Expected Intrusive Phase 2 "Geo and Contamination" Survey Costs?

[waylanderUK]

Does anyone here have recent experience of costings for intrusive geo (mining) and contamination costs? Its going to be a condition on our planning application sadly

 

We've been given a few quotes ranging from £23K - £30k!

 

In January I'm going to phone around a few more companies, but just thought id ask here for additional help. Plot is approx 400 sqm on a 1.5 acre field if that makes any difference - I'm not sure to what extent they actually do the drilling/sampling. Its in a general area of mine workings, but phase 1 showed nothing untoward under the plot itself.

 

Many thanks for any help!

[Conor]

2 minutes ago, waylanderUK said:

but phase 1 showed nothing untoward under the plot itself.

 

Many thanks for any help!

 

 

So what's the justification for a phase 2 site survey? I'd be pushing back and appealing.

[nod]

16 minutes ago, Conor said:

 

 

So what's the justification for a phase 2 site survey? I'd be pushing back and appealing.

Plus 1

Planners are great at spending your money 

 

Ours where concerned about flooding 

Due to inadequate sea defenses 

12 miles away 

 

Id go down the bring an expert in route It won’t be cheap But far less than what you have been quoted 

[Alan Ambrose]

Can you give us an idea of the spec you're looking at for the investigation? I just got a quote for £5K + VAT for 6 boreholes to 6m including all the detailed testing and analysis. I didn't think that was crazy cheap considering it would probably be a team of 2 or 3 onsite for max 2 days.

[waylanderUK]

Hi all - reason is that there is a mining entrance at the bottom of the field according to historic records and indeed in the area.

 

Also within 500m there's a old dumping ground for waste which added more weight to the contamination requirement.

 

Spec wise I did see 4m holes mentioned 4x and also  3x 30m boreholes for coal investigation works, gas monitoring, the quote was provided by my architect. It also had gas testing in it too... soil sampling... 

 

Architect says a lot of the houses in the area have the same survey requirements as its a coal/clay mining area.

[LSB]

we had one done in 2021 and it cost circa 6k.  Some just dug and taken away, 3 monitored for 3 months for anything bubbling through.

There was nothing otherwise it would have been remedial works next.

This was a condition post planning, the phase 1 was a desk study.

 

[Gus Potter]

5 hours ago, waylanderUK said:

We've been given a few quotes ranging from £23K - £30k!

That's a nasty number!

3 hours ago, waylanderUK said:

Spec wise I did see 4m holes mentioned 4x and also  3x 30m boreholes for coal investigation works, gas monitoring, the quote was provided by my architect. It also had gas testing in it too... soil sampling...

Ok this seems a bit back to front but bear with me. It's a bit of a long read but hopefully you can pick out some bits that help you.

 

You know what you want to build, how big it is as you have the plans. Do you know how you are going to build it? What are you preferred founds? What's the Architect's / your thoughts - strip founds, raft etc? From what you have we can make a quick approximation of the load the house is going to apply to the ground and how.. raft.. low pressure, strip founds a bit higher etc.

 

Next is to go back to the beginning and find out exactly what the planners are asking for (you need to be reasonably confident you'll get planning in the bag before splashing too much cash) and really look closely at the level of information they actually require. On the plus side here there seems to be pattern for your local area.. the planning computer says no unless we can tick the boxes. You may be able access other reports for the area to see what approach does work and then how you tweak that suit you best. 

 

Now we look at what we need to know to build the thing in terms of geotechnical information.

 

We have a mining element, will come to that later. Also potential gas from land fill and associated with that is contamination of the ground water. We would also check for Radon as a matter of course. The gas from the landfill.  Risks are.. it forms pockets of gas under your house, explodes and blows your house up,  asphxiates you in bed or gives you sick building syndrome (high mortgage rates can have the same effect) .. but the land topography and ground may make that impossible. Just because it is 500m on plan may not mean anything. Just say you have a railway cutting or other topographic barrier between you and the land fill.. how is the gas going to get to your house? If you can identify something like this and make a reasoned arguement then no need for gas monitoring? Gas monitoring is tricky to install and needs "monitoring" = cost thus we want to avoid if at all reasonably justifiable.

 

The ground water contamination carries risk. Just say you have some to make a good story!

