New Build in Herts with Basement

[MarkM]

Hi all,

Quick into. I have planning permission for a small house in Hertfordshire which includes a basement. The house is L shape above ground and squared off underground in the basement. The plan is for the superstructure to be SIPs

I'm struggling with the basement and getting quotes and designs that are comparable. The only certain part is the requirement for a contiguous piled support structure.

Anyway, I'm going to have a good read through the basement section of the forum and see what information I can glean. 

Regards

Mark

 

[joth]

Welcome! from a fellow Herts-based self builder (well, customer renovator).  I can't help with your question, but when you do have an answer I expect @puntloos will be interested. ?

[Dreadnaught]

Welcome. You're just south of me, I'm in Cambridge. 

[Red Kite]

Welcome Mark, we are with you on thinking basements are great and we are building a basement which is fully in ground at the back of our sloped plot and out of the ground at the front. So, not wishing to prescriptive or 'know it all', and you may have already thought it all through, but here are some of our thoughts - hope they help :-

 

Your most important element with a basement  will likely be waterproofing: and for waterproofing you will need (for building regs, BS 8102 etc) 2 out of 3 methods :- (i) external tanking, (ii) waterproof concrete and (iii) internal drainage/waterproofing. We went for external tanking and shuttered and reinforced structural waterproof concrete (btw steel reinforced concrete is often shortened to RC) as it seems like admitting defeat letting the water in and then draining it internally and pumping it out again. However internal drainage is often preferred by warranty and lenders since you can see it (and fix it) after you have finished. We will have a 20 year guarantee for our waterproofing from Cementaid / Caltite and they seem to be very thorough in checking the work our contractor is doing. Our contractor - who is a specialist groundworker / RC expert and is fantastic - reckons our external tanking is unnecessary except to get the warranty as the concrete is 100% waterproof on its own (providing it is properly done). Interestingly our structural warranty provider (LABC) was not prepared to cover the basement element. With ours - since we are out of the ground at the front, hydrostatic pressure in not a big issue - which it can be when you are fully in ground - so is much less critical and we have an externally drained French drain around the outside. Odd though it may seem, one of the issues is that in-ground basements tend to want to float (if you are below the water table at any time of the year) so you need to watch that!

 

Not sure what your need for a contiguous piled structure is (retention of some form?) - you may well be able to use the basement walls for this structural element (if they have enough steel and concrete) - and perhaps temporary sheet piles while you do the dig and construction - we have done some of this and it has worked fine - and it may be cheaper that permanent piling plus basement walls. But you certainly don't want everything to collapse into your nice newly dug hole, and the last thing you want is one of your contractors buried in it (btw not a joking matter - holes can be very very dangerous place). Also you may want to consider having a basement footprint that matches the house above - depends on cost/budget - but can simplify things and may not be that much incremental cost. There can be an issue that if you mix a basement with traditional foundations they both need to go down to the same depth to avoid any differential movement - but talk to Structural Engineers on that. Tip: if you want to build extra basement, but dont have PP, then the extra bits you build are technically an 'undercroft' that you might convert at a later stage (subject to PP / PD).

 

Our advice would be to first consider the use you want from a basement - storage (easyish)  or fully habitable (much harder). Then look at the warranty and funding side and see if this constrains you on the methodology. Next step is a topo survey plus a good soil survey (this will be money well spent - and if a basement contractor is prepared to quote without seeing a soil survey then they are likely clueless or naive) and some careful thought about site drainage (while building and on-going) - which is really important if you are below ground.  Armed with all that then talk to a few Structural Engineers and/or Architects and see what they think is best - they may not focus on cheapest but you likely will! Also talk to a few of the competing basement product providers for you chosen waterproofing - they should have approved contractors you can talk to. You may also want to look at ICF as it can be a good option for basements and is probably your only option if you want to do it yourself - we are not experts in this area but there are some on the Forum who are, and have done it all themselves (hats off!).

 

You may also want to consider a few other things - like fire protection and exits (note that 3 story buildings get much more complex from a fire standpoint and basements have a few fire quirks all of their own), natural light, foul and grey water drainage, access, insulation, and of course how your SIP structure will sit on the basement (and link to the basement insulation without cold bridges). Also consider muck away - we generated 600m3 and this is really expensive if you cant 'loose' it somewhere. Site access may be a big issue on a small site as you will likely need lots of machines and lorries - oh and look at any overhead cables and underground services - these can cause havoc and delays that you might not believe - we can tell you the saga of the sewer if you ply us with drink!

