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Drilling in granite


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We are being required to fix lots of metal plates to the original granite wall, with bolts of 16 and 24mm diameter.

I don't agree that it is all necessary or appropriate, but have little choice in the matter.

 

I'm used to drilling in concrete but not into granite, mostly boulders but some ashlar. 

 

 

How hard/brittle is it? Any tips?   

 

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“According to Moh's Scale of Mineral Hardness, granite is typically a 6-8 on the scale (1 is the softest and 10 is the hardest).”


 

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Granite is best drilled with diamond core drills especially if the holes need to be pretty clean. Granite shatters with percussion drilling and as above, it’s hard! 
core drilling is vibration free, no splinters and much easier on the operator and equipment. 

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Larger holes with a diamond core drill seem easier.  We have used granite tiles on jobs but the whining from the plumbers etc about drilling them drove me crazy.  Can you get a separate contractor to do the drilling or specify by others?

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Start off small ( 6mm SDS ) and go up in 4mm increments. Should be easy enough that way tbh. 
When you get within 6-10mm of the desired size, use a diamond tipped hole saw to start off the hole, so as to prevent the surface breakout, and then put the same size SDS in to complete. 
Use a hozelock pump up spray bottle to keep the diamond bits soaking wet slushy cutting. Will the the remaining surface be on show afterwards? 

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I was thinking diamond, but how long will they last? the ones at screwfix etc are fine for tiles and worktops.  But through multiple stones of,  very approximately, 200mm?

I have had commercial drillers do maybe 60 holes for foundation bolts with the same bit, and very quickly. Perhaps worth buying something of that class, as it will be too drawn-out  to use a specialist.

 

Also I know from experience that the hole must be just big enough for the bolt, or epoxy disappears by the box.

Most of the holes reuired are thriough the 200 inner skin, then through 200 rubble and then fixed only to 60mm of the outer skin...  I fear that real life may get in the way of drilling through rubble with 40% mortar....and how to get the epoxy, even with a stent, to the outer skin......insert tube perhaps?

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1 minute ago, Nickfromwales said:

within 6-10mm of the desired size,

Sounds worth a try as an experiment at least..  Not sure what you mean about starting with diamond then going sds.  

In most cases the hole and fixings will be concealed by inner stud construction. THe intention is 2 hold walls together at unbonded corners, and to tie a facade back to an existing floor.  More gripe about that later perhaps.

  On 2 cracked lintels they will be very seriously on show....I may ask separately about that.

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I have drilled through 1M thick granite.  Only small holes for cables.

 

You want a heavy weight SDS drill probably heavier than the little one I have, and good sharp drills, and as @Nickfromwales says start small and drill in steps.  I have always got a clean hole when I have drilled it.  If drilling all the way, the rubble core is the worst bit, it has a habit of bits shifting and jamming the drill.  The first time that happened I thought I had stripped the gearbox in the drill, it was the clutch operating so it does not twist your arm off.

 

How deep do you have to drill? I am guessing not all the way.

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6 minutes ago, ProDave said:

How deep do you have to drill

The wall is 600. Plate on the inside, hole but no epoxy on the inner 200 skin, ditto the 200 rubble, 50mm into the outer skin. All-up that is approx 450 of hole, of which the last 50 is the fixing. But the stones are anything but regular, especially on the inner face.

In reality the only way we will know if we have hit the outer skin is if it feels like solid stone and is approx at 400mm, but the skins vary in thickness a lot.

It isn't what I would do, as I can see much more practical methods, but the SER has to be be allowed through or the warrant will take forever......

I am seriously hoping for a pragmatic Building Officer, as I don't cut corners but do know about reality as compared to drawing office (been in both, currently in the former)

For a 24mm hole, the NickfromWales method will need a lot of £50 drill bits from Highland Industrial Supplies. Might be the way though, and the bits will be used again. 

Torque.....lots of it? or sds?

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I've edged my front path with Granite kerbs and inset some LED lamps in 8 of them. They needed 32mm holes through the 150mm thickness. I really regretted not getting them pre-drilled, which the supplier offered, it was an awful job.

 

I used diamond tip core drills that were £65 pound a pop and got two or three holes out of each drill. Had to take it slow and use water to keep it cool. If you go too fast the tool just melts. I'm sure it took me over an hour each hole.

 

Good luck.

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SDS max of the correct diameter I’m a decent drill

 

start with 10mm as a pilot to ensure accuracy and to let the hole fill as you go larger.

stop and clear debris on a regular basis

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33 minutes ago, joe90 said:

is this to tie the wall together?

