Do we need an engineer?

[Flyfisher]

Has anybody done this themselves? The trench will be 1 storey deep and about 3 feet wide, and probably 30 feet in length across the 3 sides of the building.

No, but we had an extension built on a previous house and the foundations needed to be two metres, with some of it hand-dug due to access problems. The builders used scaffold boards to shore the sides with bits of 6x2 keeping them apart. Seemed to work OK.

I suppose you could do something similar between the earth pile and your house wall, using scaffold boards to spread any load and horizontal acrow props, because we already know that the house wall is strong enough, but any such support arrangement would, of course, get in the way of the digging.

But I reckon the design and build of the eventual retaining wall is the bigger issue. The house wall is braced at both ends (and perhaps by inner walls as well) whereas you're looking at building a something new and, as we all know, a brick wall is not very strong when subjected to a pushing force.

I've see retaining walls with metal plates that, presumably, attach to metal rods that extend into the material being retained and, again presumably, attached to some big lump of something (concrete?). But, with your house effectively in the way, I can't see how you could drive a long rod into the mound you're trying to retain.

Your earlier mention of those interlocking steel sections driven vertically into the ground is beginning to seem like a good idea - albeit a rather big one. It's what they've used to widen the M25 around Rickmansworth, as I noticed last weekend, but you should see the size of the machines they use to drive them in! :shock:

Perhaps a 'domestic' version of the principle could be implemented with RSJs driven into the ground with a suitable spacing to accept railway sleepers between them. A friendly farmer with a big tractor and post-knocker might be able to drive the RSJs before you start the digging and they alone would provide some stabilisation while you dig out enough space to insert the sleepers (or those reinforced concrete fence panels?).

But I've no idea how to do the structural calcs that would allow me to sleep at night.

 

[Penners]

Has anybody done this themselves? The trench will be 1 storey deep and about 3 feet wide, and probably 30 feet in length across the 3 sides of the building.

I never did it. But I spent enough time flogging Volvo construction machinery to men wot did to know that trenching to that depth is definitely not to be undertaken lightly. The risks are significant.

I yield to no man in my contempt for the worst excesses of the Elfin Safety Police, but in this case I do think you need to think v-e-r-y carefully about whether to undertake the work yourself.

 

[Andrew Bodenham]

The work you are proposing is beyond the scope of a diy project. It is civil engineering. Any such scheme will require professional investigation and design, even if some of the actual work is carried out by yourself.

An excavation to that depth in disturbed (backfilled) ground will probably be highly unstable and would certainly not be self-supporting. Especially if you sit an 8 tonne excavator at the top of the trench. A safe way to carry out such an excavation would be to create a retaining wall first, either with driven interlocking steel piles or with bored and cast in-situ concrete piles, into the undisturbed ground below your final intended level. The material between the pile wall and the house could then be removed. Such a sceme would need a detailed geotechnical survey to determine the soil conditions so that an appropriate design could be developed. This is where you need the services of a civil engineer or structural engineer.

Others have hinted a issues around insurance and safety, and I echo their concerns. Whilst the basic concept of what you want to achieve is straightforward, the technicalities of achieveing a succesful long-term outcome without meeting a disaster along the way are complex.

Given that you want to retain the upper level as parking / vehicle access you might consider other alternatives to your excavation scheme. For example it may be possible to create a de-watering scheme. Essentially this would collect and pump groundwater out of the backfilled area in order to lower the local groundwater level against your wall. The boreholes or excavation required to do this could be at some distance from the house. There may be other alternatives which an engineer or surveyor would be able to suggest.

 

[d49xc]

As a chartered engineer (not civil though) I concur with all the words or warning here: especially the latest about the likely unstable nature of the infil.
I've seen a motorway embankment in process of being built suffer a mega landslip/collapse due to the unstable nature of the rock strata below upon which the embankment was being placed. That required a lot of excavating down and piles to stabalise the sub-strata before the embankment could be rebuilt.

The OP does indeed need the services of a civil engineering firm - who may wish to drill some pilot boreholes to ascertain the nature of the infil and try and determine the "natural" land countours. Any water paths through the infil may have created slip planes in the mass which will make it even more dangerous once it starts to be removed.

 

[another_richard]

Would you accro up the inside too or is that going a bit too far?

 

[FamilyWiggs]

Andrew and D49xx - many thanks for your posts,

i think on reflection it might be better to think about ways in which we could live with the occasional damp and alleviate it during the winter - ie by installing the rads in the lounge so we're warm enough to keep the dining room door open for ventilation. Anyone able to recommend a good dehumidifier that won't break the bank?

There are greater calls on the purse just now, like shoring up the front wall of the outbuildings.

