Sandstone boulder wall

[ownerbuilder]

Just briefly:

• we've had the house raised
• because of slope of land, we are getting a retaining wall built across the back yard & down sides of house
• we're using large sandstone boulders (cut into blocks) - need heavy machinery to set these in place
• wall is 1.5m high at highest point, rocks will be 3 high
• we're using aggy pipe with sock & textured membrane to stop fine silt from getting into the pipe

My questions:
What level should these rocks sit in relation to the level of our proposed concrete slab?
Can someone explain how deep should the first level of rock sit in the ground? If the boulder wall does not to sit into the ground, is it still stable?
How much gravel to put at the base before the rock gets placed?
Any other construction tips that can help with drainage (problem in '94 when water seeped from previous retaining wall and flooded the rumpus room under the house)

[johnc]

What size blocks and do you intend to put mortar or pointing in the joints? Rather than how deep in the ground have you considered putting in a footing to prevent the wall settling into the ground over time. Is soil going to be piled up to the height of the wall at the rear or does it face back to the slab?

[TallStreak]

ownerbuilder,

Not withstanding council/engineering approvals etc (walls over a meter high) previous research on a number of your questions for myself led me to the following design diagram from a local (to brisbane) company that does rock walls

Hopefully this should answer some of your questions at least:

Bottom layer embedded 200mm
Granular fill behind full height of the wall
http://www.scottysearthmoving.com.au...s/Pict0002.JPG

[ownerbuilder]

What size blocks and do you intend to put mortar or pointing in the joints? Rather than how deep in the ground have you considered putting in a footing to prevent the wall settling into the ground over time. Is soil going to be piled up to the height of the wall at the rear or does it face back to the slab?

Thanks for your reply. Here are some further details:

• It will be a free standing rock wall.
• Thanks for your suggestion on footing.
• Yes, soil is going to be piled up to the height of the wall, but will be mostly back-filled with 40mm gravel.
• Concrete house slab is to be separately located 1.5m away - just wondering about levels....we will have house slab, patio slab/pavers, want to ensure drainage stays away from house.

If you can add anything further advice, it would be appreciated.

[ownerbuilder]

ownerbuilder,

Not withstanding council/engineering approvals etc (walls over a meter high) previous research on a number of your questions for myself led me to the following design diagram from a local (to brisbane) company that does rock walls

Hopefully this should answer some of your questions at least:

Bottom layer embedded 200mm
Granular fill behind full height of the wall
http://www.scottysearthmoving.com.au...s/Pict0002.JPG

Thanks very much, TallStreak!

Very useful diagram & one I will use for reference. And thanks for your tips as well. All being taken on board.

[johnc]

There are probably a number of ways to tackle the job depending on among other things your soil structure. If there are no trenches or other works on the ground you are fixing the wall to you can dig a trench put in some packing sand, and go from there. In the past you would go down to solid clay and build up from that, this however is a retaining wall not the pyramids and removing the grass etc and the top layer of soil may be enough for your needs. Any stone sitting on old fill or trenches will subside.

If you intend to leave the joints open and the blocks are sawn nice and square I would suggest the following, after you set out the bottom row of blocks which can be set in a trench, on a packed gravel base or concrete and a bit of mortar would help stop any slippage you can easily move onto the next course. If you cut small 25 mm squares of sheet lead, these should be placed in each corner as the block is lowered into place and packed or adjusted by folding to take out any wobble. The lead not only provides a buffer but allows the stone to expand and contract with out going for a wander.

If you are not going to use mortar or grout on the joins then a bit of movment doesn't matter the wall will just move about a bit by itself. If you are setting the joins then you want a solid base otherwise your joins will crack. Early on the bottom row of stone blocks would be larger bearers and act as foundations and would usually be below ground level, these days we have concrete which is a lot more civilised. Give me an idea of the actual block size and what you have in mind for joints and I will give you some ideas, but in the end speak to however is doing your building approval to make sure you are legal. My very early background was a monumental mason, which is not quite stone walls but in the general ballpark. We have a bluestone retaining wall in the back yard made of 9' x 12" x 18" kerbs and two of us slid them in off the back of a trailer and set them on a single brick either end and packed underneath and they have not moved noticeably, your version is not single layer and you need to make sure the extra courses are secure and will not flatten some unsuspecting bystander (i.e. toddler). Your main issue willl be the drainage from behind and making sure moisture can get away easily. I am assuming we are talking about largish blocks up to .5m in height and plenty of weight.

John.

[ownerbuilder]

There are probably a number of ways to tackle the job depending on among other things your soil structure. If there are no trenches or other works on the ground you are fixing the wall to you can dig a trench put in some packing sand, and go from there. In the past you would go down to solid clay and build up from that, this however is a retaining wall not the pyramids and removing the grass etc and the top layer of soil may be enough for your needs. Any stone sitting on old fill or trenches will subside.

