Retaining Wall Design

[cooperp]

Hi. We currently have a wood sleeper retaining wall which is starting to collapse. It was built by the original residents of the house I'd say, probably in the very early 1980's. Anyway, we want to replace it and I've tossed around many many designs in my head.
There is a 16m section of the wall which will be exactly 1m high so that a permit is not required as this section of wall is not within 1.5m of a boundary, or slightly higher (which will require a permit) because the 8m side length is within 1.5m of a boundary, and so if I have to get a permit for this, then I may as well do it right and get it for the whole length and make the back section 1.3m which is what I'd prefer.
So, now comes the design question. For the back section of the wall I could use interlocking bricks, with reinforcing etc. But the advice I've had so far regarding the side wall is that I'll have to excavate into the neighbour's yard (oh yeh, like that'll happen) and use geofabrics etc.
After many many interations of design in my head and discussion with non-experts (mates), I've come up with the following.
H-beam (or nested back to back C section) uprights. I will then create formwork from post to post (25mm ply or similar, reinforced along the middle) and cast in-situ high strength (say 45Mpa) concrete with reinforcing for good measure (and maybe as a necessity). The wall depth would be 100mm to 150mm. Because of access, I’m likely to get a concrete pump in, had to do this before, what a bummer.
The concrete would be cast in one go.
The ground is fairly hard, maybe 50% clay/50% loam.
As I said, across the back, the wall will be about 16m at 1m to 1.3m high although I’ll be building some steps in, around 1m to 1.5m wide at some point.
The side 8m section actually starts at 1m to 1.3m high but over it’s length reduces to around 500mm high, so is not purely structural all the way.
I plan to lay a concrete foundation for the drainage pipe and use no-fines backfill. I was even thinking this morning I might put a drawstring/pull-through in the drain pipe so I can keep it clean of any silt that does get in there, and possibly wrap some geofabric around the backfill.
Backfill will probably be around 600mm deep.
There is a gentle slope, perhaps 1 in 20 to 1 in 40 providing a surcharge on the back 16m length of wall. The side wall has no such surcharge.
I DO NOT want this wall to move anytime in the next 100 years.
So, to my questions.

Question 1
Will this work and be permitted?

Question 2
How far apart should I set the H beams? I guess how think I make the wall and what reo I have in the wall will have an impact here.

Question 3
How deep do I need to place the H beams into the ground and what diameter hole? I was planning on 1m deep and 600mm holes.

Question 4
Anything I haven’t asked that I need to?

[Terrian]

Question 1
Will this work and be permitted?

If no permit is required then I see no reason why not.

Question 2
How far apart should I set the H beams? I guess how think I make the wall and what reo I have in the wall will have an impact here.

A standard sheet of reo is 2.4mwide & 6m long, so make the uprights 2.2.2m centers, that gives you 100mm each side to bend the reo down and weld it to the uprights
f62 reo would be (will be!) my choice. We will be adding 2 y12 reo rods welded in between the uprights (1/3 from top, 1/3 from bottom)

Question 3
How deep do I need to place the H beams into the ground and what diameter hole? I was planning on 1m deep and 600mm holes.

that will be fine, I would dig a 100mm wide x 150mm - 200mm deep trench between the uprights, rather than have the concrete wall finish at ground level.

Question 4
Anything I haven’t asked that I need to?

25 - 32mpa concrete will be more than up to the job IMO

I use to work in garden supplies, we have built quite a few bins with concrete walls, usually 2.2m high, 200mm in the ground, 6m long walls (ie: standard sheet of reo). never used anything stronger than 32mpa, and I am thinking those walls took heck of a lot more stress than the retainer wall you are going to make

[cooperp]

Wow, such a quick reply. Nothing to do on New Year's Eve? I'm working...

Thanks for the reply.

[Batpig]

Dear Cooper,

May I first make a suggestion...

Have you thought about the possibility of using a proprietary precast concrete sleeper system? If the market down there is anything like it is up here, you should be able to find a system of either semi-circular or rectangular cross-section precast concrete sleepers in just about any colour, or texture (ie. woodgrain, pebble-finish, etc) that you want. Final product will look great, would be far easier to build, and you would also have the advantage of access to engineered drawings that will make getting council fairly straightforward...

