retaining wall & drainage behind it

[tim]

My goal is to remove the slope that ends about a foot from the side of my barn which will allow for easier access around that side of the barn as well as drainage. My plan is to excavate the dirt up to 8ft from the barn wall and install a rock wall with drainage behind it. The wall will vary in height - at most 36" and allow the rain water to run into the drain and out to my ditch. The total length of the wall will be roughly 60ft. I was planning on excavating to level (if not slightly higher at the barn wall) and then dig down tocreating a 12" deep trench

12" wide against the earth wall. In the trench, I will lay down a couple inches of drainrock and then lay in the perfpipe (planning on 4"-6" pvc) fill with some more drain rock a few inches higher then lay down some mesh to keep dirt out of the pipe holes, then backfill with more drainrock. At this point, I would lay the first course of basalt quarry rock (2 man small's) just in front of the trench and backfill with mixed 5/8 minus. i would then repeat this setting the next course back somewhat towards the slope. More backfill and so on until the 36" height is met.

My questions are how does this plan sound overall, is there anything you would do differently or add? Should I add any sort of landscape cloth behind the rocks or plastic to keep the water from seeping out or would that create too much pressure? On a wall of this height should I place any rocks perpendicular that would sit back to the ground formation and backfill over them? I guess this should add more strength to the wall..

Thanks for any feedback, bigballer

[Michael Daly]

What you need to do depends on the slope and the type of soil. It can also depend on the water table depth and rainfall.

If you build a retaining wall to hold up flat-topped soil, you are only holding up the soil closest to the wall. If the surface is sloped enough, however, you are holding up a potentially big chunk of the hill.

If the soil is prone to loosen up a lot when wet, then it could come down. At one extreme, soils can liquify in an earth tremor and your wall has to be a dam. I doubt you have that situation, but you have to consider that.

Removing the toe of a slope can cause the water to drain and wash out the base behind your wall. This is especially true if the water table is usually high. Another worst case scenario, but a vertical filter fabric can avoid some of that.

Buttressing the wall into the hill is always a good idea IMNSHO. That or some kind of crib arrangement makes for a more solid wall. It can be cheap insurance.

A phone call to a local foundations engineer could tell you if there are problem soils in your area. If not, you plan seems relatively sound. If the hill is quite steep, though, you might want a pro to look at it.

Mike

[tim]

This is the pacific NW so plenty of rain falls in winter, though now it's dry 2 feet below the surface. Last winter it seemed water was coming out of the toe about a foot lower then grade.

makes sense, my property has a natural slope coming down from the hill above

not sure how loose it gets but it can rain quite alot late fall, winter and early spring. ;-)

is a vertical fabric something like whats shown in the last picture on this page? -

I'm guessing buttressing is laying a few of the rocks perpendicular to the run of the wall and into the hill. how would you crib the rocks?

was considering this as well, will have one come out and give me an assesment.

Thanks!

[tim]

This is the pacific NW so plenty of rain falls in winter, though now it's dry 2 feet below the surface. Last winter it seemed water was coming out of the toe about a foot lower then grade.

makes sense, my property has a natural slope coming down from the hill above

not sure how loose it gets but it can rain quite alot late fall, winter and early spring. ;-)

is a vertical fabric something like whats shown in the last picture on this page? -

I'm guessing buttressing is laying a few of the rocks perpendicular to the run of the wall and into the hill. how would you crib the rocks?

was considering this as well, will have one come out and give me an assesment.

Thanks!

[Art]

Probably the best way of getting an idea of how to do the job right is to visit the web sites of a few manufacturers of special blocks engineered for retaining walls and see what is involved and then realized that your stone wall has to be designed even better since it is not using engineered blocks. Celtic and Anchor are 2 popular blocks. Or get a book on retaining walls and hire an engineer. You will be surprised by how much is involved in doing it right. The drainage gravel needs to be wrapped in special fabric to keep dirt out. I also wrap the pipe too. And you will need horizontal anchors (possible fabric) to keep the wall from coming down. Also when considering wall height, the underground portion is counted so your wall will be probably 4 feet high.

[Michael Daly]

You'd probably want to make sure a filter fabric is behind the drainage just in case. That will hold back the finer material and keep it from washing away and keep the drainage crushed stone from clogging.

Yes. You'll see it used in a lot of places - highways construction for example

- whenever they want to drain. If you see a recently constructed rock wall, you can often see a bit of fabric sticking up a couple of inches at the top.

Yes - interlock the rock like brickwork if possible.

Sorry, I should have been clear - a crib would be made using wood or concrete

Not what you're planning, but something I consider a good idea.

Mike