Hilldes

Soil Investigation - £4K+ is high?

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Background is the site has clay of some form and Oak tress in close proximity, so we need a soil investigation. The SE has given this spec to two soil investigation contractors:

 

4 No. WS. 1 No. BH.

 

One company came in £2K cheaper originally. Until I told the SE that a piling company had mentioned to me that NHBC would require soil investigation for piles to be "around 20m deep". Hence I now have two quotes at £4K+ ☹️

 

Any views on how I can get what I need from the soil investigation at a reasonable cost?

Screenshot 2020-05-15 at 16.53.45.png

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We have clay and trees of all kinds 

surrounding us

We used deep strip as SE doesn’t like mini piles 

Though 20 metres sounds deep 

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We have clay, an oak tree and two ash trees within 10m of the house. 

 

Soil investigation was 4 trial pits to 2.5m cost £1330.

 

Soil investigation confirmed piling was best option (to SE). Piling company weren't that interested in the soil investigation - said they wouldn't know how deep they need to go until the piles go in. In the end the 50 piles were between 5.0 and 6.0 metres deep. 

 

 

I don't see anything in this NHBC document that mandates a depth other than trial pits being at least 3m - http://nhbccampaigns.co.uk/landingpages/techzone/previous_versions/2011/Part4/section1/default.htm#D5

 

Are you planning to have an NHBC warranty?

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I noticed your other post on your piling cost, that looks rather high unless you are building a car park, there are lots on here with a piled foundation and the general cost is normally half what yours are. Have you signed with a company yet as I would be getting some more quotes. 

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 I got a quote form a Kent based company and it was £1390 exc VAT. I do not need any NHBC certificate though.

My area is clay (I think) and surrounded by trees. Some within 2.5 mtrs of the foundations.

 

I have spoken to a pile company and they too were not interested in a report. They said they would just go as deep as they had too.

I am now trying to get some piling quotes and find the best way forward.

I can either save the money on GI and just go direct to piles or I could get a GI report done and hope that is says I can use an insulated raft without the need of piles. A bit of chicken and egg scenario at present but hopefully all will become clearer when I start to get more quotes back.

If anyone has some advice on this situation, then please let me know.

 

Thanks 

 

@Hilldes PM me if you want details of the firm I got a quote from. 

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2 hours ago, Andrew said:

I don't see anything in this NHBC document that mandates a depth other than trial pits being at least 3m - http://nhbccampaigns.co.uk/landingpages/techzone/previous_versions/2011/Part4/section1/default.htm#D5

 

Are you planning to have an NHBC warranty?

 

Not selected the warranty provider yet. I did some research that I shared with the SE because he'd not heard of any soil investigation requirements from warranty providers...

 

From a quick look at NHBC requirements I could see no mention of specific depth of trial holes. They appeared to reference BS EN 1997-1:2004 EN 1997-1:2004 (Eurocode 7: Geotechnical design - Part 1: General rules), and from that (a very quick browse), it gives this for piled foundations:
 
"7.5.2.2 Trial piles 

(1)P The number of trial piles required to verify the design shall depend on the following: 

  • -  the ground conditions and their variability across the 
  • -  the Geotechnical Category of the structure, if appropriate; 
  • -  previous documented evidence of the performance of the same type of pile in similar ground conditions; 
    the total number and types of pile in the foundation design. 
    (2)P The ground conditions at the test site shall be investigated thoroughly. The depth of borings or field tests shall be sufficient to ascertain the nature of the ground both around and beneath the pile tip. All strata likely to contribute significantly to pile behaviour shall be investigated. 
    (3)P The method used for the installation of the trial piles shall be fully documented in accordance with 7.9. 
"

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34 minutes ago, Russell griffiths said:

I noticed your other post on your piling cost, that looks rather high unless you are building a car park, there are lots on here with a piled foundation and the general cost is normally half what yours are. Have you signed with a company yet as I would be getting some more quotes. 

 

This £4K is just the soil investigation.

 

Got a couple of tentative quotes for the piling, both assuming 27 piles:

  1. Piling only, 450mm diameter CFA bearing pile, Avg depth 12m = £13.5K
  2. Piling plus ring beam, 300mm diameter SFA piles, Avg depth 9m = £32K (includes beam shuttering and heave protection) 

Both quotes are provided in advance of the soil analysis (although 1 above did piles for a new build diagonally opposite ours).

