Wet weather springs on hillside- how to dry up?

[mmolaison]

I'm dealing with a piece of property that, in a normal year, would likely be a high-and-dry hillside that drains to an intermittent stream and wetlands. I'm in Florida panhandle- so underlying geology is most likely limestone bedrock with red clay then sand deposits on top. Topsoil is very sandy, and drains extremely fast!

However, due to this insane amount of rainfall this fall/winter, this hillside (where I'm trying to build a house and barn and clear pastureland) has wet-weather springs seeping out all over it! The grade is roughly 5% on average- very gently sloped- and the hilltop is on another piece of property, 20-30' in elevation higher, still at around a 5% slope to the top.

SO- question #1- assuming I build a home on a monolithic cement slab, or on a crawl space, with no basement, is it a problem to build where a wet weather spring may occur during extremely persistent wet weather?

Question #2- can I dry it up? I'm venturing out of my area of expertise here- but I know enough to be dangerous Could we dig a ditch along the upper elevation area where the wet area begins, and route the water around the area we need dry, and let it drain on off down the hillside that way? Just an open ditch, as opposed to a real (and costly) French Drain system? Any SWAG about how deep said ditch would have to be to intercept the flow of water from the higher elevations?

It needs to be something we can handle economically- funding is limited.

I was told by an old farmer in the area, that if we just go in and clear the land (it's currently a mixed growth "forest" with thin tree cover as it was partially logged a few years ago) and establish grass, it will dry up on its own. I don't believe it- I think it's going to flow until the rains relent and the ground water table subsides a bit.

 

[oldestguy]

First off get the words "French Drain" out of your vocabulary. Too many of those consist of coarse stone backfill that easily plugs up. Before I'd do any major fixing, I would want to examine the subsoil by some form of investigation, such as back-hoe pits, auger borings, etc. I'd also install some method of observing ground water levels with time. That easily can be done with post hole digger and lengths of downspout.

Protecting the house from excessive ground water usually involved a perimeter and some interior drains. KEEP IN MIND THAT ANY DRAINS LIKE THAT MUST BE DESIGNED SO AS TO STAY CLEAR AND ABLE TO CARRY WATER. Thus all pipes and drainage backfill must be designed as a filter. Coarse gravel is not a filterr!!! However,installing a series of drainage systems up-hill may be better in the long run.

OK. let's say you did find that you have some form of barrier down there keeping water from infiltrating and thus gradually following the slope and find you want to lower the water table. Think of your house as a castle with a moat around it. That ditch you mention may do it, but that may silt in, fill with brush and be useless in time and be in the way. A more positive approach would be buried FILTERED drains, sloping to some discharge area down hill. A common and low cost drain is a 4" corrugated plastic pile, slotted, possibly with a filter sock, and surrounded with a sand filter. The best all-'round filter is ASTM C-33 fine aggregate, or concrete sand. You can economize by using it mainly around the pipe and then a cheaper bank-run sand above it. With this sand a sock is not needed. I simply call these "sub-drains".

Some may com on here espousing use of geotech filter cloth surrounding rock. It works, but is darn labor intensive. Impossible to install in caving ground.

In caving ground it is necessary to be right in back of the trencher installing the pipe and some of the filter sand. Start from the outlet and work up hill ACCORDING TO SOME PLAN YOU HAVE PREPARED FIRST. Just diving in and digging may do it, but I'd want to know a whole lot about the situation before digging.

A small part of a book put out by Armco Steel some years back (way back) is still available at Amazon for only a few bucks. I had given my 1952 copy away and recently wanted a copy, getting one like new for under $10. They show their pipe, but the corrugated plastic stuff works fine. "HANDBOOK OF DRAINAGE AND CONSTRUCTION PRODUCTS" is the book. Texts concerning agricultural drainage also may apply.

 

[mmolaison]

Thanks- we are going to do some test holes to see what the exact soil types are and if there is, in fact, hard pan (likely in my area). Right now the water table is literally AT the surface (which I guess explains the flowing springs). We dug a hole and it immediately filled to the ground surface! Haven't checked the drier areas yet but I'm guessing it's still going to be very close.

I had been reading other threads about installing drains as you describe, and so I'm not surprised for you to say that's the best option. I'll order a copy of that book and read up.

I'm also looking for an expert in my area I can have consult on a design- I realize spending a bit up front to get it right will save us money and heartache (and possibly lots of backbreaking labor!) in the end...

 

[oldestguy]

While a geotechnical engineer may be an expert you might use, the subject of this form of drainage seldom is taught in their courses. It has been my experience that the usual drawdown, as to wells, does not happen theoretically assuming uniform soils. I'd just plan that that water table comes along at its former position before the trench is dug and drops into the trench. Do not depend on much of any drawdown up-hill from the trench. The diagram in the book probably does not happen in your case. Unless your expert has had experience actually seeing what happens, I'd go on the basis of not expecting any drawdown, but that the down hill elevation of the water table EVENTUALLY is same as the pipe elevation, hopefully dropping a little with distance from the pipe. The aim is to protect the area down hill from the trench, not up-hill.

 

[beej67]

Depending on which water management district you're in, mucking about with the water table levels could very well be illegal. Florida is very sensitive of its water resources. In SFWMD, for instance, you are prohibited from doing anything land development wise that changes the SHWT, or "seasonal high water table." They're also very sensitive of water withdrawals (wells).

In the 00s, land development hydrology in the panhandle was permitted through FDEP directly because NWFWMD wasn't really funded yet. I haven't done any Fl projects since the new NWFWMD got started, but if they're up and running, and they've built themselves on the SFWMD and SWFWMD templates, then you've got some research to do.

Then again, you may fall under agricultural exemptions with which I'm less familiar.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[civilman72]

I would add to OGs post that while you may not significantly lower the water table, a properly installed subsurface drain will lower the hydrostatic pressure at your house, which is the root cause of many failures under slabs and moisture issues within crawl spaces/basements.

 

[mmolaison]

Lucky me I am covered under agriculture rules I have timberlands and pastures for livestock, saves on taxes and makes mucking with wetlands and ponds and so forth much much easier!

The goal would be to stop seepage downhill caused by water hitting the hardpan and running out downhill... give it an easier path of drainage to get it off the hill. Assuming we find that is the case.

So I've got my work cut out for me, plenty of research and hole digging to do! Never thought I'd find myself wishing for drought but I swear, this constant rain is a nightmare. I guess the positive is, we discovered this seeping problem BEFORE building!