Cellar dig out with underpinning

[Zatoichi]

This stuff is big, scary and expensive.

I'm reading up lots and thought I might as well ask here for any knowledge sloshing around.

Uk building regs, structural engineers and buckets of cash.

Any wisdom to share?

[Bagnaj97]

ik9000's probably your man, I'm sure he'll be along soonish with more information than you can possibly digest!

[ik9000]

This stuff is big, scary and expensive.

I'm reading up lots and thought I might as well ask here for any knowledge sloshing around.

Uk building regs, structural engineers and buckets of cash.

Any wisdom to share?

Sorry, I don't understand are you wanting to build one? Are you saying you have buckets of cash? If so don't mention that to a contractor ever. They'll happily play crazy taxi with you if you catch my drift. Or do you mean it costs a lot? Yes, yes it does. At least 3 times more expensive to go down into the ground than build up from it. And far more contentious planning-wise these days depending on where you live. The quality of design and the quality of contractor is absolutely vital. Skimp and you WILL regret it. Either in leaks and cracking, or potentially like this poor lady. And that is just one I could direct you to, it is not a one-off.

https://www.dailymail.co.uk/femail/a...rt-ordeal.html

shortly before building work began, she asked Knott if there was a risk that digging into the basement would lead to the house falling down.

'He told me that never happens,' she recalls.

Be very very sure you are appointing properly qualified people at every stage. And that costs I'm afraid. But is it worth bankrupting yourself over trying to save a few quid?

Note the classic errors:
1) not notifying your insurers of work in advance
2) appointing a builder who was actually more of a plumber by background to do complicated construction work
3) believing said builder when he dismisses concerns with a gung-ho "it never happens" attitude. WALK AWAY
4) a lack of proper temporary works design, nor construction sequence. No proper methodology to follow on site. (Which is ultimately why that collapse happened - as ruled on in court)
5) though the article doesn't say it - NOT APPOINTING THE STRUCTURAL ENGINEER TO CONTINUE SERVICES AND MONITOR WORKS DURING CONSTRUCTION.

The latter became a landmark ruling for structural engineering in this country based on this case. The judge confirmed in case law that temporary works were the contractors responsibility, and that, unless specifically appointed, a structural engineer is not automatically expected to have an over-arching supervising, or even monitoring, role during construction.

So the quality of contractor really really is key. Never accept the cheapest tender. EVER. And if there is too much variation in tender prices, stop, go back out to market. Also employ a QS to tell you in advance what it should really cost you, so you know when someone is being cavalier.

Assuming you get the big hole built successfully then you have to deal with water. It always wins. Don't believe any company who tell you otherwise. If you want to live in the space it is best to go for a cavity drained solution. Yes, it loses you some internal area. Yes, it requires sumps and more drainage, probably pumped. Yes, it is expensive. But you will have a space that is actually nice to use. And when the leaks happen - and they will - you are not having to redecorate, move out, get more builders in etc etc. "But, but, what about the warranty they provide on that lovely membrane tanking system that is so cheap and thin," you say? It covers repair of the membrane only. Not your finishes and liner walls. Not your inconvenience, or redecoration, or having to move out for 3 months while they do the work. Not compensation for further noise and disturbance to you or your neighbours. Nor drying out/replacing the floor which it seeped under for 6 months before the staining rose high enough up the wall to be noticed. etc etc

Edit, one last watch it - buoyancy. Sinking things into the ground can allow them to start to float with ground water. There have been cases of basement buildings literally rising from the ground. I kid you not. Basements rising out of the ground by over 1m, and it would have kept going too. Worse it was doing it at slope because one bit had less weight than the other bit. As a neutral observer I wish they'd left it so we could have seen how bad it got, but I imagine that would be considered dangerous in an urban area and not appropriate, even in the name of science. That and the owner probably objected to living in a listing building. TLDR - employ properly qualified engineers and architects to do your design. Any idiot can dig a hole. But you want to know they're doing it right before the spades start flying on site.

[ik9000]

some better images in this report: https://www.dailymail.co.uk/news/art...ly-labour.html

[ik9000]

oh and one last thing - the article doesn't seem to mention it at all, but that work should have been notifiable under the Party Wall Act. Don't do anything without serving notice to all affected parties under the act for the appropriate time, and appointing Party Wall surveyors accordingly. NB the person doing the work pays for all of that. Their surveyors and the adjoining owners' along with any external engineers' reviews. Again, not cheap.

[ik9000]

ik9000's probably your man, I'm sure he'll be along soonish with more information than you can possibly digest!

[Zatoichi]

Cool. Good words.

Yes I have a 5 foot cellar that will be converted and, No, I dont have buckets of cash.
I'm trying to rough out the job before getting finance.