 

Clearly we need to keep any contamination away from people and animals but also certain types of contamination will drive the selection of the type, grade of concrete and steel reinforcement cover if we are reinforcing the founds. Things like sulphate content, acidity and mobilty of the ground water can drive the concrete design and this would be associated with contamination at a shallow depth. Also, if you have a public water supply the supply pipework needs to be able to resist contaminated water (comes at a cost), and if you are going for a private water supply... well you really do want to know about contamination.

 

I find the mining fascinating. I've done some jobs where we have bored for mine shafts and shallow workings on small sites where the Client was happy to fork out the cash.. the outcome was that we never found many of them.. but that is not to say they weren't there.. all I learnt was they were not where we drilled! I was convinced they were there at times on some sites but unless you absolutely pepper the place you can't be sure. In the round I often made the argument that on balance of probablity there was not a huge void under the structure, made some assumptions about the type and size of void that could reasonably be there and designed the founds for that.

 

The mining. Now on the above we didn't just turn up on the day with a drilling rig. I carried out a really comprehensive (phase one) desk top study and spent at least a day walking over the site, the surrounding area, looking at the trees, the topography of the ground,  and chatting to local folk.. who are often really helpful and sometimes will show you old photos they and their family have kept. Next is re examine the mining records (can be hard) and public available borehole records. I try and find any historic books on the area that maybe relate to industrial activity, old geological records. Sometimes you find nothing at all and sometimes you find a nugget of information that can save you thousands. Now you can imagine that I'm not going to do this for free! but there is good reason for this approach, it makes you think and by doing that you manage risk.

 

All of the above gets coupled up with a typical search (historic maps, floor risk area, general mining risk etc) and info from say Ground Sure or other companies that provide these types of online data services.

 

But the value lies in showing (and just knowing in your own mind) that you have followed the correct procedure in terms of identifying risk, what you do know,  what you don't, there is the "what we don't know we don't know".. but life is exciting enough as it is.

 

The objective of this is two fold. It allows me to make a list of all the things that need to be considered, then apportion risk and prioritise each of them. It also lets me plan the intrusive (investgative part) of the site investigation and make best use of the funds available to do that. The funds can vary depending on how much the Client is risk averse.. some Clients just don't listen enough to make their own informed decisions and regret later.

 

To come back to the mining risk. As a quick over view there are a few different types of shallow mines you can encounter. Don't forget that more modern mines had ventilation and escape / ventilation shafts. Early mining acts for example 1862 prohibited single shaft mines. As a quick run down. From early times folk just dug up coal where it outcropped from the ground. Then you had say the Monks that dug bell pits, 12th centuary onwards. Ok a lot of these these have probably collapsed / settled a good bit but there are a few examples localish to me where you can see the depression in the ground from collapsed bell pits.. not for building on at sensible cost. Mind you, I have often wondered if I'll find someone who wants to dig it back out and make a bell pit basement.

 

Jumping ahead you had drift mining.. they dug from ground level and followed the coal seam which is usually sloping. Later these were widened using say timber props, you have the pillar and stall method where they would leave pillars of coal, dig out the coal, chuck the spoil into the redundant bits and when they abandonded the mine they removed the pillars and let the thing fall down as they progresssivly left the mine.

 

Laterally in some places they started using a method called long wall mining. Here they used a big machine to excavate the seam and just let it progressivly collapse behind them. This created a rolling wave of soil movement at the surface which damages houses. But the main thing here is this type of ground collapse happens quite quickly..  a few years, a couple of decades to settle in once the mine is fuly abandoned and the ground water recovers, which lubricates the particulates of soil, encouraging settlement. Older types of mining - some can take 100 years or more before they throw in the towel and cause problems at the surface.

 

All the above relates to shallow mining in general. Deep coal mines tend to have many layers of rock over so the tiny extra bit of weight from a house is negligeable.

 

Now the brief history story is nearly over. Next step is to try and identify where the coal is (if in Cornwall say then there is a lot of mineral mining but the same basic rules apply with a few tweaks), the way the rock and coal seams are sloping (they tend not to be flat and level and early miners followed the seams), how thick they may be.. and where the ground water lies. Now the good thing is that in a lot of the UK there is ground water extraction which tends to lower the water table. Many mines have been shut since the 1980's so the pumping has long ceased. Therefore the ground water level we see now is probably lower than what it was historically, give or take. This can give us the "worst case reasonable depth" of some types of mines.

