services trench Services under a stone building

[saveasteading]

I am coming to realise the difficulties  challenges in working with thick, solid stone walls.

 

Tell me if I am on the right lines here please?

 

The walls are 600th, with high quality, largish  but random blocks outside, decent but smaller blocks inside, and then rubble  and lime infill between the skins. They extend about 300mm into the ground where  they sit on 200mm or so of a sand-lime bed. Under that is virgin sand, dense and clean.

This sand-lime bed is strong but will be diggable. 

The sand is a bit low for starting drains off.

 

My plan for getting drainage pipes outside is to expose the bed on both sides, choose a nice long or big base stone that can act as a lintel, with similar to the other face, then form a horizontal hole underneath, just big enough for 100mm pipe.

Then in will go a length of pipe on a slight slope, and promptly infill with sand-lime, packed into place.  Perhaps for once, cement will be better than lime. 1m or so of pipe will also act as  a joggle pipe in case of movement.

How to form a neat hole? either with a drill and long chisel, and long trowel. OR will a 150mmbe the answer 

There will be about  7 of these to do, so the ease of doing it matters, as does keeping the height up. I am reasonably content with the structural implications, but am open to comment.

 

[TonyT]

Suppose it depends, you could kango a hole insert pipe and rebuild quicker than it could take to core the hole, depends on the stone, but that’s my experience with my 600mm thick whinstone walls.

 

 

[saveasteading]

5 minutes ago, TonyT said:

my 600mm thick whinstone walls.

Explain further please? Did you cut a hole through the stone or under it?

This picture shows the wall and where the footing has been exposed. in case not apparent, the lighter colour at the bottom is the virgin sand and a puddle, then there is darker colour below the stone,  which is the vertical bed. All we have removed was manure, so this situation has been standing for decades. We will be protecting and filling that eroded edge.

 

 

 

 

[TonyT]

I cored a hole for the flue and it took 12 plus hours( borrowed all the correct gear from work) when we first bought the house

 

i kango’d a hole for a waste pipe years later when we did an extension and it it was done in an hour!

 

so coring for me had no making good, kango/img the hole required some making good but was still quicker 

[Cpd]

Just done this on my old wall that is exactly the same as yours, I just picked a big stone and dug under it and did the same on the inside, put my 110mm soil pipe under with a swept bend coming up on the inside and then refilled the hole with stone and cement until it was back up to wall height. 
 

Previously I have core drilled (I have a proper petrol core drill) straight through but you can get into difficulty with the rubble in the middle of the wall…. 
 

and on another building I just used a drill hammer + chisel to remove the mortar around a stone, pulled it out and repeated on the far side, removed the rubble very carefully so as not to dislodge excess amounts and fed a 100mm extractor fan pipe  through and re cemented the hole. 

[saveasteading]

18 hours ago, Cpd said:

I just picked a big stone and dug under it

That was what I thought the masonry hole cutter might be good for, ie sailing through the sand/lime bed with a very neat hole. 

Would need to contrive an extremely long bit to extend it though.

How did you do it?

 

[jfb]

I wouldn’t bother trying a core drill. For that width you really need someone who has all the right kit to do it. 
 

much easier to dig out.

[TonyT]

You get extension bits for core cutters or you get 1m long masonry bits and you could chain drill a hole

[saveasteading]

jfb    So just long bits, as straight as possible, and a trowel on a long arm?

TonyT     Will try with bits first. I now see that extensions for core drills are readily available, so is an option.

 

Nothing happening currently, but I wanted  to know before deciding other construction methods and programme.

Getting the pipe in precisely the right place will be a challenge, with existing stones dictating the position. Even on new build I tend to allow some swivel room by boxing out the slab.

This is much harder than new-build or refurb.

[Gus Potter]

Hello saveasteading.

 

That's interesting what you're doing so would like to hear more!

