Patrick:

A better question is "why do retaining walls fail?". Start with that and work backward.

https://theconstructor.org/structural-engg/retaining-wall-failure/14230/

I attribute it to surface erosion cause by a loss of a cover crop of vegetation.
About 20 years ago a the house was remodeled from 2 story to 3 story and a room extension and deck were cantilevered out over the cliff off of the existing house at the 2nd story. I think the house cast enough shade over the ciiff to kill off the low growing vegetation, leaving some scraggly Japanese honeysuckle and a few weeds. It only gets an hour or so of sun in the AM. The house is sitting on a limestone slab that I am afraid will eventually be undercut by the erosion.

http://forums.pelicanparts.com/uploa...1584056443.jpg

Yes, you want to solve that problem. Get 2 more quotes on different retaining wall structures with plenty of diverted drainage around the slope. I have seen those interlocking stones go as high as 30' un-reinforced and they planted ice plant right into the wall. Once you have a structure, then figure out what ground cover might work. I don't know if ivy thrives in your area but it doesn't need much sun. The PNW rain forest is covered with ferns, vines, ivy, and whatever, and they only see sun July and August.

There are 2 issue here.

Has the foundation of your house settled in that area noticeably in the past 10 years? If so, then its a whole different ball game.

If not, then you simply need to aggressively deal with water. I would first ensure that all water from the yard above and the house roof is captured and diverted around the errosion. I would skip the ground cover and go with large crushed rock. Think rail road ballast size.

I don't see what a 3 ft wall is going to do for you in this situation.

I didn't realize the house is on top of this.

Sorry but you are going to have to bite the bullet and pay some very compitant engineer type person to work something out.

It will be cheaper in the long run to get the bank/problem sorted than renew the foundations etc if the house starts sagging.

Google "Incan Terrace farming"

I like the idea of the gambion baskets at the bottom of the slope for a couple of reasons. It will protect the slope from erosion when the creek floods as well as support the slope. Hard to find a contractor for that kind of thing in this area, let alone get a quote. This may turn into a DYI. :mad:

Here is a picture of the house - its a lot better than my drawing. :)

http://forums.pelicanparts.com/uploa...1584142980.jpg

How about something like this for retaining wall. Gonna need a Bobcat to place them though... :D

http://forums.pelicanparts.com/uploa...1584148002.jpg

Don't forget to buy a llama. He'll feel right at home and you won't have to cut the grass anymore.

http://forums.pelicanparts.com/uploa...1584154354.jpg

I'm not an engineer but gambion baskets at the base won't avoid erosion will they? They may catch the silt runoff but won't stop it.

How much water comes down the driveway and eventually to that slope?
Hard surfaces are bad that way, unless you can divert the water along the way.

Also, water absorbed at the top by the house puts pressure on the bottom as the soil gets wet

An option besides a retaining wall is to level out the slope some.
I'd also look into large crushed stone dumped down the slope

It will allow me to change the angle of the slope so there isn't so much erosion. Down near the bottom there is enough light to grow some ground cover.

None. It is all diverted to a part of the slope that is 100 feet from the house and not as steep.

Where do I get those? They look better than a gambion wall.

A lot of concrete plants make those out of left over concrete. I'd call a few concrete plants and see if they know someone who has them.