Foundation for shed on wet ground

BoatMad

Hi, I will be involving a structural engineer in this , but just wanted a general opinion

Im looking at a garden shed, ( quite big ) 8m x 3m , in cavity block. ( single story ) MY garden is however very wet with marl 1foot below surface which is essentially waterproof )( the subclass is almost like potters clay ) and if you dig a hole , you get standing water at 1'6"

The house is built on this , ( but Im not sure whats exactly the foundations of the house are) .


can I put conventional footings into this ?

onrail

Two issues here: soft ground which may cause excessive settlement of a foundation; and water ingress which might be troublesome during construction.

The soft 'marl' wouldn't be suitable as a foundation stratum - but do you know what the material is like deeper than 1'6"? My inkling is that you've possibly hit a granular water-bearing layer at 1'6". This might be ok to found on.

Re water ingress: a contractor should be able to deal with this during construction through sump-pumping or similar, depending on the rate of ingress.

Very hard to know without seeing the site, but you'd need a deeper hole than 1'6" to assess properly. Get an engineer on board, and possibly dig a couple of deeper holes say 2-2.5m with a mini-digger. If the ground is granular or a stiff-clay at this depth, there should be no problem with conventional foundations.

Disclaimer: this is for information only - no warranty provided

T-Maxx

And make sure the finished floor level is at least 150mm higher.

BoatMad

Very hard to know without seeing the site, but you'd need a deeper hole than 1'6" to assess properly. Get an engineer on board, and possibly dig a couple of deeper holes say 2-2.5m with a mini-digger. If the ground is granular or a stiff-clay at this depth, there should be no problem with conventional foundations.

high 2.5m , surely the foundations wont got down there to 2 metres ??? or is this just inspection


The other option is in essence an above ground slab approach , with a integral ring beam under the walls

Disclaimer: this is for information only - no warranty provided

its the Internet, disclaimer fully understood !!

Calahonda52

This is tough to do without a looksee at the site but one thing for the engineer to consider is that if whatever work you do changes the water table too much, you may cause problems with the house foundations.

Gumbo

BoatMad wrote: »

high 2.5m , surely the foundations wont got down there to 2 metres ??? or is this just inspection


The other option is in essence an above ground slab approach , with a integral ring beam under the walls


its the Internet, disclaimer fully understood !!

It may be required because of ground conditions and water table level.
We had to go down 1.2m with a single storey extension just to match the formation level of the existing house.

Calahonda52 wrote: »

This is tough to do without a looksee at the site but one thing for the engineer to consider is that if whatever work you do changes the water table too much, you may cause problems with the house foundations.

+1
The effect of this could cause damage to adjoining structures, boundary walls and any other sheds that your neighbours may have in the immediate vicinity.

onrail

BoatMad wrote: »

high 2.5m , surely the foundations wont got down there to 2 metres ??? or is this just inspection

possibly on the high side, but I'm speaking with my consultant geotechnical engineer hat on - Depth of (d + 1.5b) needed to meet EC7 requirements!

kceire wrote:

We had to go down 1.2m with a single storey extension just to match the formation level of the existing house.

Reminds me of another option: If you have the space, and with a narrow trenching bucket, dig an 'inspection pit' to expose the underside of the foundations of the existing dwelling. Take measurements, and repeat. It's unlikely the shed loading will be greater than that of the house

Gumbo

onrail wrote: »

possibly on the high side, but I'm speaking with my consultant geotechnical engineer hat on - Depth of (d + 1.5b) needed to meet EC7 requirements!



Reminds me of another option: If you have the space, and with a narrow trenching bucket, dig an 'inspection pit' to expose the underside of the foundations of the existing dwelling. Take measurements, and repeat. It's unlikely the shed loading will be greater than that of the house

Sorry should have confirmed that we built a full with habitable extension.

air

Do you really need a block structure?
A lightweight timber frame on a raft foundation might make a lot more sense given the site conditions.

BoatMad

air wrote: »

Do you really need a block structure?
A lightweight timber frame on a raft foundation might make a lot more sense given the site conditions.

considerably more expensive , upwards of 2-3 x ( I know cause we just finished building one in timber frame ) even with some severe cost savings , its still way way more expensive than simple 150mm cavity wall.

BoatMad

Reminds me of another option: If you have the space, and with a narrow trenching bucket, dig an 'inspection pit' to expose the underside of the foundations of the existing dwelling. Take measurements, and repeat. It's unlikely the shed loading will be greater than that of the house

thanks , i will for the next step involve a local professional structural engineer for advice

BoatMad

The effect of this could cause damage to adjoining structures, boundary walls and any other sheds that your neighbours may have in the immediate vicinity.

nothing within 60 feet , no other sheds etc , walls are wooden fence and post etc

but again just seeing the views here , Ill get a professional opinion

my 2 foot hole has never dried out in the last week at any stage , lovely clear water

maybe a sunken pond is in my future

Calahonda52

BoatMad wrote: »

nothing within 60 feet , no other sheds etc , walls are wooden fence and post etc

60 feet in this context is akin to a metre in a light year!:)

BoatMad wrote: »

but again just seeing the views here , Ill get a professional opinion

Good idea!

air

What was the cost breakdown on the raft Boatmad? Was it for a house or a shed?