomorrow. at some point, you've got to chalk it up to "mother nature". we're always careful with how we word settlement related issues and unforeseeable groundwater levels but perhaps that verbage should be stiffened up in light of the current drought.
thanks for the advice. i'll look up the papers you mention.