Ground investigations - how to scope?

[Raine]

 

I've recently had planning granted for a new build + mods to existing house, and am trying to find a structural engineer.  Some of them are asking for ground investigations to be completed before they will quote, as this will dictate what type of foundations they need to design, and therefore the amount of work they'll need to do.

 

The geological drift map shows alluvium on one side of the site (underground stream, now converted to Thames Water storm drain) and clay on the other side.

 

I'm now stuck, as I don't know precisely what investigations to commission and how to scope these.

 

What would you guys do?  Ask the SEs (who are still at presales stage) to advise? Contact some geotechnical companies and ask them? Something else??

 

Edited September 23 by Raine

[Raine]

 

Sorry, should have researched more before posting the above!

 

I found this thread: https://forum.buildhub.org.uk/topic/36759-soil-and-ground-investigation-options, and @OwenF's advice on the BGS maps.  As with the OP on that thread, I also didn't know this info was publicly available.  Thanks so much for this info!!

 

There were two nearby boreholes, the nearest being ~85m away, and on similar ground by the looks of the map:

 

 

 

Looks like alluvium to a depth of 4.3m, then a thin layer of sandstone, then stiff sandy clay, right?  Borehole 1 is similar but further away.

 

I'm concerned about ground water, as parts of the back garden get waterlogged after heavy rain, but I'm not sure how to interpret the water data shown in the borehole record?

 

The former underground stream (now converted to Thames Water storm drain) that I mentioned above is located here:

  

 

 

Would this type of ground generally require piles, do you think?  Or standard trenchfill + beam & block?

  

It’s been a while since my last new builds (2011), and perhaps things have changed since then, but neither of them (both of which were on clay soil) required full Ground Investigation Reports, either for the structural design or for the warranty.  In fact, I didn’t even need any geotechnical analysis done – the  building inspector just examined the open trenches and asked for us to go down a bit further in one corner. 

 

I'm perplexed as to why all the SEs I've spoken to so far seem to be being so very cautious by either saying that I'll need Ground Investigation Reports, or piled foundations, or both.  I'm on a tight budget and can't afford to throw money away if it's not necessary.  😞

 

 

[Declan52]

They are just being safe by asking for all the information so they can correctly give you a solution. 

Are you any way handy with a spade and a set of post hole diggers?? 

Easiest way to figure the first step out is to dig down at the side of your house and see what's there. Remove around 1m square of material to give you enough room to get at the foundations.

You should either find rows and rows of blocks then a ledge of concrete. 

Or a ring beam with reinforced concrete. 

I used to do this it's not that hard and will take an hour or so.  

You do look like you have enough room for trail pits which could be an option. 1 day hire of a digger and dig a few holes. Ideally if access isn't an issue a 5 ton digger will get you 3.5m deep. Which might be enough as you will be into the firm clay going by the borehole logs.

The different water strikes are just the depths the guys doing the drilling hit water. Nothing unusual with that.

 

By getting a SI done you will know what method is available. It's really the only way to find out exactly what's there and include data to let the SE perform his calculations correctly.

[Alan Ambrose]

>>> I'm perplexed as to why all the SEs I've spoken to so far seem to be being so very cautious

 

Well, looks like it's quite soft going to 2m or so. The problem is that your ground may not be exactly the same as the ground 85m away. Your soft layer could be at 1m or 4m. Which it is, is important, of course. The route of the storm drain may also be an indication of extra water local to your plot.

 

So, you probably need at least a couple of bores / digs near your proposed foundations. The alternative is to just start digging your foundation trenches and be prepared for BC to say 'you need to go down another metre or two' or to have to order in an expensive pump on the fly.

[Raine]

Thanks both for the replies.

 

So I get holes dug or bored - then what?

