Improving soil bearing capacity

[Bramco]

Hi,

 

Our foundations are being designed by the timber frame co.  Currently they are stipulating that we excavate between 900 and 1200mm below the level of the insulated slab that they will be installing and that we should remove and fill any soft spots when the excavation is down to the right level.

 

The soil survey shows about 0.5m of firm made ground above 0.3m of firm brown clay (90KPa - so above the 70KPa advice on foundations). Below that it's firm, red brown silty clay down to mudstone at 3m or so.  The issue is that at 1m the clay is 65KPa and at 1.3m 60KPa, so it goes from firm at the top, to soft lower down before getting firmer until the mudstone at 3m.

 

Instead of excavating to nearly 1.5 m below ffl, would a system like the Geosynthetics Elotex geotextile and Tenax grid mean we could reduce the amount of excavation.  Has anyone used a system like this to improve the soil bearing capacity of their foundations?

 

Simon

[saveasteading]

Unless very soft, it is usually better to leave it alone.

But, as you say it is 'made' ground. That perhaps needs to come out and be rolled back in layers.

 

Perhaps the company is being cautious, and it is not their money.

An Engineer should advise you on this. (I am but am not working for you or insured to do so).

As you go deeper, the concentrated loads from the house spread further, so the top being strong is a good thing.

eg a road is very strong, built on lesser ground.

 

Adding textile is unlikely to be much use.

[Carrerahill]

6 minutes ago, Bramco said:

Hi,

 

Our foundations are being designed by the timber frame co.  Currently they are stipulating that we excavate between 900 and 1200mm below the level of the insulated slab that they will be installing and that we should remove and fill any soft spots when the excavation is down to the right level.

 

The soil survey shows about 0.5m of firm made ground above 0.3m of firm brown clay (90KPa - so above the 70KPa advice on foundations). Below that it's firm, red brown silty clay down to mudstone at 3m or so.  The issue is that at 1m the clay is 65KPa and at 1.3m 60KPa, so it goes from firm at the top, to soft lower down before getting firmer until the mudstone at 3m.

 

Instead of excavating to nearly 1.5 m below ffl, would a system like the Geosynthetics Elotex geotextile and Tenax grid mean we could reduce the amount of excavation.  Has anyone used a system like this to improve the soil bearing capacity of their foundations?

 

Simon

I'd speak to a local SE.

 

Slabs or rafts were partly brought in as a means of solving issues like this one, in honestly so house builders could rattle houses up on poor ground or ground that was maybe not ideal for building, the idea being that it was just a big raft floating on the ground (as usual top soil etc. removed obviously).

 

The issue you often find with TF companies is that they sort of just have an idea in their head and that is what they go with every time. 

 

We had family member who wanted a sunroom, builder said it needed to be a 1.5m deep trench fill footing - the existing house was on a slab - the family member who is a civil engineer and a specialist in concrete was shocked at the proposal on the grounds that his house is sitting on a big raft,  then this little sunroom would connect to the house firmly seated with huge concrete boots on - they fell out big time over this. Transpired the local builder discovered this always passed BW so would just spec it every time clearly unware how founds and connections etc. all work together.

[Bramco]

1 hour ago, Bramco said:

Geosynthetics Elotex geotextile and Tenax grid

 

Interestingly spoke to a technical guy at Geosynthetics and they reckon the Tenax grid can reduce the amount of hardcore depth needed by 30% to 50%  which could be quite useful for us.

 

And my brother in law who trained as a carpenter and then started building extensions etc. said the first foundations he had to do, he thought he had to take down to 3ft.  About halfway down he hit bedrock and kangoed it out to 3ft!!   He didn't do that again after BC pointed out that bedrock was solid enough.

 

Simon

[saveasteading]

Yes it can reduce the thickness in roads where the wheels are very high point loads. Also some heavy Civil Engineering solutions.

Thought I should check, and the tenax site says this.

