Perched Water Table

[rimshot13]

Hi. I am not an engineer but I have a problem and from your very impressive site, it looks like you may be able to help me.

I am building a home in Camden County New Jersey. 4 Months ago we demolished a small home over a basement which had been dry. The pit sat full of water for much of that time. We're moving the new house farther back on the lot - about 20 feet. A recent soil boring has discovered a perched water table at 36 inches on a hard clay layer at 41 incehs. We believe the real water table to be no higher than 5 feet.

I want to excavate to at least 4 feet for the new basement. Is this a situation where we could do some additonal excavation around the foundation and re-fill with material with better drainage or what can be done about the perched table to relieve or remove it?

Thanks so much for you thoughts.

 

[BigH]

You get this a lot in the NJ area where you have some sandy soils overlying the marl. The perched level will "always" be there - I would suggest that you install a "footer" drain at the basement wall just below the interface of the sand and marl - then take the drain to the storm water sewer - it is likely greater than 4 ft depth due to frost requirements. You could contact a local geotech - there is one there on 73South in Maple Shade.

 

[ashjun]

I am sorry, but I could not understand your question. Could you please rephrase your question please?

Thanks!

 

[rimshot13]

To rephrase the question: I want to build a house on a lot that has a perched water table 3 feet below the surface. I want the basement floor to be a minimum of 4 feet below. How to remove or relieve the perched condition to avoid water problems in the future? Essentially, how do I put the foundation "Through" the perched table?

 

[ashjun]

It appears that a confined aquifer sits on a layer of impermeable clay that too at a shallow depth. This is not all that bad. You could easily have a small pit dug slightly away from the proposed construction. Have all the water flow into the pit while you pump the water out and away from the pit. At your site, probably you may not need pumping throughout as the ground water table sits on a impermeable layer and there may not be a lot of water percolating through during the dewatering process.

: Cheap and very effective

Good luck!

 

[jheidt2543]

Once you punchure the marl layer with your basement you will attract a lot of water, basically your basement excavation becomes the drain for a good sized area around your house.

I look at this as two problems to solve. The first is constructing the basement in the dry. Ashjun suggests one solution for that, ditching and sumps and it is cheaper than a dewatering system of well points or deep wells.

The other problem is how to keep the basement dry after construction. BigH has the solution to that, with perimeter drains that either run directly into a storm sewer or are pumped there from a sump in you basement. I would add that a good waterproofing membrane should be applied to the exterior of the basement wall before backfilling. Not the roll-on, black mastic dampproofing materials, but a real membrane. They cost a little more, but over the life of the house they are well worth it. Check with manufacturers like Sonoborn, W.R. Grace or R.W. Meadows, they all have good products. Also, you can check with your local membrane roofing contractors, most of them install these systems. Be sure to backfill the basement wall with a wide strip (4' wide or more) of granular material to help drain the water away from the wall and into the drain tile below. In our area we use the native, bank run sand & gravel.

 

[Focht3]

First, hire a local geotechnical engineer - his/her advice will be invaluable in the long run. Failure to do so could really cost you!

Beware of a common mistake: treating the walls and ignoring the floor! While you believe that the area water table is deeper than 5 feet, you cannot be certain without a lot of information that you have not provided (and may not know.) You are building for 50 to 100 years, not 20. Take the long-term view. Plan on a subfloor drain system to supplement the wall drains. Construct a sump outside the building lines so that it can be dug up without destroying your home, if necessary. Have a float-actuated sump pump with water level alarm, a back-up pump - and provisions for an emergency generator should you find it is needed.

Don't guess on this one - you will likely regret it.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[Mccoy]

Just one thought which occurred to me after reading this post:

As a general concept, it may be possible, but that would be strictly a geologist's job, to map the spatial extension and configuration of the permeable shallow layer, estimate its permeability, build a rough map of the hydraulic gradient around the building site and figure out the groundwater flowrate thru a sink or intercepting structure (trench, hole, basement and so on). Is the aquifer spatially limited and hence going to exhaust rapidly after each rainfall (and which is the size of the expected 20, 50 years storm)? By the way, if the ground has some steepyness, it may be possible to drain the water by a permeable trench excavated in the upslope side. The problem appears to be pretty complex. However, if any doubts persist as to the nature and size of the water flow, I'd just stick very carefully to the suggestions above dispensed by the top experts of this and correlated fora. Focht3's attention to practical safety details is particularly remarkable.
Best regards to all.