Excavating gable end, precautions?

[metzer87]

Hello
I am new to the forum as i am new to having a period property. We have recently purchased a lovely early 1800's cottage (converted to houses on a welsh hillside. It has had extensions in the 70s and renovations through the years and is in quite good repair. However unfortunately it does sometimes flood or suffer from water ingress in the heaviest of rains. The ingress is through the floor. I have been advised by a civil engineer to excavate the ground around the house to 6 inches below floor level, creating a dry moat at the back of the house. This will reduce the hydro static pressure underneath. Most of the way around the house this is easy and unconcerning as its only a few inches to lower. However on the gable end the house is dug into the slope and I will have to remove a depth of about 1.7 meters in places. As this gable end is of stone construction and will have no footings to speak of I am wondering what precautions would people recommend?
Would people think underpinning would be necessary? If so in what form and to what degree?
Any advice would be much appreciated.
Kind regards
Konrad

 

[Keithj]

Is the gable end the "uphill" end? I assume so from the description. Does the rain flow down against that side of the house, or are you dealing with water below ground?

I've had similar experience on a Welsh hillside, but the advice from a very experienced local was that the "uphill" part was solid rock from about a foot down. A shallow trench found the rock, and channelled the water round to where there was a drain. It might be worth having a dig to see what's down there.

The "dry moat" sounds like a French drain (named after a Mr French, not the country). I'm a great fan of those - one cured the damp problems in the Tudor house where I live, and another fixed the 12th Century church that I worry about.

 

[metzer87]

Hi, thanks for your reply.
No the back end is already mostly dug out. The gable end is on the side but the way the ground cambers and from the positioning of an old culvit could have water coming at the gable end. The water issue is underneath the ground as I am dealing with surface water further up.
The dry moat could be partialy back filled with a french drain, but initially the plan was to leave it open to see how things behave.
I have dug a small sample by hand and not hit rock yet, but its certainly more than a possibility up here.
When you were digging out yours how deep did you go before hitting rock and did you support the wall in any way?
Thanks Konrad

 

[Flyfisher]

Did the civil engineer not offer any advice about the implications of undermining the foundations?

 

[Feltwell]

Did the civil engineer not offer any advice about the implications of undermining the foundations?

Quite! The area to avoid digging is one extending out at 45 degrees out from the base of the wall - this is where the wall's weight will be bearing. This is the general rule of thumb, avoid that and you should be OK. But, to be absolutely sure, you need someone who is experienced and who has been on site to see the exact situation.

If you need to remove as much as 1.7m, do not whatever you do leave that trench edge unsupported - either batten the trench back (i.e. dig the sides at a good angle rather than just straight down) or make sure the trench is lined with well propped timbers. If space allows batten it back, even though that means digging out more than you ideally want to. If a trench that deep collapses it can be fatal for anyone in it.

Are you looking at leaving this area permanently open? If so, how will you support the soil?

If you search for French Drains on here you will find quite a few old posts.

 

[Penners]

Did the civil engineer not offer any advice about the implications of undermining the foundations?

I agree. The CE is the person to advise you. He/she has obviously examined the house and should be familiar with the way that it is founded and the way that the water is behaving. The critical question, as Keith has observed, is where the water is coming from. Is it surface run-off from the hillside or is it upswelling groundwater? The solution to the problem may be very different for each of these.

 

[discobarry]

Interesting read, we have similar issues but i'm on a pennine hillside !

How did you get on?