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22 minutes ago, Field_of_Dreams said:

 

I have spoken to a pile company and they too were not interested in a report. They said they would just go as deep as they had too.

 

Here is what one of the piling contractors stated: " 

we require a geotechnical bore hole investigation report for all piled projects to provide factual data of the soil strata to depths exceeding the deepest pile toe depth. The bore hole investigation report will provide the design line criteria for our engineers to correctly design the piles based on the loads required for your project. Our offer is based upon information taken from The British Geological Survey website and our local knowledge of the ground conditions.

"

Maybe I have the Rolls Royce contractors where I just need a Ford?

 

Will PM you for the piling contractor - thanks.

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20m if you were on peat but i wouldn't have thought so on clay

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10 hours ago, Hilldes said:

This £4K is just the soil investigation.

One option you may want to consider is to take the samples and get them analysed yourself.  We had to get a soil survey on our sit and used Ashdown Site Investigations. They offer a soil analysis service at £45 a sample. In our case I dug a trial pit to 2M and took samples at each layer I came across, noting the depth for each sample. In our case this was just 4 samples.  The SE was happy with the sample reports.

We are on clay and have gone the insulated raft route. We dis get a couple of soil investigation quotes and they were very expensive and it was debatable that the detail of information they would provide was required for our site.

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If your SE will accept the results from dynamic probing then you could do away with the 20m hole and this means the site investigation company could use a much smaller rig which in turn reduces your costs.

The smaller rig will easily get to 6-8m giving you samples of what type of clay your sitting on down to this depth. Once that's done they use 1m length rods and basically drive the rod down to whatever depth you need. Unless it's  soft clay they won't get to 20m anyway. They count the number of blows it takes for each rod and from this can then work out the what size and depth of piles you would need.

But if they require samples of the type of clay at 10/12/15m etc then there ain't any other options. At most a 20t digger isn't going to get you more than 7m and that's going to be a massive hole.

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It may be worth talking to Building Control as they will probably be familiar with the soil type and typical pile type in your area.

 

A Mackintosh type probe would probably reveal if the clay continues to full depth.  It is a manual device and would be far cheaper to do.

 

You can use the NHBC depth calculator to work out strip footing depth or whether a specialist foundation is required.

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46 minutes ago, Mr Punter said:

It may be worth talking to Building Control as they will probably be familiar with the soil type and typical pile type in your area.

 

A Mackintosh type probe would probably reveal if the clay continues to full depth.  It is a manual device and would be far cheaper to do.

 

You can use the NHBC depth calculator to work out strip footing depth or whether a specialist foundation is required.

 

Thanks a useful app - especially the foundation design graphic and heave protection required. It can even measure the high of the tree and distance from foundation. I had previously calculated manually from the NHBC web site, both basically need the soil Volume Change Potential Med = 2.5m trench fill, High="specific assessment by engineer required". Based on 20m Oak tree 6m from foundation.

 

I did leave a voicemail earlier this week for local BC for some advice, but the've not got back to me 😲 

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Posted (edited)
2 hours ago, Declan52 said:

If your SE will accept the results from dynamic probing then you could do away with the 20m hole and this means the site investigation company could use a much smaller rig which in turn reduces your costs.

The smaller rig will easily get to 6-8m giving you samples of what type of clay your sitting on down to this depth. Once that's done they use 1m length rods and basically drive the rod down to whatever depth you need. Unless it's  soft clay they won't get to 20m anyway. They count the number of blows it takes for each rod and from this can then work out the what size and depth of piles you would need.

But if they require samples of the type of clay at 10/12/15m etc then there ain't any other options. At most a 20t digger isn't going to get you more than 7m and that's going to be a massive hole.

 

This was what the £2K (Inc VAT) quote included:

 

"Drilling of 1No. Windowess Sampler Borehole to 10.00m bgl at front of property and 4No. Hand- Held Window Sampler Boreholes to 4.00m bgl across other areas of site. SPTs at 1.00m intervals, SHDP to depth if refused. Geotechnical testing to determine volume change potential and concrete class."

 

...but that was before I spooked the SE with the alleged NHBC requirement for 20m depth.

 

I'm going to go back to the SE and say forget the warranty provider requirement for 20m as we have no evidence this will be a constraint.

