Basement sequence and soil investigation

[Alan Ambrose]

OK so we just put in for planning with two alternative designs and specified for each that 'we would include a basement if soil conditions allow'. I would like to answer the 'if soil conditions allow' question nice and early.

 

The site is rural meadow on clay. The designs are both oak frame with timber cladding and pantiles - so reasonably light. One design is single storey, one is 1 and a half. The basements would both be simple oblong concrete boxes the whole area of a floor, maybe with light pipes, but almost certainly with an outside pit / stairs / entrance. Mostly 'non-habitable' and used mainly for storage, plant room, workbench etc.

 

Given that we don't know which (if either) design will be allowed and are therefore we're not ready to actually have the basement designed by an SE ... what's the best way of getting a soil investigation done?

 

I guess I want to include ground strength / chemical analysis for muckaway and concrete / permeability for drainage and/or soakaway / water table for ease of build. I'm hoping to actually get the basement built winter '23 / spring '24 

[Alan Ambrose]

For interest, here are a couple of old BGS borehole logs each about 90 year's old and within about a mile of the site - the  land is all fairly level around here, so I'm not expecting a wild variation from these.

 

 

[Alan Ambrose]

>>> what's the best way of getting a soil investigation done?

 

I realise I was not very clear - I'm really asking 'how do I get the soil investigation specified exactly right given I'm not ready to instruct an SE yet'? I appreciate that once I have it specified I can go out and contract someone to do it.

[Declan52]

You basically phone up a few companies that do this work and specify what exactly you listed above as your needs. They then send out the guys with the drilling rig who will go to whatever depth you require, 10m+ easily for what you are looking. As they drill down they  perform spt tests which give the bearing capacity of the ground at the various depths which is the information a SE will use to give you your drawings. 

Unless there has been something nasty on your site in the past then I don't see the need for chemical analysis. The drillers will record the water table depths when they strike it on their logs.

[George]

A mile may as well be a lightyear when it comes to UK geology.

 

Speak to a SE - get them to advise on what geotechnical information is needed (risk of going straight to a GI company is you get overspec investigations). They may charge a few hundred quid to write a spec but they don't need to do a full structural design to know what ground info they will need. Then get the ground investigation done up front. 

 

It'll pay for itself - the same information will help on the foundation design with or without a basement. This will then help by either reducing the grounds works required, and/or avoiding abortive work early in the project, and/or de-risking the groundwork for the contractor when they price.

 

I always always advise that for new builds, get a GI done nice and early.

Edited April 20, 2023 by George

[Gus Potter]

1 hour ago, George said:

A mile may as well be a lightyear when it comes to UK geology.

Too right!

 

On 19/04/2023 at 20:05, Alan Ambrose said:

what's the best way of getting a soil investigation done?

Hi Alan.

 

Hope this helps... there is a lot of prep work you can do.. seems like you have already started raking out a borehole log so well done. We call this a desktop study, often folk slag this off but what they don't know is that it forms part of the BS design codes to site investigation and good practice.

 

Try and find out as much info as you can online, look at the BGS viewer to see what kind of bedrock you have and superficial deposits. Speak to the neighbours (very good to do as we weave this local knowledge into a site investigation report), well I do as it carries weight.. believe it or not. Ask say about flooding, were there any landfill sites, was it in a bad corner of a field that the farmer filled in? .. think of the things that might be relevant to your site, be friendly and get them talking. Sometimes they produce old historic photos and give you pointers, history of the site and real nuggets of info that can literally save thousands later on.

 

Other stuff.. have a look at the services, if rural... then where are they, what services will you need.. drainage is a biggy. If in the town.. where do you think they might be and how deep to the sewer in the street, if rural have you available space for say a soakaway or easy access to a water course that does not dry up in the summer..

 

Next thing to do is to have several walk overs of the site and wander round about. Try and understand how the topography has been formed and importantly what has been changed over time. Are there culverts, how does the land drain, where are the water courses, are there big trees, if so which way are they leaning.. If you see the power company / water folk digging up the street stop and ask them about the ground! many of them are keen to tell folk what they know!.. sleep on stuff for a while.. be curious.. think of everything you can and try and work out why the things you observe are the way they are.. and write it down.

 

The objective of the above is to try and encourage you to look outwith the site and understand what is going on round about. Do your best here and put all the info you have into a document with some photos. Now you have the makings of a desktop study and off the back of that you then plan the intrusive site investigation. Your SE and investigation company will meet you with open arms if you turn up well prepared and won't make a fool of your efforts.. if they do then find another SE GI company pronto.

