I own a house that is situated in front of a slope. The part of the
slope that is directly behind my house is moderate (rises about 15-20
feet over about 30-40 feet) behind that slope, there is a heavy
slope (rises about 40 feet over 40 feet).

When I bought the home, there was a room filled with mud that ran
across the length of the house which faced the hillside. The mud in
this room started at the wall facing the hillside at the ground level
of the dirt on the other side of the wall. The mud in the room ended
at the ground level of the bottom story of the house. Basically, I
think this room was designed to take all the water that entered the
room and divert it under the house.

Anyhow, since then, we have remodeled the house to take advantage of
the space that the mud-filled room took up and added three rooms to
our home. The problem now is that our contractor ditched the job and
didnt install the linear french drain that was supposed to go in the
back (which is the side of the house facing the hillside). We have
since gotten estimates and have decided that it was too expensive to
get done and that we would have to do it ourselves.

We have started digging, but I'm trying to figure it all out. I've
looked online but have found that this particular case is very unique.
The ground level of the mud outside is about 6 feet above the ground
level of the three new rooms and I don't know how deep I should dig,
how far away from the wall I should dig, what kind of rocks to
backfill the trench with and such.

My plan at the moment is to dig about 6-7' down right against the side
of the house (exposing the concrete); just enough to get to the ground
level of the bottom story of the home. I want to make sure I grade the
trench about 1" for every 10' and cover the side of the house with a
thick plastic. Then I plan to spread a 1" layer of sand (as an elastic
cushion for the pipe) and lay the filter-fabric covered 4" perforated
pipe. After that, I want to fill 2-3' of the trench with 3/4" gravel,
and the rest with 2" pebbles. On the top of the 2" pebbles, I want to
put cobblestones that are stacked on top of each other so that they
have a slope that is down and away from the house.

My question is if I am attacking this problem correctly. If I am not
please help me correct any mistakes that I am making.

I have heard a lot of people saying that I don't need to dig that
deep. The people that gave me an estimate though said that they were
going to dig 1' below the footing of the house. My concern for me to
dig that deep is that the house will become unstable. Please address
those concerns as well.

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Clarification of Question by neilgandhi-ga on 15 Jun 2006 19:09 PDT

About the three rooms that took the place of the mud-filled room,
those rooms are basically underground...The hillside facing wall of
the rooms is about 6-7 feet below the topsoil outside. Basically if a
7' man was standing in one of the newly constructed rooms, he would be
at eye level with the dirt outside and at the same level as the
garage.

One more thing to note is that there are other rooms on the bottom
story of the house, and a whole second story that is above the new
rooms. The rooms on the bottom story used to be separated from the mud
room by a wall and a hallway. Now the hallway has doors to access the
new rooms (not that it matters). The old rooms on the bottom story are
pretty much at ground level with the outside dirt. The top story is
unchanged.

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Clarification of Question by neilgandhi-ga on 16 Jun 2006 14:20 PDT

thank you for the comments. I really appreciate the community taking
the time to do this.

Is there anything I can do, take pictures of, or take measurements of
that cound help?

Hire an engineer that can examine the site and tell you exactly what to do.

Hi neilgandhi-ga,

I think you have to have someone come on-site. Even if you are going
to do all the work, get an engineer to specify it.

One problem you may find is that the engineer may be over-cautious and
over-specify. After all, they'd rather have a customer dig much more
than they need to, than to have a customer complaining that it doesn't
work! But it's really the only option.

Regarding how deep to dig: it's a law of diminishing returns. You can
get a relatively big benefit from a relatively shallow French Drain,
yet depending on your local conditions (soil type, foundation type,
rainfall level, catchment area etc) you may need more than that.

Another option that might help is a V-shaped drain uphill of the
house, to divert some of the water away before it even gets to the
house. It probably won't avoid the need for a French Drain, but it
might avoid the need for such a deep French Drain. But again, without
going to the site, it's impossible to say.