 

 

FYI, I already had a trial pit dug while I was purchasing the house, at a corner of the existing 1970s extension (the house itself is 1930s).  The hole revealed 525mm (7 courses) of below-ground brickwork, on 675mm of concrete.  I had a 50mm hand auger done to 3m depth at the same time, which filled with water to about 1.4m depth in the couple of hours that the hole was open. 

 

The guy who dug the trial pit kept the soil that you see in the below pic.  Should I send that away for analysis?  Would that be enough?  Trial pit was on the right-hand side of the plot, and my plans also include new foundations about 17m away on the left hand side of the plot.

 

I also have a hole currently open on the other side of the site, which was needed for a soakaway test, but that's only to 500mm depth.  I could dig it out further, but again, then what?

 

 

 

Photo shows the drier, shallower-depth soil at the bottom of the picture, and the wetter, deeper soil at the top of the picture.
 

 

 

Edited September 24 by Raine

[TommoUK]

Will you be getting a new build warranty? Check with your proposed warranty provider before you proceed as they will.probably have minimum requirements and also a method statement/scope.

[Declan52]

You can't use that sample for anything as it's classed as a disturbed sample. They can check it for organic materials is about the height of it so waste of time. 

When you do a borehole you use a hollow pipe/cone to perform a spt which is basically counting how many blows it takes to move a set depth. If it takes under 10 blows the ground makeup is very soft and on the other hand you can get over 50-60 blows and even refusal where the tool just bounces, usually when you hit rock. You can see on the logs above they have performed 2 tests, with 7 blows and 50 blows. So a massive difference in the bearing capacity of the ground in a few metres. 

Did building control come out and look at your trial pit or just you and the digger man??

[Raine]

On 24/09/2024 at 20:53, TommoUK said:

Will you be getting a new build warranty? Check with your proposed warranty provider before you proceed as they will.probably have minimum requirements and also a method statement/scope.

 

Yes, I will be getting a warranty, but I haven't organised that yet.  My previous self-build warranty providers were happy to accept SE foundation designs and BCO sign-off, but I guess not all of them will be.

 

Speaking of which, does NHBC Solo no longer exist?? (I am aware that there are other providers, but that was a big one)

 

 

On 25/09/2024 at 14:18, Declan52 said:

You can't use that sample for anything as it's classed as a disturbed sample. They can check it for organic materials is about the height of it so waste of time. 

When you do a borehole you use a hollow pipe/cone to perform a spt which is basically counting how many blows it takes to move a set depth. If it takes under 10 blows the ground makeup is very soft and on the other hand you can get over 50-60 blows and even refusal where the tool just bounces, usually when you hit rock. You can see on the logs above they have performed 2 tests, with 7 blows and 50 blows. So a massive difference in the bearing capacity of the ground in a few metres. 

Did building control come out and look at your trial pit or just you and the digger man??

 

Thanks for this info Declan, I didn't know any of that.  

 

 

I just want to find the least-cost route of getting foundations that are good enough, without being over-engineered, and of course the SE and warranty providers both have a vested interest in me taking the opposite route.  It's hard to navigate.

 

[Alan Ambrose]

>>>

I'm now stuck, as I don't know precisely what investigations to commission and how to scope these.

 

What would you guys do?  Ask the SEs (who are still at presales stage) to advise? Contact some geotechnical companies and ask them? Something else??

<<<

 

An SE will spec your GI for you for a smallish fee, say £250. Check yourself that it seems to make sense.

[kandgmitchell]

Most SE's I've dealt with have specified what information they want from a site investigation before they will consider designing foundations..........

[Declan52]

On 26/09/2024 at 17:51, Raine said:

 

Yes, I will be getting a warranty, but I haven't organised that yet.  My previous self-build warranty providers were happy to accept SE foundation designs and BCO sign-off, but I guess not all of them will be.

 

Speaking of which, does NHBC Solo no longer exist?? (I am aware that there are other providers, but that was a big one)

 

 

 

Thanks for this info Declan, I didn't know any of that.  

