How do you deduce water table?

[flanagaj]

Currently contemplating adding a basement to to a new build.  Only reason is that the site is quite small and the current plans cannot increase the footprint of the building nor to add a garage.

 

Apologies, if this is a daft question, but I read that before even considering having a basement you need to find out the water table for the site.   Does this simply require bore holes to be drilled on site or can the information be obtained from known geological surveys that may have been done in the area? 

[Mr Punter]

Dig or drill a hole.  If you are doing a basement you should do a proper geotechnical survey to establish soil type, strength cohesiveness and water table.

[Ralph]

Also remember that the level changes during the year, with us there is a huge difference between now and January

 

[Bitpipe]

It's more that just water table - you need a geotechnical survey to the satisfaction of your SE who will be designing the basement structure.

 

This will be a mix of desk survey (historical investigation) and onsite ground survey (this element should be zero rated for VAT as it it involves 'tools' and is materially related to the construction, per HMRC guidelines) to determine:

 

- Ground composition : required for both structural design and also construction design, i.e. do you need retaining structures during build etc. Also required to calculate muckaway as different materials bulk up differently when out of the ground and you pay by m³ vs weight

- Load bearing capability : fundamentally required to structurally design the basement, will a slab suffice, how thick etc, piles?

- Soil contamination : may be requested by your LA if there is any historic likely hood. Also required to know how to dispose of the soil - needs WACS testing & classification

- Water levels : required for your waterproofing design (and maybe also build strategy if water level is higher than your excavation).

- Gas levels : required for your gas permeability design (membranes)

 

The other factors for basement design will be

- intended use (sleeping, living, storage, plant etc) which will drive BC requirements most obvious of which is alternative means of escape or fire suppression.

- full or partial footprint : is the basement the foundation also (like ours) or do you have a hybrid approach? Is the house above designed so the SE can take the loads imposed into consideration?

- insulation : our basement sits on 300mm EPS and has 200mm EPS to sides (all EPS grades calculated by the SE) so it needs no heating and is always @ 20^o ambient temp by virtue of the other heat sources in it (appliances, plant, people).

- light : natural light is great for basements but needs lightwells to be designed (can be structural or bolted on GRP etc).

- services: will you want wet services (toilets etc) in the basement, if you you will need a sump and pump or macerator to surface level foul drainage. 

- ventilation: if you're having MVHR, it will need to serve basement. If not, how will you get fresh air to circulate?

 

Note that adding a basement to a 2 story house makes it a 3 storey house and so the fire regs step up, minimum FD 30 doors to all rooms off the central stairways, possibly hire grade fireproofing.

 

Now, that all sounds a bit intimidating but it's just more detail to contend with and the only expense you need to commit to is the survey to understand if you can build a basement within your budget.

 

We had a full technical survey that comprised 10m probes and a few 6m cores.

 

For one of the cores, they sleeved it,  put in a gas and water monitor and capped it. Returned after 3 weeks to take measurements - no water found (and this was in October).

 

Given basement excavation was 3.5m that gave confidence that WPC and a good drainage strategy (to a soak away 4.5m deep) would do the job. No piles required but chalk substrate meant the slab needed to be a bit heftier than ususal.

 

[flanagaj]

@Bitpipe I appreciate this will vary according to the works required, but the sort of questions I still would like to answer is 

1)  What if anything can you do on the site if you haven't exchanged contracts and the current 2 storey building plans are just slightly too small for your needs and the basement is a must.  Even if it was just 50% of the footprint?

 

2)  What is a ballpark figure for the surveys / SE.  The footprint of the property as it stands if the basement was the size of the house is 9.5m * 7.5m  so quite small.

 

3)  When people talk about the m2 costs of building a basement does that cost include excavation, footings, floor, cast concrete walls and tanking up to ground level so that the next step of the build process would be putting in a beam and block floor?

 

4)  What are current estimates for a basement.  I appreciate that is difficult to answer?

 

Thanks

[Bitpipe]

9 hours ago, flanagaj said:

@Bitpipe I appreciate this will vary according to the works required, but the sort of questions I still would like to answer is 

1)  What if anything can you do on the site if you haven't exchanged contracts and the current 2 storey building plans are just slightly too small for your needs and the basement is a must.  Even if it was just 50% of the footprint?

 

2)  What is a ballpark figure for the surveys / SE.  The footprint of the property as it stands if the basement was the size of the house is 9.5m * 7.5m  so quite small.