 

What kind of mining has possibly taken place in the past? If we can find a bit about the ground water average level then we could assume the Monks, earlier methods of pillar and stall were no deeper as they didn't have steam pumps say to get the water out. Many millions of years ago the part of the UK below the Highland boundary fault was well south of the equator before it joined up with the top part of Scotland to make the UK.. our island. There is more to it but for another day. The coal was laid down when the bottom part of the UK was tropical / sub tropical. Later though when we all got glued together the magma in the earth forced it's way up to create what we call magma (granite)  intrusions / dykes. This hot material burnt the coal measures and these are often marked as "barren coal". In other words it's of little use. It is a wel known fact that barren coal measures were avoided by the miners. In places dykes can be quite closely spaced and are often identified in the records. Mind you you don't want one under half your house as they are nightmare to ecavate and cause hard spot which is problematic, not just for founds but getting drainage routes established..

 

Now if your site has say a high water table, some granite dykes then we want to look at that as we may be able to say.. hey this coal is barren / high water table so it probaly has not been mined... thus no need to indulge in expensive driling. Just be aware though. It's not just coal. In many parts of the UK they also mined fire clay and other minerals so we would need to watch out for that.

 

All of the above is the phase one investigation, thinking and research. If you assume you are going to get planning approval in one form or another I would be inclined to plan the site investigation around gathering the information we really need to get the house built. I would try and avoid gas monitoring, drilling if possible (make a big saving) and spend a bit of the savings on maybe more trial pits, some porosity testing, gruond water (especially is you have a basement) to make sure you can get rid of the water easily. On the other hand if it turns out you do have a real mining / contamination issue then we need to know that as soon as possible so we can design for it at the outset and manage cost.

 

It may be worth doing both phase one and two (intrusive investigation/ analysis) at the same time. Then tailor a report with plenty technical detail for the planning submission. Say to the planners, if there is anything you disagree with let us know. At the end of the day if we are going to design the structure to be safe and seviceable then surely that should satisfy the planners? I think the planners want to know that what they are giving concent for is not going to create a legacy problem. In some ways you are just a custodian of the house.

 

Hope this helps a bit, keep us updated and all the best.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[waylanderUK]

15 hours ago, LSB said:

we had one done in 2021 and it cost circa 6k.  Some just dug and taken away, 3 monitored for 3 months for anything bubbling through.

There was nothing otherwise it would have been remedial works next.

This was a condition post planning, the phase 1 was a desk study.

 

Was this full contamination and mining checks including boreholes then? 6K sounds more palatable!

[saveasteading]

A desktop search of mining can bd rather approximate. Maps can show pits and shafts in completely the wrong place.

 

Just 3 things to suggest.

1. Is the ground suitable for supporting your building? Engage an SE to consider what tests are necessary: no more than necessary.

2 contamination. Assume the worst in terms of gas and chemical attack. Protect the building using radon barrier and venting. No need to test.

 

3.Plus soil tests to see if contamination is present that could harm children or pets  or get into vegetables. Replacing soil would cost a fortune, so best find out promptly, then most likely, relax.

 

Fees ? Plus some 6m bore holes. £4,000?

[waylanderUK]

Yeah I suggested to the architect that we just use a barrier and therefore fewer tests but apparently he’s tried that before and the planners insisted on the tests anyway.
 

honestly for price of mind a few grand isn’t an issue but 20k would which is why I’m here really. All good input so far and I’ll be speaking to my architect/planning consultant again to see what we can cut down.
 

thanks again! 

[saveasteading]

The planners are not Engineers, and should accept a proposal from a suitably qualified one. This is Civil Engineering with a geotechnical / environmental slant  so you'd have to ask around.

[Declan52]

The drilling rig most companies use to get to 6m would be a small tracked machine that fits in a large van and realistically you should be able to do 6  boreholes to 6m in a single day. This would  include any samples that need taken plus geotechnical testing if required and a standpipe put in to monitor gas and water. 

But to go to 30m your into bigger rotary machines, doubt shell and auger type rig would be able to get to that depth in a hard ground. That's why the cost increases quite a lot. These rigs need a lorry to transport it to and from site and a large compressor all which cost more. 

