Geolotechnical Analysis for Piled Foundations

[Randomiser]

As mentioned before we are likely to be using piled foundations, so I am hoping I can sense check a suggestion being made to me.

 

We previously engaged a Structural Engineer to do work on the foundations for us.  As part of that they wanted us to get some basic analysis done on the soil type, water table, etc. which we did.  Having had this input the Structural Engineer prepared a piling plan with a required resistance (not sure if that is the right term) for each pile which we used to get quotes from various companies for piling.  We have been on pause for a while for a number of reasons, but now getting back on it.

 

It's no been suggested that we should get a geotechnical survey done.  But I am confused as to why this is required if the piling plan is already done.  I wondered what experience other who used piles had around this, did they get geotechnical surveys done and if so what information did it give and how was it used?

 

Thanks in advance. 

[Declan52]

The first survey is just a general investigation into what's below the ground. A geotechnical investigation will involve a full site investigation using a drilling rig to perform a series of tests at different depths to give you a set of figures from which you can work out the bearing strength of the ground. From this your piling company will be able to say you need X amount of piles to be driven in at least 8/9/10m whatever it will be. They can then work out a more exact quote knowing how many and how deep they need to go.

[Randomiser]

41 minutes ago, Declan52 said:

The first survey is just a general investigation into what's below the ground. A geotechnical investigation will involve a full site investigation using a drilling rig to perform a series of tests at different depths to give you a set of figures from which you can work out the bearing strength of the ground. From this your piling company will be able to say you need X amount of piles to be driven in at least 8/9/10m whatever it will be. They can then work out a more exact quote knowing how many and how deep they need to go.

OK, so essentially it is risk mitigation.  Piling could go ahead without doing it, but that would mean having much less idea of what the total cost would be.  Having it done won't change the answer, but it could stop you going ahead with something you can't afford / is uneconomic.

[Andrew]

Our piling company were quite clear we wouldn't know how deep the piles were going until the first one was driven to set (and even then the conditions could change across the site so whilst the others would most likely be in the same ball-park they may differ).

 

The cost of mobilisation and the first pile will most likely be around the price of the geotechnical survey - you can always say stop if the first pile goes down to 30m. I wouldn't waste my money on the survey. 

[ToughButterCup]

Hmmm, @Andrew.... our piling company wouldn't come on site without a proper site geotech. survey and two trial pits.

 

Reason?

Risk reduction. 

To get a 45 tonne rig on site is expensive....

[Andrew]

5 minutes ago, AnonymousBosch said:

Hmmm, @Andrew.... our piling company wouldn't come on site without a proper site geotech. survey and two trial pits.

 

Reason?

Risk reduction. 

To get a 45 tonne rig on site is expensive....

 

Depends on the piling method I guess. Our mobilisation cost was £1200. 

[Bitpipe]

For our basement we had an initial survey which comprised 5-610m probes (steel rods hammered into ground by a rig, penetration for each 'whack' measured) and a few cores (soil composition, gas & water monitoring).

 

Results were inconclusive and suggested piling may be necessary due to historical near 'made ground' i.e. filled in gravel pit.

 

We paid for a more intensive follow up survey (10-12 probes, 5-6 cores) and got better data that suggested that piling not required.

 

While it pained me to pay the extra £2k (discounted as they made a few errors in the first survey) I was happy we had solid data to move forward with.

 

Up to you ultimately, you could crack on and see what happens but at that stage you're committed (if you can find a firm to do it without strong data).

[Declan52]

If it was me I would want to know exactly how much I was going spend on piles which means getting a proper site investigation done. Once you have this then you might think no it's going to cost to much or yep that's a good price and proceed.

@Bitpipe the wacking of the rods is called dynamic probing and will give an indication of the bearing strength of the ground. You measure out  100mm sections and count how many blows it takes to drive the rod that depth. The more blows the better. But as it's just a beating session there is no way of knowing what is actually down there so that's why they take the cores.

