Settlements due to dewatering
Settlements due to dewatering
(OP)
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
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





RE: Settlements due to dewatering
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.
RE: Settlements due to dewatering
Dik
RE: Settlements due to dewatering
RE: Settlements due to dewatering
RE: Settlements due to dewatering
Dik
RE: Settlements due to dewatering
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.
RE: Settlements due to dewatering
GPT nailed it.
RE: Settlements due to dewatering
thanks,
RE: Settlements due to dewatering
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.
RE: Settlements due to dewatering
RE: Settlements due to dewatering
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
RE: Settlements due to dewatering
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
RE: Settlements due to dewatering
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!
RE: Settlements due to dewatering
RE: Settlements due to dewatering
RE: Settlements due to dewatering
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!
RE: Settlements due to dewatering
Intrusion Prepakt
www.marineconcrete.com
RE: Settlements due to dewatering