The innovative 3D GRID are geogrids specifically designed for road and railway applications, where the load applied to the soil mainly develops longitudinally.

 

You have very simple vertical loads. Strength and thickness of each layer is what matters.

[ToughButterCup]

2 hours ago, Bramco said:

.... Has anyone used a system like this to improve the soil bearing capacity of their foundations?

 

Yes.

Almost the same spec ground as ours (glacial till). Whatever the engineer specifies will be fine: the business case is a no-brainer. This is the stuff we used. Saved us a good few hundred quid.

But just a quick heads up - while on the one hand its brilliant during the build up of your foundation - it's the Devil's Own Assistant when, later, much later you need to dig through it by hand (in our case to lay the foul dain). Like I had to during the first lockdown. 30 meters of it. 

 

Which is one of the main reasons why I say - If ya can't take a joke, don't start a self build

If you need to put anything under it, get it sorted out before you lay the stuff.  Got some spare if you need it. 

 

 

[IanR]

Advanced Foundation Technology have some innovative solutions for poor ground conditions. ie. a Suspended slab on piles.

 

Luckily Olof is a better Structural Engineer than web designer, you may struggle to find the details on his website, but here it is:

 

https://www.advancedfoundationtechnologylimited.co.uk/wp-content/uploads/2019/01/aft-groundshield-designs-2018.pdf

 

While your timber frame company is supposed to be doing the raft getting AFT involved may be a lot better than a 1500mm excavation.

 

I have a standard Insulated raft from AFT. They're happy to do design, supply and install or just design and install, or somewhere in between, and can include the UFH as well.

[Bramco]

@IanR and @ToughButterCup thanks for the links. More research!!  ?

 

[Gus Potter]

2 hours ago, Bramco said:

@IanR and @ToughButterCup thanks for the links. More research!!  ?

 

 

Hi Bramco.

 

Have a look at your soil report. It may mention what is called the soil shrinkage potential also known as the volume change potential. This a rough estimate on how much it will shrink and swell between say winter and summer.. it can be very little or a lot.

 

Here is a link to the NHBC guidance, plenty diagrams etc to give you a feel for things.

 

https://nhbc-standards.co.uk/4-foundations/4-2-building-near-trees/4-2-10-heave-precautions/

 

There is a bit on how trees can influence the design. Once you have got the basics, then you should be better placed to look at identifying a good solution particular to your site, recognising that you have a stiffer "crust" over a softer layer.

 

For plenty of domestic stuff ground bearing type slabs can be made to work above an allowable bearing capacity of 50 kPa without spending a fortune. It's the swelling / shrinkage and things like trees that can put a bit of a spanner in the works.

 

 

 

 

 

 

[Bramco]

13 hours ago, Gus Potter said:

It's the swelling / shrinkage and things like trees that can put a bit of a spanner in the works

 

Fortunately we're about 30m from the nearest trees.   One thing less to worry about!

 

Simon

[ToughButterCup]

@Bramco, I notice the term Made Ground in your post. That sets the Caution Bell ringing in my head. 

 

Please get a Structural Engineer to give you an opinion about the foundations.  We sought the opinions of two.  One, using the same soil data as the other, saved us many thousands of pounds. He recommended Ground Improvement Columns (a much cheaper piling system to you and I) where the other SE wanted steel sleeved concrete piles.

It was a post on BH that alerted me to the fact that not all SEs are equally informed about all aspects of Structural Engineering.  I mean who knew that eh? ?

[Bramco]

13 minutes ago, ToughButterCup said:

I notice the term Made Ground in your post. That sets the Caution Bell ringing in my head.

 

As I understand it that's 'earth' to you and me. The plot was allotments at the time of the war and since the sixties has had an occasional pony on it and few 'lawn mower' sheep borrowed from a local famer - who used to thank my father in law with some of the proceeds - a lamb, once a year.