 

If you come to me and say Gus.. I want to build this house.. I'll say.. to do it right and do our best to avoid things coming back to bite us we need to start with the above. This way we take all reasonable steps to mitigate the risk to you and we can then focus our effort on the different designs knowing we have started out doing things the right way.. done the leg work.

 

The above is the starting point of planning an intrusive investigation that reduces the risk of wasting money while at the same time providing useful design information that can unlock options for the building / basement.

 

 

 

 

 

 

 

 

 

[Alan Ambrose]

Thanks Gus, for another of your generously detailed answers.

[Phil Sacre]

Indeed Gus, very generous and informative.

 

I would add that the basement and timber frame might be buoyant.

 

It is building control who decide about a fire escape. If the basement is seen as a space that could be habitable accommodation in the future it will need building with a fire escape. You mentioned an outside door but not an internal staircase.

 

I have found soil investigstion specialists quote prices all over the place. I would begin by phoning half a dozen at least. If you phone Gus and he can't cope with his workload he will stick a big lump on top. If he has nothing on he might knock 10% off.

 

Your basement will displace water the area of the basement plus outside staircase times the depth beneath that where water flows through the ground. This might be 3.3m to base of excavation less the top of the yellow clay shown at 6 feet deep on the second log. It could be 1.5m but it needs to be known.

 

The concrete is likely to be 300mm thick floor slab plus 300mm thick around the perimeter 2.95m or thereabouts high. Add all that up and, at 3.5 tonnes per cubic metre, it needs to equal the mass of water displaced. You should have 10% spare mass but I haven't included the timber frame etc.

 

If you are nearly there but not quite; make the floor slab bigger (adding a heel) and backfill with heavy stuff that will press down on the heel adding mass to resist uplift.

 

If you are nowhere near heavy enough then anchor piles that would need to be sucked up with the basement would tend to be the solution. I don't think you are remotely in that territory.

[Alan Ambrose]

>>>

Your basement will displace water the area of the basement plus outside staircase times the depth beneath that where water flows through the ground. This might be 3.3m to base of excavation less the top of the yellow clay shown at 6 feet deep on the second log. It could be 1.5m but it needs to be known.

 

The concrete is likely to be 300mm thick floor slab plus 300mm thick around the perimeter 2.95m or thereabouts high. Add all that up and, at 3.5 tonnes per cubic metre, it needs to equal the mass of water displaced. You should have 10% spare mass but I haven't included the timber frame etc.

 

If you are nearly there but not quite; make the floor slab bigger (adding a heel) and backfill with heavy stuff that will press down on the heel adding mass to resist uplift.

<<<

 

Thanks for outlining that calculation and solution - it makes a lot of sense. My concern atm is where the water level will be - the plot is 38m above sea level and as far as I can see (I need to study the contours a bit more) the top of a fairly flat 'peak'.

 

 

[Nickfromwales]

A basement gets considerably more complex when you can walk out of it, but into a pit with stairs. These require sumps and pumping sets, and if they fail, then the basement floods.

Why not go for a well thought out and as naturally lit as possible internal stairwell, and save yourself a bunch of grief, cost, and liability? Do you really need that pit / stairwell that badly? There's no way I do that if / when I get the chance to build, and I will defo look at a basement with all my might, but not one which turns out to be a PITA and a huge risk. 4 sides round and waterproof will be the order of the day.

[Alan Ambrose]

>>> Do you really need that pit / stairwell that badly?

 

My understanding is that if there's any thought of 'habitable space' (now or in the future) then there needs to be two means of egress. Seems sensible too.

 

As an engineer (at least, of sorts ), the prospect of draining any rain out by sump and pump doesn't fill me with too much concern. I guess this could be mitigated, if desired with say a glass roof to keep most of the rain out.

 

Also the ability to get stuff down there fairly easily for the plant room / workbench is appealing as I was only planning a spiral staircase otherwise.

 

[Bitpipe]

We have a 120m2 basement which contains 4 rooms plus plant (kids dens, gym etc). BC wanted sprinkler or external egress, so we went with latter.

 

+1 on getting a SE to spec the ground investigation works. Depending on the site history it's normally a bunch of probes to 10m or refusal around the footprint of the excavation to determine bearing strength, 1-2 6m boreholes for determining soil composition and perhaps some gas/water monitoring left behind in the boreholes for 6 weeks.

 

You only want to do GI once, we had to repeat due to sloppy work by the original on site supervising engineer and had to pay for some of it as the GI firm argued it was all part of the unknown nature of GI. 

