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Old 20-01-2020, 08:34 AM
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The_bluester (Paul)
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New obs build

So, I am finally making a start after years, a pretty basic start.

On the weekend I made a final decision as to where my proposed RORO shed will go and used that to pin down the spot for the first pier block to go in. Taking advice received here on IIS and elsewhere I am aiming at approx 1 cubic meter of concrete in the ground. Thankfully I have been able to give myself a head start as I have a post hole auger on a tractor so I bored a number of holes down to about 900mm (The limit of where the auger will plunge to) so I can then break out the soil and clay between them rather than having to manually do the whole lot.

It has led to a couple of questions in my mind. I have tossed the idea around for a long time and decided to set threaded rods in when casting the block (Three, not four unless someone can give me a good reason why not as three mounting points makes for far easier leveling by shims as required) What material are people using for the threaded rods, gal or stainless?

The second was how extensive a reo mesh cage are people putting in? I was thinking it does not need to be all that extensive as the purpose is really to make sure the block stays together in the ground, rather than trying to make it strong enough to hold up a bridge. I was thinking of a pretty basic light reo cube some 400mm or so smaller than the hole, with the threaded rods attached to the top of it, and the whole lot supported by a form that the threaded rods will be bolted to so that it is all suspended away from the bottom and walls of the hole during the pour.

If all this works out then with the currently planned shed size I might end up with anything up to 5 blocks, 3 for piers and two that will be double poured (Leaving the ability to remove the top section) and brought up to finished floor level to have somewhere stable to plonk down dobs or visiting tripods.

Based on previous advice a wooden floor will be built over the blocks and cut in around the piers with a space for some foam rubber seal to avoid the thermal mass of a concrete floor.
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Old 20-01-2020, 08:42 AM
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Day 1 progress. Hopefully the bit of rain we have had will soften things up a little without leading to problems later. I would want a nice dry hole before the pour to make sure I was not going to see any shrinkage in the ground later.

Do people enlarge the hole at the bottom to create a locking profile where the hole tapers down in size to the top?
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Old 02-02-2020, 06:25 PM
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So, posting in the hope of seeing what others have done in terms of reo and mounting bolts.


Unless someone says something otherwise I plan to build a rough cube of light reo mesh about 700 mm on a side. I reckon 12mm bolts should be plenty to attach the steel pier and I am going to weld them to a plate which in turn will be welded into the top of the reo cage. Then to pour the block I will make a timer frame that will screw to the boxing up for the pour and will support the reo cage via the mount bolts so it won't come out of the concrete anywhere except for the pier mount bolts. The hole has ended up just under a cubic meter, I ran into a rock in one corner so I am stopping there!

I am tossing up if I should use four bolts or three, three should provide plenty of strength and makes for far easier leveling via shims than four bolts do.
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Old 02-02-2020, 08:23 PM
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pfitzgerald (Paul)
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Hi Paul

Sounds good so far. My ROR Obsvy. is 'still' in the planning stage. From the research I've done over the last several years it sounds like you're on the right track. Your idea for the cage inside the concrete plinth is sound based on what I've read. The advice I got from Steve (he of the SVAC spit!) was to go for three bolts for mounting the pier - for the same reasons you give. I actually have six holes drilled into the base of my pier - if you like I can post a picture. The holes are 25 mm in diameter. The top of the pier has no rat cage - instead it has three steel blocks that have been cut and drilled to take the top of a HEQ5 Pro tripod head.

Hope that this all helps - but keep in mind that although I have the pier made (and it's sitting in the garage), it doe not yet have a proper home!

Paul
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Old 02-02-2020, 08:46 PM
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I made up a pier with materials gifted to me by Andrew C so that is ready to go. No rat cage here either. I michined (Which huge effort as it is a bit big for my lathe) a round base big enough to put an EQ8 plate on and bought an EQ6 pier adapter that I will bolt down once it is all in place and I can do a solar noon to align the plate pretty accurately. I welded the round plate on top and cut an access hand hole in the side of the pier to get at the central bolt for the EQ6 mount then fitted a wooden panel inside the pier so I can't drop the bolt to the bottom.