Well, some progress has already been made with our wall - a trench has been excavated and the base is now all solid clay. The guy doing the job has laid down a gravel base.

If you intend to leave the joints open and the blocks are sawn nice and square I would suggest the following, after you set out the bottom row of blocks which can be set in a trench, on a packed gravel base or concrete and a bit of mortar would help stop any slippage you can easily move onto the next course. If you cut small 25 mm squares of sheet lead, these should be placed in each corner as the block is lowered into place and packed or adjusted by folding to take out any wobble. The lead not only provides a buffer but allows the stone to expand and contract with out going for a wander.

Our blocks are squarish but not perfectly cut - more rough. Have been told that wobbly bits have to packed tight with thick slivers of rock.

If you are not going to use mortar or grout on the joins then a bit of movment doesn't matter the wall will just move about a bit by itself. If you are setting the joins then you want a solid base otherwise your joins will crack. Early on the bottom row of stone blocks would be larger bearers and act as foundations and would usually be below ground level, these days we have concrete which is a lot more civilised. Give me an idea of the actual block size and what you have in mind for joints and I will give you some ideas, but in the end speak to however is doing your building approval to make sure you are legal. My very early background was a monumental mason, which is not quite stone walls but in the general ballpark. We have a bluestone retaining wall in the back yard made of 9' x 12" x 18" kerbs and two of us slid them in off the back of a trailer and set them on a single brick either end and packed underneath and they have not moved noticeably, your version is not single layer and you need to make sure the extra courses are secure and will not flatten some unsuspecting bystander (i.e. toddler). Your main issue willl be the drainage from behind and making sure moisture can get away easily. I am assuming we are talking about largish blocks up to .5m in height and plenty of weight.

John.

Block size 500mm high x 500mm deep x length varies - up to a metre long. Yes, extremely heavy - have to use machinery to do the lifting and placing of each rock. Have spoken to a few people as well. We already had aggy pipe (with sock), have now purchased geo-fabric, so are going to be laid behind the wall and we're using a good amount of gravel to fill in directly behind the rocks. I think we've figured it out with the certifer's input & Woodwork Forum, so thanks very much for your very useful advice, John!

[johnc]

All under control then, hopefully they can get themselves some stone dogs which make placing the blocks dead easy. That size is pretty easy to move with a pinch bar to get into position.

John

[ian]

Here are some further details:

• It will be a free standing rock wall.
• Thanks for your suggestion on footing.
• Yes, soil is going to be piled up to the height of the wall, but will be mostly back-filled with 40mm gravel.

• Sorry to be a Cassandra, but your wall is not free standing.
  A free standing wall would be a stone fence. You're describing a retaining wall.

• Concrete house slab is to be separately located 1.5m away - just wondering about levels....we will have house slab, patio slab/pavers, want to ensure drainage stays away from house.

[ownerbuilder]

Sorry to be a Cassandra, but your wall is not free standing.
A free standing wall would be a stone fence. You're describing a retaining wall.I'm unclear where the house slab is located.

Yes, you're right - wrong use of terminology on my part!

Is it 1.5m away from the top of the wall or 1.5m away from the base of the wall ? If it's on top it's a surcharge load for your wall, I hope you've sought professional advice.

From base of wall.

If it's at the base, one issue will be keeping water from the pavers and such around your house away from the base of the wall to avoid saturation of the wall's foundation.[/list]ian

Yes, that's my concern. The retaining wall has now been built and the certifier has said it is sound and has passed the inspection. As you've mentioned, we'll just have to make sure the paved/concreted patio area next to the house foundation is properly drained. The retaining wall extends along the sides of the property for about 3/4 of the length of the house. If you have any advice on further steps we can take to improve the drainage, that would be appreciated.

[Bleedin Thumb]

I know this is all a bit too late but what you are building is called a gravity wall. As the blocks are not tied into each other the walls strenght is derived from its own weight and the friction created between the stones is greater than the forces exerted by the backfill/hydrostatic pressure.

Because the wall is not tied together a footing is not required to provide structural resistance to prevent the wall from toppling over and the gravel footing just is a bedding layer.

If I get the picture right, you have a concrete slab 1.5m away from your wall you shouldn't have a problem with water getting into the slab.
1. Use 32mpa for the slab ..it helps with waterproofing
2. If your really concerned get the concretor to add Xipex to concrete (expensive)
3. Place a dish drain at the base of the wall to pickup runn off.
4. Place a vertical "Atlantis" agg type drain at the edge of the slab to act as a membrane