MMH
Gold Member
I need to build a 8 foot tall retaining wall. What is the most cost effective construction method?
8' Retaining Wall
- Thread starter MMH
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MikeInEburg
Veteran Member
MMH said:
Eight feet is fairly high. If it must be straight up, you'll need a high quality design as well as safety rail at top. Better would be a terraced arrangement.
Mike
forgeblast
Elite Member
versa lock? the base is going to be very important.
Chuck Oppermann
Member
Just curious, is the ground level that it is holding back, or is it on the side of a steep hill and trying to hold that back? Is the ground sandy or solid. What is on the down hill side of the wall? I think we need information on what the surroundimng conditions are before an adequate responce can be given.
CurlyDave
Elite Member
In every jurisdiction I know of, an 8 foot retaining wall must have the design stamped by a registered engineer.
Most places 4' is the absolute maximum for a self-designed retaining wall, some places it is 3'.
The main reason for this is that the forces on a retaining wall increase as the cube of the height. An 8' wall is only twice as high as a 4' wall, but has eight times the overturning force.
In Oregon, my neighbor had a self-designed retaining wall which fell down in its first winter. I tried to show him a better design, but he insisted on having a guy with a backhoe come out and re-stack the 2'x2'x4' concrete blocks, and it fell down again in its second winter. The third time he was more willing to listen and the wall was restacked in a different manner, which has been stable for several years.
Most places 4' is the absolute maximum for a self-designed retaining wall, some places it is 3'.
The main reason for this is that the forces on a retaining wall increase as the cube of the height. An 8' wall is only twice as high as a 4' wall, but has eight times the overturning force.
In Oregon, my neighbor had a self-designed retaining wall which fell down in its first winter. I tried to show him a better design, but he insisted on having a guy with a backhoe come out and re-stack the 2'x2'x4' concrete blocks, and it fell down again in its second winter. The third time he was more willing to listen and the wall was restacked in a different manner, which has been stable for several years.
hr3
Veteran Member
Well Curly Let us in on what his "yours" last design was that worked
jdonovan
Gold Member
I'm a fan of keystone wall units. They stack with out mortar, and interlock with fiberglass pins.
Here we need a permit if we go above 2'. So my walls are built as terraces. 2' wall, 3' setback, and then another 2' wall. Repeat until tall enough. With a 150% setback the county views them as separate walls. Therefore each may be 2' tall without permits. This also assumes a flat surface behind the wall for 150% of height.
An 8' wall has HUGE overturning forces. I absolutely would not attempt to build one with out an engineered design.
I did build a 6' wall and the design required a geotextile to tie the block into the hill every 2 courses and the tie back was 6 - 8' long. Every 6" of backfill had to be compacted to 95% density. It was a huge project and I'd really not recommend it to a novice wall builder.
The county was highly skeptical of a homeowner doing this work, but I took my time did it right, and 8 years later it and all my other walls are still standing, no settling, or creep.
Army grunt
Platinum Member
I built several in Ct. an had good luck with 6x6 pt.The trick is to build in dead men into the wall, also back filling with tires & crushed stone with drain pipe. In the north your fighting the frost, then the heavy wet soil of the spring thaw.Plan ahead an the walls will work great an stay.There are walls still straight an where i put them over 20 years ago
Army Grunt
Army Grunt
Steve_Miller
Veteran Member
We produce these products, may be much larger than your looking for.
Retaining Walls : Shaw Pipe
Steve
Nova Scotia
I am putting an addition on to my house w/ a full basement & want a walk out basement. The ground slopes from the back of the house down to the front. At the back of the house the ground is level w/ the first floor (i.e. walk out of the kitchen). At the front of the house (the length of the addition will be 46 ft.) the ground level drops by 5 feet (so 5 feet of the basement at the front part of the addition would be above ground). I want to excavate along the side by an additional six feet wide for the walkout and this is where I need a retaining wall. Towards the back I would need the retaining wall to be 8' tall & this would taper off to the front where the retaining wall would be only 3' tall.
Perhaps a terraced arrangement would work. I'll have to look at the lay of the land & check out the gas line easement...
CurlyDave
Elite Member
Well Curly Let us in on what his "yours" last design was that worked
1. You must drain the back of the wall properly. Drain rock wrapped in filter fabric, with a perf pipe at the very bottom. At least 12" thick and you must either provide weep holes through the wall at the bottom, or some other way for water to escape the perf pipe.
2. If you have done (1) you may consider the drain rock behind the wall as a fluid with and effective density of 40 lb/cubic foot. (This is clearly not the real density of the material.) If there is a surcharge above the wall, i.e. material slopes up away from the top of the wall, this method does not apply and you need to consult a local registered engineer. If you have not done (1) properly you must use an equivalent density of 102.4 lb/cuft. I typically round this up to 105.
Now we come to part which depends on the particular design of the wall.
If the blocks are such that they lock together well, you may:
3. Integrate (this is a calculus operation) the force on a small piece of the wall times the distance from the bottom of the wall to this small piece from the bottom of the wall to the top of the wall. This will give you the overturning torque the wall will see per unit length.
4. Tilt the wall back, into the earth behind the wall, at an angle such that the overturning torque the wall sees back into the earth is greater than or equal to the overturning torque from (3).
If you do not feel comfortable doing these calculations yourself get an engineer, who may be able to advise you on less expensive ways to achieve the result you want.
In the situation I described, the material had already been purchased and was on-site. I was determining the way to use it, with the least additional cost. Very different than starting from scratch.
In other situations a different methodology may well be superior.
My point was and is that unless you have an engineer design a high retaining wall, things that look perfectly reasonable to most people can undergo a catastrophic failure.
beenthere
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Helped build a RR tie wall from used but good ties. It was about 100 ft with the highest portion 12'. Built it with a good lean back to the hill, and about 2" setback for each course. Dead man ties were dug back into the hill and anchored at each third course. The hill behind the wall was sandstone. Drainage was added every third course as well. Long spikes were placed in each tie into the course below. The first 15 years, that tie wall held up very well. My friend moved from there 5 years ago and I've not been back to see how it is doing.
The ties did make a nice looking retaining wall, as there were a few well-placed plantings of flowers and some flowering bushes planted there.
The ties were cheap, and sorted out from pulled-up track. Only the most recent ties were taken for the wall. I'd expect they'd last 30 years in the wall, as they were creosote treated hardwood ties.
simonds
Bronze Member
Go with poured concrete wall. The best and last as long as you live.
Railroad ties will rot over the years. That's why they take them out replace them after so many years and get rid of them
12" cement blocks with lots of reinforcement, maybe, but why take the chance
Whatever, make sure you have a footer below frost level and a drain tile behind it.
General Contractor for 41 years
Clayton
Railroad ties will rot over the years. That's why they take them out replace them after so many years and get rid of them
12" cement blocks with lots of reinforcement, maybe, but why take the chance
Whatever, make sure you have a footer below frost level and a drain tile behind it.
General Contractor for 41 years
Clayton
radioman
Super Member
yup - I have a railroad tie that is 3 feet high that came with house I bought. It fell over few years later. 8 years later I rebuilt most of it with railroad ties again. so far its holding, but I'll never use railroad ties again.
Based on a couple of responses, seems like poured concrete will the way to go. Where can I find some design guidelines? I am envisioning a monolithic pour w/ an oversize footer that offset - the cross section would look like 'L' with the backfill then pushing on the bottom to resist the overturning moment. Perhaps some integrally poured gussets if strength at base (once again, the overturning moment) will be a concern? Obviously, I will plan on backfilling with stone & draing pipe for good drainage.