 

We're sort of in the hands of our timber frame people who are contracted to do the insulated slab - the bit at issue is the bit we have to do under that.  We're expecting their foundation engineer to send through the plans any day now based on the SE's input on line and point loads.   I've sent them the info on Tenax and Tensor - will be interesting to see if they poo poo it, or hopefully can reduce the depth of we have to excavate, get rid of and fill with hardcore.

 

Simon

 

Note to self, must update the thread once we've got a final plan....

[deancatherine09]

On 18/05/2021 at 13:03, Bramco said:

 

As I understand it that's 'earth' to you and me. The plot was allotments at the time of the war and since the sixties has had an occasional pony on it and few 'lawn mower' sheep borrowed from a local famer - who used to thank my father in law with some of the proceeds - a lamb, once a year.

 

We're sort of in the hands of our timber frame people who are contracted to do the insulated slab - the bit at issue is the bit we have to do under that.  We're expecting their foundation engineer to send through the plans any day now based on the SE's input on line and point loads.   I've sent them the info on Tenax and Tensor - will be interesting to see if they poo poo it, or hopefully can reduce the depth of we have to excavate, get rid of and fill with hardcore.

 

Simon

 

Note to self, must update the thread once we've got a final plan....

Hi - we are going through very similar at the moment (probably with the same TF company!) and wondered if you ever got a resolution to this?

[Bramco]

On 18/05/2021 at 13:03, Bramco said:

Note to self, must update the thread once we've got a final plan....

 

Well that note to self didn't work did it!   We moved in on top of the slab a couple of months ago!

 

@deancatherine09   We ended up just digging to the depth the TF Co specified and now have a mighty bund down one side of the plot.

 

The whole process with the TF Co. stalled for what seemed like a few months while their SEs didn't seem to be able to give us a straight answer. Somehow it seemed as though they normally would make the decision as the foundations were being dug, i.e. based on what the actual site conditions were.  To cut a long story short though, we had to dig down about 1m and lay down 500mm of compacted hardcore onto which the TF Co put their 150mm of hardcore, the sand blinding layer and then the EPS basin for the slab.

 

Weirdly, they didn't put the drains in until the sand layer was down, so everything was dug out along the line of each drain and then re-laid.  We'd have thought that they would have put the drains in place and then done the 150mm and the sand blinding layer after that.

 

If it is the same TF company (I've DM'ed you about who we used), they will also have specified land drains - ours went in some time after - when our groundworkers were doing the rest of the drains (rainwater and foul).  I guess they could go in as you are doing the 500mm of compacted hardcore that you will be responsible for.

 

Off topic, I would insist that the team erecting the frame are not a subcontracted company - ours were and they created a lot of problems.  If we'd known that the erection would be subcontracted out, we'd have employed a project manager to make sure that the work was done to spec.  Although having said that, even though there were some mighty hassles, we're very pleased with the end result. 

 

Simon

[Mr Punter]

49 minutes ago, Bramco said:

Off topic, I would insist that the team erecting the frame are not a subcontracted company - ours were and they created a lot of problems.  If we'd known that the erection would be subcontracted out, we'd have employed a project manager to make sure that the work was done to spec.  Although having said that, even though there were some mighty hassles, we're very pleased with the end result. 

 

I have had hassles with TF erection crews.  It is luck of the draw.  With some of them it seems like they found them at a bus stop.  Others are pretty skilled, but generally it is a smash and grab type business.

[Temp]

I'm no SE but I thought the width of your foundation trench could be increased to effectively spread the load over a wider area when the ground was a bit soft. The NHBC have this but pushing in fingers doesn't seem very scientific..

 

https://nhbc-standards.co.uk/4-foundations/4-3-strip-and-trench-fill-foundations/4-3-7-safe-transmission-of-loads/

 

[George]

It's either thumb pushing or a geotechnical investigation unfortunately (although I'd love to get a hand shear vane, it's only applicable to certain soil types and not much better than a thumb)