Building on alluvial soil on raglan

ixedoc

Hello,
We have seen a plot of land in South Wales and the ground survey has shown alluvial soil on raglan with ground water at 1 meter depth. The plan is o build a 4 bedroom house on the plot. Can you please advise what type of foundation we need to put in and will it be more expensive than standard foundation
Kind regards
Ixedoc

marksoton

No, you need professional design advice.

But with ground water at 1M you may have problems. You need to submit the full GI report to a reputable designer.

Preferably one versed in those local ground types.

D_M_E

I don't know what anyone else thinks, but to my mind "alluvial soil" means right in the middle of a flood plain and, given that your ground survey says "water at a depth of a meter" I would think that you either need to put in piles which reach well below the water table and support your property above any expected flood, or build your proposed property on some sort of raft to cope with any future flood.

That's my thoughts on putting a building weighing several tons on alluvial land which is only a meter thick on top of water, anyway.

ixedoc

Thank you so much for your replies!
D_M_E u are absolutely right. The plot lies in a flood plane but the flood defences have been recently erected and flood consequence assessment said there is a one in a hundred years chance of flooding. Could anyone please tell me what the price difference is between conventional foundation and piles foundation
Regards
Ixedoc

AnotherJoe

I've got two observations.

First, with water at one metre, I think a good foundation would be a boat.
Second, what on earth are you doing asking on the Internet when it's patently obvious an architect, or possibly a boatbuilder, will be required ? I assume you werent just planning to head off and start building on the basis of advice you got here ? (If so, you'll need two masts and a good bilge pump)

marksoton

ixedoc wrote: »

Could anyone please tell me what the price difference is between conventional foundation and piles foundation

Very large i'm afraid, dependent on the method used and size of structure.

The design will need proper calculation for groundwater uplift.

We deal with this all the time and as alluded to by other posters this is not something you should attempt on the cheap.

D_M_E

Given the number of reports over the last 12 months of flood defences being overwhelmed, either by sheer volume of water or inadequate drainage (same thing) or flood defences being rendered useless because of things like bad maintenance you need to be very sure that any defences put in will do their job.

Also, there's been quite a few storms already this year and the long range forecast is more of the same and probably more frequent.

As for foundations - well, a normal concrete raft - not a joke, I think that's what the concrete base is called - might only cost 2 or 3 thousand or so, but to put piles in, well, the piles have to be long enough to get below the groundwater and rise above any expected future flood level, even after allowing for any flood defences.

Having said that, the piles MIGHT need to be 10 meters or more in length - that's just an example - you could need 20 or more and putting them in place doesn't come cheap.

Once the piles are in, they then have to support the concrete base which would either be built around the piles or the piles would support some sort of structure on which the concrete foundation would be poured.

This could cost any amount - my guess would be up to 20 or more times the cost of a "normal" concrete raft, but as others have said you need specialist advice on this.

That's my guess for what it's worth.

HouseBuyer77

one in a hundred years chance of flooding.

So assuming you're planning this as your forever home a reasonable chance of seeing a flood at least once (say'll you'll be there for 35 years that's a 35% chance). Plus that is a statistic based upon an assessment of new defences rather than based in the history of the area. As D_M_E says there's been a lot of problems with inadequate flood defences recently so maybe your 1 in 100 years is on the optimistic side!

csgohan4

The other thing is insurance, will it cover the building?

G_M

Are you by any chance a bit mad?

Do you really want uninformed opinions from a barely relevant internet forum?

If you must use a forum, use one where specialists/professionals hang out!

But better still, pay a profesional to advise you.

marksoton

G_M wrote: »

Are you by any chance a bit mad?

Do you really want uninformed opinions from a barely relevant internet forum?

If you must use a forum, use one where specialists/professionals hang out!

But better still, pay a profesional to advise you.

This ^^^^^

But make sure you insist their reports are summarised in coherent English.

I currently have 4 on my desk that mean nothing to me because they're too technical. And i'm no thick idiot when it comes to GI.