Settlements due to dewatering 1

[geomtl]

Hello,

I have a question, in my experience in the field of geotech engineering we always warn of the danger of settlements to adjacent buildings caused by local dewatering (deep excavation below the water table adjacent to existing buildings on standard spread and strip footings.)

I recently had a case where excavation for the basements will require lowering the water table approximately 10 - 15 feet. In this case we recommendeded a waterproof excavation, however sheet piling may not be possible due to the boulders in the soil, and slurry walls are very expensive.

The question came back from the client, what was the worst settlement that could be expected? We are in a sandy silt, with cobbles and boulders, with some more pervious sand layers. Is there an acceptable method to determine the settlement that may be caused by dewatering this layer, and also in consideration of duration of construction as well?

thanks, looking for some general pointers.

ck

 

[GeoPaveTraffic]

You should be able to calculate the settlement based on the increase in effective stress caused by the dewatering. Apply an appropriate fudge factor for unknowns and you will have an estimate of the expected settlement.

The length of time that the excavation is dewatered will only effect the amount of settlement if the soils consolidate slowly relative to the dewatering time. Given that the soils are sands and silts, time will likely not be a factor and the settlements will happen quickly.

 

[dik]

Dewatering has cause subsidence of several feet (10-12) in Mexico City.

Dik

 

[geomtl]

thank you for the responses, in Mexico city what was the drawdown and what are the soils?

 

[LCruiser]

You're not drawing the water table down significantly, and what you are in effect doing by excavating for a basement is decreasing the surcharge.

 

[dik]

I understand that the drawdown was caused by pumping for wells. Soils I don't know... but some second floor levels became main floors...

Dik

 

[DRC1]

Mexico City from my understanding is soft silts and clays. Although dewatering for public wells has resulted in significant settlement, some dramatic examples, such as the national art museum I thought were due to liquifaction.
Dewatering raises the effective stress of the soil from 65 psf to 120 psf +/-. Analyzing the soil in layers will yield approximate settlement. Patrick Power's book Construction Dewatering is excellent reference. What exactly is waterproof excavation? I am curious because I have never seen an excavation below watertable that did not require pumping.

 

[Ron]

The Mexico City thing has taken about a hundred years to occur.

GPT nailed it.

 

[geomtl]

thank you for all your responses, sorry, called it waterproof to refer to either a slurry wall or sheet pile type shoring as opposed to wood lagging and soldier piles.

thanks,

 

[VAD]

I presume that the situation you are describing is one with the adjacent buildings on spreads and strip footings.In that case GeopaveTraffic approach is what would be normally used. Curiosity Questions: How far away would your excavation be to the existing buildings. What type of buildings are adjacent. Does full scale dewatering have to be done.

The Client in asking about the settlement expected of the adjacent buildings appears to be the owner of those buildings.

I presume that you have done some drilling or test pitting in the land to be excavated. How were the inflows in the test pits - rapid, slow etc. Was seepage at all levels of test pits

The reasons for asking these questions is that some times unless there is a complete understanding of the ground conditions we may make some decisions that may not be necessary and could lead to expensive solutions. Indeed while we at first thought realize that it is a bad idea, we must after look for clues within the ground that would allow us to ensure that our judgement is well founded.

Very often we are taxed in this direction by the type of question that the client has asked. Makes the job interesting and requires one to think about both theory and practice.

The above is not intended to detract from the advise given so far by others.

Final question Would you be concerned if the adjacent buildings had been constructed with basements.

 

[Ingseven]

Back to Mexico, it is really a different issue. Soft clay and many years of pumping. You won't have 6 to 7 ft settlement in your case where sand and silt are involved.

 

[geomtl]

In rsponse to your questions:

1) we have performed 5 boreholes to 70+ feet across the site. We appear to be in a faulted rock, with solid bedrock dipping steeply (70-80 degrees) across the site. The soils range from solid breccia limestone to faulted rock (boulders and cobbles with sand and silt in the voids., overlain by a variable thickness of glacial till (silty sand to sandy silt)

2) Due to artesian groundwater conditions we performed a pumping test below the proposed excavation level, monitoring the groundwater lowering across the site in piezometers. The permeability is quite high and groundwater lowering by means of a few deep holes appears to be an opton during construction, but the cone of depression is quite large, hence the concern of dewatering vis a vis the adjacent buildings)

3) my client does not own the adjacent buildings, hence his desire to know what the settlement could be, i.e if it will be on the order of 10 mm or so then he will just go in and patch any cracks that appear due to his activities.

Have a good weekend all, and thanks again for your comments, great post.

ck

 

[VAD]

Thanks for the info. I read that your boreholes were done to 70 ft depth. If that is the case, at what depth did you encounter the artesian conditions.

Reason for asking is to assess the relationship between the excavation depth and depth of artesian layer. I presume that you may have looked at this. If so just curious of your evaluation.

Regards

 

[fattdad]

Look at the soil/rock conditions, compare the dewatered phreatic surface to the static water surface, determine the change in effective stress at various points (it will vary as you get further away from the sheeting) and calculate the likely compression based on the change in loading. This approach requires that you have some idea of the soil/rock modulus value. I'm assuming that primary consolidation is not a factor (for this project - i.e., being a rock site and all). Somebody mentioned the idea of looking at the off-loading from the project excavation. You can look at that if you want to, but just bear in mind that you will complete the dewatering in advance of the excavation, so damage may have already been done (something to check).

From the sound of it, if this is an area surrounded by rock, I'd be more worried by the blasting then the dewatering.

f-d

¡papá gordo ain’t no madre flaca!

 

[VAD]

For a better appreciation of the problem can you provide the stratigraphy of the site from existing ground level.Thicknesses of the various layers are required.

 

[civilperson]

Patching of the cracks may not be the adjacent building owners choice of remedy. Could have experts recommend destruction and rebuild.

 

[fattdad]

I should have added: You should really do a flow net for this problem (if settlement/compression is a measureable factor). You see if you just look at the dewatered surface you may not fully appreciate how the pore pressures are distributed below the phreatic surface. The change in effective stress is total stress minus pore pressure - in a flow net you may find that the pore pressure does not add up at z*gammaW.

Good luck. I've worked through these problems in the past and you just have to think through it - that's all.

f-d

¡papá gordo ain’t no madre flaca!

 

[Prepakt1]

Possible solution is to grout below the existing stuctures either through their floors or from the side and solidify any areas that are liable to wash out.Any settlement at all could as per the "civilpersons" comments lead to some very expensive repairs or lawsuits. All this calculation is fine but the ideal is not to have settlement .Before starting work,support the adjacent stuctures properly.There are a variety of methods.

Intrusion Prepakt

http://www.marineconcrete.com

 

[Donrolla]

I agree with Prepakt1, your better off assuming that your site activities will result in settlement of the adjacent structures and mitigate by proper underpinning/shoring. Not quite sure why your cone of depression would be large based on sand/silt fractured bedrock and artisean conditions....anyway underpinning is the way to go.