Advice on foundations (and potential for work)

[Swhitehe]

Hi all,
We are planning a 2 storey timber frame self build in Gloucestershire and think we will hit problems with the foundations. The plot is level but on made up ground. The soil has clay and alluvium and so preliminary advice from a geo firm has indicated piling will be required.
House is a square 1100 sq ft footprint.
Had a quote back for 8m piles and foundations of £20k.

Survey firm also wants 5k to dig test holes for building control confirmationa as well as more money to tests for soaks ways.

Looking for advice on next steps, potential alternatives and whether there's anyone on here who might want the job?

Regards

Sean

 

[^woody^]

You already have your answer. Although the £5k test holes seem too much. A few hundred for a borehole will do.

You're better off using an independent engineer to report, specify any tests, and design the foundations, rather than go with some random quote for unkown design.

BTW, the test holes are for the engineers confirmation not building control's.

 

[RonnyRaygun]

I take it you've got some window sample / borehole logs in your geotechnical report? What reasons have they given for going for piles? Even if you have made ground as long as it's not too deep trench fill / strip footings shouldn't be out of the question if you get down to virgin clay / gravel.
You could easily go 2.5m deep to get to good ground and still spend less than on a piled foundation. Even if the clay is relatively weak the footings shouldn't be excessively wide for a timber frame. The firm that wrote your report should be able to give you a bearing pressure of the virgin ground from their window samples. The SPT test results from the report will give an indication of likely bearing pressures for the soil in each borehole.

A raft foundation is another alternative that might work.

Soakaways generally won't work in clay or where there's a high water table - your report should tell you the level where ground water was encountered in each of the boreholes / window samples.

 

[^woody^]

With all the material, work, shoring, and general risk, piles start to become viable at around 2m in crap ground.

 

[RonnyRaygun]

With all the material, work, shoring, and general risk, piles start to become viable at around 2m in crap ground.

Sometimes they do, but crap ground doesn't necessarily mean unstable. It can mean it has a particularly low bearing pressure, liable to dissolution, or simply made ground while being perfectly stable for excavation. I'm doing the design for such a site at the moment where there is deep made ground yet we are using trench fill.
The trial pits would give a good indication or the soil's stability.

 

[bernardgreen]

If the timber frame is true post and beam then pad foundations are ( should be ) acceptable and can be better than a raft or trenched foundation provided the posts are not fixed but merely rest on the pads. This allows for some horizontal movement of the pads relative to each other and the timber frame is ( should be ) flexible enough to allow for slight sinking ( or heave ) of one or more of the pads.

Hence piling can be more economical than other forms of foundation when building a timber framed house.

( based on my experience of DIY self building a Walter Segal designed house )

 

[RonnyRaygun]

If the timber frame is true post and beam then pad foundations are ( should be ) acceptable and can be better than a raft or trenched foundation provided the posts are not fixed but merely rest on the pads. This allows for some horizontal movement of the pads relative to each other and the timber frame is ( should be ) flexible enough to allow for slight sinking ( or heave ) of one or more of the pads.

Hence piling can be more economical than other forms of foundation when building a timber framed house.

( based on my experience of DIY self building a Walter Segal designed house )

There shouldn't be any relative horizontal movement of the pads/piles if used as they will be tied together by the ground beams which are essential for building the masonry off up to dpc. Settlement should be limited by the pile designer to a similar amount to trench fill.

 

[^woody^]

This allows for some horizontal movement

There should not be any movement of foundations or allowance, and you certainly should not be expecting a structure to twist to accommodate foundation movement just because it can. Unless the OP is building a pagoda in Japan.

 

[bernardgreen]

I am talking about milli-metres of movement over several years.

building the masonry off up to dpc

A post and beam frame building ( Walter Segal method ) doesn't have masonry and the "damp proofing" is by sealing the ends of the posts with lead sheet where they rest on the foundation pads

 

[^woody^]

I am talking about milli-metres of movement over several years.

But no-one designs foundations for any movement, not even millimetres. They are designed to be stable, and to resist movement.

Segal buildings are a specific and integral design of structure and foundation. OK if you like their appearance, but the construction methods are not transferable to traditional build.

 

[RonnyRaygun]

The OP hasn't said what type of timber frame he has, but it seems unlikely it will be a Walter Segal type considering how rare they are.

 

[bernardgreen]

but it seems unlikely it will be a Walter Segal type considering how rare they are.

Houses designed and supervised by Walter are not that rare, Over the past thirty years since his death in 1985 his method has been developed and enhanced by several architects and has been used for many houses and several other buildings such as community centres and small commercial units. Often credit is not given to it being based on Walter's method.