 

[Me!]

The trench will be 1 storey deep and about 3 feet wide, and probably 30 feet in length across the 3 sides of the building.

Just an idea, but what about digging a trench with a JCB about 1 foot wide but about 3 or 4 feet away from the house and to the depth you want. Then lower into it a frame of steel reinforcing bars and fill the trench with concrete. Then once it has set, remove the bit of ground between your new concrete wall and your house wall. You could then face the concrete with local stone to make it look better.

 

[FamilyWiggs]

What a good idea....

 

[Schoolmarm]

Would you get enough "shoring up" lateral force with a wall?I can just imagine it tipping over. I think a lot more lateral force would be needed and quite a wide angled wedge. If you found out the requirements of a retaining wall for that depth perhaps it could be done.

 

[Feltwell]

Just back from holiday and catching up on posts...

I was a Civil Engineer - the caveat being that I said "was", I did my degree and did my time as site engineer before coming to my senses and going off to do something completely different :wink: However I did supervise construction of a good number of retaining walls for road and tram projects in my time, as well as designing a few. I'm far too rusty to do your calcs though!!

Existing building - should be fine by the sound of it, if there is no lateral force from the other side that the area you want to dig is opposing then I don't see any issues.

Digging up against the wall - as others have said, a 360 degree excavator with a knuckle joint just in front of the cab, at the bottom of the arm, is what you need - so rather than the arm pulling the bucket back through the ground towards the cab, it works at (almost) 90 degrees to the cab. We did this for my garage recently - it's not easy to operate the digger with the arm like this though, be warned! We dug to within 12" of the neighbours wall and then dug the last bit out by hand.

The collapse of excavations is something that should NEVER be taken lightly - trench collapses can happen suddenly and without warning, the crush injuries that result can be fatal. Generally you either need to shore the excavation up (you can hire trench support systems) or - and if at all possible I would do this - batter back the sides to a 45 degree angle and backfill afterwards.

The problem with retaining walls is that you have a lateral force which is applying a turning moment effective towards the base of the wall - which is a way of stating the bleedin' obvious that the ground behind the wall is trying to push it over. The bad news is, there is no cheap & easy way to construct a retaining wall of any decent size. This Wikipedia article is actually quite good:-

http://en.wikipedia.org/wiki/Retaining_wall

For heavy Civils work, Interlocking Sheet piles are probably the cheapest method – these are profiled steel sheets, often used as temporary retaining walls on construction sites but they can be left in-situ. They are driven into the ground - which usually means a vibratory hammer, a heavy hydraulic device lifted into place with a crane that clamps to the top of the pile and literally shakes it into the ground. An impact hammer can be used as well. Neither of these are something that I would want that close to my house, they really do make the ground shake.

Bored in-situ piles are a much gentler alternative. Here you auger a large hole in the ground with a drilling rig, put a steel reinforcing cage inside the hole and fill it full of concrete – drill another right next to it and so on, forming a wall. The depth of the pile below your excavation depth is what stops the wall from being pushed over. Cost can depend on ground conditions, it’s cheaper if the ground is self-supporting enough for the pile to be drilled and filled with concrete without collapsing in on itself – otherwise a steel tube needs to be inserted as it is drilled and then withdrawn as the concrete goes in.

Reinforced earth systems come in many different types, basically they usually consist of a front face of wall tied back into the ground with straps that the ground itself retains and stops from being pulled out. Usually these systems are used where you are making an embankment of some kind, i.e. you’re starting off with a flat surface and building up – not trying to retain an existing hill.

Digging a trench and just filling to with concrete won’t work, unless you went far below your house floor level and incorporated a reinforcing cage – it needs to be reinforced concrete, not just mass concrete, unless you really make it a very thick wall where it’s sheer weight will be the factor (which will be expensive on concrete). Where I have built retaining walls going up to bridge abutments, they have been an inverted T shape and constructed from reinforced concrete, poured in-situ. You’ll often see with new bridges that the retaining walls next to them appear to be brick – this is nearly always just for decoration, behind them is a reinforced concrete structure or piles.

These are all systems for “heavy” Civil Engineering works though and are almost certainly over-the-top for what you need. An ordinary masonry wall will be pushed over unless you can buttress it. I bought a house some years ago with a 5’ high retaining wall at the back, holding an extension and patio up. That retaining wall was failing and starting to lean over – I solved the problem by building another new wall 2’ in front of it, and tied it back into the existing wall at regular intervals with brick piers. The brick piers were then buttressing the existing failing wall and preventing it leaning any further – that existing wall stayed in-situ and was buried, so was supporting all the soil behind it. My new facing wall was therefore only having to retain 2’ of new fill material.