Failing this, Terrians idea of spacing the posts to suit a sheet of reinforcing mesh on it's end is right on the money. Makes so much sense if you weld it at each end... Keep it closer towards the front edge of the posts with - say - 50mm of cover out to where the finished surface will be, and with the horizontal wires outermost. That should be sufficient cover for your environment, as well as give you enough room out to the formwork for good workability of the wet concrete inside. A thickness of 150mm sounds like more than enough and is a good match for F62. Any thicker and you will start to need F72 mesh - not for strength purposes, but for crack control.

600mm diameter for the footings is overkill. If you can get an auger on a Bobcat that is more like 350mm in diameter, that will do the job, but the final diameter should also depend on how big in cross-section your posts are, so that you end up with about 100mm cover of concrete around the steel posts all the way around. In general with sleeper wall footings, you are far better off with more depth than diameter, with the actual required footing depth being largely dependent on the type of soil that you will be retaining. For the soil type that you are suggesting, I reckon that a Structural Engineer would probably suggest that the footing depth in your case should be equal to about your wall height plus - say - 300mm. For safety's sake, though, I suggest that you snoop around the web a bit until you can find some sleeper retaining wall tables that you can get a feel for footing depth from, based on your given set of variables. Perhaps if you find out who the local manufacturer of treated sleepers and posts is down there, and then see if they have a website with a link to some tables. The variables to be considered are retained soil type, foundation soil type (can be two different things, since low side may be in cut, and high side may be filled), wall height, post spacing, footing diameter, backfill gradient, and surcharge (ie. will there be a car parked up there all the time?...)

Regarding concrete strength, you'll have to go 32MPa for durability purposes, so that the saltiness of the rain doesn't make the concrete crumble in a few years time, nor start to make the steel inside corrode with the result being "spalling" out of the concrete at each reinforcing bar. You certainly wont need anything stronger than 32MPa for strength purposes. 32Mpa and 50mm cover are your lucky numbers for your situation, but make sure the horizontal bars are outermost. The rest of your proposal sounds quite thorough. I wouldn't bother with the No-Fines concrete around the Ag Drain - I'd just get an Ag Drain with a geotextile sock around it.

Good luck with it, but also have a think about my initial suggestion of using one of those proprietary pre-cast systems. Will cost you more in materials, but site works will cost you less. And I'm not sure how council works down there, but how are you going to go about getting the wall certified if you follow your own proposal, which might sound good but hasn't been engineered? At the end of the day, you'll still need an engineer's certificate wont you? However good your proposal is, an engineer just wont be interested in putting his number on your design. They just don't charge enough for retaining wall designs to do anything other than rubber stamp a pre-existing standardish design. And if you ask one of them to do some custom numbers on it, it will cost you so much extra that you would be better off pouring that money into a pre-engineered proprietary system instead. Catch 22...

With a precast system that used separate sleepers, the join lines where the sleepers met would give your wall a real finished look - like weatherboards on a house. Otherwise you might end up wanting to render the thing with some sort of texture because smooth plain concrete can look pretty boring. If you do get the urge to look into some precast systems, try ringing up the local branch of the National Precast Concrete Association of Australia - they should surely be able to point you in the direction of some manufacturers.

A Happy New Year to you, and Best Wishes whichever way you go.
Batpig.

[cooperp]

That was an awesome reply, thankyou. I have considered the concrete sleepers you are suggesting, but 'Wife' isn't into it, plus 'Wife' wants to build a couple of curves into the wall to parallel the pool shape (wall is above pool) which I admit would look good.

I'm also considering that once the walls are cast (if I go that way) then we'll probably have Wife's nephew build a nice loking proper brick wall in front of it which will hopefully add another couple hundred thousand to the value of the house!

Plan on getting a Dingo to do the earthworks, with an auger on it so will make digging erasier thats for sure. I'd hate to be the resident of the house in the future (once we move out) that tries to move the wall!