 

I was wondering if that stone you have is Chert. Not sure though, but the way the face splits looks a bit conchoidal. If so it will be pretty hard as you know. Also, the surface is smoother so the lime mortar gets less of a grip thus although a hard stone each piece can come loose easily once you start say coring with the associated vibration.

 

I've read a few of your posts (and responded after a fashion) and appreciate your knowledge. I wonder. If you have seven holes to do then it's sods law that some will locate under parts of the wall where you are changing the loading a bit and thus introducing more concentrated loads.. say by widening doors, near piers between windows and so on.

 

"We will be protecting and filling that eroded edge."

 

Have you worked out how you are going to protect the exposed edge of the wall? This could well be your starting point so if you nut that out a solution for the service penetrations may just offer it's self up. In an ideal world you want to leave the origonal stone in place.. maybe just because it's a good moral thing to do and keeps the ethos of saving a steading?

 

Here is a few thoughts for all.

 

Often on BH you hear much about clay soils which shrink and swell, they can be prone to long term settlement and so on. Trees for example can remove moisture from the ground causing the clay to shrink. When you cut down a big tree the moisture content of the soil can rise.. the clay swells up, often with great force which can lift things in an "undesirable" way.

 

Sands and gravels are a different animal. Being simplistic (big subject) they are not generally prone to swelling and shrinkage.  I'll leave frost action out as this is also an expansive subject but important if you have a BC officer who insists on a certain foundation depth to prevent frost heave.. So sand and gravels..can be good news but there is often no free lunch, well there can be but..

 

Sands and gravels derive much of their ability to bear load, say from a house foundation by the "friction" action between the particles. Each particle has a mass and this rests on the one below and presses / interlocks against the one each side so friction / interlock is generated. In the ground the stresses act in three dimensions, up.. down, left and right. If you fill a glass with sand and push down from the top it will bear weight. Take the glass away and the sand column will collapse. This is often called "confining" . In other words for a foundation the ability of the sand / gravel to bear load is partly determined by not just what is below but also to the sides and above. Importantly if you lower ground level you loose the dead weight of what is above (surcharge) and this can have a significant impact on how much load you can put on the soil.

 

Now to generate friction between the particles we need mass and this comes from the density of each particle. But we know boats float..hopefully.. Archimedes. Now if you have a house and the water table is well down then you get the full mass of each partical of sand acting on it's neighbour. Raise the water table and now the relative density of each particle is roughly halved.. thus half the bearing capacity. SE's use this as a rule of thumb and it's one of the first questions asked.. where is the water table (and where could it be in the future)  if building on sands and gravels.

 

That's a bit of theory but applicable to say an old steading and also virgin ground. You also can get a bit of a "cementing effect". Here for example minerals will weave their way between the particles of sands and gravels and create more of a bonding effect so you get friction and bonding. It's very hard to test for this as testing is often intrusive and breaks the bonds and by the time it reaches the lab say it's lost it's mojo! We know these old buildings have stood for years but it's harder to prove why from basic tests. But we know they stand up. On a positive note.. a building will use every alternative load path to stay up before it falls down..but don't erode this factor of safety.. it's last resort stuff.

 

Turning back to the practical side and in particular a typical Scottish steading construction. These were buildings to house animals / sometimes folk too and thus you can imagine that when they were build no one could forsee that they would have the value they now do.

 

You have a "water proof" outer skin, random rubble interior and an inner leaf of stone that is less dressed and often less well bonded. The walls vary in thickness depending on their height and amount of buttressing from the interior walls. For these walls to work in simplistic terms they have to function in a number of dimensions. When you load the wall from above the load spreads out sideways. Often the vertical load is not central on the wall head so this causes a shearing effect between the layers as they work to share the load. When the wind blows the walls bend and this also causes a shearing effect between the layers in both a vertical and horizontal direction. For the keen this is sometimes called "complimentary shear". Remove material from one side of the wall and you loose the surcharge.. you invite movement between say the inside and outside of the wall. Vertical shear effects are introduced and the wall may protest.

