Footings dug out - flooded, now a moat!

[jat147]

Howdy,
I'm replacing a retaining wall in my garden with a bigger one - the garden above is sloping towards it. I dug down half a metre and stopped when the soil became wet, ready to pour contrete onto a hardcore base.

The next day 2 inches of water were at the bottom of the trench, and has steadily filled since. It's now right back up to the original level with all the rain that hasn't stopped since I dug it out.

The soil was full of clay to start with and now seems unlikely that it soak away, at least half a metre of it. How can I ever hope to pour contrete footings now?
Any ideas??

 

[RedHerring2]

1. 500mm is not very deep, even for a garden wall. How high, long and thick is your wall going to be?

2. You don't need hardcore under your footings. Just go down deep enough. Perhaps others more qualified and experienced will advise.

3. Typical problem at this time of the year. Just dig a sump (a depression in the bottom of the trench, at least 250mm deep and at least 300 X 300mm) so that you can sit a pump in there to pump out the water.
Do it daily and on the morning before the concrete is poured. If the sump is still very wet before the concrete goes in dump a bag or two of cement and some half bricks in to fill up the sump.

I have also sprinkled a bag or two of cement along the bottom of the trench before now to 'take up' the bit of water remaining in the bottom.

Also, if it's clay, as you say, avoid walking along the bottom, you'll just soften the clay even more along the bottom.

A BCO will probably ask you to scrape the bottom dry'ish before pouring, especially for building footings.

 

[jat147]

Thanks.
A friend of the wife has a builder husband, he advised to go 600 deep.
I only got to about 500 when the water started to appear, so stopped.

The wall will be about 1500 high, made with 7N blocks layed on their sides - as directed by the friend. Length of wall will be about 4m.
Cheers

 

[r896neo]

Is this wall near any buildings or boundaries? A retaining wall at that height really needs designed if it is. If it's not then you can get away without the engineer but frankly a simple 9 inch block wall is not really adequate.

Also don't put hardcore in your footings.

What are you doing about drainage?

 

[jat147]

The wall is seperated from the back of the house by a patio, and
is about 4-5m away.
There was no drain there originally, so I'm planning on fitting a drainage channel at the edge of the patio.

It is a 50's semi - but i cant think why no drain was put in to start with?

 

[Nige F]

The wallIt is a 50's semi - but i cant think why no drain was put in to start with?

maybe because it was just fields around when it was built - or if part of a coeval development- they just didn`t bother

 

[r896neo]

Err i was talking about the absolutely essential drainage behind the retaining wall.

Are you sure you are on top of this project? How wide are your proposed foundations and what wall structure are you planning?

 

[jat147]

On the soil-side of the wall? ...yeah i knew that...

This is where i expose my ignorance, what are the pipes called that are found along the base (at intervals) of such a wall to let water drain thru?
Is that the drainage that is needed?

The foundations are about 500 wide, the wall should be 1500ish above ground. There was a wall there when we moved in, but was just a dry stone wall. I've removed that and dug backwards about 4m to give enough room for a little patio in front of the new wall.
thanks again.

 

[DIWHY]

Weep holes, plastic/copper pipe, at the bottom, into a layer of open structured hardcore/gravel approx 1 metre spacing, fabric over ends stops them clogging.
Advised to tank wall above weep hole level with plastic sheet.
If youre flagging the top of this new area rather than grass then most of the surface water should be drained away to gulleys anyway.
Or you could let the patio fall to the edge and run the water to spitters (gargoles my favourite) over the edge, maybe not strictly to b regs but it works and looks nice.

 

[wavetrain]

How to build retaining walls is covered in "Building Construction Handbook". IMO your design would soon start to lean from the weight of earth against one side. I would not consider digging footings at this time of year. Read the above book and start up the project again in April. The above book also covers the subject of draining ground water during construction and width of footings needed for a given load.

 

[r896neo]

You really need a wider footing for a retaining wall that size. 9 inch blockwork is not up to the job and if you don't want piers to the front i would advise stepping the wall out at the back so the base is say 27 inches for the first 2 feet, then step into 18 for the next 2 then finish off in 9 inch.

Maybe wait for one of the engineer types on here to weigh in too as this is just a suggestion.

Sorry forgot to add, the most important detail of drainage. You need to either put a land drain/ french drain behind the wall and take it to somewhere or incorperate a planned arrangement of weep holes. To avoid staining and efflouresence due to constant water saturation of the wall it would be best to cover the back in dpm. Liquid or just normal polythene sheet.

 

[dipwing]

Howdy,

The next day 2 inches of water were at the bottom of the trench, and has steadily filled since. It's now right back up to the original level with all the rain that hasn't stopped since I dug it out.

The soil was full of clay to start with and now seems unlikely that it soak away, at least half a metre of it. How can I ever hope to pour contrete footings now?
Any ideas??

I had the same problem with the footings for my house extension. I needed to dig down 1000m below ground level. Local BCO wanted clean sides and base to the trench. Ended up buying a pump from Screwfix, draining it out and then digging out the sludge.
Ended up 1200mm deep.
At this point the clay went from the normal orange colour to a grey, more dry texture.
BCO turned up to inspect, said "why'd you go that deep. Get it filled asap!"

All that work for less than 5 mins site visit.

Nightmare of a job but done now and on with the floor.

 

[jat147]

Thanks very much for those tips
I'm grateful for any little pointers, my enthusiasm
often outweighs my ability

 

[wavetrain]

It's not uncommon for first-timers to digging footings to become very alarmed when their trenches fill up with water and you may hear the phrase: "Oh youve hit the water table mate". But all they have done is to dig a drainage channel that allows the surrounding ground to drain its mosture into the channel. Of course, there are some dryer winters in the UK where if you are quick, 2 metre depth footings can be dug in London clay and they stray dry for a while, but we have had a lot of rain this winter.

Mind you, think of digging footings over in Melbourne right now, now that would need a very powerful pump.

 

[Deluks]

Mind you, think of digging footings over in Melbourne right now, now that would need a very powerful pump.

They'd probably go for a 'raft' foundation.