Most cost effective option for foundation -

[Zak S]

I am thinking foundation for about c330sqm foot print. I have received the GI report which is not encouraging and points to the need for piled foundation. there are multiple options e.g. piles with voided raft, piled with ring and beam, screw piled foundation. What do you think would be the cheapest options?

 

Please is the GI report and property expected foot print (yellow border). Also if any one has experience of screw piling existing foundation (are they cheaper than new foundation?)  

 

Also the GI report mention that given the high water table the storm soakways is not recommended. What other option I am left with to divert rainwater away from main drainage system? Thanks all in advance.

 

 

GI Investigation Report.pdf

Edited January 12, 2022 by Zak S

[Mr Punter]

If this is for demolish and rebuild, it looks like you will need to install piles to 10-12m.  From the report the existing foundation is not suitable for additional loading.  Unless there is obvious subsidence I would either leave the foundations and building loadings as they are or demolish and start again with augered concrete piles and ground beams.

[Gus Potter]

The GI report make for interesting reading.

 

What are you proposing to build, how many storeys and do you have a rough floor plan?

[Zak S]

Just now, Mr Punter said:

If this is for demolish and rebuild, it looks like you will need to install piles to 10-12m.  From the report the existing foundation is not suitable for additional loading.  Unless there is obvious subsidence I would either leave the foundations and building loadings as they are or demolish and start again with augered concrete piles and ground beams.

Yes. Thanks @Mr Punter the plan is to demolish and rebuild but it would depend all ok the costing hence trying to investigate the cheapest option for foundation for the new build using traditional  or helical screw piles. The quote from Abby Pynford was 65-75k for 250sqm voided slab foundation (with no insulation inside the raft). For 330sqm would it be proportionally higher? Not sure if screw piled foundation will be any cheaper but I am getting a view from a specialist company in that respect. Ring beam with 55/60 concrete piles was quoted at 52-60k. Any views on these numbers?

 

Is there no option at all to salvage the existing building for building on top by strengthening the existing foundation (via piles/underpinning etc)

 

Also what would be your advice on soakways as council insist to create soakways. If water table is high and soakways is not recommended does it mean it's a good thing I.e. cost can be avoided or would the council insist on alternative to soakway (I am  not sure if there is an alternative). Thanks.

[Mr Punter]

If you cannot use a soakaway you may still be required to have attenuation tanks so that water does not splurge into the sewer as soon as there is a downpour.

 

Get some more quotes for your piling.  Underpinning may be no good as you need to go deeper.  If you keep the existing foundations and strengthen with screw piles or whatever make sure you get a proper guarantee against failure.  Once you have had underpinning, future buyers will be very cautious.

[Zak S]

21 minutes ago, Gus Potter said:

The GI report make for interesting reading.

 

What are you proposing to build, how many storeys and do you have a rough floor plan?

To me rebuild could only be justified if I build a standard 2 storey plus a loft kind of house. Given the foot print it could easily reach 600sqm but I onky need 450-500 sqm. But want to have decent size ground floor.

 

[JohnMo]

When we doing our house I asked the architect about screw in piles and firmly told they would be expected to last the life of the building, so would be rejected by the council.

[Zak S]

1 hour ago, JohnMo said:

When we doing our house I asked the architect about screw in piles and firmly told they would be expected to last the life of the building, so would be rejected by the council.

Hi @JohnMo thanks. Just to clarify did you mean to say that screw piles would  NOT last over the life of the property hence not allowed?

[Mr Punter]

2 hours ago, JohnMo said:

When we doing our house I asked the architect about screw in piles and firmly told they would be expected to last the life of the building, so would be rejected by the council.

 

The warranty firms are not keen.  I understand corrosion protection is an issue as ground conditions have a big impact on this.

[Zak S]

35 minutes ago, Mr Punter said:

 

The warranty firms are not keen.  I understand corrosion protection is an issue as ground conditions have a big impact on this.

I have been speaking with a company and they say that they are approved by warranty providers and there solution get approval for building regs buy apart from warranty (which only last for ten year), there is a question of impact on insurance as well ability to sell and longevity of screw pile play an important part in it. Given the ground spoils ph is seems to be within acceptable range meaning the corrosion will be not material. Any view on this?

[JohnMo]

I will possibly use screw piles on an out building in our wood, but not on a house that could be there for a century or more

[saveasteading]

17 hours ago, Zak S said:

screw piled foundation.

In my considered opinion, which may be out of date, screw piles are suitable for huts  and  decks where loading is low, and deflection isn't a great loss, and short term use eg site cabins or injection stations.

Also I can see they are ideal for pylons, masts etc where access is difficult.

 

For 'proper' buildings, they are too near the surface, don't carry enough load, and then there is corrosion.

 

I did look into this seriously as the reps were saying what they could do...but they couldn't really.

 

For a house I would first look at a precast driven pile system where they also install the edge and cross beams, and I would be immediately out of the ground.

Can be to odd shapes easily enough.

 

Bullivant is one, and the project we did with this system went very well.

Edited January 13, 2022 by saveasteading

[Zak S]

2 hours ago, saveasteading said:

 Bullivant is one, and the project we did with this system went very well.

Thanks. You confirm the reservation I had which is much appreciated.

 

I have asked Bullivant and they said in the email "With ground conditions not getting to anything significant until 8/9m then this may not be enough to warrant a pile design."

 

well if piles is not an option then have I run out of options ??

[Makeitstop]

Bullivant are one piling specialist, but there are numerous others, such as Arsleff, Foundation piling, Van Elle and so on.