I was originally going to use four bolts to mount the pier to the block but was thinking better of it for leveling reasons, four mount bolts is a pain and three is easy! There is space on the base plate to make it three and for subsequent piers (Andy is going to want one too) I will get a triangular base plate cut.

I can see the point of up to five pier blocks and in a 6M X 6M roll off section (With a 3M X 6M warm/store room) I reckon I can make space for that many, not that I would expect to ever see that many piers in place at once, but three would be easy to see with a concrete base near the warm room to plonk a dob on a movement free surface.
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Old 11-02-2020, 02:50 PM
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The function of a concrete pier is to get a footing down below the influence of weather on the ground . So deeper, within reason is always better.

Cantilevering out of the ground to resist lateral live loads like a telescope mount , again , deeper offers a larger resisting arm. Deeper is still better given the same weight on flat ground.

Creating a bell on the bottom would only be useful in soft ground to spread large vertical loads as a little area makes a big difference.

Having said that the cantilever arm can also be horizontal , so widening the top of the pier with a small slab level with the ground is an easy way of adding more overturning resistance and weight in solid ground with less digging. It would also make use of your reinforcing which other wise is simply stopping the pier cracking around your bolt plates.
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Old 11-02-2020, 03:09 PM
Sunfish (Ray)
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Ps ,
Check Marcus pier here:
https://pbase.com/gailmarc/image/169751663

http://www.iceinspace.com.au/forum/s...=178080&page=6

The widened top with a 45 degree edge is very good I think.
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Old 12-02-2020, 09:57 AM
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I probably can’t make a batter and flat top work for my planned obs build. I intend to put in a raised timber floor so the block needs to allow for footings to support the joists. By the time I add another 400mm or so to the top of the block I might be getting a bit wide?

I might just go a little deeper. Some movement over time might be inevitable given about 700 of the 900mm or so is in clay so probably reactive soil. Although once an obs is built over it there should not be much variation in moisture to contend with.
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Old 12-02-2020, 02:02 PM
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clay wetting/drying

Paul, I think you are optimistic about the ob increasing clay stability. I think lateral water flow is quite substantial. My situation is on deep clay.


My pier sits on somewhat more than 1 cubic metre of concrete and the ob on a 3 metre by 3 meter by 100mm slab with plenty of reo. The pier has a 25 mm gap between it and the slab.
Four observations about this set up.
1) When it used to rain here moisture came up the gap and flowed out drainage slots cast in the floor, not much but just detectable.
.
2) due to the above I had to place interlocking plastic grating beneath the foam tiles on the floor


3) The slab and the pier move differently


4) the pier moves by a couple of seconds over a couple of months and I use Sharp Cap to re-align the mount. After a few years the floor had developed a noticeable

slope.



Chris
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Old 12-02-2020, 02:53 PM
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I am planning on adding some drainage around the obs to help combat that sort of issue. More or less a 500 or so deep aggregate and slotted pipe drain around three sides out from the walls to intercept water coming from the high side and keep it away as best I can. A couple of meters on the low side is a dam so the water will end up in there.

Beyond that I will just check PA periodically and adjust as required and shim the pier on the block if needed. I am planning a suspended wooden floor for the obs, so no slab as such to worry about differential movement.
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Old 12-02-2020, 07:55 PM
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Chris has demonstrated the reason for piers in clay. His slab moves around as it is not founded on deep piers but in the top layer. Not a good base for any building you would like to stay level.

Irrespective of the size and weight of the pier, one pier by itself will heave and move around with the soil over time. You can only found it deeper or add a number of piers anchored into rock, but as you say , it is not a bridge.

The idea of drainage well away from the pier may help isolate the top layer of wet soil outside from the inside dry material as long as it is sealed and does not work in reverse and let water deeper into the top layer. 500 sounds pretty deep. For that amount of digging you may as well replace the soil under your observatory with a 200 bed of hardcore which is unaffected by water or fill the slot with concrete and put your observatory on it.
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Old 12-02-2020, 08:40 PM
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500 deep for a drain is just a guess, it might end up shallower but I don't plan to be digging that by hand that is for sure. So long as I get the fall right so that any water entering drains away it should help at least to some degree. As much as anything else I would be aiming to help keep the under floor dry, along with ventilation.

