Relation of existing ground water level and the maximum drawdown of groundwater by dewatering

[ynnuseel0106]

I am a structural enginner and hope to seek for some advice on geotachnical concern. In typical strutted cofferdam, dewatering inside the site is required and we need to control the amount of groundwater drawdown outside the site to avoid excessive ground settlement. Does anyone know whether a higher or lower existing ground water level will induce a more critical groundwater drawdown outside the cofferdam due to dewatering?
Thanks so much.

 

[PEinc]

If you pump from inside a cofferdam, how much the water level drops outside the cofferdam depends on the permeability of the soil and whether or not the SSP is toed into an impermeable layer. I suggest you talk to a local geotech or a contractor that specializes in dewatering.

http://www.PeirceEngineering.com

 

[SlideRuleEra]

There are steps a Contractor can take to minimize surrounding groundwater drawdown:

1. Don't dewater the cofferdam to full depth until work inside is ready to begin.

2. As work inside the cofferdam progresses, dewater only as deep as needed for work to proceed safely. Actually, raising the water level inside can improve safety - the distance a worker can fall is reduced (to the water level) and better to fall into water (wearing a life jacket) than onto, say, a concrete footing.

3. At night or on weekends, allow the cofferdam to partially fill. Even a small increase in water level will noticeably reduce groundwater inflow. Obviously, don't do this if the work inside will be harmed by a higher water level. However, never allow the cofferdam to completely fill. Maintaining a higher hydrostatic pressure outside will ensure that the sheeting, wales, struts etc. remain where they are supposed to be.



We did have an unusual situation during construction of an electric generating station that may be of interest. A large, open-ended, existing underground pipeline submerged in groundwater (10 feet inside diameter, 1/4 mile long) needed to be dewatered. Groundwater pumped from the pipe was injected into the (shallow) water table a distance away from the pipe. We approached the appropriate groundwater regulatory authority with this proposal. They had not encountered a request to inject ground water into ground water. After a timely review, approval was given - the work proceeded without complications.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[Panars]

The greater the change in the groundwater level, the more risk there is for ground settlement. So to answer your question, it is generally a higher existing groundwater level which is drawn down will cause more settlement. However, there are also other aspects to the situation that will affect things, such as the permeability of the soil.

 

[oldestguy]

The deeper you can drive the sheets, the less the nearby effect and likely less quantity of water pumped per day . Also, there is less chance of creating a quick condition inside and the resulting loss of bearing capacity inside.