concrete pier sizing

[aidank]

my brother wants to hang gates for this driveway, to keep his two toddlers in off the road. - Nothing too problematic there, we will be making the gates ourselves. Will be using farm gates for now I think, however in the future we will probably fabricate nice gates. Gates will each be 10ft long and ill guess 150kg weight.

Its a new driveway, and there is a good 3ft of well compacted backfill bring the driveway up to the same level as the road. The field through which the driveway passes was orignally 3ft under the level of the road. Topsoil was removed first before the backfilling started. A lot of lorries gone in and out over the last 3years have looked after the compaction.

I guess to sink a pier for such a gate the foundation needs to be put into the original field to a depth of at least 1m even 4ft.

Thus the length of a prefabricated concrete pier will need to be as follows. 1m even 4ft in the ground, + 3ft of pier to offset for all the backfill and then nearly 5ft for the gate itself. 4+3+5=12ft in total.

Anyone know of a concrete calculator where I could estimate what size of a pier would be necessary. 12inch by 12inch maybe and a lot of reinforcement in the pier. ? guessing.

 

[Agrismith]

A seven foot deep hole is going to be quite a mission to dig. Hand digging it will not be fun and diggers do not like digging small diameter deep holes. Rather than going down into virgin ground just dig down until you hit virgin ground and make the foundation a bigger diameter so the weight of concrete supports the gate.
There isn't a calculator for reinforced concrete as such. that would be a job for an engineer. Is there any reason why you would prefer concrete over an rsj. You can still clad the the rsj to match whatever aesthetic your going for.
It will be cheaper and considerably easier. For a correctly built reinforced concrete column you will have to have the reinforcing pre bent to do a good job. which will be expensive for a small order. You will then have to get the concrete in the top as a concrete lorry won't discharge 5ft in the air. And if you mix yourself then you still have to get it up there. The dimensions you list is already 1m3 of concrete. So near enough £200 if you pay for a part load on a lorry and about the same for bulk bags and cement from builders merchants. Plus reinforcing and shuttering and time. A universal beam to do the job will be around £100 plus half the amount of concrete. Could probably do 1 bulk bag ballast and a few cement. no shuttering or reinforcing to pay for and a quicker installation.

 

[Erie Fred]

I thought they make pre-cast ones.
My old books from the UK precast association (or whatever) showed a few different styles.

 

[Agrismith]

I thought they make pre-cast ones.
My old books from the UK precast association (or whatever) showed a few different styles.

I've just googled and it appears they do. I have never seen one. They appear to be a product in dry stone wall country. Although they might be a bit short for OP as they are only designated for a standard field gate. Could possibly drill and install rebar into the bottom and use a mass poor foundation to give the extra strength to hold a heavier gate.

 

[Agroshield]

A lot of lorries gone in and out over the last 3 years have looked after the compaction.

Lorries will only compact the ground where they touch the ground, which is generally two lines, one on the left and one on the right. That is not where a gatepost will be.

The post cannot close its eyes when a 100kg person swings on the end of the 150kg gate.

A lot of reinforcement in the pier will make post-fixing a hinge difficult, so cast-in fixings accurately located may be an alternative.

 

[aidank]

A seven foot deep hole is going to be quite a mission to dig. Hand digging it will not be fun and diggers do not like digging small diameter deep holes. Rather than going down into virgin ground just dig down until you hit virgin ground and make the foundation a bigger diameter so the weight of concrete supports the gate.
There isn't a calculator for reinforced concrete as such. that would be a job for an engineer. Is there any reason why you would prefer concrete over an rsj. You can still clad the the rsj to match whatever aesthetic your going for.
It will be cheaper and considerably easier. For a correctly built reinforced concrete column you will have to have the reinforcing pre bent to do a good job. which will be expensive for a small order. You will then have to get the concrete in the top as a concrete lorry won't discharge 5ft in the air. And if you mix yourself then you still have to get it up there. The dimensions you list is already 1m3 of concrete. So near enough £200 if you pay for a part load on a lorry and about the same for bulk bags and cement from builders merchants. Plus reinforcing and shuttering and time. A universal beam to do the job will be around £100 plus half the amount of concrete. Could probably do 1 bulk bag ballast and a few cement. no shuttering or reinforcing to pay for and a quicker installation.

We would be making the piers precast down on the ground first using our mixer. Loader to lift and digger dig the hole.

We are handy at bending the reinforcement bars.

 

[Redsailor]

Think I would dig a suitably size foundation hole, concrete fill to ground level with possibly cast in situ anchorage studs. Then gate post would be a heavy wall box section with welded gate fittings.

Think of how you would install a two post car lift, that has to bear heavy side loadings.

 

[Islander71]

Putting a wheel on the end of the gate - spring loaded if the ground isn't perfectly flat - would greatly reduce the strain on the post. I've done this on a wooden driveway gate, which otherwise often sag and have to be lifted to open.

 

[Turbo]

You will have to clad the field gates in mesh as toddlers will climb through the gate.

 

[Gareth J]

We would be making the piers precast down on the ground first using our mixer. Loader to lift and digger dig the hole.

We are handy at bending the reinforcement bars.

Concrete railway sleepers?

They don't need to be down into virgin ground, that's really deep for you.

You'd probably get away with digging the holes a sensible (3-4') depth and tamping the removed earth back in around with a bar. While the earth around the hole may not be that well compacted, a well compacted mass around the post will behave well. Or just concrete the posts in, the extra weight will provide ballast.

 

[Agrismith]

We would be making the piers precast down on the ground first using our mixer. Loader to lift and digger dig the hole.

We are handy at bending the reinforcement bars.

Im not quite sure I see the advantage of making them like this. You won't get the finish on them of precast from a factory. More cost than an rsj. And no strength advantage of cast in situ.
And even with a digger they do not like digging deep small diameter holes. Going as deep as you suggest you will be lucky to dig much less than 1m in one direction by the width of the bucket. And you will end up with a tapered hole which isn't ideal for the stability. Vertical sides are stronger.
The reinforcing will have to be quite accurate as well. its a bit more than bending some rebar to get the links right as they are only going to be 200mm square.

 

[Turbo]

A prestressed concrete post will be many times stronger than one you cast yourself.

But I would suggest using a galvanised steel post for you gate. You can buy them at a good agri merchant, made for hanging gates. There are brackets made for the posts for your gate.

I would use a post hole borer on a mini digger to dig your hole. You could hire one if your digger is piped for aux hydraulics.

 

[jmp49]

I cheated, excavated out about a metre down by whatever the smallest jcb bucket I had at the time was (probably huge, its been a few years), rsj sunk in and bars inserted in a x pattern horizontally and ran mains power cable in trunking then backfilled with concrete and stone rubble, and then we dropped nice looking precast hollow pillar blocks around it at ground level up, and a bit more rebar in a box formation bought in, then hand filled and rammed it down. I built a dry stone wall either side and put in rebar ties to the pillar blocks for good measure when I was doing it.
Havent moved in about 10 years. So glad I put the power in too. Gate openers can be put on later easy enough that way.

Just curious about the sizing you specify as that reads as 20ft wide of total gate opening with the two gates open together. That's rolling gate territory normally not a pair of fanfold gates and you could probably drive a smaller bus sideways through that size opening...