Hopefully the idea pilfered from someone else to drive star pickets well down (Think in terms of a meter driven in) inset from each corner by maybe 250mm and use them to hang the reo off will help to some degree by keying the block into the subsurface, because the only way to avoid clay around here is to hit rock! I have plenty of both. Even driving the pickets may be pot luck between driving in easy and hitting rock after six inches.

Either way, I can only do what I can do and deal with any movement later by monitoring PA and adjusting as required.
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Old 16-02-2020, 08:56 AM
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Turns out the rock I found in one corner extends under almost all of the current hole, so I will get a rotary hammer and bore half a dozen holes in it to pin the block to it with reo. There are plans for more pier blocks, hopefully the rock does not interfere with those! Or maybe it would be good if there was a house sized monster under there to pin them all to.


For the sake of simplicity, unless someone convinces me otherwise I am going to build a reo cage, supported from the top The support I will remove just before it is covered, after there is no chance of the reo moving down and coming out the bottom of the block) and I am going to drill and grout unithread in with concrete epocy for the mounting bolts to save the inevitable chance of the bolts moving around during the pour.

Last edited by The_bluester; 16-02-2020 at 09:59 AM.
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Old 18-02-2020, 07:37 AM
Sunfish (Ray)
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A giant floater increases the depth and weight of your footing. As you only have one pier it is not a bad thing. Chemset anchors are great. Good enough for that bridge if the holes are blown clean. You could chemset a couple of short deformed bars into your floater.
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Old 18-02-2020, 08:50 AM
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I had a couple of lengths of the right diameter galvanised unithread on hand so I used them and will tie off my reo cage to those. I bought a nylon brush and dragged my compressor out to clean the holes properly too. I have no idea if it is a little floater or something the size of the house under there. It extends more or less under the entire hole and I did not push through the bottom of it anywhere when going the full depth of a 250mm masonry bit.

Only question left in my mind is what mpa concrete to order up. I am really hoping to get it poured on Saturday so I can leave it sit for a few weeks to cure well before I go boring holes in it and loading it up. Apart from anything else I don't like leaving holes open even though I am covering it when I am not actively working in it.
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Old 18-02-2020, 11:00 AM
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I always order the highest standard strength as I think you may as well go for a little strength. 20mpa or 25mpa standard and is ok. As long as the concrete does not have too much water added and is properly compacted with a bar when placed. You can keep it covered and wet.

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Originally Posted by The_bluester View Post
I had a couple of lengths of the right diameter galvanised unithread on hand so I used them and will tie off my reo cage to those. I bought a nylon brush and dragged my compressor out to clean the holes properly too. I have no idea if it is a little floater or something the size of the house under there. It extends more or less under the entire hole and I did not push through the bottom of it anywhere when going the full depth of a 250mm masonry bit.

Only question left in my mind is what mpa concrete to order up. I am really hoping to get it poured on Saturday so I can leave it sit for a few weeks to cure well before I go boring holes in it and loading it up. Apart from anything else I don't like leaving holes open even though I am covering it when I am not actively working in it.
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Old 18-02-2020, 12:32 PM
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Yeah, I will work it into the corners with a shovel to make sure it gets out there and also has a decent chance of expelling the air, I just need to buy or find a bit of steel to use as a screed for the final surface. I am bringing it up about 100mm above ground level to give me a bit of freeboard to level and gravel the ground around it.

The plan is still to have multiple piers over time so I will use a string line from the top of this one to make sure they all end up at the same height.
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Old 18-02-2020, 09:40 PM
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Great. Sounds like you will have yourself a solid pier. I only mean that with multiple piers for the same beam , a floater can get in the way and lead to unequal bearing. With a single pier for telescope mount it will be a bonus in any case.
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Old 19-02-2020, 08:48 AM
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I do plan for anything up to five pier blocks (The rest I will hire an excavator to dig, but we lost my young blokes terrier recently to a snake bite and I needed something suitably menial to do to clear my head) but I don't plan for them to be connected to each other, hopefully this rock is as it looks, only under that first hole so I don't end up with any issues by tying half of them to what turns out to be the same floater.
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Old 20-02-2020, 11:18 AM
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No pic update. Concrete pour is booked for Saturday. It will be really nice to have a pier after many years.
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