how to build a tall retaining wall?

[Mia]

The county didn't tell us we would have to build a retaining wall, but I really don't expect that 8 foot bank of raw dirt to stay there, behind our house-to-be, once the serious rains are going.  I've been reading about retaining walls.  I expect we will have to have an engineer design it.  What I am looking for is info about various types of materials we could use to build the wall.  Budget is a major factor of course.  I didn't allow for the cost of a tall wall when estimating the cost of this home.  

[glenn kangiser]

In an old posting you mentioned a basement - is it in addition to the 8' retaining wall needed making a total depth to bottom of excavation about 16'?  How far behind your house to the slope?  No solutions - just trying to get a better picture of what's going on.  Sounds like an engineer's help may be in order.  

Also a diversion terrace with a ditch at the back to  take water off to the side above the excavation can help stop water  (from up the hill) from going over the slope and washing it down.

[Daddymem]

As a civil engineer, I wouldn't touch a wall over 4-feet in height...leave those to an engineer with retaining wall design experience.  There are many options out there, the most popular now is modular blocks.  You can go with rail road ties, landscape timbers, reinforced concrete, gabion baskets, or a steep slope with riprap or geofabric to elimate the wall entirely.  Keep in mind that any kind of segmental reatining wall (SRW, ones made with modules not continous like a poured wall) will require "tie backs" of some form.  For example, modular block walls use geofabrics, landscape timbers use deadmen. The distance back has to do with the height of the wall and the quality of the material behind it.  Basically a wall holds back a large wedge of soil starting at the base of the wall all the way up to the ground surface.  The angle of that line is the variable that you need an engineer  for those taller walls.  

As important as the design is...the most important factor in an SRW wall is the contractor.  If you can read plans and understand how walls are constructed watch the contractor like a hawk, soooo many contractors cut corners on wall construction which can lead to catastrophic failure.  If you pay them to build it, make sure it is built right.  If you can't evaluate the construction yourself, make sure you budget in the engineer for a few site visits to watch over the construction if you can.

[glenn kangiser]

As an example of what Daddymem is talking about, last year I jackhammered out a wall made by a friend of mine that was not over 4 feet high, had drains at the bottom -was into undisturbed soil about a foot.  It was at the toe of a slope I would say was about 30% and went back about 60 feet or so - the high point of the  slope was probably about 15 to 20' higher than the toe.

I think gopher holes caused winter rain to soak into the clay causing the whole bowled out area to de-stabilize.  The 8" thick 30' long wall with footing was  moved over tightly against the corner of his garage.

We removed the wall - re-routed the water above the site,  packed the slope so water went on top instead of  under (temporary until next gophers move in )  This year with double the rainfall there was no problem.  There is a sufficient space cleared out at the back of the garage to prevent damage in case the earth decides to have another movement.

[Amanda_931]

Gabions (somebody told me once that wasn't what this was, but if you run a search you find these) can do a lot.

Haven't been back to Shiloh in a couple of years but here is a set intended to protect a tall river bank--and the Indian mounds on top--from barge traffic and floods.  The baskets are 3 or 4 feet tall.



Or they can not look too bad, like these--about 18" baskets, at least on the front, separating Wal-Mart's parking lot from the store below--Florence Al.  It's probably around 15 feet tall.  And three years later, almost completely grown over.



Another picture of the same wall, taken at the same time.  But in the interests of faster loading, here's the link.  

http://groups.msn.com/ap615/spring2002.msnw?action=ShowPhoto&PhotoID=141

[Mia]

thanks for the responses!
this wall is up the hill, set back from the house.  The grade is about 22% to 25% where we are building, a bit steeper farther up the hill (28% or so).  The hole for the basement is at least 10 feet deep on the uphill side.  then a flat area about 12 wide was cut, at the top of the basement excavation.  then, there's the rest of the hill.
and the flat area on the uphill side of the house will probably have to be cut lower, since the basement engineer specified, no more that 6.5 feet of dirt on that wall.
The soil is mostly clay.  and hard.  but no rocks, unlike where we live now, about 12 miles away, where there are lots of rocks.
I'm thinking maybe we should step back the highest up the hill wall, maybe 4' up, then 4' back, etc.  I know it will be battered.  I don't want to cut too much, but I really don't want the hill sliding into the house.

[Daddymem]

A note on stepping walls....unless they are spaced horizontally far enough apart, they will act as one single wall and should require engineering if greater than 4 feet high overall.  That line from the footer to finished grade has to be far enough away from the toe of the uphill wall.  Drainage behind the walls may become more difficult with some many locations to daylight.  Stepped walls may be more visually appealing, but now you have multiple walls to be concerned about proper construction, unless you can get the walls spaced far enough apart so they just become a vertical break in the topography holding back small amounts of earth.
Gabions can look nice if done properly; construction is crucial.  The plastic covered wire ones require care in rock placement so as to not nick or rip the plastic coating resulting in rust and failure of the basket.  If you get the galvanized you have to watch for scrathing too deep, again rusting.
It sounds as if you have a tight soil so walls shouldn't be much of a problem.  Groundwater (perched or actual) may be the be an issue.

[glenn kangiser]

Mia, have you already started excavation?  Considering budget problems, perhaps you might want to reconsider construction methods- maybe post and pier or sloped/stepped crawlspace.  The retaining walls could get very expensive.
In my area the ground is stable enough that you can make that type of a cut with no retaining wall as evidenced by old road cuts, but every place is different and has to be considered on it's own merits.

[Mia]

excavation for basement is already done.
we knew that building in the middle of our property instead of down by the road would be more expensive.  
we will have a garage, bathroom, & one or 2 rooms in the basement, plus storage.  with all the earth sheltering against the walls, ought to be very cozy.
some ways, we are saving money - from a house being torn down, I salvaged 3 pairs of solid wood folding doors, & we have 40 sheets of 5/8 plywood that used to be the roof of that house.  and, always looking out for other materials that can be reused.  (need to build some small storage buildings.  if we can find flat places.)