Possible problem of building with really heavy stones

[Tom]

Next year w're going to be reinstating the outside chimney stack at the back of our house. In the 1960's, some poor demented soul hacked it off at gutter level :shock:

The house is built mainly of Blue Lias and I've recetly bought in 8 tons from Somerset, to do the job (and some other bits). Anyway, my builder has said that he may have a problem with the stone being too heavy and squashing down on itself as the courses get higher, giving a chocolate eclair effect with the mortar. The solution may be to build 2 or 3 courses at a time and wait for them to go off before building more. However, that means I'll have to have the scaffolding for months, which is going to be very expensive. Does anyone have any other ideas of how to do this, please? What would they buiders have done originally? Thanks for any replies.

Jaqy

 

[Schoolmarm]

I presume little bits of stone in the mortar bed, carefully placed, will support the weight. Like packing. What mortar will you use?

 

[Tom]

Yes, I did think of that myself but wasn't sure if it was a valid idea. The mortar will be Hydraulic Lime and sand.

Jaqy

 

[Gervase]

Your builder's talking twaddle!
When building with stone you need to have stone touching stone, and the mortar is only there to fill the voids. As each stone is laid you whack it with a rubber mallet to squeeze out the excess mortar and strike it off with a guaging trowel on the face of the wall. Thus as the work gets higher the weight is taken by the stones beneath and not the mortar. Today I finished a rubble-stone wall at wall-plate height (about 15 feet) and there is no eclair effect. I reckon about 30 tonnes of stone went into it, and some of the individual stones weighed around 80lb.
Here it is about three-quarters built:

And here's the house before work started:

The result of a building control officer, who didn't know how stone buildings are put together, insisting that footings for an extension were dug vertically down rather than at 45 degrees. :shock:

 

[Schoolmarm]

:shock:

 

[Tom]

Wow, Gervase...yes, what you say makes perfect sense and would explain why the mortar lines on our original walls are so fine...which I love!!

Right, another lesson for my builder, me thinks!

Thank you so, so much

Jaqy

 

[MdB]

Your builder's talking twaddle!
When building with stone you need to have stone touching stone, and the mortar is only there to fill the voids.

Interesting. I'd been told the exact opposite though I forget by whom. I think I believe what you say though over them as your results speak for themselves.

 

[chippy]

Gervase, cud you explain the bit about digging footings at 45 degrees instead of vertical? nver heard of that

 

[MdB]

Gervase, cud you explain the bit about digging footings at 45 degrees instead of vertical? nver heard of that

This is my non expert view. Lets see if I get it right...

With a pile of stuff not glued together, if you put pressure on the top of the pile, the forces are transmitted to the directly below and those within a cone up to 45 degrees from the vertical. This makes sense if you think of a pile of ball bearings or similar. The same applies if the stuff is lose stone, sand, dirt, or anything else not stuck together.

If the pile of stuff is glued together, then a force on top will be directly transmitted to the bottom with no forces to the side. This being your concrete foundation. From the bottom of your foundation downwards, you are back to the stuff not glued together so the forces spread in the cone up to 45 degrees from the vertical again.

A wall made of stone and lime is considered, for the above, to be a pile of stuff not glued together as lime is very good a keeping things apart but poor at keeping them together. i.e. it is good in compression but poor in extension.

 

[Gervase]

MdB's explained it far more expertly than I could!
Essentially, because rubble stone and the like is built without foundations, you have to be very careful when digging nearby. In most cases the whole structure simply rests on the ground, so any disturbance to that ground has to be done to minimise the risk of the soil on which the wall sits being displaced. Thus, if you're digging out for a new floor inside or for an extension outside, NEVER dig vertically down. Always leave an angle, or 'batter', to maximise the 'footprint' of the wall.
Otherwise this will happen:

Thankfully what was a very pretty cottage is now well on the way to being very pretty once again.

 

[Tom]

The 45 degree angle stuff has also been discussed on threads regarding digging out for a limecrete and LECA floor, for example, for the same reasons.

Jaqy

 

[Penners]

If you're digging out a floor to pour limecrete, and you leave a 45-degree "batter" (I've got all the lingo, now!), then it follows that the limecrete slab is going to get thinner and thinner as it gets towards the edge of the building. It's already been observed that lime is not as effective at sticking together as cement, so how do you guard against the risk of the slab's cracking or crumbling near the edge of the slab, where it's thinnest?

