Soil and Ground Investigation - options...

[AppleDown]

Hi everyone. As we ramp up the next stages of our build, we're looking to get info over to a number of people to help finalise quotes and designs. The major one at the moment is a ground/soil investigation.

 

Foundation and timber frame guys both need the info on their sides, so I'm looking at what options we have there.

 

From reading on here and research online it looks like 'Phase 1 - desktop research' and 'Phase 2 - site investigation' are the go to. My questions are:

 

• Does 'Phase 1' actually contribute anything? Looks like it just brings together local soil types, water tables etc that aren't super specific to the exact location
• Is 'Phase 2' enough for structural calcs? That seems more robust and actually has specific location details
• For BC, what do they need to see?
• And finally - does anyone know someone in the SE who can do these for us?

 

Thanks as always!

[saveasteading]

SE  located in the SE.

The SE is too big an area. I suggest talk to a very local SE.

 

They will know the local soil types.

Probably they will want to see one or 2 pits  on site, so that only takes a couple of hours.

 

As you suggest, it isn't complex unless the site has history.

That is what phase 1 could show.

eg you don't want landfill or pollution. You may have done this already.

Building Control will accept the SE report.

 

Try phoning one or more for a chat about the ground and subsequent needs.

I'm wondering how you have a foundation contractor at this stage,when there isn't a ground report yet.  And is the frame design dependent on the ground?

[AppleDown]

Thank you @saveasteading. So with Phase 2 we’re still looking at a lab test to inform loads etc? 
 

No contractors signed up yet, all needed to get a solid quote so we can fully budget. Frame design isn’t dependent, but will inform the foundations. Just ensuring we’re getting each stage to a level of information we can move forward with.

[IanR]

You've not mentioned needing to do a Contamination Survey ie. a Condition on your planning that requires one prior to commencement.

 

If you don't have such a condition, then a Phase I isn't required.

 

Even a Phase II is more than you need as it will likely include gas collection, lab work, which will be looking for contamination within the dug boreholes.

 

People needing Contamination Surveys do combine them with a Ground investigation to save money, ie. if the right rig is used for the boreholes it can also give you the bearing capacity at various depths, which is the info the SE needs for the foundation design.

 

Normally the SE designing the foundation will have a specific requirement for what they need. It may just be to be onsite when the trench is dug to see where the organic matter stops, or if it's for, say, a ground bearing insulated raft, they may specify a number of bore holes in specific locations where they want the bearing capacity down to 4m or 6m.

 

Sounds like your foundation design is remote (and your timber frame, but not sure what they want from the ground investigation)

 

So you don't need another SE to do the Ground investigation, someone like Geosphere Environmental Ltd. could send a rig down and do the bore holes in a morning. If the Foundation people really need to know soil types they could also do the analysis and/or lab work.

What specifically are the foundation and timber-frame people asking for?

Edited January 9 by IanR

[AppleDown]

Thanks @IanR. Yes, we’ve gone out to a few of the insulated raft providers, so they’re all looking at it from a remote POV. They obviously need soil bearing capacity, and the only way I can see us getting that is from the (plainly ridiculously expensive) ground investigations. 
 

From the timber frame guys, it’s not that they need the same info, they just want to know what they’re sitting on from a foundation standpoint. And I can’t give a solid answer until the first bit is done.

 

I’ve reached out to Tanner Structural for the SE main designs, so waiting to hear back from them (sounds like they’re super busy at the moment).

[twice round the block]

You could try igne geotechnical services. Have own engineers and testing laboratories. 

[saveasteading]

I'd say we dug a trial pit or two on 90 % of our projects and our SE simply observed the stratum, which was usually as anticipated. Sometimes he'd go in the hole and prod at it.

It isn't necessarily very complex sometimes they'll send samples fof testing. Often not.

On the other 5% we had boreholes. 

It really depends on the natural terrain And how ptedictable it is. Hence your SE might immediately save unnecessary  testing. 