You could use the same principle – dig out and batter back, build a cheap blockwork wall behind the line that you want, with blockwork piers extending forwards to a “visible” wall made of a more visually pleasing material, if you will see it. Then backfill. It will mean a lot of digging though. You MUST put weep pipes in – bits of drain pipe going back into the soil behind the structural wall, to stop a build up of water behind the wall. A build up of hydrostatic pressure in this way can cause a sudden and catastrophic failure in very wet weather, it must have a path to drain to.

Alternatively, use Gabions (sometimes called Gabion Baskets) – these are steel mesh cages that you fill with crushed stone, crushed hardcore, whatever so long as it’s inert and heavy – basically they rely on their sheer weight to stop the soil behind them pushing them over. Crude but (relatively) cheap and effective. Again lots of digging out required.

http://www.weld-mesh.com/gabion.htm?gclid=CML047-H4qMCFYZo4wod2CmKZA

Finally, there are lots of proprietary interlocking concrete block systems out there that could also be suitable, such as this one:-

http://www.colinwellconcrete.com/pdf/retainingwall.pdf

Or from timber:-

http://www.phigroup.co.uk/solutions/gravity-retaining-walls/permacrib-gravity-retaining-walls

In fact the PHIgroup website has quite a few different solutions.

Last but not least do you really need a retaining wall? Is improved drainage not a cheaper option that might solve the problem?

Phew - didn't mean the post to be that long!!!

 

[Schoolmarm]

Great post. Loads of interesting ideas there.

 

[Feltwell]

Do let us know what you decide to do in the end Robin.

 

[FamilyWiggs]

Still awaiting the visit of t'engineer.

Think it more likely we do the outbuildings and learn to live with the occasional bit of damp in the dining room for now. There's a bit more I could do with rainwater goods and ensure the gap between concrete yard/paths and the walls is filled.

I'll update the thread as we progress.

 

[Penners]

Great post. Loads of interesting ideas there.

I quite agree. Lots of ideas, but all with the same bottom line - to do the job that Robin wants, safely and securely, is going to be very expensive.

As you've said, Robin, I think I'd be settling for a bit of damp.

 

[Feltwell]

I quite agree. Lots of ideas, but all with the same bottom line - to do the job that Robin wants, safely and securely, is going to be very expensive.

Too true. As well as rainwater goods and the gap between the yard and wall, I'd also look at a installing a French drain or similar - if you get a contractor in next summer when the ground is (hopefully!) dry and therefore more stable, you could potentially dig a trench out, lay a perforated drainage pipe and backfill with shingle all in the same day - do it quick and you may not need to support the trench, or if you do need to send anyone into the trench it can be temporarily supported (which a contractor should have the kit for).

I'd get a groundworks contractor in to do this rather than trying to do it yourself - speed will be of the essence and a knowledge of what the ground conditions are like will be vital, an experienced machine driver knows when to stop! Your engineer should be able to advise on if this is possible, I couldn't say without visiting your house.

 

[Alex]

I was watching Grand Designs last night and thought of you or rather your engineering problem. I don't know it you saw it but they used gabions, those steel cages filled with stones, as a retaining wall for their garden. I have seen these used as retaining walls on other building programmes (definetly another Grand Designs episode where the planners would only allow the house to be built if it could not be seen from a nearby road) including using 'pretty' stones on the front face and all sorts of stuff including concrete as infill as a way of getting rid of stuff that would probably go to landfill anyway and also saving money on the pretty stones that would be seen. I don't know if this would be of any use in your situation, maybe it would be worth having a look at the Grand Designs website and see if there is any information on there that could be useful to you, you never know.

Alex

Edit. Just been on the Grand Designs website and the other episode was called Amersham, The Water Tower. Photo 17 of the project clearly shows a high retaining wall, made of gabions, keeping the garden from falling into the house.

 

[LadyArowana]

I believe that this house was featured on GD and that during the groundworks they had a problem with a wall and the neighbours garden. It is for sale now http://wildoakresidential.co.uk/cecil_house although they don't mention the show in the particulars. Strange co-incidence as I was discussing it with a friend today as she was watching the episode on a cable channel.

 

[Alex]

LadyArowana, sorry but we must be thinking about different episodes. Both last nights episode and the one that I mentioned in my edit didn't seem to have any neighbours close by but I think that we crossed posts.

Alex

 

[Flyfisher]

I'd certainly want a lot more land and fewer neighbours for over £2m+ :shock:

And I think I've mentioned this before, but where do people keep all their 'stuff' in places like that?

 

[Alex]

Flyfisher, you build an underground bunker in the garden and keep it in there
Have you noticed that all these houses seem to echo, echo, echo ?