 

If your lucky and some investigation work has been pretty close by, next field, then you might get some information using this site which might allow you just to do the shallow depths.

https://www.bgs.ac.uk/information-hub/borehole-records/

[ToughButterCup]

On 15/12/2023 at 15:19, waylanderUK said:

..... 

Architect says a lot of the houses in the area have the same survey requirements as its a coal/clay mining area.

There's your answer. Make a list of all the properties that have had to have that survey done and then make contact with them and ask them how much they were charged.

[LSB]

11 hours ago, waylanderUK said:

Was this full contamination and mining checks including boreholes then? 6K sounds more palatable!

no mining in Suffolk, no hills, but was an old pig farm so looking for methane.

This was done by the SE.

 

[Alan Ambrose]

@LSB

 

Possible to tell me which one? I could do with another quote…

[LSB]

5 hours ago, Alan Ambrose said:

@LSB

 

Possible to tell me which one? I could do with another quote…

JP Chicks in Ipswich, quite commercial so not cheap, but very efficient.

 

[Bigdeadbadger]

I have just been through this very topic over the last year or so in Central Scotland. 

 

First recommendation - challenge everything.  My experience here is that the "worst" case scenario is typically assumed with limited justification especially from 3rd parties.  In our risk adverse world, I experienced people making (faceless) choices based on those worst cases vs what might actually be a more pragmatic approach.  Had I challenged more - I would have saved time and money.

 

As part of my planning application, I was required to do a phase 1 desk survey, which highlighted my plot was a low to moderate risk of unrecorded shallow mine workings (not uncommon for this area - like mentioned in earlier responses) and potential for ground gases.  This outcome then placed a requirement to do an intrusive phase 2 site investigation as part of my planning conditions - including 6x boreholes for ground gas monitoring.

 

Based on the phase 1 desk survey, my site was allocated an initial CS2 rating by my local councils 3rd party contaminated land consultant (noting my council outsource this work).  It's at this point - I wish I had challenged more, as I feel in hindsight, this was excessive.

 

Having obtained quotes for the phase 2 survey, a couple of things jumped out.  The cost was ~£6K as per examples listed above and the monitoring period was over 3 months.  Based on these costs, the potential for ground gas to be discovered anyway, and a naïve thinking I would be ready to build soon and the monitoring would hold me up, I took the decision to include a Gas Membrane in my slab design and accept a CS2 rating in Lieu of the gas monitoring.  And that is where things got really fun, as for me, it was not just a case of "just" including a membrane.  Instead, I had to:

 

1) for my building warrant, I was expected to provide a Contaminated Land Response, detailing how the Gas Membrane would be installed, and how the design met BS8485

2) not only was a membrane required to meet the points allocation needed to pass CS2, I also needed to demonstrate my slab was reinforced with minimal penetrations (pretty easy) but also then design in a passive ventilation layer below my slab (all of this is in BS8485).  Giving me a points score of 4 (target was 3.5)

3) detail how the membrane would work along side my insulated raft formwork

4) provide external 3rd party validation of the membrane installation in line with CLA:IRE guidelines

5) my BC could not answer ANY questions on the topic.  All discussions went via their 3rd party, had to be via a "design" and typically had a 3 week turnaround.

 

Based on being thoroughly frustrated at this point due to many things on my build, I actually composed document 1) myself by getting hold of a copy of BS8485 and demonstrating a compliant design.  I did not obtain costs for a 3rd party to do this, but I would expect low £ thousands as its (rightly) outside the scope of a SE and Architects specialization. 

 

All in all, by the time I include the Visqueen membrane, the labour, the 3rd party verification, the cost of the low fines gravel ventilation beneath my slab, I will have spent ~£10K. 

 

I sleep at night, by convincing myself that bore holes may have found ground gas, and I would have then spent and additional £6K on monitoring and I didnt spend extra on someone else designing the solution. 

 

BUT - some pictures below to show it can be done

 

Low fines material for ventilation (this was quite expensive) - 300mm deep, 260m2 (my house is going to be quite big - ground floor at 200m2).  Could be cheaper going for preformed venting sheets from the membrane manufacturers. 

 

 

And installed below my formwork (still to be dressed on the sides of the formwork).  Excuse the mess. 

 

 

 

 

Hope that helps

 

 

 

 

 

Edited December 19, 2023 by Bigdeadbadger

[saveasteading]

3 minutes ago, Bigdeadbadger said:

I have just been through this very topic over the last year or so in Central Scotland. 

