A question for those with more (any) experience...
I have a gently sloping piece of land (~40 acres) that gets a lot of water drainage from farther up the hill. This is not really surface run off, but 3-4 feet down is glacial hardpan so anything above this is always damp/wet and I'm trying to think of ways to drain it. My first choice would be to dig trenches that start at the upper corners of the property and come together in the middle (like the shape of a "Y") and then dig a big hole at the base of the "Y" to make a small pond. I figure the trenches will be 3' deep, and I'll install a PVC pipe surrounded by crushed stone and fabric.
My question is: how important is it to dig a super accurate slope over the course of several hundred feet? and do you think it is easier from the bottom (drain side) up, or from the "top" down to the pond area? Also, any other tips??
A little more info may help. Such as.., how much room do you have above, or rather up the hill, from where the water seems to be perking out, to make the soil damp..??
If you have enough room "up the hill", to dig down about 3', elevation wise, to reach that hardpan, then that is where you want your trench. Actually picking up the water before it gets to the point where it seeps to the top, or out. Where the water seeps out is probably close to where the hardpan/rock, or whatever..., crops out. Water follows the path of least resistance.
You don't say where you live, so I'm assuming the water is either coming off the rock, or fire clay, like we have in our area. But in our area, above the fire clay is coal... Even a small seam of "Blossom" coal will carry a lot of water.
If you have a hoe to dig some test holes would be a big help. Dig down and see what is actually there. Not any larger than you need to, just to see what is there, where you want to trench.
If it is rock, it may be laying flat like Mother Nature made it. Unless it is an outside crop that has tilted down on the outside edge, you may get some fall... Or fire clay as mentioned above. On hillsides, softer layers of shaley sedimentary rock underneath, may have been washed away, and allowing it to drop.
A few test holes near where you want the trench, and a transit or level will tell you how much fall you have to work with. Just make sure you reach the hardpan to find that level. Stay on that hardpan with your trench to catch all the water. If it happens to be fireclay ( pretty gray clay, like modeling clay) do NOT go through it..!! Stay on top, right down to it. The dirt will peel right off of it, leaving a nice clean, but slippery surface if wet.. If you break through it, you may loose you water, and will filter through some..., more than likely shale, and crop out at a lower level somewhere in the field.
You may need to modify your "Y", or if the rock is flat, just dig a straighter "Y" across, and catch the water. And collecting in the middle is your best bet. Who knows what you will find, and not be able to get the fall you need, if going full length to drain. I wouldn't go less than a 2% grade on the fall if possible.
We always started at the top, and worked down. You never know what you will find. A sudden drop in the hardpan, or a saddleback rising up. You will need to cut through whatever, to allow the water to get out. Otherwise, you've created a low spot to collect water. Yes, it will flow out, when it gets deep enough to flow over top of that high point. But if not cut to allow the water out, you've created an underground ponding of water, that will cause you more problems than you already had.., volume wise of water, in that particular area. Let alone the "head", or pressure created by the depth of the water, before reaching the point of flowing over the high spot.. This will continue to seep out, like it did before, or worse.
At least a foot of gravel over the pipe, IF this is just surface/ground water soaking down to the hardpan. We used a lot of 4", 360ｺ perforated, lined, black plastic pipe. This is not the plastic pipe that comes in rolls. This comes in 20' straight pieces. The crush strength on this is lots greater, than the roll pipe. You may do a price comparison vs. PVC. Much easier to work with, and what we used had the snap together ends. We secured with 3 screws, to keep it from popping apart, in deep trenches.
Over top of the gravel, however much you feel comfortable with in the trench..., (You just have to make sure you've picked up the water source.) we laid strips of either geotech cloth, or have used roof felt cut in strips the width of the ditch, even doubled over, to keep the dirt from filtering in.
With all we've put in, and a few we have had to dig back up, because someone didn't know how to read a stick, or didn't have one..., and caused the above "ponding" issue.
I've never seen any that had dirt that had fitered in, because we didn't completely surround the stone and pipe with geotech cloth. You'll just have to use your own judgement on this... Whatever you are comfortable with. We had surplus geotech, that contractors turned back in from large road jobs. We would cut it to width with a chainsaw, off the large roll. It kinda' half melts, and makes a mess under the chain guard, but lot's easier than cutting strips with a knife...
At the "T" in the "Y", we used a small section of perferated to get started, then into solid pipe.. This we did wrap with geotech, and covered with dirt. More or less forcing the water into the solid, so as not to leach out going down the hill. Most of our drains ran across fill material, and didn't want it saturating, what we'd just put in.
At the outlet, we slipped various types of pipe over it, to protect it from being crushed. We did have a nice stock of what our supervisor called "whistle pipe". 5" galvanized, corregated steel pipe in 10' sections. Schedule 40 6" works well too...
I always like to underdig under the last 2-3' of the outlet, and place like 6" # 2 limestone under it for something to splash on...
And just for good measure... Drive a post beside the outlet marking the drain... attach a highly visible sign, reflector, or something on it...
If you get a good stream of water coming out of your pipe, where you want to dig your pond, you can always buy a roll of the 4" to divert it away from the work area, or..., just dig a temporary ditch to divert it..., or dig the majority of your pond first, and hope it's deep enough... What ever works best for you... What seems to be a slight trickle of water, can produce a lot of water, in just 24 hrs. A stream the size of a lead pencil will produce pprox. 500 gallons in a 24 hr. period...
Oops.... Looks like I got a little carried away..!!