 

To quantify the above.. fairly recently a Black House fell down on one of the Islands. It was built on sand / gravel and lets face it although they sound great they were built with the few resources available at the time. They have an different way of dealing with driving rain permeating the walls but it's a good example of what you need to watch out for when doing up old steadings. Here apparently the builder had excavated out the inside down some 400mm, just to the level of the stone found. But removal of that small amount of surchage caused the sand to weaken on the inside, vertical shears built up in the wall, the layers delaminated and the thing fell down. Maybe the builder had a machine in, maybe there were some big boulders that were "eased" out when no one was looking" Who knows the whole storey.  Luckily no one was hurt.

 

If you've read this far then thanks and if you are thinking about doing up a steading in Scotland then I hope this helps. If you can grasp the basics it's a great journey and can give you confidence.

 

Saveasteading.. sounds like you have a fairly robust structure so you may be fine. Would be interested to see how you are going to protect the exposed part of the wall and so on. Maybe as I said earlier it's a case of starting at the end, edge protection for frost / support and working back from there?

 

Oh and do you know what stone it is and where it came from?

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[saveasteading]

15 hours ago, Gus Potter said:

That's interesting what you're doing so would like to hear more!

 

The stone is mostly granite in boulder form, but dressed at the corners. the frontage is mostly sandstone ashlar, with some serious but traditional architecture to it.

what we think is the oldest part has some enormous boulders as the base layer.

We are currently doing serious repairs that don't involve regulations, to keep the worst of the water out, as that has caused some local failures to stone and timber.

 

the exposed base will be buried when the courtyard is built up for domestic use. meantime I intend to bank stiff mortar under and onto the face to reinstate full bearing and divert water to ground. this face has been exposed for decades, other than having 18" of manure against it, so is not about to fail.

 

the ground is pure sand, rather dense and amazingly single-sized, yet angular, so is strong and porous, yet very easy to handle. The downside would be supporting any trenching and avoiding undermining.

 

I am a Chartered Civil Engineer but don't have experience of stone buildings or A frame roofs.

 

Thanks to all of you. Enough for now, and I welcome any tips on such buildings. 

[Cpd]

On 20/08/2021 at 14:30, saveasteading said:

How did you do it?

My footing stones are on stable earth and it’s easy to just dig a long deep hole from either side, given you have sand it might be problematic taking this approach, I also my core drill and with a bit of carful measuring you can get a good line up so that the holes meet, I have a mix of bits going up to 120mm. I have restored 3 old buildings or at least am in the process of restoring 3 buildings that are your typical old - very old croft houses that you see the remains of all over Scotland, it’s very challenging and expensive and given my time again I would have knocked at least one of the 3 down and got a nice new build when I had the cash but that ship has sailed long ago…..

[saveasteading]

I think you have all helped sufficiently on this for now, thanks. I know it is sensible to go under the wall, and can try with long drills and a trowel, or use a core drill for possibly a neater hole.

As there is the sand-lime bed it should hold up fine.

Whether to put 100mm pipe in straight away. or a 150 as a duct I will decide later.

 

The downside is that we will be starting the drains at minus 500 from current floor level, before anything new above. then there will be, for foul drains, a 50m route to the digester, at 1:80 that is +/- 600mm,  so getting deep. But hey., it is sand so out and back easily and cheaply.

The surface water I am planning to take 4 different routes to spread it out. Reduced volume at any one spot, better environmentally, and also cheaper. Soakaway rate is on the limit of being too fast, but as there is 10m or more of this sand, it isn't about to form a sink hole, especially with a long holey pipe.

 

I will have more questions about forming new openings. It is either very simple or very difficult according to different sources. We previously looked at similar buildings in Aberdeenshire, and found that forming an opening is called 'slapping'. That isn't just a trade term but is on planning drawings. I don't know how local a term that is.

 

Meanwhile if you have any dire warnings now is a good time to tell, as the layout is being developed.