 

So best of luck with the project and welcome to the forum - we have found that there are lots of amazingly knowledgeable and generous folks out there. If you want then take a look at our week by week video Blog ( Self Building two in North Wiltshire) but be aware that we are building two houses and the below ground elements are really complex - PM us if you have specific questions or want to chat or visit. If you want we can ask our contractor if he is interested in a job in Herts but this may be too far for him - though he does cover a big area, and ours is a smallish job for him!

 

[Simplysimon]

welcome to the club,

as with the above, why contiguous piles? that must be quite expensive. that said they should be almost waterproof

[Red Kite]

Its the almost in that sentence that gives me the shivers!!!!

[MarkM]

Thanks for the welcome everyone. I've been investigating this build for about 3 years now while the planning permission and associated appeals went through and I can't believe I haven't stumbled across this site before.

We've had a topological survey and ground investigation report carried out. The latter concluded that to batter the excavation would take 45 degree slope which would undermine the neighbours house and also take out some of our garden including trees. However, unless I got it wrong contiguous pile walls wouldn't be waterproof as they are 450mm diameter on a 600mm centre to centre pitch.

The site is quite tight and there are telephone lines that are going to have to be avoided with the piling rig.

As I see it I need to pile 10M down, add a capping beam, excavate and get rid of the muck. Then blinding and a slab plus waterproof concrete RC walls up to the ring beam.  Then some form of membrane such as Delta

My problem is the quotes I have this vary by £90K which makes or breaks the project for us. Strangely, the most detailed quotes in the lowest but, as they say, "if it sounds too good to be true..."

The use case for this to be a house we can downsize into but we would like the basement to be usable as a living room plus bedroom with en-suite (Saniflow macerator).

The project is both daunting and exciting. Mostly the former at the moment!

[puntloos]

Just two quick notes:
 

1/ If you're serious about a basement, probably better to do a ground water level check sooner rather than later. If your basement is going to have to hold back the water price jumps massively.

 

2/ Assuming ground water is not a problem, I received a handwavey quote from a builder in the area of 2500/sqm

[MarkM]

We have had a ground water check done and that doesn't seem to be an issue. They left us with an inspection point which I should check again.

The £2500/sqm seems to be about in the middle of the quotes we have had so far. Perhaps my main worry is the lack of detail in some of the quotes.

[Red Kite]

What sort of ground conditions do you have? If its not rock or boulders I would check out temporary sheet piles rather than permanent concrete contiguous piles - might be cheaper if they will do the job. We held back about  a 4m wall of wet clay with them (and the driveway + vehicle traffic) OK and then built the RC walls and backfilled with clean stone and removed them. So far everything seems to be staying put. Cost will depend on steel grade / how thick sheet you need and how deep you need to go. 10m sounds a lot - I think the rule of thumb for sheets is about as much in as out, but I think with steel you may get some gain vs concrete because they all interlock. Also depends what sort of machine you need for the hammer (we used the big digger already on site with a high frequency hammer). Might be worth talking to a specialist. Our sheet piling was about £15k for the duration (installation, hire and removal) - dont know how this stacks up with the concrete piles? Also depends if you are into re-use - I think steel piles get used many times over. Not sure if the nuisance factor for sheet is worse than concrete piles - but the vibration was a bit epic! It might be a factor if for any reason you care about your neighbours.

 

This photo is our 'short' side not fully dug yet (went down to about 4m) but you get the idea - and note the water - we needed to run a pump almost continuously, The bit further on was not piled and sat OK having been terraced / battered for the duration - but our clay was pretty stable and has a reasonable repose angle - I suppose there has to be some benefit to clay in  that it sticks to itself, and pretty much anything else it touches!

 

 

 

 

 

 

 

 

 

 

 

And this is the shuttering in behind the sheet pile which held back the driveway to the right - gives you an idea of the space you need to work in

 

 

 

 

[MarkM]

I am told there is a mixture of everything from sand to gravel to clay and a fair amount of flint. But, luckily, no water.

But, I do like the idea of sheet piles. I would worry, as you suggest, about the adjacent house. Maybe a combination of sheet piles on 3 sides and contig piles on the side running along side the neighbours. They only moved in a few months ago and have young children. They seemed very reasonable when I spoke to them. They did purchase the house after our planning permission was granted so cannot be too surprised. I just had a quick look and maybe there is an option to vibrate or push the sheet piles in rather than banging them in

The quote for the contig piles plus capping beam is £47K. Which seems a big difference to what you were seeing for the sheet piles.

I am supposedly having a sheet piler contact me and I will suggest this to him and see what he says.

Thank you very much for the idea. It's certainly worth thinking about and investigating some more.