Digging out... and retaining walls.

[FamilyWiggs]

Been a while since I posted... and I've dropped down to 9th most prolific poster on the site... :shock:

A recap. We live in a 1650s-ish Welsh stone farmhouse, which had had the usual mistreatment which we are slowly rectifying. Currently in year 12 of a 3 year project. SWMBO still doesn't understand the value of patience, planning, kitkats, the need for new tools and the fact that any job worth finishing isn't.

Current problem... one end of the house is buried in the mountain we live on upto first floor level. It wasn't built this way but some time between 15 and 50? years ago the cattleyard was concreted over (hopefully leaving the cobbles in place) and this meant filling in around one end of the house. It happens to be the dining room. If you imagine that the house is a cross shape with one long axis and a landslip has engulfed one end and that the "sides" of the slip are now concrete steps between the two levels that wrap around the end of the house by around a dozen feet, you'll get the idea. It's a bit more complex on one side where the "slip" comes as far as the central cross, but essentially the same problem.

We now have penetrating damp starting to cause a problem, 8 years after we removed the polythene tanking and replastered internally in lime. It seemed to be fine for the first 5 or 6 years and we were hopeful the lime plaster would self-regulate the moisture moving through, but damp has appeared in the past couple of years and the plaster is blowing in places. Maybe it is wetter recently and more water is moving though the mountain and under the house. It is starting to corrode electrical backboxes and cause the occasional RCD issue.

So clearly time to sort properly - by removing tons and tons and tons of earth and creating a retaining wall around the house so the walls can dry out and the ground level reduced to where it should be. We might even be able to reinstate the two windows in the dining room. The yard needs to remain usable (it is the drive). The difference in height is about 8 feet and the yard is 20 feet wide with 2-story stone outbuildings running parallel with the end of the house on the other side of the yard. 20 feet would give me enough width to have a walkway round the house, an 8-foot vertical retaining wall and enough mass to support the necessary 45 degrees upto the base of the outbuilding... I think.

All in all, a structural conundrum as I wouldn't like the outbuildings to collapse (again! - but that was another story....detailed on here somewhere).

Anyone got any tips on how to go about this? I'm assuming a structural engineer's advice would be sensible rather than just hire a JCB and start digging. I'm thinking of excavating one set of steps and digging out by hand up to the end of the building on one side, just to assess the state of the buried wall/stonework - sound sensible?

If I excavate slowly and build a retaining wall (in concrete blocks?) as I move round the house, is that one way of doing it. Or am I better to excavate a 45 degree slope down to the base of the house wall, build the retaining wall and then back fill the slope level again with the yard floor? If I do the latter I could put some draining pipes into the wall and backfill but it will mean destroying any possible cobbles under the concrete yard. How strong a retaining wall will I need for 8 feet tall?

Long post, thanks for getting this far.... anyone had similar problems or got structural expertise? It's a bit more than the usual "dig a French drain" issue often posted on here!

 

[88v8]

Hmmm. Oh dear.

Lime will only do so much. Was at Lacock a couple of weeks ago, the old abbey, the lime render had come off for a few feet above ground, been repaired, come off again.

Eight feet is a pretty serious wall, especially in wet ground, and with a dead load of outbuildings sitting at the top. It's going to need a very substantial foundation.
The shallower you can make the slope above it, the better. But that of course eats up some space.

I'd start by digging a narrow trench into the fill to ascertain what it is and how stable it is. Dig as far as you will need to, to insert a battered retaining wall perhaps two feet thick at the base. You mentioned 'vertical'. It may be vertical on the front but unless you use ground anchors I think it will have to be battered on the back, so that's going to be a lot of stone. I'd lean the whole thing backwards.

Be careful the sides of your exploratory trench don't collapse on you.

Some stuff on retaining walls here
https://www.thisoldhouse.com/ideas/engineering-retaining-wall

And a comment from another site:
In every jurisdiction I know of, an 8 foot retaining wall must have the design stamped by a registered engineer.
Most places 4' is the absolute maximum for a self-designed retaining wall, some places it is 3'.
The main reason for this is that the forces on a retaining wall increase as the cube of the height. An 8' wall is only twice as high as a 4' wall, but has eight times the overturning force.
In Oregon, my neighbour had a self-designed retaining wall which fell down in its first winter. I tried to show him a better design, but he insisted on having a guy with a backhoe come out and re-stack the 2'x2'x4' concrete blocks, and it fell down again in its second winter. The third time he was more willing to listen and the wall was restacked in a different manner, which has been stable for several years.

I'd be getting some quotes from structural engineers. Even if you don't go with their proposals it will give you some ideas.

Whatever you do remember that the wall must have plenty of weep holes to release the pore pressure.

Not sure where this stands in relation to Building Control. Let's not go there.

Ivor

 

[FamilyWiggs]

Thanks Ivor, that's a very helpful reply.

Looks like some exploratory digging is the first step and the wall is going to be a significant structural undertaking and I will need to be resigned to losing a fair bit of the yard.

The frustrating thing is further along from the house on both sides are very old unmortared stone retaining walls which seem to be doing the job ok. Just annoying that the house was submerged at some point.

I will report back when SWMBO has authorised the trial dig (or gone out for the afternoon and I can crack on unobserved!)