 

What I would say, is that if you have a problematic site, then using a seriously decent outfit is the only way to go. From my own brief experience with piling contractors, many seem to suggest "solutions" that arguably suit themselves and their own gear more than they do to suit your actual ground conditions.

 

Ask many Questions and double check everything. The variation in methods and pricing can be startling.

 

From a brief read of your ground, it doesn't look straightforward. 

 

Good luck with whatever you do.

[Zak S]

I just noted from the report that they only went to c11m when the brief was to go to 15m depth. I just re read the quote and it said as follows:

 

"Geology for the site is likely to be a mixture of Clays, Sand and Gravel with the quote being based on the excavation of up to 2no hand dug trial pits to expose foundations to the existing dwelling, maximum pit depth 1.20m and for 2no combined sampling boreholes utilising dynamic and rotary augur drilling equipment with provisional borehole depths of 15.0m or refusal*, whichever is the shallower, being included.  Refusal* is achieved when >50no blows produces <300mm penetration during a standard penetration test"

 

It's the "refusal" bit I am not sure about. Is that standard? Bullovant saying given he did not go to 15m they will not accept the boreholes and it's likely building regs/warranty provider will say the same and would ask the new investigation. That's means that SI was not properly done. Please could you help? Thanks.

[saveasteading]

33 minutes ago, Makeitstop said:

Bullivant are one piling specialist, but there are numerous others

I am suggesting them specifically because they do the precast edge and cross beams at the same time, and that recovers some cost relative to dug foundations.

 

The others may offer something similar, I don't know.

23 hours ago, Zak S said:

proportionally higher

No, There is a big lump for turning up, perhaps £10k. take that off, pro-rata what is left, and add it back.

 

3 hours ago, Zak S said:

With ground conditions not getting to anything significant until 8/9m then this may not be enough to warrant a pile design."

Doesn't make sense, as that is what piles are for. Anything more than 3m and piling is the answer.

I have one other suggestion but will PM you if you don't mind.

[Zak S]

5 minutes ago, saveasteading said:

have one other suggestion but will PM you if you don't mind.

Thanks. Please do.

[Zak S]

6 minutes ago, saveasteading said:

Doesn't make sense, as that is what piles are for. Anything more than 3m and piling is the answer.

My guess is that this is specifically for forecast piles which may go to only certain depth but I might be wrong.

[saveasteading]

2 minutes ago, Zak S said:

for precast piles which may go to only certain depth

They bring the right lengths of piles to site, and chop off any sticking up once it has reached a set.

They are banged in until the correct resistance is reached, which is encouraging.....unless they keep going down.

[Zak S]

26 minutes ago, saveasteading said:

They bring the right lengths of piles to site, and chop off any sticking up once it has reached a set.

They are banged in until the correct resistance is reached, which is encouraging.....unless they keep going down.

 

I provided the detail about 'Refusal' of rig to go any further and he very helpfully stated:

 

The rationale is that they have stopped the investigation at 11.45m as soon as the SPT’s have become greater than 50 blows for 300mm or less. They have not continued using a core sample and hence the strata they have refused on could simply be a hard band of clay with softer deposits lying beneath which could cause potential issues of “punching”. 

 

I personally think given the two house opposite only went to 10/11m for their piles we might be ok but this means RB would be a bit reluctant to accept this SI report. 

 

Nothing in life is simple...? 

[saveasteading]

If the survey is inconclusive then the survey may be at fault.

 

But they have stopped testing when the ground becomes hard enough for a pile. Of course it could be different lower down! But a local SE will know what to expect.

[Zak S]

1 minute ago, saveasteading said:

If the survey is inconclusive then the survey may be at fault.

 

But they have stopped testing when the ground becomes hard enough for a pile. Of course it could be different lower down! But a local SE will know what to expect.

Thats makes sense.

[Makeitstop]

The stp (standard penetration test) is what determines the amount of resistance the ground has at a given depth. If it states that there is no more "give" than 30cm from 50 blows with a drop hammer / chisel, and, that in meeting that level of resistance that the ground is adequately supportive for the likely given loading for a proposed structure, then that would be the approximate depth the piles will need to get to in order to provide you with a reasonable bearing.

 

However, if bullivant say the depth achieved from s.i boreholes is inadequate for them to offer you a correctly assessed solution, then you need to find out what exactly they would require to enable them to price the job for you. As was mentioned above, any level of resistance "must" show significant bearing capacity, and, the contractor needs to be pretty sure that what they reach is not simply a hard band of sub-strata, where they may be a soft layer beneath. Of course, local knowledge will be hugely important, but ground can be massively variable over just a few metres of separation, so it's all somewhat open to guesswork to some extent. The boreholes simply provide a very good indicator, which is why its crucial they are to correct depths.

Edited January 14, 2022 by Makeitstop

[Zak S]

1 hour ago, Makeitstop said:

 

However, if bullivant say the depth achieved from s.i boreholes is inadequate for them to offer you a correctly assessed solution, then you need to find out what exactly they would require to enable them to price the job for you. As was mentioned above, any level of resistance "must" show significant bearing capacity, and, the contractor needs to be pretty sure that what they reach is not simply a hard band of sub-strata, where they may be a soft layer beneath.

 

Bullivant is saying they need to go 5m below the depth of pile to provide the warranty. What is it I need in the report which is not there which would confirm that there is nothing under the sand stone which caused the Refusal. Is it drilling a new bore with different equipment. I really want having to pay for the GI report twice. I have not yet settled the invoice but the current GI covered by stating the Refusal in the quote and me being not technical did not understand the implication. Any thoughts?

[joe90]

Coming to this late but could you consider a raft and timber frame  build, less ground  loading and probably less than the existing build which I guess has not moved so far?