Strip, raft or piles

[Andrew]

We've finally received our soil investigation report (wish we'd have done it much earlier) and it's not great news. The ground is mainly soft sandy clay with low bearing capacity. 

This is what the soil investigation report said :-

 

Quote

Taking account of the low HSV (Cu-kN/m2) values, foundation stresses on soils at 1mbgl should be limited to 30kN/m2 and hence traditional strip foundations are not recommended. Wide reinforced strip foundations may not be economic since a line load of 50kN/m would require a width of 1600mm.

 

It may be feasible to consider a pseudo raft foundation incorporating edge and internal thickenings under structurally loaded walls, limiting stresses to 40kN/m2, and subject to rigorous preparation of the subgrade. 

 

A piled foundation solution may be preferred with either bottom driven steel cased cast in-situ or square pre-cast concrete piles being the recommended piling solution. Piles would support a 450mm x 450mm reinforced concrete ground beam.

 

Our structural engineer has reviewed the report and this is their comments

 

Quote

We wouldn’t recommend the raft foundation option given in the CC Geotechnical report. A raft foundation deals with differential settlement around the building however there would still be a potential issue with overall settlement and possibly tilting. Penetrations for utility services can also be problematic. A piled solution is more reliable and likely to be more cost effective. 

 

I've also spoken to a local friendly builder, older chap, seen and done it all, constructed most of the houses in the village built in the last 50 years. He's just finished the foundations on a house a few hundred meters down the road. His view is why waste your money on a raft or piling,  I'll do some lovely strip foundations for you,  building control will check them out. it'll be fine. 

 

My preference would be a raft foundation but there's some conflicting advice above. I'm pretty sure the local builder can be discounted but the SE seems to have some reservations. 

 

Would welcome any thoughts on the above and specifically the SE's concern regarding tilting of the raft. We are using timber frame mostly timber clad if that makes any difference. Thanks. 

[ProDave]

I am puzzled by this as we are on "Sandy Clay" and the structural engineer that did the soil sampling declared strip foundations were fine. The only stipulation being we dig through the black organic top soil into the sandy clay, and put rebar in the foundations strips.  All done at 600mm wide.

 

 

[Mr Punter]

Maybe have a chat to find out what Building Control and the Warranty provider are looking for.  Strip footings are often the most economical. Does Mr Builder do beam and block or concrete ground bearing for the ground floors?

[Andrew]

9 minutes ago, ProDave said:

I am puzzled by this as we are on "Sandy Clay" and the structural engineer that did the soil sampling declared strip foundations were fine. The only stipulation being we dig through the black organic top soil into the sandy clay, and put rebar in the foundations strips.  All done at 600mm wide.

 

This was one of the four trial pits.

[Andrew]

7 minutes ago, Mr Punter said:

Does Mr Builder do beam and block or concrete ground bearing for the ground floors?

 

Normally concrete ground bearing but I think he'd do either. 

[Declan52]

3 minutes ago, Andrew said:

 

This was one of the four trial pits.

I would say it's the presence of the organic matter, roots and rootlets, that has them worried. 

[nod]

8 hours ago, ProDave said:

I am puzzled by this as we are on "Sandy Clay" and the structural engineer that did the soil sampling declared strip foundations were fine. The only stipulation being we dig through the black organic top soil into the sandy clay, and put rebar in the foundations strips.  All done at 600mm wide.

 

 

We are exactly the same as you Dave We are opposite a sand quarry and surrounded by trees

He told us that he would be happy to help us spend our money on piles But strip would be more than adequate 

With clay boarding around the edges 

[Russell griffiths]

When I started thinking about foundations I walked around all the neighbourhood and looked for anything that was newly built, I talked to all the owners to find out what they had done for foundations, I asked 3 different neighbours and got the same answer from all of them. 

 

Why dont you do a bit of legwork, and ask locally. 

[Andrew]

1 hour ago, Russell griffiths said:

Why dont you do a bit of legwork, and ask locally. 

 

This is the perspective the builder is providing - he knows what's been built locally and it's all strip foundations. 

 

It feels like science (investigations and engineers) vs the builder's nose. 

[PeterW]

Any trees in the area ..? Large ones..?

[Declan52]

Where you present when they did the testing???  I take it they just used a hand held cpt test to get the test results on your trial pit document. How did they get the test result at 2m depth.

Would it be worth it to get them back and do 1-2 boreholes to 6m to gain more information.  It will either confirm the need for piles or hopefully give more accurate results which might allow strip or a raft type foundation. It will also allow you to contact those companies/engineers who specialise in raft foundations and give them this data and let them figure out if it's a possibility to use this method. 

Plus if it has to be piles then you will have more data to give to the piling company which will let them design the piles to suit your site. 

Just because the  field next door is good for strip foundations doesn't always mean that yours will be.  You can have rock in one field sticking out of the ground and peat in the next. 

[Russell griffiths]

 

 

Just because the  field next door is good for strip foundations doesn't always mean that yours will be.  You can have rock in one field sticking out of the ground and peat in the next. 

This is also very true, 

with my situation I found my closest neighbours had piled, and the house500m away was on piles, and 200 new holiday homes being built where on piles, so it sort of dawned on me that’s what was going to happen. 

[Simplysimon]

in the test bores we had done it was clay down to x metres then basalt. when excavated all foundations (strip) landed bang on whin within 700-1000mm, house ain't going nowhere. one of the bores didn't hit basalt.