 

 

I just want to find the least-cost route of getting foundations that are good enough, without being over-engineered, and of course the SE and warranty providers both have a vested interest in me taking the opposite route.  It's hard to navigate.

 

Anything basically below finished floor level has always been referred as the great unknown. Nobody really knows what's under that lovely green grass until the digger comes in and starts working. You could be lucky and hit hard earth/rock pretty shallow or end up with 3-4m of trench fill or piled to 15m. That's why you never get a definite price for a job until your at FFL. 

If you get a SI done and they come back with bad news that it has to be piled to beyond 6m would this make the build a no go?? 

Have you contacted a SE to see if a series of trail pits to 4-5m would be suitable. Would require a much larger digger 8-10t but would still be cheaper than a full SI.

[Raine]

On 24/09/2024 at 08:27, Raine said:

 

 

 

Still pondering this.  I know the local borehole that I mention above (85m away) is unlikely to be 100% representative of my ground conditions, but it's the best info I have to work with at this theoretical stage, so I'd like to be able to understand and interpret it.

 

I think it's showing me this - have I understood correctly?

 

§  0.1m topsoil

§  1.5m of soft brown grey silty sandy clay (to total depth 1.6m)

§  0.9m of soft light grey silty sandy clay with decayed roots (to total depth 2.5m)

§  0.3m of soft to firm grey silty sandy clay with decayed roots (to total depth 2.8m)

§  1.5m of firm grey silty sandy clay (to total depth 4.3m)

§  0.2m of hard sandstone (to total depth 4.5m)

§  5.5m of stiff sandy clay (to total depth 10m (bottom of borehole))

 

Is there anything else noteworthy in that borehole report?

 

[Alan Ambrose]

3 water strikes plus standing water at the surface - no indication how much. Surprising to get standing water in July, but maybe it just rained.

 

Standard Penetration Test value of 7 at 1.6m? Cone Penetration Test value of 50 at 4.3m? Surprised to see both in the same bore.

 

https://en.wikipedia.org/wiki/Cone_penetration_test

https://en.wikipedia.org/wiki/Standard_penetration_test

 

So suggest v. soft @ 1.6m, quite hard at the thin sandstone layer.

 

I think you need to just get on and either run a proper SI or use a digger as suggested and/or ask local BC and/or engage an SE. There's only so much you can accomplish sitting at a desk.

 

Personally I would do the last three to start with. You find out in a matter of minutes what's going on down there with a digger. Even better if you have BC or SE there while you dig. Doesn't preclude geting proper bearing numbers / particle size analysis / shrinkage / chemical analysis using an SI later, but gives you a good clue what's going on with groundwater and you also get a rough idea how firm the ground is.

 

What founds are you thinking of? Strip?

 

[Gus Potter]

On 23/09/2024 at 16:07, Raine said:

I've recently had planning granted for a new build + mods to existing house, and am trying to find a structural engineer.

Although folk on BH may disagree, SE's and Architect's often work on quite a tight business model, especially if they employ folk.

 

I offer from time to time offer a service where folk pay me for an intial SE consultation. But as I used to be a building contractor in my last life this turns in to a general over view. This starts off with a phone call and a general chat, I ask a few questions but mostly try to listen, then I identify what initial info we need to make some basic decisions. I try and explain why we need to do things and answer any questions you have.. the objective is to inform you as much as I can and give you the tools to make your own decisions.

 

Part of this process allows me gauge how knowledgeable your are and where you may need help. You can cover a huge scope in a couple of hours on the phone.

 

 

On 26/09/2024 at 17:30, Alan Ambrose said:

An SE will spec your GI for you for a smallish fee, say £250. Check yourself that it seems to make sense.

My approach is to say up front.. here is my hourly rate for; general advice, a supportive chat and advice on the process of selecting other advisors / designers and how you put together a document that specifies what you think you need and importantly where you think you may need help. At the end of the day I always make sure that folk feel they have got value for money, usually this results in over all savings and that justifies my fee.

