Soil investigation for basement - cable percussion AND percussive window sampling rig?

[Alan Ambrose]

Hi,

 

SE has asked for 6 boreholes - 2 cable percussion and 4 percussive window sampling. Does it make sense to use both methods?

 

Whole spec below. This is on clay with sand starting maybe 5-15m down (from nearest BGS logs).

 

Ta, Alan

 

• Formation of 6 no. 6.0m deep windowless sampling boreholes.
• Soil logging of all 6 boreholes.
• SPT testing @ 1m intervals on 4 of the boreholes.
• Undisturbed samples collected for lab-based shear box testing (6 no samples @ 1m intervals).
• Disturbed samples collected for Particle Size Distribution testing (PSD) (allow for 6 no samples).
• Atterburg limit testing on any fine-grained material encountered.
• Installation of groundwater monitoring well (SI company to allow for 3 subsequent site visits).
• Geotechnical report to document all findings.

[Blooda]

Similar to our requirements, except we had a gas monitoring [CO₂] instead of Ground Water Monitoring.  

A shy over £3k inc Vat in the Summer of '19.

 

 

[Alan Ambrose]

Ah, my lowest quote £5K so far. The questions is - does it make sense to use 2 drill rigs?

[Declan52]

The smaller rig probably a dando won't get much past 6/7m deep. It's just not powerful enough. This rig uses plastic sleeves to retrieve the material so it's classed as undisturbed which you need for your lab testing.

The shell and auger is capable of going 30m deep easily so it's needed to get you to to your 15m depth. It is basically a very heavy weight on a steel tube you drop hundreds of times chipping out what's below so no good for samples. It can perform spt testing which from you can recover a tiny sample, like a big Xmas size tube of smarties.

[Declan52]

If you don't need samples after 6m you can do another process called dynamic probing which gives you data rather than actual samples. The smaller rig is more than capable of doing this to over 10m. 

It's just drives a thin pole, 40mm diameter approx and you count the blows It takes to go down each 1m section. Soft sands could be 4-8 blows and very hard clay could be 50+ and you just stop if you encounter this before you drive the rod in that hard you can't remove it. From this data a structural engineer can determine the strength of what's below. 

[Alan Ambrose]

Thanks Declan, very interesting, and you're right - the SE has only specified to 6m.

[Pendicle]

We had 6 bore holes 6m and 2 trial pits with gas monitoring and soil samples, desk top and final report came to little over 6k. Paid final invoice this week, we are also having a basement.

[OwenF]

Geotechnical engineer here.

 

It’s difficult to assess validity of a spec without understanding the desk study and proposed design. That said, it doesn’t look totally unreasonable.
I would probably spec this for a a small

multi-plot development, so not sure the size of your house?

 

Cable percussive seems overkill presuming foundations will be circa 2m deep max. In good ground window sample should get 5-6m.

 

However window sampling won’t achieve enough sample for shear box testing. Needs approx. 30-40kg of suitable material (basically means enough material with a certain grading). Double this minimum amount of material for PSDs.

 

Why is shear box necessary for sand at 5m+ depth? It’s an expensive test. An SPT in base of window sample hole, then use established correlations to get phi. Saves you £3-400 of a test, never mind the sampling.

 

I would seriously consider replacing the CP rig with trial pits if depth isn’t a concern. Do a soakaway test in one whilst you’re at it.

 

£5k for all that (incl. CP rig) sounds too good to be true. Does it include testing, reporting, supervision? 
Watch out for ‘delays’. Is it lump sum or day rate. Also lump sum never means lump sum in GI world! 

Edited February 9, 2024 by OwenF

[Alan Ambrose]

Thanks @OwenF. It's for a potential basement, hence 6m. I imagine excavations for the actual basement will be to say 3-4m. Goodness I still find geo a strange art - a module on soil mechanics in my engineering degree 40 years ago obviously didn't provide much enlightenment .

