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 Monday at 15:11 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 Tuesday at 16:19 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.