 

What I do is different but it does work for some Clients and me.

 

Try if you can to find an experierienced SE who will spend a couple of hours chewing the fat with you.

 

 

[saveasteading]

On 23/09/2024 at 16:07, Raine said:

Ask the SEs (who are still at presales stage) to advise

A local one who has been around a while and  knows the ground. 

[Raine]

On 03/10/2024 at 11:41, Alan Ambrose said:

3 water strikes plus standing water at the surface - no indication how much. Surprising to get standing water in July, but maybe it just rained.

 

There is a former underground stream which has now been replaced(?) with a Thames Water storm drain.  I don't know how much of the underground stream still remains in the wild, but I was expecting to see water here.  I presume that pushes us more towards piles than trenchfill?

 

 

On 03/10/2024 at 11:41, Alan Ambrose said:

Standard Penetration Test value of 7 at 1.6m? Cone Penetration Test value of 50 at 4.3m? Surprised to see both in the same bore.

So suggest v. soft @ 1.6m, quite hard at the thin sandstone layer.

[..]

What founds are you thinking of? Strip?

 

I guess hard sandstone would give a high SPT number thought, right?  But it's a very thin layer (and may not even be present at my site), with stiff sandy clay below, to the bottom of the borehole.  

 

I was hoping I could have <2m trenchfill (but I've budgeted for 5m-deep piles) but if the ground is soft down to 2.5m, would that make piles more economical? I heard that anything deeper than 2m generally works out cheaper to have piles than trench??

 

Would the stiff sandy clay provide sufficient bearing for a pile, or would I need to go deeper and find something even firmer (which would likely be deeper than 10m, based on the local borehole)?  Or is it a case of it might be sufficient, subject to calcs?  Friction piles would be the thing here, right??  In which case, I presume the amount of friction needed would be based on the weight / load per pile, and that would then inform the required pile depth?

 

[saveasteading]

What do neighbouring houses have for foundations? Talk to neighbours.

Do their houses look OK?

[OwenF]

On 05/10/2024 at 10:28, Raine said:

 

I guess hard sandstone would give a high SPT number thought, right?  But it's a very thin layer (and may not even be present at my site), with stiff sandy clay below, to the bottom of the borehole.  

 

Firstly, glad you found my link to BGS useful. Welcome to the exciting and detailed world of engineering geology.

 

On the above, it’s is geologically impossible for sandstone to form above a Holocene clay. Therefore your ‘thin layer of sandstone’ is 99.9% a cobble. Ignore it for design, but factor it for digging.

That SPT (incorrectly labelled CPT) is refusing on the cobble E.g. 50 blows for 15mm.

 

So other than the SPT of 7 shallow you don’t have any insitu data. The U100 results would be useful, but unlikely to be attached to the log.

 

 

I haven’t digested every detail of this thread, but I suggest (as others have done) you find a SE and get their advice.

 

I design piles regularly (for bridges and large structures, not houses) but even with nearly 20 years of experience, I couldn’t teach you foundation design on t’internet.

 

the SE will know how to navigate Building Regs, which I’d suggest is more relevant than your perceptions of ‘sufficient bearing’.

 

My 2c for a ‘feel’ before spending money : dig a slit trench to 2m (your proposed trenchfill limit) then:

1. See how long it takes for water to fill it

2. if it stays (relatively) dry get a sample from the base and try to stick your finger in it. If you can, probably too weak at 2m depth

if it floods quickly, that makes trench fill less appealing

 

(H&S warning: dig with excavator if possible and don’t get in the pit. If you have to hand dig, make it wide and step the sides out)

 

[Jilly]

As you’ve budget for piles and you might end up there, bear in mind it’s worth getting several quotes as I found a vast difference in price. 
The muck away is astounding, you’ll save a lot of you have somewhere on site to get rid of the spoil. 