[Temp]

Our SE recommended 2m deep trench foundation and we've had no problems. House a few doors down was recommended 3m deep trench foundation by different SE. As a result they got estimates for piling which were slightly cheaper. Problem is when they started piling they found they had to go much deeper than expected and it ended up costing quite a bit more. Don't think it occured to them to do a trial hole.

[Randomiser]

1 hour ago, AnonymousBosch said:

Hmmm, @Andrew.... our piling company wouldn't come on site without a proper site geotech. survey and two trial pits.

 

Reason?

Risk reduction. 

To get a 45 tonne rig on site is expensive....

Did the geotechnical survey give you an estimate for the depth of the piles?  If so, was that reasonably close to the actual depth of the piles in the end?

[ToughButterCup]

34 minutes ago, Randomiser said:

Did the geotechnical survey give you an estimate for the depth of the piles?  If so, was that reasonably close to the actual depth of the piles in the end?

 

Yes. And yes, it was bang on for each pile. Except one ? ( refusal required about 200Kn.)

The last one. You couldn't make it up.

 

[Randomiser]

5 hours ago, Andrew said:

Our piling company were quite clear we wouldn't know how deep the piles were going until the first one was driven to set (and even then the conditions could change across the site so whilst the others would most likely be in the same ball-park they may differ).

 

The cost of mobilisation and the first pile will most likely be around the price of the geotechnical survey - you can always say stop if the first pile goes down to 30m. I wouldn't waste my money on the survey. 

One of the points that has just been made is that the warranty company is likely to need the information from the Phase 2 survey so they can get comfortable with the pile design.  I see from your blog you are using Protek for your warranty, have they raised this?

[Andrew]

16 hours ago, Randomiser said:

One of the points that has just been made is that the warranty company is likely to need the information from the Phase 2 survey so they can get comfortable with the pile design.  I see from your blog you are using Protek for your warranty, have they raised this?

 

Protek asked for the specification of the piles and a schedule of piling. I supplied the piling plan which showed the number and location of the piles and ground beam along with the details of the piles we used (150mm tubular steel driven to set for 150kN). They also asked for details of any party wall agreements required which didn't apply in our case. Once the piling was complete they wanted to see the piling log. 

 

Just to clarify my comments above, we did have a soil investigation carried out which involved 4 trial pits down to about 2.5m. I am assuming this is what you've already had done for the soil analysis. In our case it was very clear from these that piling would be necessary. If we were borderline and a further investigation could possibly remove the need for piling I would have carried it out. Moreover if the SE or piling company would have said further investigation were necessary I would have had it done.

 

 

I think it's also worth mentioning that we had a selection of piling quotes, all of which quoted a price to a certain depth and then an additional cost per meter per pile thereafter. All of them made it clear there was no discount if the piles didn't go as deep as their initial depth. With the quotes we gathered the one who quoted the smallest initial depth was also best value at deeper depths but I imagine there may be situations where one company is cheaper at shallower depths and another cheaper at deeper depths and in this case it may be worth having more investigation up front to determine which route would be best value. 

 

Finally there's no guarantee that the piles will go to the same depth across the whole site and there can be large variations. I'm not sure how may boreholes a typically geotechnical survey carries out but even with this data there's still a chance the actual piles will need to go deeper (or less deep) in parts of the site as the conditions vary. 

 

 

 

[Bitpipe]

22 minutes ago, Andrew said:

 I'm not sure how may boreholes a typically geotechnical survey carries out but even with this data there's still a chance the actual piles will need to go deeper (or less deep) in parts of the site as the conditions vary. 

 

In my limited experience, as many as you pay them for.

 

Ideally you want to get some representation of conditions across your building footprint- this is where dynamic probes are better than inspection pits as you can easily move the rig around and as the probes are quite thin, you're not making swiss cheese of the site.