[pls8xx]

Hello from the USA. I've been checking out the site for a few days; looks like you guys have a lot going on. It's a little hard for me to understand everything; you have some different names for standard tools and equipment. And I think in feet and inches, I'm a little lost with meters.

Anyway, I'm a home owner that needed about 130 meters of retaining wall done real cheap. I've been working on this project for two years. Things go slow when your crew is only one old broken down bloke, me.

I looked at doing timber walls, but they don't last and some of this wall supports a building addition and a parking area.

I looked at using the new SRW blocks (manufactured concrete units), but to get the wall height, I would need the heavy expensive ones. I didn't think my old body would hold up to all that much lifting.

I decided on poured in place concrete. What I needed was a forming system that a one man operation could do from standard building supplies. And I also needed several step units.

The photo below is some of the wall. After I complete all the wall, I plan on rendering some of it, some will get a veneer of natural stone. The wall ranges from 0.5 meters up to 1.8 meters.

[Terrian]

Hello from the USA. I've been checking out the site for a few days; looks like you guys have a lot going on. It's a little hard for me to understand everything; you have some different names for standard tools and equipment. And I think in feet and inches, I'm a little lost with meters.

feet & inches are my first measurement type, metric is second, but I tend to jump from one to the other quite a lot, even mid sentence )

Anyway, I'm a home owner that needed about 130 meters of retaining wall done real cheap. I've been working on this project for two years. Things go slow when your crew is only one old broken down bloke, me.

if nothing else, you have the claim of doing it your self...

The photo below is some of the wall. After I complete all the wall, I plan on rendering some of it, some will get a veneer of natural stone. The wall ranges from 0.5 meters up to 1.8 meters.

ok, so you had formwork up, with bolts going from the front sheet through to the back sheet judging by the holes, a bit of render with some nice colour and it will look a millions dollars. Nice job.

[Ivan in Oz]

Hello from the USA. I've been checking out the site for a few days;

I knew there was someone looking over our shoulder

And I think in feet and inches, I'm a little lost with meters.

Hey! I get lost just thinking, Metric or Imperial

G'Day pls8xx,

Welcome aboard and Happy New Year.

And to all you others,
Happy and SAFE New Year;
Thank You for being here

[cooperp]

Your wall looks great. I agree with Terrian, will be amazing once rendered etc as you say.

The different between your wall and my proposed one is that you seem to have cast your wall with a front and sides, the sides providing support, whereas I'm going to use the H beam to support the wall. Is that right?

Yes, its a shame that you still use imperial (british) measurement, something left over from the red coat days. Metric (new british) is so much easier.

[Batpig]

Looks superb pls8xx,

Have you done the part of the wall near the building extension yet?
For that matter, have you done the building extension itself yet?

Besides the obvious need for the building footings to be sitting on either firm natural ground or properly compacted select fill, also make sure that if the bottom of your proposed retaining wall is any higher than a line drawn down and outwards from the bottom of the building footings at an angle of about 30deg below horizontal, that you either put some concrete piers underneath the bottom of the building footings, or construct the retaining wall to take the extra surcharge from the building. Some of those segmented dry-stacked concrete units that I think you might have mentioned, held back by reinforcing grids that run through properly compacted backfill soil, would be ideal for creating a platform on which you can build. Something like "Keystone" for example... Unlike normal reinforced grout-filled block walls, it actually likes - no, make that needs - the backfill to be compacted for the wall to work. Other brands are available, and the concept in general is the only practical way to go if you're wanting to build close in on the upper side, without having to pier. Isn't afraid of heights, either. Only limitation is that it really becomes impractical when trying to retain an embankment on a boundary where your neighbour is on the high side.

What you've done looks great. Good Luck.
Batpig.

[pls8xx]

Thanks for the warm welcome and the kind words for my project.

Though my hands-on work with concrete is a recent thing, I have been around design and construction of civil projects all my life. I have come to some general conclusions about walls. For walls up to 2 ft. or 0.6 meters, homeowners often build a wall that will last. Ask an engineer to build a 2 ft wall and he will over build it, the cost will be 4 times what the homeowner did.