 

To be clear, the SE asked for the list above, the £5K quote was for "track-mounted percussive window sampling rig, dynamic probe tools and equipment". I'm not sure why the geo quote ignored some of the requirements. I have asked them to explain and also to talk to the SE directly. 

 

Frantically reading Craig and also Atkinson but it's still black art heuristics to me, give me a clean CFD problem any day

 

 

 

 

Edited February 9, 2024 by Alan Ambrose

[Gus Potter]

13 hours ago, OwenF said:

Geotechnical engineer here.

 

It’s difficult to assess validity of a spec without understanding the desk study and proposed design. That said, it doesn’t look totally unreasonable.
I would probably spec this for a a small

multi-plot development, so not sure the size of your house?

 

Cable percussive seems overkill presuming foundations will be circa 2m deep max. In good ground window sample should get 5-6m.

 

However window sampling won’t achieve enough sample for shear box testing. Needs approx. 30-40kg of suitable material (basically means enough material with a certain grading). Double this minimum amount of material for PSDs.

 

Why is shear box necessary for sand at 5m+ depth? It’s an expensive test. An SPT in base of window sample hole, then use established correlations to get phi. Saves you £3-400 of a test, never mind the sampling.

 

I would seriously consider replacing the CP rig with trial pits if depth isn’t a concern. Do a soakaway test in one whilst you’re at it.

 

£5k for all that (incl. CP rig) sounds too good to be true. Does it include testing, reporting, supervision? 
Watch out for ‘delays’. Is it lump sum or day rate. Also lump sum never means lump sum in GI world! 

Lots of agreable points here @OwenF

 

7 hours ago, Alan Ambrose said:

Thanks @OwenF. It's for a potential basement, hence 6m. I imagine excavations for the actual basement will be to say 3-4m. Goodness I still find geo a strange art - a module on soil mechanics in my engineering degree 40 years ago obviously didn't provide much enlightenment .

 

To be clear, the SE asked for the list above, the £5K quote was for "track-mounted percussive window sampling rig, dynamic probe tools and equipment". I'm not sure why the geo quote ignored some of the requirements. I have asked them to explain and also to talk to the SE directly. 

 

Frantically reading Craig and also Atkinson but it's still black art heuristics to me, give me a clean CFD problem any day

 

 

 

 

To paint a bit of a picture I think Alan has in mind a single storey relatively light weight timber framed structure going on top of a basement, say a formation depth of 4.0m, insulation on that with a concrete box on top.

 

This is a bit different from a heavily loaded raft (basement slab) say where you may want to recognise that the pressure bulb under a raft can be quite extensive and extend to a considerable depth, just say 2 times the maximum slab dimension.. Atkinson et al? 

 

Being pragmatic you could take the view that what you are digging out to form the basement is heavier than what you are putting back, thus the stress at formation level is similar or less. Now let's also say that the local geology and superficial deposits are pretty well understood. But due to the natural variation in soils you could end up being unlucky and build on the only rubbish bit. Now if that was to be the case then you could maybe even pick this up during a walk over survey.. notice a depression in the ground say as the soil has been loaded more histrocally than what you are intending to put back.

 

If your formation is a 4.0m then investigating to 6.0m should cover most risks given the loads at formation level in this case.

 

I often lean towards trial pits as you can really get down and dirty with the soil, also you expose a larger surface area of soil that you can "feel", look at, smell (if confident ground is not contaminated) and get a better idea of the strata etc. Also great point for @saveasteading on another post.. you can do some soakaway tests etc at the same time.

14 hours ago, OwenF said:

Why is shear box necessary for sand at 5m+ depth? It’s an expensive test. An SPT in base of window sample hole, then use established correlations to get phi. Saves you £3-400 of a test, never mind the sampling.

Jury is out on this one, can only suggest this may be a requirement of a specialist basement designer, best thing to do is ask what they are going to do with the results. They may have a perfectly and sensible reason for doing so on this kind of design.

 

On something like this I would want to try and understand as much as I can about what is lying say up to 2.0m below the formation as a perched water table / local artisian pressure is always a risk. For self builds the cost can run away if you need to indulge in extensive bracing of the excavation and have to manage ground water. If in good ground you could assess the stand up time, and try to mitigate the temporary works cost. With a fair wind you could just batter back the excavation.. simple if you have the space to do so.