 

The mind boggles at what a "Hand- Held Window Sampler Boreholes to 4.00m bgl" might be. Got this picture of a man with an SDS drill and a 4m long drill bit 😀

Edited by Hilldes

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It's a big Jack hammer that drives the hollow tube into the ground using a hydraulic pump. You then take off the hammer and attach a lift and clamp it onto the rods and pull it up 800mm at a time. Pretty standard operation.

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If you Google hand held window sampler you get a pic of me having a fun day at a power station in Derry.

 

IMG_20200516_112406.jpg

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Hi there - we had a soil investigation done with 2x boreholes and a depth of 10m on each borehole which cost us £1400.

 

As it turned out, due to the clay we need piles and the structural engineer charged us £1200 and there are around 30 piles specified.

 

^ to give you an idea of cost.

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Seriously consider an insulated raft at this point.   You would need to do some ground investigation but not to the depths you’re talking here. 
 

Also, a 20m Oak will have a canopy diameter of about 15m or so - are you building under the edge of the canopy or are you measuring from

some other point ..??

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Posted (edited)
2 hours ago, PeterW said:

Seriously consider an insulated raft at this point.   You would need to do some ground investigation but not to the depths you’re talking here. 
 

Also, a 20m Oak will have a canopy diameter of about 15m or so - are you building under the edge of the canopy or are you measuring from

some other point ..??

 

Thanks @PeterW Here is a plan showing proximity of trees.. Most of the larger trees are 20m high. Canopy diameter is approx 18m for the larger tree shown to the right who's canopy extends to the existing house corner.

 

Would an insulated raft be an option even with the 'worst' type of clay?

Screenshot 2020-05-16 at 14.57.50.png

Edited by Hilldes

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Sorry but that makes your trees 9m from the foundation not 6m..??


And yes, insulated raft can be used on lots of different surfaces. 

 

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13 hours ago, PeterW said:

Sorry but that makes your trees 9m from the foundation not 6m..??

 

 

@PeterW Smaller Oak tree is 6m for corner of new wall, larger Oak tree is 10m from same corner.

 

Thanks, I'll do some more reading on insulated rafts.

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@PeterW From Touchwood website, pros and cons of insulated rafts: "Not suitable for certain ground types – shrinkable clay for example"

 

I can see no mention of such limitations on other sites. 

 

MBC state this, so looks like the soil investigation will be required for all foundation types...

 

 

 

Screenshot 2020-05-17 at 08.55.02.png

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Posted (edited)
8 minutes ago, Hilldes said:

"Not suitable for certain ground types – shrinkable clay for example"

 

I am having an insulated raft over shrinkable clay, albeit with screw piles. It is currently being designed for me by a specialist engineer in Ireland.

 

Edited to add: oh, and I inherited a soil survey from the seller of the plot, and everyone concerned with the foundations has asked to see it.

Edited by Dreadnaught

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From the Kore Design guide:

 

"Suitable for all ground conditions..."

 

"Great care also needs to be taken when building on shrink- able clays (prevalent in parts of the UK). These clays also expand and shrink with changing moisture contents (not to the same extent as top soils) and are greatly influenced by the presence of trees in the vicinity of the building. Foun- dations in these clays generally need to be taken to greater depths than normal to minimise variations in moisture con- tent over time that will cause clays to shrink and expand, thus causing the foundations and building to move, possibly causing damage. In some circumstances, piled foundations may be required or be the most economical way to avoid ex- cessive movement in the foundations. Piles for lightweight structures such as houses, do not need to be large, but they transfer the superimposed loads to a much greater depth, where moisture contents are practically constant and thus shrinkage and expansion of the clays is not an issue. An al- ternative to piling that is also worth considering is the use of Vibro Stone Columns. These are columns of stone that are vibrated into the ground, consolidating the ground around them and providing columns of highly compacted stone that transfer the superimposed loads to greater depths, as with piles. Each option outlined above is compatible with the KORE Passive Slab. In shrinkable clays, a suitably qualified engineer should be consulted to advice on the most appro- priate foundation solution. Further information on shrink- able clays can be found in Building Research Establishment (BRE) Digests No’s 240, 241 and 242."

 

10 minutes ago, Dreadnaught said:

 

I am having an insulated raft over shrinkable clay, albeit with screw piles. It is currently being designed for me by a specialist engineer in Ireland.

 

Thanks @Dreadnaught I was kind of hoping that an insulated raft would remove the need for piles, but perhaps not.

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