It is.  There is a cross-wall of the same construction, but it is not bonded (new term for me 'risband joint'), so the Engineer we have had to engage has said it must be  tied. The solution is a 1m long x 75 plate bonded to the inner wall, then a short return onto the outer wall that has a bolt through it and glued to the outer skin. 

Then there is a similar butt joint at a main corner, where the same is proposed. In my mind this will simply transfer any loads further along the wall, but it isn't the biggest deal.

 

Then there is a facade with arches that have joggled to a new shape. the proposal is to us similar ties linking the existing first floor joists to the facade, either within the arch stones or just above them.   My understanding is to leave arches well alone as they adjust  to movement....but hey ho.

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1 minute ago, TonyT said:

start with 10mm as a pilot

a 450mm long 10mm bit is likely to bend a bit as there is only perhaps 6mm of common core after deducting the spiral. Perhaps there are stiffer ones....bend before you buy.

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Posted (edited)
9 minutes ago, TonyT said:

for a resin bolt

It is for a resin bolt, glued to the 50mm penetration in the outer skin....somehow.

Actually, 50mm is very shallow as you discount the first 25mm as standard practice, and so the pull-out load capacity wont be great.

Edited by saveasteading
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Having worked on rubble filled walls, how the hell are you going to get the resin all the way to the outer skin and past all the dust and rubble in the middle 🤔

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If resin anchoring into hollow walls etc. You drill the hole and insert a mesh sleeve, this directs the resin and acts as reinforcement. Sleeves can be purchased or self formed by rolling plastic or metal mesh into a tube and folding the end over to stop resin being pushed straight out of the end

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2 hours ago, markc said:

insert a mesh sleeve

450 long into a hole that is filling with bits. Perhaps possible with  a rigid tube first to guide the mesh in. but then we are leaving a much oversized hole behind.

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Sorry, my comments were on the assumption the surfaces were on show after drilling, whereas that is not the case. 
The best investment here is a set of very high quality ( Bosch / DeWalt )1000mm SDS 

drill bits at 10, 16 and 24mm sizes for provisional drilling. 
Drill the first side with the 10, then the 16, and then send the 16 through the rubble fill and into the second side. The 16 will stay relatively straight whereas the 10 will not.

Go to next size up accordingly, and best of British with the rubble fill cave-ins. 
One trick is to drill the 10 and 16 and then get a closed cell expanding foam, push the nozzle in as far as you can get, and inject a punt or do of foam. Squirt some water into the hole first to soak the rubble fill, which will make the foam cure faster and spread better, and then leave for an hour or two. 
The cured foam will keep the rubble fill in check whilst you drill through and install the resin.

 

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26 minutes ago, Nickfromwales said:

the rubble fill cave-ins. 

Serious practical advice there, thanks. 

I might invite the specifier or his boss to do the first fixing: show me how it is done.

The core at that point might have a nice big stone that can't be drilled because it displaces, or be all mortar and bits.

 

There are so many other unknowns though, such as what does the back of that outer stone look like. With some serious tape measuring we can assess that there is a substantial stone there, so that we are not too near an edge. But we can have no idea what depth or contour it will have. Indeed will it be intact or perhaps cracked. 

Will it be thick enough to take a 50mm hole without punching through the outer face, or pushing the stone out.

 

In any case I don't think it is good building practice.  . Where there is any sign of movement we may be shifting the stresses somewhere worse.

Where the risband joint shows no distress we are encouraging stresses, and the wall is not built as a structural wall, simply as a divider...If there was no wall there, there wouldn't be an imagined issue.

 

If we are to do this, I would have no issue with simply fixing to a selected big inner stone. The bond created because of the 3 layers is surprisingly strong, to the extent that one area of local collapse by damage, still has an overhang of rubble wall. 

 

There are far bigger issues with the design we have received, but this is one hurdle before dealing with the others.

 

 

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To expand the same subject, we have 2 ashlar concrete lintels which have cracks. they are in line with the outer skin of masonry only, with the remainder supported by very hefty oak lintels. My proposal was to jack the stone up the 1mm or so it has displaced, then drill through the oak into the granite and bolt them together (no rubble).  The Engineer's proposal is to drill up through the crack and bolt it back together.

Again I am reluctant but not so much so.....if it works. 

We have found a good mason, so he might have experience or views...he hasn't seen this......but there are more of you and  with a variety of experience.

Any thoughts?

1645424327_lintelrepair.jpg.f4e9ee26318c3dbceb77a091d0431485.jpg

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