 

3)  When people talk about the m2 costs of building a basement does that cost include excavation, footings, floor, cast concrete walls and tanking up to ground level so that the next step of the build process would be putting in a beam and block floor?

 

4)  What are current estimates for a basement.  I appreciate that is difficult to answer?

 

Thanks

 

1) You can do a desk survey and look at BGS borehole data to get a feel of ground conditions. You can also get a feel for site conditions, access etc. Where will basement sit, will you invoke party wall etc.

 

2) Depends but your SE does not need to be near you so you could contract someone in a cheaper location. I used Tara @ build collective in Bristol - maybe they can give you an indicative cost. GI cost me a lot (£12k but I made some mistakes and so did they). You shoyld be able to do it cheaper - ask the SE what you need (holes & probes) and then shop around the national GI companies.

 

3) Yes, normally excludes the fit out. Some include the basement lid if that's part of the design.

 

4) depends  £1500-2k / m2 would not be a bad start, you may pull it in cheaper once conditions are known or it may get more expensive.

 

[flanagaj]

11 hours ago, Bitpipe said:

GI cost me a lot (£12k but I made some mistakes and so did they). You shoyld be able to do it cheaper - ask the SE what you need (holes & probes) and then shop around the national GI companies.

 

Wow.  I wasn't expecting 12k.  I assume that is also not VAT exempt.

[Bitpipe]

3 hours ago, flanagaj said:

 

Wow.  I wasn't expecting 12k.  I assume that is also not VAT exempt.

 

Ours had to be repeated due to inconsistent results which turned out to be errors by the GI company but was unable to reduce the fee. I also paid VAT when a large portion of the works would have qualified as zero rated as they required tools, and were closely related to the build. Was not able to reclaim that either.

 

You may well get what you need to satisfy the SE with much less investigation but depends on where you are and what local conditions are.

 

You can look up BGS borehole data for free and that normally gives you a good start.

 

You will need to do some soil analysis (WACS) for contamination to enable your groundworker to quote disposal.

 

There are really too many variables to guess the price but talk to local groundworker firms, NOT basement specialists, and they should give you some ideas.

 

N.B. If your site is small and close to neighbouring property, you may not be able to 'batter back' the excavation safely and may need sheet piling which adds more cost.

[Temp]

4 hours ago, flanagaj said:

 

Wow.  I wasn't expecting 12k.  I assume that is also not VAT exempt.

 

It can be zero rated to you but only if its done after you get planning permission. That's implied in VAT708 where it says..

 

Services in paragraph 3.3.4 are standard-rated where, for example:

* site investigation or demolition work is carried out before planning permission for the construction of a building that qualifies for the zero rate has been granted

 

 

 

 

[Bitpipe]

8 hours ago, Temp said:

 

It can be zero rated to you but only if its done after you get planning permission. That's implied in VAT708 where it says..

 

Agree, but I didn't have the benefit of this forum back then so didn't know. They also refused to refund when I brought it to their attention a few years later.

[Thorfun]

6 hours ago, Bitpipe said:

 

Agree, but I didn't have the benefit of this forum back then so didn't know. They also refused to refund when I brought it to their attention a few years later.

the GI company I used wouldn't budge on charging VAT. no matter how much I argued HMRC said it was zero rated they wouldn't have it. for us it was about £500 in VAT so I decided to not bother arguing too much, nor to bother spending the time trying to find another company to do the work to save that £500.

 

just a word of warning to those who assume GI work is zero rated by everyone.

[WWilts]

On 23/06/2021 at 09:07, flanagaj said:

you need to find out the water table for the site.   Does this simply require bore holes to be drilled on site or can the information be obtained from known geological surveys that may have been done in the area? 

We were asked by planners to show that the water table was at sufficient depth for surface water infiltration (not identical situation to yours).

 

However, we then ordered a site report from British Geological Surveys. Included the water table but also a lot of other useful information (clays, bedrock permeability, soil stability, soluble rock content & collapsibility, etc). All for about £20 a couple of years ago. 

 

Perhaps a good initial investment, might save you a few quid down the line.

[flanagaj]

On 04/07/2021 at 19:45, WWilts said:

All for about £20 a couple of years ago. 

 

 

Thanks.  That does sound like a very good investment and I will have a look.

[Bitpipe]

8 minutes ago, flanagaj said:

 

 

Thanks.  That does sound like a very good investment and I will have a look.

 

It's a good initial investment but no substitute for a full GI on the footprint where your basement will reside.

 

Should give you a good idea of local conditions.