Robin

 

[Feltwell]

I'm having trouble picturing the situation Robin (pictures would help :wink: ) but I seem to remember suggesting ages ago using gabions? They are very simple and you can certainly go to 8 feet / 2.4m high - see below, gives design examples for 2m and 3m high:-


https://www.gabionbaskets.co.uk/gabion/gabion-wall-standard-design

Fill the baskets so they are faced with local stone (any old rubble can go behind!) and do some creative planting with climbers etc and they can look good - and minimal digging out for foundations - you only need to go down about 20" usually.

 

[FamilyWiggs]

Feltwell - thanks ever so much.

I had completely forgotten I had posted this question on here many years ago. It was a really informative read and has helped remind me of many of the issues. We didn't proceed at the time with the work, as shortly after, the front wall of the outbuildings collapsed and that took a bigger priority. We'd lived with the minor damp for the intervening years, but it is now getting worse.

The gabion baskets certainly look the probable (and relatively cheap) way to go - and that website was really informative. Having just spent a week with a digger doing many other jobs, I reckon this could be DIYed - if the trench is dug and battered back shallowly enough. Gabions put in place and filled. Backfill restored and then the last skin of earth by the house removed by hand once the retaining wall is nice and secure. My only concern about the gabions is that the base round here is either limestone, sandstone or clay. No way of knowing which until you did down. If it is clay it might be a bit soft for 2.5m of rock? Mind you, it's supported a 2-story stone house for 400 years, so maybe I needn't worry.

Do I need to install weeping pipes with gabions or are they porous enough? I'd still need a drainage channel in the base of the trench to carry water away around the sides of the house.

Next step is to get an engineer out for a look - is it structural or civil I need (and is there a significant difference)?*

Oh - and here is the pic form that old thread... it's quite old now (the owner of the blue toy is now a hulking teenager!), but you get the general idea. The outbuildings are just out of shot on the right.

*Apologies if I have just offended all civil and structural engineers in the land!

 

[FamilyWiggs]

Oh - and have I just come across you on another "FUN" forum?

We've just bought our first and joined.

 

[Feltwell]

Do I need to install weeping pipes with gabions or are they porous enough? I'd still need a drainage channel in the base of the trench to carry water away around the sides of the house.

Plenty porous enough as they are! The great thing with gabions is they are so simple there's really not a lot to go wrong, so long as they are filled properly and wired shut tight when installed. Eventually the back of the cages will rot, but that should be long after you care about them!

Next step is to get an engineer out for a look - is it structural or civil I need (and is there a significant difference)?*

Yes - Civil are more handsome, virile, witty, charming and well, civil. Guess which one I was (am? I suppose I'm still qualified even if it has been 25 years!)

Either is fine, and in fact if you study Civil Engineering a significant chunk of it is Structural Engineering anyway and many would describe themselves as both, Structural is taken as a subset of the wider topic of Civil. A general small engineering practice is what you want, your query is pretty simple. Building Control may want to be informed - not sure - but if they do they'll almost definitely want the engineer's calcs. I don't for a second think they actually understand structural calcs, it's more for arse-covering I think if they sign it off as being built to the stated spec and something then goes wrong.

An engineer may well give you other options as well - there are plenty of different ways of holding earth back.

Trivia of the day - why "Civil" engineers? Because originally engineers were either Military or Civil - all these other young upstarts such as Mechanical or Electrical came along later.

Oh - and have I just come across you on another "FUN" forum?

We've just bought our first and joined.

Shhhh! The dungeon is supposed to be a secret! Damn internet, can't keep anything quiet for long.. :wink:

If on the other hand it's about Motorhomes then yes, you've found me....I'm helping to bring the average age down over there ;-) One reason why Feltwell Towers is on Year 11 of the 3 Year restoration plan with still no end in sight.....

 

[Mickey]

Hi Family Wiggs,

this is completely off topic but seeing your photo I noticed you have the same roof set up pretty much that we do i.e. parapets, coping stones and slate. We are still battling water ingress onto the head of our wall resulting in damp. Did you reroof and if so what was the specification where your slates meet your parapets? We are looking to finally remedy this, this summer current set up is lead flashing into back of parapets, membrane under coping stones following refilling of voids within wall with lime slurry and back of parapets have lime filet. Still it leaks!!!
We are facing probable reroofing because the previous owners reroofed by counter battening over what was originally covered in clay pantiles all meaning the upstand behind the parapets is minimal at the very top and the roof dips down meaning water is directed at the back of the parapets! Think I've just answered my own question but just wondered if you had re roofed and come across a specific specification, I am not a fan of parapet as a design feature, or roofers! 7 years on and roof still leaking....

cheers
Mickey

 

[Flyfisher]

I can't really offer any advice but just wanted to say 'welcome back' to FW

Beyond the fairly obvious point that the digging will be the easy bit, the real challenge is restraining the rest of the mountain and for that I think I'd be seeking some sort of professional advice - especially if it was my house that depended on the restraint actually working!

Other than that, I'm pleased to hear that your project is proceeding along the same general lines as ours, i.e X years into a Y years project where X > Y, or possibly more accurately X >> Y :lol:

 

[88v8]

.........this is completely off topic but seeing your photo I noticed you have the same roof set up pretty much that we do i.e. parapets, coping stones and slate. We are still battling water ingress onto the head of our wall resulting in damp.

If you start a new topic - something like Failed Leadwork at Rear of Parapet - you might get lucky and have Doug come along to tell us how the junction should be leaded.
Seven years of leakage is not good for your roof structure.

Ivor