 

[Tom]

Penners, when we did ours we dug down about 6" and then started to angle it in. We didn't go as far as 45 degrees though because our new floor went in straight away. The good people on here had advised that nothing would start to move that quickly and that everything would have gone off by the time any part of the house started thinking about moving.....and, fortunately, that proved to be correct

Jaqy

 

[Andrew Bodenham]

Penners, that can be a real problem. In many cases though the house walls will go down at least some distance into the bearing surface before stopping, so it may be possible to excavate to a suitable depth for a floor slab. In some cases where a building has very shallow footings then excavation may simply not be possible.

It's not quite as simple as the 45 degree rule discussed. Every granular material (soil, sand, gravel etc.) has a natural angle at which it forms into a pile - think about running dry sand through your hand to form a cone. This angle is called the Angle of Repose (thought you'd like that). For dry sand / gravel it's about 40 deg. Clay soils, about 45 deg. Wet sand, 22 deg, ball bearings practically zero. The presence of water in the material will reduce the angle of repose. In practice this means that if you're digging a footing in sandy soil with a high water table then the safe slope angle could well be less then 20 degress. So it's hardly surprising that these old walls collapse when people start digging vertically down next to them. On the other hand, compacted, undisturbed soils will have a higher angle of repose, and the size and shape of the hole is also important. This is why you can dig a grave in the churchyard, but a continuous trench of the same section would probably collapse. Before digging near your footings it's also worth pondering just how heavy a building is - they really are quite weighty things and do need to be nicely held up.

 

[Penners]

Thanks Andrew, although I must tell you that The Angle of Repose is something that I'm very familiar with.

 

[shelli]

that was beautifully explained Andrew - even I understood it!!

 

[chuckey]

My lime built cottage and barn was (I think ) built with a continous bed of lime under most of the rough limestone blocks. Over the years as the lime has degraded ( they only used soft sand from the river Ure), some blocks have moved leaving block to block contact and some have cracked in half! The builders round here lay all their limestone blocks on a bed of soft sand cement mortar, albeit a dryish one. Though if they repoint an old building some use sharp sand + cement mortar(?)
The problem with block to block contact is that you don't know what (if any) load the mortar is taking. So the wall may be balanced on a load of stones, each with a point contact with the one underneath. So the whole structure is actually only balancing. This has been brought home to me when a section of a pig enclosure wall fell down.
The wall was built with lime and roughish limestone/sandstone/millstone grit blocks.It was about 5' high, 2' wide and unusually was built on a radius of about 8'. The straight sections of the wall were Ok, but when a builder clambered on to this curved bit a "lump" about 3' long and 3' deep fell out of it (outwards). When I removed the loose bits it was clear that the first underground stones (2' X 2' X 6" thick) were rocking. When they were removed there was no lime between them and two of them had a roundish bottom. The stones under them were also big and flatish. So basicaly the corner of the wall was actually balancing on these rocking stones. Time and tide weakened the bonds to the stable parts of the wall, hence the destruction.
Frank

 

[Tom]

What you say, Chuckey, makes sense....so now I'm really confused!! :?

Also, the stone I've bought in is already dressed for coursing and is quite flat, so in some places there would be no room for any mortar at all.

I think I'll play safe and build a few courses at a time, with fine joints but not overly-fine...if that makes sense :?

Jaqy

 

[Gervase]

Dressed stone and ashlar does have a different laying technique to random and rubble stone, and is generally laid bedded on mortar. Although, to be honest, even with dressed stone, you ideally want to use snecks and pin stones to keep each face apart long enough for the mortar bed to cure (which, in the case of a fat-lime mortar, could be months). Often what looks like a thick bed of lime is simply just the pointing, and underneath you'll find blocks with irregular tops and bottoms and a lot of pinning to ensure stone touching stone. If you put mortar under the block and then knock it down until you can hear that there's contact between the high and low points of the stone, the rest of the mortar will eventually set and form a uniform bed, so the idea of stone just 'balancing' doesn't enter into it. It's the knocking down that's important - it compacts the mortar beneath and ensures that there can be no rocking or movement later. At its simplest you can use another stone or a club-hammer and a rag, but I find a rubber mallet best because it avoids cracking the stones.
One reason for collapse in rubble-stone walls is that they were often built using mud and clay rather than a lime mortar, with lime just used for pointing. Once the pointing fails, the mud and clay is easily washed out and then you can get 'rockers' and loose stones.
As for the new wall, a lot depends on how flat 'quite flat' is. If it's like brick - flat and square on every face - then be all means bed the stone like brick (noting the above caveat). Otherwise, interleave the stones and treat as rubble stone, relying on stone-to-stone contact with the mortar used primarily to fill.
But I stand to be corrected. If I'm wrong, however, then every wall I've built or altered is also wrong, and so is every builder in West Wales going back for the past 500 years!

 

[Nemesis]

And no cement.