Once you get above a 3 ft or 1 meter wall, the home owner fails to understand that the forces are many times greater than that against a low wall. With time these walls fail. An engineer understands the forces involved and will design a wall with an acceptable safety margin. Expensive yes, but necessary.

What the homeowner often does not understand is the relationship of wall height to weight of soil retained. In the graphic below I start with a slope at the maximum that is stable. A low wall is built and the soil retained is shown in brown. Next the wall height is doubled. Notice that the weight of soil retained is 4 times as much. And last I show the toe of a long slope cut away and a wall built.



cooperp, if I understand your proposed wall correctly, I feel a little uncomfortable. I'm not sure I can explain my concerns. It seems you will depend solely on H beams at intervals along the wall to hold it up.

There are two basic movements to be controlled; the lateral movement of the entire wall and the tilt or roll over of the wall. The deep H beams should control the lateral movement but it is the tilt that I question.

Think of a section of the wall in profile as a lever. The most secure un-moveable position of the H beam is at the bottom and thus this can be considered as the fulcrum.

In the graphic below I show the structure in profile, then the the face view of a wall section and a single H beam that must support the wide wall section. It is easier for me to visualize the force by rotating the structure to the horizontal. Shown is the fulcrum point, the red cross hatched area is the soil mass retained, and the narrow area of the H beam in ground. Notice that the soil mass is at the end of the lever shown by one blue arrow. This force is multiplied by mechanical advantage down the lever to where the narrow soil at the H beam counter acts the force as the second blue line.



Around here, most of the homeowner built walls of this type that were over 3 ft high have failed. Something to think about.

[Batpig]

Dear Pls8xx (& a "Rest Assured" for Cooper),

Your theorising is not all that bad Pls8xx, but there's one thing not readily discernible that makes those embedded posts work in your typical sleeper-wall setup, namely that the type of pressure generated by the foundation soil in resistance to the footing moving towards it, is of an entirely different and much higher magnitude than that generated by the retained soil wedge when it is in the process of failing and trying to push the wall over. Both types of pressure increase as a function of soil depth squared, but the coefficient for soil pressure in resistance to movement can be typically ten times that of the soil pressure that is generating the movement. You therefore don't need a continuous width of footing in a sleeper wall in order to match the width of the retained embankment. Do a search on Active and Passive Soil Pressure if you're really keen and wanting to learn more...

Another thing too, just as a curio: the footings don't actually end up rotating at the base, but rather at a point somewhere further up that is mathematically difficult to describe. There's a zone of Passive Soil Pressure in front of the footing from the top down to this point of rotation, and then another zone behind the footing from this point down to the bottom of the footing. Together they do the job of preventing the wall from overturning. Another comment that might be worth making after looking at your diagrams is to always "bench" any slopes (ie. cut horizontal platforms into them) that are behind your walls before backfilling. There is also another mode of failure, besides the common two of overturning and sliding that you mentioned, that can "put paid" to a wall: it is called "Global Failure" or "Slip Circle Failure". It is a theoretically part-circle failure plane that starts a fair way behind the wall, goes under the wall, and then comes out in front of the wall. Quite rare, but starts to become an issue in certain peculiar circumstances, such as where the ground in front of the wall keeps sloping downwards.

But rest assured Coop, the theory is there that your type of wall can work if done right. It's just like anything else, though - if you underdo things and cut corners, it will fail. Perhaps it would be a good thing if you try and sniff out some of those tables that I was talking about previously, in order to make sure that you're in the right ballpark. If you're going to need certification at the end, you might be better off lining up an engineer earlier in the piece, just to play it safe and avoid a very big engineering bill.

Best Wishes,
Batpig.

[dazzler]

That was an awesome reply, thankyou. I have considered the concrete sleepers you are suggesting, but 'Wife' isn't into it, plus 'Wife' wants to build a couple of curves into the wall to parallel the pool shape (wall is above pool) which I admit would look good.

Then let 'wife' do it