Retaining Wall Design

cooperp · Dec 31, 2007 10:49 am

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?

DeKa · Dec 31, 2007 2:12 pm

I wouldn't be using interlocking bricks for anything you want to last more than 5 years, let alone 100 years.

The interlocking brick retaining wall near Boral (don't they make the things?????) in Clunies Ross St Prospect (Sydney) which has huge sections bowing out is a great example.................... I presume Boral built it, so it was built the right way.

In regards to retaining walls, there is one simple rule. In equals out. This means you need 2 to 2.6m long posts, depending on the height of your wall. It is a cantilever wall, so you need to ensure there is the same amount of support below the ground as the maximum potential load on the wall above ground.

If you want my honest opinion for a wall which will last, use 2.4m and 3.0m 200mm round treated pine posts and then either 200x75 sleepers or 150mm winged splits (cut from a 175mm round log). Pick whichever you prefer the look of (square look-sleepers or rounded look-winged splits). Materials for that wall would cost you under $1600 I would say.

cooperp · Dec 31, 2007 2:20 pm

Thankyou. We don't like the look of sleepers or logs, and actually plan on laying bricks in front of the boring concrete slab to make it look nice.

While this will still cost a bit, it will cost a damn site less in engaging a professional landscaper to design and construct.

DeKa · Dec 31, 2007 3:56 pm

cooperp

Thankyou. We don't like the look of sleepers or logs, and actually plan on laying bricks in front of the boring concrete slab to make it look nice.

OK - that's different then

Stick to the in=out rule with your posts and all should be well.

davinci · Jan 01, 2008 11:18 am

the type of retaining wall you are talking about will be quite expensive even to do it your self
conc at 45 mpa is $200 plus per q/m 1 q/m at 100mm will equate to 10sqm
the reinforcment you would need is y12 deform bars at 2500mm cross intervals
y12 6m lengths are around $15 - $20 each as you can tell the reinforcment will add up
you will need a footing with all of you vertical supports and reinforcment rising out of it
a wall at 1mtr high is not the sort of thing a non tradesman can form up
wet concrete per q/m weighs around 1.5 tonne the vertical and horizontal pressure on form work is huge so if you are not a concrete or form worker and know how to calculate the pressures and form boxing to suit you will end up with concrete on the ground collapsed form work and twisted reo i would suggest to not to do this your self if you havnt done it for a living
i worked on a two storey concrete pour several years ago 860 sqm the builder set all the props etc, it collpase while we were on it, wet concrete has a massive weigth
a sleepers wall when built properly with internal liners etc will last 30 + years, keeping in mind the aqua pressure affect in regrads to liner placment, drainage etc