Clay filled lawn with bad drainage, needs solution

Question

We just bought 28 acres of land with a 30 year old existing house in the center on about 1 acre cleared, the rest is completely tree filled.

There is a natural slope only a few degrees heading from the back of the house towards the back yard, downward. About 100 feet back is a large drop downwards that goes out for at least 1000 feet and down about 150 feet. It is at least a mile or so of very slight downgrade to the nearest neighbor behind that.

As with most houses in my area, the ground stays slightly wet until late March because the "water table" is only 1.5-2 feet below the surface, because my yard is almost 100% clay with hardly any topsoil and very little grass, mostly moss. This confuses me that the water doesn't follow the slope downwards and away from the house. My guess is that it can't travel horizontally through the clay even with the downgrade.

I want to make a level yard with nice grass and no water issues. This requires me to put a lot of fill towards the back of the yard to level it (because of the natural slope).

The yard will be about 150' wide by 80' long from the back of house.

My project thought was this:

1. remove 1.5 feet of the entire top layer of yard (all clay) and move it to the back part of the yard that needs to be raised. This would give a nice sturdy base to the back end of the yard
2. when removing the clay, keep a slope going away from the house at 'one inch drop per ten feet horizontal run'
3. put 4" (suggest a different amount if not correct) of #57 stone (suggest different if something better) to create a natural water way, away from the house to the back part of my yard that drops down
4. fill the rest of the yard back up to desired level with 1 foot of topsoil on top the stone
5. plant grass and enjoy :)

I would like the yard to be level, but the stones underneath to be sloped for water runoff, so less topsoil close to the house and deeper topsoil as approaching the back of the yard.

Any suggestions/advice would be greatly appreciated.

I am no expert, but was a heavy equipment operator in the Seabees a few years back and feel comfortable on a backhoe/front end loader that I can rent locally.

I do realize that the cost of the stone/topsoil will be a lot, guessing around $10K (more/less?).

I will also be putting an automatic watering system in at the same time. I will also be running PVC pipes from the downspouts of the house past the end of the yard.

Answer 1

This sounds like a lot of work. The picture you show of the backyard looks nicer than some other lawns I have seen where people have a maintenance firm do the fertilizing and cutting.

Just some points to note:

• if you raise the grade anything more than an inch or two this will kill any trees inside the raised area. Trees don't take grade changes well.
• to do this kind of work a backhoe is more practical than a bobcat or smaller loader as the bucket capacity is larger
• As the knowledgeable Ecnerwal points out you could get a drier lawn in the back with a lot less digging. Some french drains and a few trenches with drainage pipe with sleeve will let the backyard dry out sooner in the spring at a fraction of the cost
• if you do decide to add or remove soil in bulk when calculating what the volume to remove or order delivered add 30% to the volume. When you remove soil it occupies more volume after removal. New soil will compact up to 30% after being spread.

Answer 2

You will want 3-4" diameter perforated pipes in the stone layer (also sloped 1"/10ft), and you will probably also want filter fabric on both sides of the stone layer, or it will become a layer of clay with rocks embedded in it soon enough.

You might also want some much deeper "french drains" to deal with the water, rather than only having drainage 1-1.5 feet below the surface. You're doing major earthmoving anyway, so that should not be overly costly on top of what you are contemplating. At least below frostline, (which varies with where you are) and preferably also below the basement floor (if there is a basement.) You want those to run "out to daylight" on the slope.

As usual when digging, call dig-safe (or local equivalent) before starting, and also locate your own private services (well, septic, etc) which dig-safe won't know about and can't mark for you.

The moss is generally a sign that you have shade (lots of trees around) and acidic soil (typical for clays in my area.) If you were open to a less-drastic approach at much reduced cost, some tree cutting at the forest edge, a LOT of limestone and/or gypsum (do a soils test) and just doing french drains as Kevinsky suggests might get you there eventually. You might start with tilling the whole yard and planting cover crops for green manure, even. The lime will take time to work, and will also probably be an annual maintenance exercise. Your description of the current slope of the lawn does not sound like a huge problem unless you are putting in grass tennis courts or lawn bowling...

Answer 3

If your looking to make sure the grass actually performs well, you should be sure to amend the soil with a decent amount of compost. I'd recommend at least 1 inch, but if it was me and I was making such a large investment in time and money with the other aspects of the project, I would go with 2 or even 3 inches . At 1 inches depth, simply multiply the area in square feet you need to cover - 12000 sqft - by 1/12ft (1 inches high) and this will give you the cubic feet you need 1000 cubic feet. Then divide the cubic feet by 27 and this will give you the cubic yards you need -37 CY-, which is how compost is sold. Now 37 CY is a fair amount of compost, it's about a transfer truck full so you're going to want to make sure you didn't return that front end loader just yet.

The compost is going to be important for a few reasons. First, when you're scraping off all that clay, you're removing the A horizon which has all the nutrients and if you put down sod right onto the B horizon it's not going to do well. When state department of transportation agencies like TxDOT, Caltrans, WSDOT and other states need to establish vegetation on a roadside cut for erosion control, they'll apply a blanket of compost.

Second, the compost is going to improve the drainage so there's less risk of the sod sitting in standing water. It's hard to know exactly what's causing that high water table without knowing where you are exactly. If you look at this soil map of the US http://casoilresource.lawr.ucdavis.edu/gmap/, you can find the type of soil your house is situated on and why exactly there is a high water table. The map is zoomed into Davis California, but you can scroll to wherever you are in the US. If your soil is poorly draining because of the heavy clays, you can apply gypsum with the compost and this will help to flocculate the clay and improve drainage.

I'd rather have a good front end loader than the backhoe/excavator for moving everything around but if you can get a front end loader with a backhoe than I guess you'd have the best of both worlds. 4" over that area is an enormous amount of rock. You're looking at 148 CY. That's several semi trucks. You might want to think about amending the soil with compost a little bit more, using less rock over the entire area, but then putting in good network of french drains.

Also, at the back end, where you're just dumping the clay, I would watch out for erosion. It sounds like you get a decent amount of percipitation so you should have something to retain back end of this yard. Otherwise you're going to start losing the back end down the slope.

I'd echo Kervinsky in that it looks like you're yard is already pretty nice, but if you really want to level it out and have sod you have to maintain, the most important thing to remember is to add quality compost.