 

Oh and the other 5%? I was cocky enough to not do any trial pits because it was 'obvious'. Once I was expensively wrong though.

 

[IanR]

3 hours ago, AppleDown said:

They obviously need soil bearing capacity, and the only way I can see us getting that is from the (plainly ridiculously expensive) ground investigations. 

 

Then you just need a plate bearing test at the surface rather than a bore hole.

 

Don't know them but these have a an office in Southampton

 

https://www.earthenvironmental.co.uk/plate-bearing-test/#contact

 

An advantage of bore hole tests though is that it will also identify if there's any ground water passing under the site, within or close to the depth of the foundation. I'm on clay and the bore holes showed there was water passing through seams in the clay at about 600mm down. While it was below the dig level I still took the French drain I'd planned around the perimeter a little deeper to make sure I kept any water away from the sub-base.

 

Are Advanced Foundation Technology on your list? Or are you specifically going with Tanner as you plan to use MBC?

Edited January 10 by IanR

[OwenF]

I’m a geotechnical engineer and GI is my day job (though typically on large infrastructure)

 

Previous replies have covered some points, but I’ll add/clarify some more.

 

- Planning conditions usually only ever focus on contamination assessment. As said, if it isn’t a condition in you planning, you don’t need a GI for this purpose.

 

- Phase 1: desk study. These range in scale from a quick look on BGS website, to £30k worth of work. Again, if not a planning requirement, don’t pay for one. Search BGS viewer and you’ll find the mapped geology (superficial and bedrock) for your location. You can also turn on the ‘BGS borehole’ layer and see nearby historic boreholes to get a feel for thicknesses of strata.

 

- Phase 2: intrusive investigation should be validating outcomes of the Phase 1 e.g. proving presence of sand depth for bearing/ presence of water as risk, etc (not comprehensive).

Your SE should be explicit on what he expects and provide a brief / scope for GI (usually based on the Phase 1 desk study) if he’s expecting you to engage and procure a supplier. To be honest, bigger SEs will work with geotechnical consultants regularly and would seek them to do this. However I’d expect for a house building SE this should be routine for them to spec and recommend a basic scope for you (or minimum what info they need!)

 

GI Techniques: depending on what your SE needs and the ground risks for your site, I expect the most you’d need is a couple of window sample boreholes with SPTs and a trial pit or two.

Day-rate for windowless sampler rig is between £600-£1200/day. They’ll do two shallow holes in a day easy.

 

Your SE should specify if he wants an interpretive report or if he’s happy taking the factual data. The former will add cost (probably £800-£2000 depending on outfit).  Tbh, I think overkill and most SEs worth their salt would know an SPT result and be able to make conservative foundation design decisions.

 

All above is simplified for (what honestly) is low risk Geotechnics. The discipline is a skill as much as structural design and it’s easy to spend money that isn’t needed.

[AppleDown]

2 hours ago, OwenF said:

- Planning conditions usually only ever focus on contamination assessment. As said, if it isn’t a condition in you planning, you don’t need a GI for this purpose.

Nothing noted on that, so hopefully clear on doing this.

 

2 hours ago, OwenF said:

- Phase 1: desk study. These range in scale from a quick look on BGS website, to £30k worth of work. Again, if not a planning requirement, don’t pay for one. Search BGS viewer and you’ll find the mapped geology (superficial and bedrock) for your location. You can also turn on the ‘BGS borehole’ layer and see nearby historic boreholes to get a feel for thicknesses of strata.

Just spent the last 30 mins going through this - had no idea it existed, so thank you. Below is a strata table from a borehole about 200m away (for a well at a golf course). Lots of clay. The other one about 400m away is from 1941 for similar well and readings. Really interesting!

Really appreciate the input. I'll go back with a few more questions and see what they say. As always, this site is invaluable

[saveasteading]

Well done finding that. So you are 99% sure of sitting on London Clay that is 258 feet thick.

Routine for your SE.