Footings and ground prep - General advice + questions

[Whitepj]

Hi all. If I could get some advice on a project, I would be really most grateful. It is turning into a headache...

In short, I am planning to add a small retaining wall to our front garden - approx 5m long, and dropping the level of the ground surface by 550mm (although the wall itself will probably be in the region of 750mm high). We are based in North Shropshire, where the subsoil is clay - although having spent time digging out trenches for drainage elsewhere by hand I would not class this ground as the hardest or most heavy ground I have worked.

First question: Do I need to have a structural engineer to work on this with me, for this height? The plan at the moment is to have a brick-and-block construction,with a wall thickness of 300mm at the base, tapering to 215mm at the top.

I'll leave the other questions, and the littany of problems that have occurred so far for another time; suffice to say that the project is already over 18 months old, and it looks like I am going to have to do most of the work myself...

Thanks

Phil

[lutonlagerlout]

I would say even with this relatively low retaining wall the key issues are foundation width ==> wider is better
also a land drain and weep holes to make sure water cannot build up behind the wall
hear a bad story at the weekend about a 30m wall 1.5m high built with concrete blocks laid flat, 3 months old and the whole lot went over,builder is in a world of trouble

cheers LLL

[GB_Groundworks]

The rule of thumb for retaining walls is 2/3rds their height in foundation width

Often see them built on a strip like a garden wall

As tiny has said drainage is really important need to be free draining to stop hydraulic pressure build up

[Whitepj]

Well, that's giving me a little optimism We're finally making progress here, and I was hoping to avoid more delay. I'll post a little more detail then...

Drainage already planned (in fact, that was the first stage of the project) and partially done. 110mm twinwall. Question: This will be wrapped in geotextile - but do I choose woven or non-woven?

Next question: Can I pour the footing mix on to bare ground, or do I have to put a layer of MOT1 down first? If I can work out how to upload a graphic, I'll upload. In the meantime, the current plan is 125mm thick concrete footings 500mm wide (probably fiber reinforced), then 1 course of engineering brick which will define the height of the path on one side (maybe block paving, not decided yet), with block/brick for the rest. (125mm footing thickness chosen because that + brick + 10mm = 200mm, which I gather is the correct thickness for block paving...)

Potential problem: if I need an MOT1 layer, I may have a problem with depth. With the current plan, I am just above a glazed clay drainage pipe. This carries surface water off to a soakaway, and runs pretty much slap bang where I want the wall...

[r896neo]

Firstly never use any kind of hardcore or compactable material under a concrete footing, pour straight onto the sub grade. (unless a specific engineered solution is being employed)

Second, even thought the wall is not big, in my opinion your footing is too thin it should be 150mm min.

As for the fibres, they are more for slabs, in your case some steel mesh or simply lengths of rebar would be much more effective.

As for geotextile you want needle punched non woven, something like terram 1000 or equivalent.


Re-reading your post i am concerned that the footing trench seems very close to the surface?

How deep into virgin ground are you? It reads like your trench is only 125mm deep

Also does the pipe you mention run along the length of the wall?

[seanandruby]

You will have to re-route the pipe if runs pararrel with the footongs, if it goes through the wall then a lintel over it will do.

[Whitepj]

Thanks. Please forgive the daft questions - I am afraid that I am having to DIY with no prior experience and only the web to guide me. I didn't think I needed MOT1 down, but I do like to double check everything before I make a mistake!

Close to the surface? My apologies - I probably was not clear before (how do I upload a diagram?)... Dunno - what defines 'close'? Currently, I am between 300mm - 600mm below original ground level, depending upon which bit of original slope I measure from. I am not yet into 'virgin' subsoil; still excavating odd bits of brick, broken roof tile, and clay pipe (the things you used to stuff tobacco into and smoke). However, I doubt that it has had any part disturbed since the house was originally built though (1930's?)

If the soakaway pipe needs rerouting, then so-be-it. That will add another 120mm to the depth of the footing in part. Again, just fill with concrete? (I am presuming that I remove the old pipe...)

Can anyone recommend a 'local' supplier of "terram 1000 or equivalent"? North Shropshire/South Cheshire/Birmingham/Stoke-on-Trent. It will save me trawling the web...
Alternatively, if someone can sell me a 24m offcut from a standard 100m roll ...?

[GB_Groundworks]

most builders ,merchants will have small roles 2m wide for paths as well as the 4.5x100 as long as its needle punched and non woven you'll be ok

you can get geotextile socks for drains or buy pre wrapped pipe but its 3x normal pipe and only available in places like burdens or keyline civil places

http://www.ebay.co.uk/itm....MKaFzJA

[r896neo]

Dont worry about the questions, you want to plan it right and have what you need to hand to do it properly.

Your trench sounds fine depth wise, just lift out the pipe and any soft disturbed material then fill the lot with concrete.

[digerjones]

Sounds like your in my area. Where are you, maybe whitchurch or drayton at a guess. There's a few places I can recommend

[Whitepj]

Hi Dylan,
Yep - first guess was spot on!

Still on the questions: I am ending up doing most of this because the last guy I hired to come in and do the job slapped a random quantity of concrete down at the wrong height and in the wrong place! (Swore blind that these footings were good enough for a retaining wall...) Consequently, I now have several blocks of concrete I have now lifted after cutting them up - all approx 500x300mm of varying thickness. Can I recycle these in my current footing trench, laying fresh concrete above and below?

[seanandruby]

Not unless you want it to fail. Pour fresh concrete to the correct height. Make sure the finished level is fairly consistent +/- 5mlish it makes it easier for the brickie.

[Whitepj]

Darn! Oh well! I'm OK with the level - if anything slightly OCD. Most of my digging is being done at night with the aid of a laser level. Best 30 quid I spent! (This is because I am daddy daycare during the day, and don;t have any other time free...)

Next daft question: This damn pipe running right under the footing. Why exactly am I removing it?
If it is to allow access to the pipe should it fail / develop a leak, can I not reroute it, and leave the old one in place? If that leaves a problem void, what about just filling the pie with concrete, but leaving it in place?

(I know - I'm potentially being a lazy arse. It is sort of a rhetorical question - but I'm interested in the why's and wherefore's...)

[seanandruby]

Lazy arse, foundations need to go on virgin ground. You need to take the pipe out, the bedding and any lose ground. To fill a pipe with concrete is no easy feat either.

[digerjones]

Hi Dylan,
Yep - first guess was spot on!

Still on the questions: I am ending up doing most of this because the last guy I hired to come in and do the job slapped a random quantity of concrete down at the wrong height and in the wrong place! (Swore blind that these footings were good enough for a retaining wall...) Consequently, I now have several blocks of concrete I have now lifted after cutting them up - all approx 500x300mm of varying thickness. Can I recycle these in my current footing trench, laying fresh concrete above and below?

A.R.Richards at Ternhill have terram and any other aggregates you need. Cheapest around