Surcharge loading on retaining wall.

[russ57]

I've been "instructed" to install an above ground pool on a paved area we have.
The area is built up, with a 900 high wall about 1200 away from the proposed end of the pool.
The wall is double brick, but not in great condition. (a slight lean the wrong way...) it's been there probably 30 years.

Is the pool far enough away from the edge, or should I replace it, or reinforce it. Will I need engineering?

Views welcome..

Russ


Russ

[truckjohn]

Was the proposed area undisturbed/virgin compacted sub-soil or was it filled in?

If it was filled in - you probably need to compact it before putting in the pool.. Or the pool will compact it for you and you might not like that result...

A soils testing company can test it for you... It may cost more than the pool to do this...

The classic way to compact soil like this is to "surcharge" it... You pile a big pile of dirt about 3-4 meters high on top and let it sit a year.. The "Fancy pants" way includes special measuring equipment to measure settling... It's generally OK to use once it stops settling.....

The cheapest/first thing I might do would be to talk with your Professional Engineer neighbor/friend from church/club/shed/bar/etc and see if he will give you friendly advice about how far away from the wall does it need to be so it won't bulge the wall when everything settles....

[rob streeper]

Hi Russ,

I'm not an engineer but we have a similar issue and a wall of similar dimensions that retains soil supporting the foundation of our home.

Things I've learned from the engineer:

A wall that is leaning is 'in failure', i.e. it has failed and will continue to fail.

Our wall is about 3m from the foundation. The engineer has stipulated a cantilevered reinforced concrete replacement wall due to the surcharge of the house. Depending on your pool design you are probably looking at a similar loading.

Thus, I think that you're probably going to need to replace the wall before you install the pool.

Nonetheless, you need to check with your local equivalent of a Professional Engineer.

Regards,
Rob

[Sam]

An engineer will tell you to build a new retaining wall no question.

[ian]

I've been "instructed" to install an above ground pool on a paved area we have.
The area is built up, with a 900 high wall about 1200 away from the proposed end of the pool.
The wall is double brick, but not in great condition. (a slight lean the wrong way...) it's been there probably 30 years.

Is the pool far enough away from the edge, or should I replace it, or reinforce it. Will I need engineering?

Views welcome..

Hi Russ

You don't need "engineering" but as Rob points out, your existing retaining wall has already failed, and with the surcharge of the pool the failure will most likely get worse.
Worst case scenario, the retaining wall fails catastrophically, your pool collapses and floods next door injuring a person living or visiting next door -- this is when you discover that your house insurance is void because the ground the pool was sitting on is fill, and the retaining wall was not "engineered".

Best case scenario. You fill the pool, the fill under one end settles and damages the frame supporting the pool walls. You then have to remove the pool, cart in additional fill and buy a new pool frame. It would be a similar situation with a gradual wall collapse.

Me thinks that getting professional advice and most likely budgeting the cost of a new retaining wall is the go.
(from what you have described, I don't think your existing double brick retaining wall is code compliant.)


There is a further issue,
In most parts of Australia, swimming pools need to be fenced to a particular standard. I strongly doubt that your existing retaining wall would support a code compliant fence.

[rwbuild]

Ian has summed it up perfectly.
I assume the pool will have a heavy duty vinyl liner and if it punctures, the ground under and adjacent turns to soup = collapsed wall = BIG trouble for everyone.
The cheapest and best solution in your situation is a crib wall that you can do yourself out of either H5 sleepers, concrete sleepers or proprietary crib blocks from any of the masonry suppliers.
Be aware that the NCC (National Construction Code) which councils must adhere to, requires an engineers design if it is over 600mm high.
Your pool will have to be certified under the requirements of councils pool register and comply in every respect.
For your own benefit, ask for a site inspection by your local council inspector or accredited building certifier BEFORE you go any further

[russ57]

Thanks all.

You've pretty much confirmed what I suspected.

I was just hoping for a 'get out of gaol' - eg, far enough away not to be an issue.

Now to find an engineer...



Russ

[truckjohn]

I think rather than starting with an engineer - I would start with a reputable pool company.... Have the guy come out to give you an estimate for above ground vs in-ground... You will know right off by his reaction to the retaining wall.... If he cuts and runs and won't return your call - you know the answer.

So... Just a fair warning...

When you talk with the engineer - it will be worthwhile to get him to Estimate the cost of the soils testing that will validate the suitability of the location....

I would not be surprised one bit to find out that the soils testing alone could easily be in for $3,000-$5,000 before a plastic liner touches the ground....

And it goes without saying that you are going to have to replace the retaining wall... 99% chance you will have to install some sort of french drain and major drainage improvement to capture and re-route rainwater and pool spillage through piping so it doesn't saturate the soil adjacent to the wall and cause a wall failure...

It may actually be much cheaper (or the same price) to do an in-ground pool... Because if you already have to excavate the wall and sort out drainage and subsoil.... You may as well put the pool in the hole.

[rwbuild]

Standard soil classification cost current as of 5 days ago $800-00

[russ57]

The area is not accessible to decent machinery, so in ground is not viable, not in that location anyway.

I guess a dingo may get there, down steps, to allow excavation for partial burial, which would reduce the load, but that would introduce a spoil disposal issue, as well as increasing the fencing requirements - an above ground pool with sides > 1200 only needs fencing around ladders etc.

I had soil tests done 20 years ago for an extension, which is right next to this area.
. Hopefully I can find the them and they covered the right area...

My plan is to install myself, at least at the moment...

There is a plan b location, but it's a long way from the house and 'facilities'.
And still no machine access.


Thanks


Russ

[ian]

Hi Rus

I recall you mentioning that the failed retaining wall held back fill material.
Even after all this time it is unlikely that the fill is consolidated meaning that the load from the pool would lead to more settlement and more load on the wall.

At the very least, you should budget on replacing the retaining wall with one that incorporates drainage and meets "engineering" requirements.

Whatever you do, DO NOT rely on the designs and advice found in the brochures published by the various masonry block wall suppliers.
Pay for professional engineering advice.

[Chris Parks]

The water board regards soil fill that is less than 20 years old not to be stable, this might give you some idea of the condition of the fill.

[russ57]

Thanks.
Any recommendations for engineers in Eastern suburbs Melbourne?
(most of the fill was there when I moved in 35 years ago, I built the wall and put another 20-30 on top plus backfill to level.

I notice the block suppliers are all very careful to provide details for walls to 1m or so, with no surcharge. Anything over that, requires consulting.
Coincidently, council regs allow a wall to 1m without permit.


Russ

[rwbuild]

I would double check with council, possibly a caveat as to distance from structures etc

[russ57]

Yes, there are lots of rules.

Unlike 30 years ago when I could happily build a retaining wall over 2m with no permit.
Now I need a permit to repair it...
(a completely different wall, he hastens to add...)

Russ