 

Maybe an approach is to do a best / worst case analysis of the piling costs and then see what a day's worth of site investigation will cost as as % of that.

 

You're really paying to remove uncertainty and to select the most cost efficient contractor - the ground conditions will be what they are - so you need to quantify what that is worth to you.

 

 

[Randomiser]

1 hour ago, Bitpipe said:

 

In my limited experience, as many as you pay them for.

 

Ideally you want to get some representation of conditions across your building footprint- this is where dynamic probes are better than inspection pits as you can easily move the rig around and as the probes are quite thin, you're not making swiss cheese of the site.

 

Maybe an approach is to do a best / worst case analysis of the piling costs and then see what a day's worth of site investigation will cost as as % of that.

 

You're really paying to remove uncertainty and to select the most cost efficient contractor - the ground conditions will be what they are - so you need to quantify what that is worth to you.

 

 

That is a good suggestion, but how does one decide on the worst case scenario?

[Declan52]

Just now, Randomiser said:

That is a good suggestion, but how does one decide on the worst case scenario?

It all depends on the site. How big it is will dictate how many holes need done. Then you have to look at the previous history of the site. Has there been something there that might need to be investigated for possible contamination. 

The main cost of site investigation is the mobilization costs. Once the rig is there its not much more to do another few holes. With a terrier rig, pretty small but will do most jobs, you could do 6/8 holes in a day down to 10m and collect a few samples from each hole. If your looking at depths of 20m + then your into big rigs with bigger gear so will be much slower and more expensive.

Has there been any building in the fields beside your site recently. This will be the best indication of how deep you need to go.

 

[Randomiser]

3 minutes ago, Declan52 said:

It all depends on the site. How big it is will dictate how many holes need done. Then you have to look at the previous history of the site. Has there been something there that might need to be investigated for possible contamination. 

The main cost of site investigation is the mobilization costs. Once the rig is there its not much more to do another few holes. With a terrier rig, pretty small but will do most jobs, you could do 6/8 holes in a day down to 10m and collect a few samples from each hole. If your looking at depths of 20m + then your into big rigs with bigger gear so will be much slower and more expensive.

Has there been any building in the fields beside your site recently. This will be the best indication of how deep you need to go.

 

Maybe I misunderstood, I thought Bitpipe was talking about the worst case for piling costs.  That seems to be driven by depth, but can you guess at what the deepest possible pile may need to be? 10m? 20m? More!?!

[Declan52]

It all depends on your own site. I have seen piles over 30m deep and some 5m deep. 

http://mapapps2.bgs.ac.uk/geoindex/home.html

Load this up and find your site and see if there are any boreholes close by. When you open it up and find your location then click the symbol at the bottom left corner then boreholes and click them. If you see a dot near your site click on it and it will bring up any info about that job.

[Andrew]

1 hour ago, Randomiser said:

That seems to be driven by depth, but can you guess at what the deepest possible pile may need to be? 10m? 20m? More!?!

 

Our contractors mentioned a recent job with 32m piles which was in their experience exceptional. They said most were between 5m and 10m, but of course it depends on the local conditions. 

 

The depth also depends on how much load you need on each pile, which will be determined ultimately by the weight of the building, the line loads and the number of piles. Ours at 150kN per pile was fairly modest, I was told. If we'd required a greater loading then the piles would have needed to go deeper to get the greater resistance on the pile to take the higher load. 

[oranjeboom]

You could alternatively go back to square one and get another SE to look at your build plans. From my experience many are over cautious and will cover their arse to the n'th degree. Obviously you don't want issues with your build, but sometimes things are way over engineered. 

 

But certainly if you need piling done, get a survey first. And then get ready for a big hit on the wallet (as you have no doubt seen from your the quotes you already have in). I'd also recommending piling in the drier months as it's no fun shifting the rig around in the quagmire and any miss-drilled piles could be at extra cost to you so check the contract carefully!