 

We were in an urban environment, demolishing an existing dwelling and records showed an old filled in gravel pit within 100m of site - as old records are not reliable, concern was part of our site could have been made ground - turned out (after 2nd survey) that all was good and a slab foundation would suffice. 

 

Contamination studies would be required if planning conditions mandate, however your muck away contractor may want reassurance that the spoil is inert for the cheapest disposal. If the site is truly greenfield then you may be ok without any of this - any previous use or any made ground in the vicinity (like old workings etc) would trigger this condition.

 

We specced a pit with external steps - water was not a concern in our build as it was below 6m so we made the main basement structure waterproof concrete (sika warranty) with below slab and external EPS insulation, the pit was a separately cast open box which butted up to the insulated main structure. It has an aco between the stairs and external door and the pit slab has a sluice drain to a soakaway - has never given us any trouble.

 

 

[Alan Ambrose]

Thanks for writing that up in detail - very interesting. Your door opening was cast in the main basement part as it was done?

 

Any chance of an anonymised copy of your GI spec for comparison - that is the bit given to the GI guys for them to quote on?

[saveasteading]

Do you have a specific need for a basement, such as shortage of land, extensive wine storage, Putin style bunker, etc? We don't need to know which.

 

I ask because the cost of a basenent is about 3, maybe 4, times that of an enlargement of the area at ground level. To this you add the in life risks of drainage problems.

 

Gus' advice is of course good.

Although a soils investigation company will be professional, they don't design the building, and you may have carried out too much or too little testing to suit an eventual SE.

 

If you build at ground level you may only need a small pit and a site inspection.

[Alan Ambrose]

>>> I ask because the cost of a basenent is about 3, maybe 4, times that of an enlargement of the area at ground level. To this you add the in life risks of drainage problems.

 

The LPA are being a challenge - they've gone a bit loopy over our 1.5 storey design, so we may need to go with a single storey design. They are obsessed with 'subservience' to the fairly dull grade II 50m away. It seems every farmhouse is listed in Suffolk - there's 4,120 of them.

 

So the basement seems a fairly easy way of getting some extra floor area. We can dump all the plant down there and have all the storage and workshop space we want.

 

As long as it comes in at less than 4.5K per m^2 - which is the rough cost of quality housing around here, then we're ahead, believe it or not.

 

I'm confident we can address the drainage problems with a bit of careful design - we'll probably do something very similar to @Bitpipe - which is why his post is very helpful to me.

 

Actually I have a question for him re:  "the pit was a separately cast open box which butted up to the insulated main structure".

 

How was the joint to the main basement done - was it thoroughly watertight, or maybe it didn't need to be if the water table is always fairly low?

Edited May 24, 2023 by Alan Ambrose

[saveasteading]

11 minutes ago, Alan Ambrose said:

we may need to go with a single storey design.

What stops you making the ground floor bigger?

[Alan Ambrose]

>>> What stops you making the ground floor bigger?

 

As I said, the LPA heritage man is very resistant. Loads of writing about 'modest and low profile design' and 'subservience' - as though we are still C16 serfs and need to touch our forelocks every time we pass 'the main house'.

 

On a practical note, the two designs have been in to planning for a couple of months and are coming up to the end of their 8-week period. Yes, I could file another design or two, but ...

 

The original planning was only obtained at appeal so maybe there's some LPA sour grapes there.

[saveasteading]

OK so get your permissions and think again.

Remember that a basement of 3m x 4m loses the stair area.

Allow an extra £30k compared to above ground construction. Plus ground and weather risks, so avoid doing it in winter.

[Alan Ambrose]

>>> Plus ground and weather risks, so avoid doing it in winter.

 

Yeah, good point - this is on clay so choosing the right weather / water table will be important.

Edited May 24, 2023 by Alan Ambrose

[Gus Potter]

On 06/05/2023 at 15:00, Phil Sacre said:

I would add that the basement and timber frame might be buoyant.

Yes good point.. like that.

[Bitpipe]

On 24/05/2023 at 13:39, Alan Ambrose said:

 

 

Actually I have a question for him re:  "the pit was a separately cast open box which butted up to the insulated main structure".

 

How was the joint to the main basement done - was it thoroughly watertight, or maybe it didn't need to be if the water table is always fairly low?

 

There isn't one, as far as I recall, the external box that has the stairs just sits next to the other structure with EPS insulation in the 200mm gap between them. Not seen any movement between them in the 7 years since construction.