Pier Footings in Wet Clay Soil - Question

[ArmyAirForce]

I'm in the middle of digging to Australia, for the footings of my pier. After the six to eight inches of hardcore under the patio, I'm down to the garden's heavy clay soil, which is very wet and soft, and stays that way for months ( one of the reasons we re-modelled the garden five years ago ).

I know I can't dig past the clay, it's about 9 feet deep, so would a wider footing be better than deeper?

I was also thinking of driving several re-bars diagonally down into the clay in the bottom of the hole, deep into the surrounding ground to help spread the loads further, and give the footings more leverage.

Thoughts???

[Davey-T]

I think wet clay is actually pretty stable, I only dug a 2 feet by 2 feet hole for my pier in my similar soil.
If you're imaging you don't want to be wandering round the pier anyway.

Dave

[ArmyAirForce]

It's 24 x 24 x 14 inches at present, so by the time I've cleaned up the bottom and added some re-bars down into the clay, it should be ok.

 

[Davey-T]

IMHO digging a huge whole and filling it with tons of concrete is a waste of effort and money, others swear by it

Dave

[ArmyAirForce]

Being so soft, it's really easy to dig, but once it is stuck the the trowel, it is very reluctant to let go!!!

[Davey-T]

Tell me about it
I dug a six foot deep 3000gal pond, once I prized out a shovel it was a case of throwing the whole thing out of the hole for my son to scrape it off into the skip, took months and every time it rained it got a foot of water in the bottom that had to be pumped out.

Hope the fish appreciated it

Dave

[Daniel-K]

30 minutes ago, Davey-T said:

I think wet clay is actually pretty stable, I only dug a 2 feet by 2 feet hole for my pier in my similar soil.
If you're imaging you don't want to be wandering round the pier anyway.

Dave

 

me too dave. i tied mine in with some rebar and it was solid. 

[ArmyAirForce]

I thought I was done with hard digging after we finished the garden in 2011. It started out just a huge slope of clay. We terraced it when we got the house in 1999, but the ground and lawns were always wet and couldn't be used, so we remodelled it in 2011 and put artificial grass down. Just in 2011, we must have moved 80 tons of soil and rubbish out, and hardcore, bricks and sand in! I'd hate to think how much we moved around in 1999 - mind you, I was a lot younger and fitter then!

[buzz]

I'm in the same situation, with London clay in Essex, in a wet corner of the garden. I have a 24x24 x 18" concrete block. It is in an observatory and I have guttered the roof and run it into a water butt. The ground is slowly drying out as far as I can tell. If there is any instability, an unbalanced system will make matters worse. I levelled my pier and mount carefully and ran a T-Point model. The results indicated it was stable, though I do expect to re-run a 'recal' every few months in the case of ground movement.

It could be worse, we could be on a fault line, my mate in NZ wants to do astronomy but it is virtually impossible with the continual shocks.

[Gina]

Clay here down a foot or so.  My pier base is about 27" square by about 20" deep and it's really solid - never a problem with vibration or movement.

[kirkster501]

Hammer some rebar into that hole diagonally downwards 2 or 3 feet on each side.  Plus three or four pieces straight down into the bottom.  It will be as solid as a rock when you pour your concrete into it.

People over-engineer their peers IMO.  I can understand why, because one does not want to be excavating concrete again!  But there's no need to overdo the size of the hole.

[ArmyAirForce]

I am, I picked some up late this afternoon.

[chouet]

My garden is solid clay and changes from boggy to completely dry throughout the year. I put a 2 foot cube of concrete in as the pier base and I haven't seen any movement in 3 years.

[Oddsocks]

We're on soft, old river bed material here, left over after the Dorset Avon spent millions of years meandering over the flood plains.

The old pier footings for my ex Skyshed Pod were 0.8m x 0.8m x 0.6m and I thought that would be enough but there is an old roadworks filled-in trench crossing the road 50 metres from my house, the trench infill has become sunken by a few inches. If I am set up and guiding the telescope completely loses the guide star when the night bus races up the lane and crashes over the old trench.

Been like it for years now and it does't seem to be a priority for the local council to fix.

Dug a bigger hole for the pier footings for the new observatory, now 1.0m x 1.0m x 1.0m with four pieces of galvanised angle iron 4 m long bashed straight down into the ground. I doubt it will make much difference though, the shock wave from heavy passing traffic being transversal will tend to sway the block from side to side, heck, it even makes the wall hung pictures in the house move!

[ArmyAirForce]

It's all done today! For what didn't appear to be a big hole, it took almost 300Kg of concrete for that and the pier.

[KY Cygni]

i dug my pier this weekend same soil type, I hammered some cut off scaffold tube into the bottom of the hole as pilings, my pier is a massive 7x7 inch gate post so I went pretty big with the cement and hardcore 

 

[chiappa]

On 03/03/2016 at 03:55, kirkster501 said:

Hammer some rebar into that hole diagonally downwards 2 or 3 feet on each side.  Plus three or four pieces straight down into the bottom.  It will be as solid as a rock when you pour your concrete into it.

People over-engineer their peers IMO.  I can understand why, because one does not want to be excavating concrete again!  But there's no need to overdo the size of the hole.

That sounds like an interesting solution!

Does it mean that rebars can be used to reduce the negative effects of clay soil expansion?

I wonder why I haven't seen this mentioned elsewhere. Looks like rebars are often just used to enforce the concrete pier itself.

Is there any reference to this or where can I find more information? TY.