[ToughButterCup]

No ground survey = worry. What if there's a .....?

Harsh, but thats what we found. Piling contractor, groundworkers (we put in our own digester)  and my instinct all clamoured for a proper survey. Locally its all glacial till, but in our site there is an (at the time) hidden incursion of sandstone. Had we not known that, the piling price would have been higher and we would have had to re-route our foul drainage. Deduct that cost from the cost of the survey. Neither would we have been able to do our SUDS program as planned. After the survey we were able to replan the SUDS provisionn and save about £2000. Deduct that from the survey cost too.

 

Forewarned, forearmed, better nights sleep. More money in the bank. Simples.

[Bitpipe]

21 minutes ago, ToughButterCup said:

No ground survey = worry. What if there's a .....?

Harsh, but thats what we found. Piling contractor, groundworkers (we put in our own digester)  and my instinct all clamoured for a proper survey. Locally its all glacial till, but in our site there is an (at the time) hidden incursion of sandstone. Had we not known that, the piling price would have been higher and we would have had to re-route our foul drainage. Deduct that cost from the cost of the survey. Neither would we have been able to do our SUDS program as planned. After the survey we were able to replan the SUDS provisionn and save about £2000. Deduct that from the survey cost too.

 

Forewarned, forearmed, better nights sleep. More money in the bank. Simples.

 

There are stories of basement construction commencing based on thin data and then discovering unexpected ground features that stop work until the contractor, and possibly SE, can resolve. Costs can spiral significantly.

 

We were prepared to walk away from our basement if the GI report suggested it was not viable economically. After the first investigation borehole suggested that we were sitting on made ground which would have meant piling and end of basement plan. We paid for a more comprehensive investigation and confirmed that ground was solid. 

 

[ToughButterCup]

59 minutes ago, Bitpipe said:

...We paid for a more comprehensive investigation and ...

... slept better, I'll bet.

[Bitpipe]

51 minutes ago, ToughButterCup said:

... slept better, I'll bet.

 

Well not really as the first report would have sufficed if the supervising engineer did not mis-locate one of the boreholes and fail to interpret the data correctly. I got a 'discounted' rate for the more comprehensive second investigation but still cost me £5k more.

 

Wrote a complaint to the CEO listing all the failures and got a 'well, that's ground investigation for you' reply. However they did send the SE a bunch of flowers to apologise for messing her about (she recommended the firm).

 

Did tick me off for quite a while.

[saveasteading]

Beware of basements. Whatever the ground report says, the circumstances can change.

New housing can put a lot more water into the drainage system, concentrating it downstream,  and everyone's water table can rise.

The planners and EA don't worry about this very much, if at all. The SUDS rules are rather lax, allowing all the water into the drains, just slowed down a  bit.

I know a village (now town) where the water table rose from minus 1.5m to minus 0.5m due to housing.

That was before the changes we now allow for increased rainfall intensity.

 

I would nearly always assume that the ground could get very wet to some height, and fully waterproof the construction. Exception for being on top of a hill and built on sand.

[Bitpipe]

5 minutes ago, saveasteading said:

Beware of basements. Whatever the ground report says, the circumstances can change.

New housing can put a lot more water into the drainage system, concentrating it downstream,  and everyone's water table can rise.

The planners and EA don't worry about this very much, if at all. The SUDS rules are rather lax, allowing all the water into the drains, just slowed down a  bit.

I know a village (now town) where the water table rose from minus 1.5m to minus 0.5m due to housing.

That was before the changes we now allow for increased rainfall intensity.

 

I would nearly always assume that the ground could get very wet to some height, and fully waterproof the construction. Exception for being on top of a hill and built on sand.

 

Well, 'beware' is a bit strong but I agree that you need at least one of the 3 waterproofing options (membrane, water proof concrete, internal membrane & sump/pump) - some warranty or BCO will require two.

 

Knowing where your water table is, and more importantly your ground conditions, will dictate your structural design (inc. waterproofing & ground gas mitigation) and site strategy and ultimately your cost to construct.

 

We have friends who built more or less in a Thames feeding stream -  they had a full dewatering system during the build and a 2 part warrantied waterproofing system. Still dry as a bone, main design challenge was preventing hydrostatic pressure pushing the basement out of the ground!

 

 

[saveasteading]

I withdraw 'beware' and substitute, 'be very cautious'.

 

Also, be aware that boreholes are a surprisingly primitive procedure, and the report depends on the operative paying attention and being interested. The Engineer is seldom present.