 

A contractor pricing this is often going to assume the worst and price accordingly as it is a one off self build. Their mind set is often.. it is a one off so if I lose a bit I'll not get the chance to get it back on the next job.

 

If you plan out the works, sequence and provide the Contractor with good information it should help drive the cost down. There is a risk (will be in the contract) in that if it all goes south you'll have to pay more. But you may not end paying any more at the end if it does go south than if you accept a price from a contractor initially based on the worst case. Also if you have good ground information you'll be in a better postion to push back against a Contractor if they hit for unjustified extras.. you can say.. I gave you the info.. it's your fault for not reading it.

 

14 hours ago, OwenF said:

Geotechnical engineer here.

I bet you've told so many people so many times.. set aside a meaningful percentage of your budget, give me the resources to do my job properly and I'll probably, nine times out of ten, easily cover my fee and could save you thousands or more..

 

If you look at the figure mentioned above we are talking a couple of thousand each way.. but imagine what it could cost if you encounter something in the ground that you could have discovered for that small extra amount of 1 - 2 thousand. That difference could get swallowed in a day or two on site with especially if you need a sudden re design.

 

 

 

 

[Alan Ambrose]

For anyone following along - the consensus was that the cable percussion rig has a bit more grunt and could deal with shale and sand easily should we find it below 5m - whereas the dynamic probing rig might run out of steam. So, the current proposal is to do 2 holes with the former and 4 with the latter. That's probably overkill as it is almost certainly fairly homogeneous clay, but 'you don't know what you don't know'. I also learnt that all of the rigs can estimate allowable ground pressure and that nearly all ground failures are shear stress failures. Also the particle size distribution tests are to gauge clay shrinkage and swelling. My education continues...  

 

Also, my house loading is ~8 kN/m^2 but dwarfed by the loading of the basement concrete itself at ~33 kN/m^2. But ... the existing soil to be dug out is presently loading at about ~51 kN/m^2 @ 4m depth. So the structure to be introduced actually weighs less than the earth sitting there right now.

[OwenF]

3 hours ago, Alan Ambrose said:

Also, my house loading is ~8 kN/m^2 but dwarfed by the loading of the basement concrete itself at ~33 kN/m^2. But ... the existing soil to be dug out is presently loading at about ~51 kN/m^2 @ 4m depth. So the structure to be introduced actually weighs less than the earth sitting there right now.

Common for basements (which I didn’t realise you had when I answered before).

Net bearing pressure = gross pressure at base of foundation - pressure of soil

removed. 
 

@Gus Potter thanks for adding meat on the bones to my hurried observations.

Yes, I am constantly striking a balance between trying to save money upfront vs. convincing a client that money spent upfront saves in the long run. 
 

The additional context from @Alan Ambrose clarifies the original spec a lot. I didn’t realise there was a basement.

 

Incidentally one of the annexes of BS EN1997-2 gives recommended depth of ground investigation below different foundation types. 
 

It sounds like he’s reached a reasonable compromise of SE requirements and not OTT with a bit of knowledge added at the same time!

[Gus Potter]

On 14/02/2024 at 18:05, OwenF said:

Yes, I am constantly striking a balance between trying to save money upfront vs. convincing a client that money spent upfront saves in the long run.

 

On 09/02/2024 at 06:49, OwenF said:

Geotechnical engineer here.

Great to have you chipping in. You may get a bit of slagging off.. but just stick to your guns.

 

Stick with it.  BH can be great fun and rewarding if your are happy to give pro bono advice.

 

Gus

 

 

[saveasteading]

For context, Why do you need or want a basement? 

Is this an extension or all new?

They're risky,  and expensive, and the remaining clay may react to the change in loading.

@OwenF and @Gus Potter will be more up to speed on the geotechnics, but I've had these tests done and also designed and built one basement  hence my question.

