dewatering effect on surrounding existing foundation/structure 3

[delagina]

Any good recommendation about this topic. I'm googling this topic also.

I have an excavation that is about 15w'x15L'x10 deep. Dewatering is installed (pipes with pump) to keep water level at bottom of excavation. Water level normally at 4'. There are foundations and structures around the excavation and question was asked about the effect on this. How do I determine these foundations will still be ok.

 

[delagina]

If excavation is properly shored, I need to determine the effect of lowering water table on surrounding foundation specifically how much settlement are we expecting.

 

[SlideRuleEra]

There is a zone of influence around dewatering pipes (wellpoints), see my sketch.
The horizontal extent of the zone of influence depends on soil type and dewatering depth.
Effect on nearby structures depends on these factor plus the length of time soil is dewatered.
Depressing the water table by 6' or 7', for a short time, is often not a big deal. If the existing structures are very close and/or the dewatering will take place for a long time, consult a local geotech.

http://www.SlideRuleEra.net

 

[oldestguy]

If you do lower the water table under nearby foundation,you put the affected soil into a non-submerged state. It no longer has the buoyancy effect, probably changing unit density from maybe 40 or 50 PCF to maybe 110 or more. That adds weight on lower soil. Adding pressure on deeper soil can cause settlements. It is a common problem that by lowering water tables, you can have foundations settle.

What can be done to avoid this? One might consider surrounding the excavation with deep sheet piling and do not use well points. Sheeting does not stop at planned excavation depth. Examining the flow pattern of water entering the excavation from below and to the side, knowing all soil layers, this might (MIGHT) negate the effect of pumping from inside the excavation on adjacent structures. Remember the sheeting has to be so deep that no quick condition occurs within the excavation base. That means upward hydraulic gradient at the base of the excavation must be well below 1.0 . I'd guess at least 8 feet deeper for the sheeting as a minimum.

 

[delagina]

Who is the better expert ask about this, a geotech or a hydrologist?
Obviously there is an effect/settlement but difficult to quantify.

What book or study you recommend about this topic?

 

[oldestguy]

This falls right into the work of a geotech. There are numerous books dealing with this as only one of many possible soil problems. You might try a Google search. In order to quantify the amount of settlement you need test borings and samples that may well require testing in a geotech lab. The drilling firm that takes the samples should be under the direction of an experienced geotech engineer, since job sampling may require more than a tech. Regardless when there is a risk of settlement, it is best to have detailed elevation information from the site of possible problems, because damages later may require some court action.

 

[SlideRuleEra]

Delagina - What is the distance from the structure to the closest wellpoints? How long will dewatering be taking place?

http://www.SlideRuleEra.net

 

[oldestguy]

delagena: This article has a bunch of ways to dewater. However, the problems related to settlement from increased pressure from this are not very common and thus no or few articles. You probably would find testimony in legal cases of dewatering caused problems.

http://www.pecivilexam.com/Study_Documents/Const-Materials-Online/Dewatering.pdf

To digress, but to show something of different conditions between saturated soil and damp, not saturated, here is a case showing the difference.

I was called once to see if I could help a house builder. He has been building houses for years in an area with nice sand and gravel soils. Houses had attached garages and basements. He "settled" the fill in the garages by jetting the loose fill with a length of pipe attached to a garden hose. In this case the weight of the saturated sand fill applied about three times side pressure to the wall next to the house basement as compared to damp soil. That concrete wall pushed in and in came a great deal amount of saturated sand. The center beam under the first floor pushed out the far side of the basement and down came the center of the first floor, walls and all. Major destruction, probably a total loss. He said he had been jetting that fill under garage slabs for many years and no problems. My reply was "Bet you never jet again". I was of no help.

Interesting that today I visited a new neighbor seasoned builder building his new house. He remarked that come summer he will jet the fill under his new garage area. His basement wall thickness in 7.5" (damn thin). I told him the above story. We shall see if I had any effect. My problem now is that with this foundation now there about one month and "no problems", but I had photos of that wall "just in case", Yesterday I wiped my camera clean figuring he "got away with filling". Now I'd have no before and after shots.

So in this post's case, taking away water and you take away water effects, but more importantly the soil is no longer submerged with a light effective buoyed-up density. Damp spoil imposes about three times vertical pressure on lower soil than when saturated. Can result in settlement. Note thre is difference between lateral pressures and vertical. so the two different cases don't quite compare here.

 

[delagina]

This inside a plant. There are underground pipelines all over, some are crossing the wellpoints. Above grade are just a bunch of pipe supports, no major or big structure near it.
We (plant) own everything around the excavation so no lawsuit.
I'm thinking as long as the excavation is properly shored to avoid soil from caving in, the settlement caused by water table lowering is not major. Water table goes up and down all the time due to rain or dry season. Also they (contractors) have been doing this dewatering all the time inside the plant for decades without settlement issues that I know of.

 

[oldestguy]

Sounds great. Past history info can't be beat. Good luck. For any upward flow to the excavation there might be loosening. So if the interior could get well points also, that risk is minimized. Much depends on soil condition, such as layers,etc. If severe, a combinations of driven sheeting and well points may be needed, depending.

 

[delagina]

I got this book "Construction Dewatering and Ground Control" by Powers. I'll read it and see what I can learn.

 

[maxim22]

With 6' (2m) drawdown in sand soils I would expect little settlement (<5mm) in the closest structures due to the change in the soil stress. I'm assuming the fines are not being drawn out by the pump. I have a current insurance case where the drawdown is 8 m+ at a distance of 10 m in sand and chalk geology and the brick property has cracking to 1-5 mm width. The damage occurred almost instantly (suggesting an elastic ground response), and there is slight rebound when they turn off the pumping (also elastic). If the structures are not so brittle then you might not notice any movement unless there is an accurate baseline. There are sometimes seasonal variations in the water level of 1 m or more. If you need a siltbuster for the water effluent then that might be a different problem of undermining.

 

[delagina]

Ive read about fines should not be drawn out and proper filter should be installed. Can I physically see this. There is a portion at the wellpoints where the pipe is transparent and I can see the water being drawn out and it looks "clear" but I'm not experienced enough on this to tell if soil fines are not being drawn out. How can I determine that fines are not being not drawn out?

 

[delagina]

maxim22,

You were able to approximate that 2 meter water drawn out at sand soil, it will have <5 mm settlement due to change in soil stress.

How did you come up with that <5mm approximation. What formula did you use? What if it's clay soil?

 

[maxim22]

Inspection of effluent is the way to check for fines. Normally they would discharge via a settlement tank into the receiving water (river/pond/sewer) and the settlement tank would get full of soil, or not.

Settlement estimate is inspection of your case was as presented, judgement and comparison to a scenario I made a proper calculation on.

2m drawdown = approximately 20 kPa increase in total stress is not a lot so settlement not a lot. Young's modulus for sands are published values. increase in stress = increase in strain. E=fl/ax.

Clay soil also has published Young's modulus but is more complex due to low permeability. You should not get much water out of clay so drawdown is not really the same either. Usually de-watering is in high-moderate permeability overall.

 

[oldestguy]

Remember the settlement estimate is in addition to any that is caused by change in density due to those added pressures, if at all.