ᐅ Excavation/Foundation Construction in High Groundwater Areas – Experiences?
Created on: 19 Mar 2024 12:01
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gregman22
Dear community,
We are facing a challenge as our earthworks were actually supposed to start since this/last week. However, our plot is located in a high groundwater area, and the earthworks are complicating things. I haven’t received clear statements from our earthworks contractor, so I would like to hear about your experiences.
Situation:
- Basement floor slab with overhangs needs to be poured
- Groundwater is currently about 1.85 m (6 feet) below the plot surface
- Excavation for the basement floor slab must reach 3.55 m (12 feet) deep → thus, the earthworks will be 1.70 m (5.5 feet) below the groundwater level
- Planned earthworks measures: 3 boreholes for dewatering; pipeline route to sewer 470 m (1,540 feet), diameter of sewer pipe into which we can discharge: 40 cm (16 inches)
- Additionally: We have installed a soakaway well on the property, reaching a depth of 8 m (26 feet), theoretically allowing further water discharge
- Weather forecast for the next two weeks looks rather positive (2-3 rainy days, otherwise mostly dry); on dry days, the water level drops about 1-2 cm (0.4–0.8 inches) per day
Further info: At the very beginning, sheet piling was considered. However, no clear necessity was explained to me, so I decided against it—mainly due to the risks of damaging neighboring houses.
Currently, our earthworks contractor is installing the pipes and planned to start pumping from Friday.
Now my main question to you: How do you see the situation? How optimistic can I be that it will work out, on a scale from 0 (very pessimistic) to 10 (very optimistic)?
Thank you all.
Best regards
We are facing a challenge as our earthworks were actually supposed to start since this/last week. However, our plot is located in a high groundwater area, and the earthworks are complicating things. I haven’t received clear statements from our earthworks contractor, so I would like to hear about your experiences.
Situation:
- Basement floor slab with overhangs needs to be poured
- Groundwater is currently about 1.85 m (6 feet) below the plot surface
- Excavation for the basement floor slab must reach 3.55 m (12 feet) deep → thus, the earthworks will be 1.70 m (5.5 feet) below the groundwater level
- Planned earthworks measures: 3 boreholes for dewatering; pipeline route to sewer 470 m (1,540 feet), diameter of sewer pipe into which we can discharge: 40 cm (16 inches)
- Additionally: We have installed a soakaway well on the property, reaching a depth of 8 m (26 feet), theoretically allowing further water discharge
- Weather forecast for the next two weeks looks rather positive (2-3 rainy days, otherwise mostly dry); on dry days, the water level drops about 1-2 cm (0.4–0.8 inches) per day
Further info: At the very beginning, sheet piling was considered. However, no clear necessity was explained to me, so I decided against it—mainly due to the risks of damaging neighboring houses.
Currently, our earthworks contractor is installing the pipes and planned to start pumping from Friday.
Now my main question to you: How do you see the situation? How optimistic can I be that it will work out, on a scale from 0 (very pessimistic) to 10 (very optimistic)?
Thank you all.
Best regards
G
gregman2220 Mar 2024 20:12Many thanks @Allthewayup. That helps me a lot and reassures me a bit.
Reading your logical explanation that the total required pumping capacity decreases over time makes me somewhat more optimistic. Would you agree with that?
Allthewayup schrieb:Can you put the pump rate of 30 l/s into perspective for me? Considering our 3 boreholes/pump shafts, how much can such a pipeline handle at peak?
30 l/s is quite substantial. That equals 2,592,000 liters over 24 hours.
We haven’t even pumped a third of that amount.
Reading your logical explanation that the total required pumping capacity decreases over time makes me somewhat more optimistic. Would you agree with that?
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WilderSueden20 Mar 2024 20:52When I look at the submersible pump from my cistern... with 120 liters per minute (2 liters per second)... then 30 liters per second is a lot. And the pumping capacity decreases with the lift height.
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hanghaus202321 Mar 2024 09:14If a pump is connected with a C hose, one pump is sufficient.
In my opinion, the well shaft works better than a deep borehole.
In my opinion, the well shaft works better than a deep borehole.
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Allthewayup21 Mar 2024 12:28hanghaus2023 schrieb:
If there is a pump with a C hose attached, one pump can handle that.You can’t say that in general. It depends on the pump’s head height and pressure, as well as its overall pumping capacity.
I had two 2-inch (5 cm) hoses (B hoses) running to the distributor. There, I increased 2x B to 1x A, then passed it through the meter, and afterwards split it again via a distributor into 2x B towards the settling basin.
As mentioned, hydraulic calculations are necessary to answer this properly.
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Allthewayup21 Mar 2024 20:30gregman22 schrieb:
Can you help me put the pump capacity of 30 liters per second (l/s) into perspective? Considering our three boreholes/pump shafts, how much peak flow can such a pipeline handle?
Reading your logical explanation that the total required pumping capacity decreases over time makes me somewhat more optimistic. Would you agree with that? What kind of comparison are you looking for? I don’t know the details of the boreholes, the pump datasheets, or the pumping head, etc.
Focus on these facts:
- Highly permeable soil with very low risk of settlement affecting neighboring buildings
- Because of the highly permeable soil, a comparatively cost-effective groundwater lowering method can be implemented
- The city or municipality has approved the project based not on hearsay but on proper documentation
- A (construction) contractor is carrying out the groundwater lowering
The only other thing I can mention: You are discharging into a sewer system, right? Are there wastewater fees involved, and if so, how much? In our case, the fees were charged per cubic meter of water.