I've been researching 6 months + so insomuch as I can glean from building regs and other case studies;
I know how much materials are needed.
I know the sequence of events

What I don't know is the exact specification of the new foundation (I know the range of possibilities though) and I'm approaching structural engineers to get that sorted.

Insurance companies was a thing that completely passed me by.
Paying for the neighbours to get their own structural survey - bugger.

Its a terrace and our neighbours on one side already have a basement dug out that was done 15 years ago. The other neighbours are nervous so I want them to be happy.

(I must be tired, I can't find the thanks button)

[ik9000]

Cool. Good words.

Yes I have a 5 foot cellar that will be converted and, No, I dont have buckets of cash.
I'm trying to rough out the job before getting finance.

I've been researching 6 months + so insomuch as I can glean from building regs and other case studies;
I know how much materials are needed.
I know the sequence of events

What I don't know is the exact specification of the new foundation (I know the range of possibilities though) and I'm approaching structural engineers to get that sorted.

Insurance companies was a thing that completely passed me by.
Paying for the neighbours to get their own structural survey - bugger.

Its a terrace and our neighbours on one side already have a basement dug out that was done 15 years ago. The other neighbours are nervous so I want them to be happy.

(I must be tired, I can't find the thanks button)

By 5ft I presume you mean the undercroft level? If it's only 5ft across, don't bother. the cost won't be worth it, and the space will be rubbish.
Lowering by a small amount is just as complicated as lowering by a large amount if the walls need underpinning as you say, but if the foundations are deep enough already you might get a cheap win. The likelihood is you'll need to alter the existing foundations or loose internal space due to projecting footings however. Impossible to say without knowing the full details and doing a few trial pits etc. Your engineer will talk you through it. Oh and watch out if you plan to use your neighbour's wall that they built - under the PW act that might be deemed "benefiting from their work" and you could need to pay a share of their construction cost. A party wall surveyor can advise further but better to be aware up-front.

Key things:
1) try https://www.findanengineer.com/ to find a chartered engineer in your area
2) Planning regs, building regs see your local authority website. Planning must be done by them, but you have the option of using an approved inspector for the BRegs. Planning is needed before work starts, BRegs is more flexible, but for basements the sooner the better.
3) Party Wall (see https://www.gov.uk/party-walls-building-works and https://www.partywalls.org.uk/
4) CDM 2015 (see HSE website, and find the guide for domestic clients) it places duties on domestic clients.
http://www.hse.gov.uk/construction/areyou/client.htm,
http://www.hse.gov.uk/construction/c...ic-clients.htm,
http://www.hse.gov.uk/pubns/indg411.htm

That's probably the key starting list, and with that over to your engineer!

[Zatoichi]

It's a Victorian terrace .
2 main spaces. 15ft square and 15x12.

Thanks for the links.

The research continues

[ik9000]

It's a Victorian terrace .
2 main spaces. 15ft square and 15x12.

Thanks for the links.

The research continues

If it's the typical masonry and timber Victorian terrace townhouse they tend to have shallow spread footings ie brick corbels stepping out to form a wider base. These are usually sat on a band of some very lean lime-crete (calling it concrete isn't quite right) presumably to give a firm level surface for the brick layer to work from.

Quite often the corbels are not very far below the ground level (or cellar level where one is present). So any reduced dig is likely to reveal the corbelling. This then leaves you with typically 150mm-450mm (depending on building height and ground condition) of projecting brickwork out from the face of the wall to deal with on all sides.

The load spread from the foundation is best shown as a series of bulbs in section, but as the image shows for ease the convention is to take a 45 degree line from the edge of the foundation (eg as referred to in the Party Wall Act).



Cutting beyond that line compromises the footing. As would breaking off the corbel. So you either (and assuming no unusual lateral loading or retaining behaviour of the existing wall element)
1) have to maintain that 45 degree line and batter the ground surface, or bench it so you have a perimeter step all round the room. That can save costs of underpinning etc, and make PW awards much easier (you still need one - work within 3m is notifiable under the act). But it's probably not what you're after, which leaves (as you've already said)....
2) underpin the wall to a lower depth, and create a footing of the same net width lower down before breaking off the corbel. The complication involved varies with the ground condition, type of soil (granular or cohesive eg clay) and whether the ground improves with depth or has weak bands within it. Always get a geotechnical site investigation done to confirm this. You do not want to be the person who underpins down to a squidgy layer. (And it's been done. Not clever.)

Depending on the depth of underpinning you have to consider the dig-out sequence and maintaing lateral stability of the wall, ie resisting the lateral ground pressure. Propping may be needed and shores, waling beams etc. That was ultimately the cause of problems in the article above. Insufficient propping allowing the ground to push the bottom of the wall inwards, and if the base goes down and in, the wall goes down and out - just like you can see in the photos above. Just because ground doesn't collapse immediately does not mean it is stable - cohesive soils (and even some non-cohesive ones in the right condition) will sit quite happily for a few hours, days, sometimes weeks before.... ! Even sands can sit for half an hour before the inevitable. Just long enough for a man to crawl into the hole and get suffocated to death: https://www.ioshmagazine.com/article...ation-collapse Big blocks of concrete will slide quite happily under ground pressure. The forces are larger than you'd think.