 

First recommendation - challenge everything.  My experience here is that the "worst" case scenario is typically assumed with limited justification especially from 3rd parties.  In our risk adverse world, I experienced people making (faceless) choices based on those worst cases vs what might actually be a more pragmatic approach.  Had I challenged more - I would have saved time and money.

 

As part of my planning application, I was required to do a phase 1 desk survey, which highlighted my plot was a low to moderate risk of unrecorded shallow mine workings (not uncommon for this area - like mentioned in earlier responses) and potential for ground gases.  This outcome then placed a requirement to do an intrusive phase 2 site investigation as part of my planning conditions - including 6x boreholes for ground gas monitoring.

 

Based on the phase 1 desk survey, my site was allocated an initial CS2 rating by my local councils 3rd party contaminated land consultant (noting my council outsource this work).  It's at this point - I wish I had challenged more, as I feel in hindsight, this was excessive.

 

Having obtained quotes for the phase 2 survey, a couple of things jumped out.  The cost was ~£6K as per examples listed above and the monitoring period was over 3 months.  Based on these costs, the potential for ground gas to be discovered anyway, and a naïve thinking I would be ready to build soon and the monitoring would hold me up, I took the decision to include a Gas Membrane in my slab design.  And that is where things got really fun, as for me, it was not just a case of "just" including a membrane.  Instead, I had to:

 

1) for my building warrant, I was expected to provide a Contaminated Land Response, detailing how the Gas Membrane would be installed, and how the design met BS8485

2) not only was a membrane required to meet the points allocation needed to pass CS2, I also needed to demonstrate my slab was reinforced with minimal penetrations (pretty easy) but also then design in a passive ventilation layer below my slab (all of this is in BS8485).  Giving me a points score of 4 (target was 3.5)

3) detail how the membrane would work along side my insulated raft formwork

4) provide external 3rd party validation of the membrane installation in line with CLA:IRE guidelines

5) my BC could not answer ANY questions on the topic.  All discussions went via their 3rd party, had to be via a "design" and typically had a 3 week turnaround.

 

Based on being thoroughly frustrated at this point due to many things on my build, I actually composed document 1) myself by getting hold of a copy of BS8485 and demonstrating a compliant design.  I did not obtain costs for a 3rd party to do this, but I would expect low £ thousands as its (rightly) outside the scope of a SE and Architects specialization. 

 

All in all, by the time I include the Visqueen membrane, the labour, the 3rd party verification, the cost of the low fines gravel ventilation beneath my slab, I will have spent ~£10K. 

 

I sleep at night, by convincing myself that bore holes may have found ground gas, and I would have then spent and additional £6K on monitoring and I didnt spend extra on someone else designing the solution. 

 

BUT - some pictures below to show it can be done

 

Low fines material for ventilation (this was quite expensive) - 300mm deep, 260m2 (my house is going to be quite big - ground floor at 200m2).  Could be cheaper going for preformed venting sheets from the membrane manufacturers. 

 

 

And installed below my formwork (still to be dressed on the sides of the formwork).  Excuse the mess. 

 

 

 

 

Hope that helps

Good work. 

I've done several jobs on methane sites. In most cases I've specified a perforated pipe network beneath the slab, surrounding  in gravel, then venting outside the building.

There are branded systems at about 4 times the cost.

I have the advantage of the qualifications, which the bco should accept. 

Your blanket of gravel clearly allows gas to escape too, ( for a small building, so you must have detailed it very well.

It needs to be open at different ends of the building to get wind suction. I assume you have this.

[Bigdeadbadger]

Thanks saveasteading,

 

Re the ends of the building - yes, I have trenches in my design around the perimeter of the building that will allow the wind/venting up to the surface (2000mm2 per linear meter off the top of my head ) , but can only get to that as I look at backfilling in/around the slab at a later point.

 

Cheers

[saveasteading]

To reduce the chance of your gravel clogging with muck and weeds over the years, could you put a perforated  pipe in the gravel and vent it upwards? Then cover the gravel with slabs?

It would work better if the pipes vented either higher, or distant to the house, allowing the breeze/ differential pressures to evacuate any gas.

 

Despite my interest i have never  heard of there ever being an explosion in a house  ( or anh building) from ground methane. Might as well do it properly though.