[saveasteading]

28 minutes ago, Jilly said:

The muck away is astounding

as in 'cost'.  some methods do not require earth removal. make sure these are considered if appropriate to your ground.

 

31 minutes ago, Jilly said:

I found a vast difference in price. 

all the more so when different solutions are being considered

 

On 05/10/2024 at 10:35, saveasteading said:

What do neighbouring houses have for foundations?

anything?

[Raine]

On 05/10/2024 at 10:35, saveasteading said:

What do neighbouring houses have for foundations? Talk to neighbours.

Do their houses look OK?

 

Yes and no!😞

 

The houses in the road were built in the 1930s, and all look good and solid. However, as you can see from the BGS map, the ground conditions on the site are likely quite different from the rest of the road:

 

On 24/09/2024 at 08:27, Raine said:

 

 

 

The neighbour immediately to the south (which looks like similar ground, according to the BGS map) did a rear extension a couple of years back.  I have asked them what foundations they had, but they've not got back to me, and I suspect they won't.  It's only single-storey anyway, so won't have anywhere near as much load as my two-storey + rooms in roof.

 

My existing house was built in the 1930s and is good and solid with no sign of movement.  Attached to the house is a 1970s two-storey side extension that has a distinct DIY feel about it, which has moved significantly away from the main house (I got a big discount on the agreed purchase price because of this). The exact cause of this movement isn't yet known.  Here's where I'm at with that:

• Previous owners claim this was settlement and hasn't moved for years (they would say that though!).
• Massive ash tree ~3m from the extension felled when I bought the house last year. 
• Trial pit dug exposing the extension foundations. 
  • Bottom of concrete @ 1.2m depth
  • Very large tree roots pushing up against the bottom of the concrete
  • Standing water at 1.4m depth
• We suspect the extension hasn't been properly tied to the main house, which I am hoping to see when we rake out the crack and get a proper look. 

I am hoping that the movement will stop now that the tree has been removed, and when the extension gets tied properly into the main house (Helifix). Worst case, the extension needs demolishing and rebuilding.  😞

 

Hmm, maybe I should get the helifix done now rather than waiting, and see what happens over the next few months?  If I get that Helifixed and the foundations are still insufficient, where am I likely to see the issue reappearing?  At the helifix location, or cracks elsewhere in the extension?

 

 

Edited October 8 by Raine

[Bancroft]

From my recent experience on this topic I would ask your chosen Structural Engineer for a detailed explanation of exactly what information he/she needs in order to correctly design your foundations/house.

 

But still be prepared for some vacillating from them.

 

We needed to have some ground investigation done (planning requirement) regarding percolation rates, drainage etc.  I found a suitable company and got a quote from them for the work.  I then sent that quote to the SE and said "Will this survey be sufficient for you to do the foundation design work?".

 

Yes, was the answer.

 

When the results came back, I sent them to the SE and told him to crack on.  He then said he didn't have all the information he needed and suggested I might need another survey...

 

A short re-briefing ensued, I told him to speak directly to the ground investigation company, and harmony has now been re-established.

[Daniel H]

Just in case it's helpful - as this conversation seems to have petered out a bit, our experience was that we knew we needed a ground Investigation for any future engagement with a Structural Engineer, and also for the water connection pipe selection (local water company stipulated on their website) and that the LPA might have some questions as would our neighbours who are uphill on two sides from us.

 

I sourced 10 companies and heard back from a fair few then rang around and had conversations with 3 and went with one who knew the self-build context and he helped a lot in terms of the brief. We might have overpaid (it was not cheap) but I felt super confident with him, and we have as far as I can tell everything we need. In the absence of a SE already (we are not yet at that stage) I wanted a GI so this seemed to be a good way to do it. We asked a SE friend to check the brief for us.

 

I totally get it if you have a SE and they can offer to detail a brief for you - would probably mean you only commission what you need. 

 

Anyway, my experience in case it's helpful!