Using boulder as pier footing?

[AstroApprentice]

Hi DIY OBS folk,
I’m thinking about setting up a permanent pier, and possibly OBS around it in the future. Bringing in large amounts of concrete for pier footing etc will be difficult on my property. Mega anchors look good, but I need to do more research on them. There is, however, a large boulder, likely 2-3 tonnes in a good spot. Just wondering what you think about drilling into the boulder to mount a pier and then building a deck over rock & around pier?
Drilling may be very difficult, but another boulder has a drill hole through it so it can be done. I’m not even sure how I would anchor into holes?
Plus Would thermal issues from rock radiating heat be too much of a problem, even with deck shading it?
I welcome any advice or criticisms on the idea.
Thanks

[doppler]

If you have hard ground I don't think you need to go overboard with the concrete. Here's a pic of mine, well a temporary one I made 18 months ago it doesn't vibrate and hasn't moved yet, just a wheelbarrow of concrete below.

[bojan]

Depends on how big (high) the boulder is... Do you have any photos ?

BTW, 6 years ago I drilled a hole in the ground with the help of auger (bunnings), 75cm deep and 30cm in diameter (widened the original hole with shovel, as temporary solution for a concrete slab for pier (I think I used ~10 bags of pre-mix)... it is still there being used and it did not move a bit.

[AstroApprentice]

Hi guys,
The boulder is approx 1.5 m long x 1 m wide, with a relatively flat top. It looks to be at least 1 m deep in sandy soil, with 50 cm above ground.
I was thinking I would dig a hole and use premix concrete for pier, but then I noticed the boulder and thought maybe I could use its mass as a solid footing...

[bojan]

50cm above ground is not too high... however you may want to extend the top a bit (with decking?) to have more space, 0,5m around pier is a bit to tight.
I would go for it.

Try to drill couple of holes for dina-bolts.. and see how it goes.

[Kunama]

Using a diamond grinding disc make a flat and level area about 300mm, mark out the pier base plate and the holes, drill the first hole to suit Ramset 'TruBolt' of at least T12100 size, you need a 12mm masonry bit and you need to drill 100mm deep plus the thickness of your base plate.

Once you have the first hole drilled install your pier (which should have 15mm holes drilled in it to allow for installation. Drill your second etc holes through the holes in the pier base as otherwise the drill will want to wander about in natural rock. After each hole is drilled install that TruBolt and then drill the next one...

(that's how i would do it..... )

[AndrewJ]

I wouldnt even worry about grinding the rock flat.
Just drill the holes so you can anchor the pier and use a small 1" packer in the middle to allow it to be set level.
Once level, just use dry pack grout to fill the space under the baseplate and when set, fully tension the hold down nuts.

Andrew

[Peter Ward]

Did exactly that, some 24 years ago. BIG and 1/2 submerged boulder in our back yard. I built a suspended deck over the boulder, and after drilling the rock and inserting some re-bar into the holes, made some form work, then poured a small concrete footing for a bick column up floor level of the deck.

Filled the brick column with concrete, leveled the top in preparation for a steel pier for the Paramount. Dynabolted the pier to the concrete. Works a treat.

Thermal inertia of the boulder is a non-issue, as it is shaded by the deck and effectively the same temperature an the earth it is half buried in.

[Wilso]

The best and most stable foundations are either sand or rock for any structure.
Ramset have chemical anchor and threaded rod for use for uneven surfaces and is available from hardware stores.
Then use structural non shrink grout to fill the gap. Or otherwise grind the rock as Kunama said.
Hardness of the rock could be your nemesis in either applications.
Best to try before you commit!

Good luck!

[AstroApprentice]

Wow Peter, that sounds impressive - looks like I can expand my simple plan!
Thanks for all of the replies, it’s good to know the boulder idea can work and might save me the footing concrete at least. Always good to hear what others have done and recommend. I’ll do some test drilling at the weekend...

[The_bluester]

I would worry a little about using a rock. Perhaps it is just the geology of the last two rural properties I have lived on but in my experience things like rocks move out of the ground over time with the ground expanding and shrinking with moisture. In theory you would reckon a concrete footing would too but they don't seem to. Perhaps the square shape makes the difference, or perhaps the ground being covered by an observatory would do the same with a rock (Avoiding moisture changes)

[Imme]

Agree with Paul it would move a small amount with moisture changes in the soil however it is still a solid foundation and suitable for a pier in my opinion.
The changes would be gradual and occur over time.....it's just a matter of checking your alignment/levels every few months.

.....from experience I can say concrete footings move as well.....I had a pole concreted in the ground right next to a house I once owned, it would gradually get closer/further away from house through each season.

[SkyWatch]

There is unlikely to be too much movement in sandy soil, so it should be fine. Rocks can move out a bit, but more in expansive or clay soil.

A rock that size will act just as well as a good deep foundation.

Have fun!

- Dean

[xelasnave]

Franky pile it.
Pipes or rods driven deeper into the soil and conected to rock with concrete.

You may find bed rock with your pipe or rod but even if you do not that system will reduce movement.

But the main thing is the rock is a good base even if it does move..it would not be much and you just have to adjust polar...

alex