Dewatering my Excavation

[SharkswithLasers]

Say I have a rectangular excavation 120 feet by 80 feet by 20 feet deep (below grade surface) in the shallow water table in sandy soils (K = 10E-2 cm/sec). Sheet piling/deadmans are holding the sides of the excavation. Water table is at 10 feet bgs.

How can I estimate the pumping rate (gpm) for pumps placed in the dewatering wells?

As a simplified example (to see if I have any clue about dewatering at all), can I take the hydraulic conductivity (10E-2 cm/sec or 1.181 ft/day) and multiply by the area of the excavation bottom (9,600 SF) and convert to gpm units? I would then get 59 gpm (seems high for real world applications).

Does the head (10 ft above excavation bottom) or gradient come in to play in the calculations? How?

Now if I add wells, say 10 total wells along sides of excavation, how does this affect the calcs?

Trying to understand dewatering...

 

[GeoPaveTraffic]

You are asking the wrong question. You need work backwards from where you want the water level to be, then determine the number, size, depth, etc. of the wells, then determine the size pump needed.

Based on the information you provided, 59 gpm does not seem like near enough pumping capacity for the excavation. I would strongly recommend you contact a geotechnical or dewatering engineer with experience in the geographic area where the project is located.

 

[dicksewerrat]

First of all you have 10 feet of water in the hole. Then you have to see what flow rate throught he sand will give you a 'quick' condition in your hole and place the wells deep enough to counter this. The wells will be outside the hole probably around 5 feet from the sheets and about 20 feet deeper than the sheets. Try those numbers and draw some 'drawdown' curves. See what that gives you. You should talk to your well driller about his thoughts on this.

 

[greggers]

I would definitely consult an engineer with extensive experience in dewatering.

GeoPaveTraffic is right. You need to start where you want the water level to be, which will probably much lower below the excavation that you may think at this point. You also must be cognizant of area around the dewatering - ie do survey of existing structures.

 

[Focht3]

Yup. That 59 gpm pump rate won't do much given the excavation dimensions, soil properties and water/excavation levels!

You need a geotechnical engineer with good dewatering experience.

If the sandy soils are 80+ feet deep, then the dewatering system may not require too many wells. On the other hand, if you've got a clay layer at 30-40 feet, expect a LOT of wells.

Also be aware that the soils aren't uniform - so don't expect the bearing surface to be really dry unless you are willing to pay for redundancy - and a well or two within the excavation itself.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"