 

[Alan Ambrose]

My planning applications went in last February - so this ship sailed long ago. The two alternative designs both specified ‘basement if ground conditions allow’. The plot is next to a farmhouse (say 75m away) - one of Suffolk’s 2,000 ‘heritage’ assets and the LPA have medieval ideas about ‘subservience’.

 

Unless you want to argue this out with the planners on my behalf and successfully resubmit my planning applications for me  … I am where I am.

[saveasteading]

2 minutes ago, Alan Ambrose said:

am where I am.

It would help to know why you want a basement.

[Pocster]

Christ ! Basement feasibility seems like an expensive game from when I did it !

[Alan Ambrose]

Ah I thought I had explained - the planners want our buildings to be 'subservient' (i.e. as small as possible above ground) and even though we have 1/3rd acre that means we either have to have a house smaller than we want or build a basement. Property values are high in Suffolk and the economics work out.

 

In the Highlands you may have as much land to build on as you care to and maybe your planners are not so fussed and would happily let you build out rather than down. Fantastic. But the circumstances are different here.

 

We could put in more planning applications for more footprint, but (a) they don't have a high probability of being approved and (b) that would set us back 18 months.

 

Unlike Germany, we don't do basements much in UK, so they're expensive and more risky than space above ground. But just like building over 2 storeys, building down is a way to get what we want.

 

There's one other economic factor - with clay our founds are likely to be deep-ish. Beyond a certain point, you might just as well have a basement anyway as the incremental cost is not going to be so high compared with say building on good ground. 

Edited February 17, 2024 by Alan Ambrose

[joe90]

2 minutes ago, Alan Ambrose said:

There's one other economic factor - with clay our founds are likely to be deep-ish. Beyond a certain point, you might just as well have a basement anyway as the incremental cost is not going to be so high compared with say building on good ground. 

Good point 

[saveasteading]

1 hour ago, Alan Ambrose said:

with clay our founds are likely to be deep

Only if there are thirsty trees nearby or your clay is very soft.. Most founds on clay are 1 to 1.2m.

Otherwise it makes sense.

So the planners haven't asked for a basement. Could subservience be provided by low profile extensions. Catslide roof? That makes it bow down before the other buildings.

[Alan Ambrose]

Just to report back: the ground turned out to be quite hard boulder clay and the window sampling rig, which turned up first, only got down to 2.5m. So, the cable percussion rig was a good call and we moved 2x GW monitoring to those holes instead.

 

We also dug a percolation test pit by hand and the 300x300 hole in the bottom started slowing filling with water - and then kept on going. So, a sort of reverse perc test. A week later it was at 67cm in the main 1m deep hole! (It is protected from direct rainwater entry.) Our theory is it's using the slight sandy deposit to convey surface water. So, we'll need v good surface drainage for the winter and we're fairly close to proving that soakaways are out of the question.  

[saveasteading]

51 minutes ago, Alan Ambrose said:

close to proving that soakaways are out of the question.  

Don't assume that yet. What level did the groundwater rise to?

[Alan Ambrose]

Well I hope it's from the surface or 'perched' rather than GW, but 67cm of the 1m cube depth in 6 days

Edited March 21, 2024 by Alan Ambrose

[saveasteading]

1 hour ago, Alan Ambrose said:

67cm of the 1m cube

Sorry, I see you had said already.

330mm below ground level Is very high but it took 6 days to fill.

Great fun for foundations or even any site strip.

I think do this again after a drier spell of weather.

 

The water coming out of a digester is not in large quantities so maybe it can be made to work ( site specifics and pragmatic bco depending).

But where will rainwater in modern vast volumes, go?

[Alan Ambrose]

>>> But where will rainwater in modern vast volumes, go?

 

There's a culvert / ditch at each end of the plot which we can use for drainage - so it's just a matter of convincing a BCO that we've tried hard to do SUDS I think.

 

The clay does this in the summer, so I'm expecting that it'll be OK to dig, but quite hard.

 

Edited March 21, 2024 by Alan Ambrose