Mulling on this in my insomnia last night, if no-one has mentioned it to you yet there's the classic "where are the drains?" conundrum to remember too. Best to get an asset search from the water company and appoint someone to do a thorough CCTV and mapping survey ie following every route from every manhole on your property back to their origin and also tracking the outflow to the mains sewer. This is to make sure your neighbours don't have anything linking in that might be under your house.

Terraces tend to have drainage that goes under the house from rear to front since the roads tend to hold the mains sewers. Sometimes this is done at every house, but sometimes they pick every x houses and take everything together under that one. Sometimes they even shoot the rear drainage off down the garden and under the opposite buildings (especially on hills). Awkward if it comes where you want to dig. Especially if it's shared drainage, which is now classed as an adoptable (i.e. public) sewer and under the ownership of the water authority. So then you'd need licence to divert and/or build-over agreements. Those can take ages to get approved, and again dent the wallet. And with a terrace it's not like you can divert sidewards around the house, so it's either lose some internal space and pop it against the wall, or else go deeper and pump it back up (if the run can't be taken all the way to the main sewer by gravity). Which for shared drainage would be resisted by the water company, and is not very wise for foul/combined drains in a domestic situation.

That said, if you only want to go down a small way you may find the drain is low enough that you can bridge over it with a suitable concrete slab - fine so long as it's not shared you can do what you like. If shared there's a good prospect that the water company would accept this, but they would want some details to show why it's ok and how the pipe is definitely protected. Either way you need to ensure the foundations bridge the pipe and protect it vs load it and crack it either via pressure or inducing settlement that dips the pipe.

It all comes down to getting a good level of site investigation done. And then a good level of design. You always end up paying for it in the end whether you have one or not, so just bite the bullet and pay for a good one up-front and minimise the likelihood of the delays and costs later on that so often occur when the gamble didn't pay off.

[ik9000]

I should probably add I'm not trying to put you off getting the work done. Sorry if it all sounds a bit negative. Doing a basement is entirely reasonable provided it's well thought out and done correctly.

Sadly it's all too common to see designs (eg as party wall submissions) from people - including engineers who should know better - who clearly haven't thought it through and are proposing (or worse have started on site) things that are ultimately going to damage their, and their neighbours' buildings. Some are even downright dangerous to the point of needing external intervention from the powers that be.

I always think it is far better to inform early-on of the watch-its and pitfalls so you have the opportunity to ensure your designer has addressed them before you start the costly process of appointing a builder or sending designs for your neighbours' review. There is nothing like a protracted party wall review to get one neighbour arguing with another over spiralling costs and perceived obstructiveness, when actually the surveyors and their engineers are legally obliged to act impartially and for the good of the wall i.e. to make sure the technical details are correct and safe. And until that is so, the award doesn't get signed.

Edit: Semi-signing off for a while now as I'll only have intermittent access for the next few weeks. Don't be offended if I don't reply punctually. The best thing to do is appoint an engineer, and consider also an architect to do the wider building regs and waterproofing details etc. NB insulation requirements and U-value targets, flood risk assessments and the like. (what you can do for planning depends on the flood risk).

Best of luck, and I look forward to hearing how things progress.

[Bagnaj97]

Definitely a compliment and with 7 posts in this thread so far, some of them very substantial, I wasn't wrong!

[Zatoichi]

ik9000, dude.

Thanks!

I'm at the ground assessment stage.

The cellar is 5ft of corbels sat on dirt (probably compacted by horses in 1890)
The street is a hill (we're at the bottom so our drains are through the back gardens)
The neighbours on the other side already have a dug out cellar (although 15 years ago)


So I need to assess the sub soil beneath what ever the Victorians did as well as the extent of the neighbours underpinning.


Thermal regs are a pain in the arse. I need to dig out soil to fill it up with what they need.

I think last time i ran the calcs I'm looking at 25m3 of excavation. (+20% for fluffing up after being dug)
There's one structural wall that was knocked through on the ground floor and a loft room to add to the forces involved. I'm very aware of the need to get this right.
https://imgur.com/rDQiSPK

Not taking your comments as negative, I want to do this right.

I'm very Johnny 5 at the moment.

[Zak33]

man... this scares me too much to condsider myself. I'd be looking at moving home tbh

[Zatoichi]

man... this scares me too much to condsider myself. I'd be looking at moving home tbh

It excites me.

[OHLDavies]

Potentially a longshot given the time that's elapsed but did you end up going ahead with this Zatoichi? I'm in a similar 'scoping out' position and am curious how your project went...