Floor level below ground level

[Steve law]

Hi 

I have some building experience but not a ground worker or bricklayer by any stretch of the imagination. 

My question is as you can see from the design, the extension is lower floor level than the original ground level to achieve more head height in the loft. I foolishly thought I could go down 1.5 meters with floor level but after consulting some people found that to have a finished floor level of 1.5 would prob need to dig down 2.5 meters causing massive costs of sheetpiling extensive soil testing and samples. Without just going ground level and putting up with very little head height in loft level is there a happy medium of depth and method of detail that I can dig footings build some kind of foundation blocks then use a liner and or tanking to go down a reasonable depth but without the need of sheet piles. The water table in this area is around 8 meters deep and and a inspection hole of 3 meters has been dug and seems to be permible clay that drains pretty well.

 

[Dave Jones]

be cheaper to take the roof off and use attic trusses. be in the dry in less than a week.

[joe90]

@Steve law is asking about foundations not roof trusses 🤷‍♂️

[Marvin]

Hi @Steve law

 

You must love taking on a challenge....

 

Digging deep foundations requires thought about the depth proposed, the proximity of other buildings and the extent to the site is owned in my opinion. Start with all that information and you should get the best relevant responses.

 

Good luck

 

Marvin

 

Oh and of course nearest trees and service runs... and? 

 

Edited August 24 by Marvin

[Alan Ambrose]

I think that's an interesting problem and situation. You're on a bit of a slope and you're extending behind and you want to make the extension two storey, taking advantage of the fall? That gives you a kind of semi-basement - a bit like a bunch of Nth London houses which have a lower ground floor about 1/2 a storey deep and then an elegant flight of steps up to the front door ? You can excavate from the rear garden fairly easily as the drop there from GL is maybe 1m?

 

Bearing in mind I'm not a groundworks expert (or in anything much 😃) but we are planning a basement, I wonder whether:

 

+ get in a couple of groundworks firms on spec for an hour or two for their advice and thinking on the basis you will give them the opportunity to quote.

+ check what other support methods might work that are cheaper that sheet piling. Is the extent of you plot shown on the drawing or do you have scope to batter back at a lower angle. Low enough angle and you need no or minimal suport. if you have room, sometimes you can step the excavation walls to avoid expensive propping.

+ the minimal SI you might need is a load test and an angle of repose test - not crazy expensive. The latter will give you safe angles of batter back for the excavation. Local BC might be able to short circuit the need for those if they know the ground and that kind of desgn has been used locally a bit, so you could ask them.

+ that might all become moot as the design and installation of the waterproofing will need to be done well, so you need either an SE who knows waterproofing or foundations guys that are familiar with the manufacturer-guaranteed systems (Sika etc). Find the latter and they will answer most of your questions and know what to do.

 

 

[Steve law]

Thanks @Alan Ambrose that is a very helpful response. It seems to be a very difficult as if you go to architect they say you need to talk to a structural engineer but it's very hard knowing what you want to achieve in the build ie more head height in loft/first floor without blowing the build out of the realms of possible on any budget. Because when you start talking a 1.5 meter basement that's not a 1.5 meter dig that's more like 2.5 what would require so much calculation tests and technical drawings before you even break ground. It also doesn't help I have a unusual shaped plot which is long and skinny with a drive one side but only a meter the other side from boundary fence. I can easily get in to garden and use 8 ton digger and have field to dispose of waste soil. But the architect didn't really advise me very well and now I'm just trying to go down as far as I can without it getting stupid expensive. You can see by the pic the thick mortar level is the floor level and there is about 400mmdrop over the 12 meter extension so you can see that I don't want to go just 400 below ground level at start because I will be at ground level by the end of the twelve meters and achieve zero drop so nothing like the design picture

[Alan Ambrose]

I don't know if it helps your thinking any -  my full basement design has a 100mm screed (with UFH), 300mm slab, 300mm XPS insulation on 50 sand blinding & 100mm MOT. So, that's 850mm. Full dig depth is about 4m.

 

But that's maybe way over the top for you depending on your insulation requirements. You could probably cut most or all of the screed and power float or polish the slab and then depending on SE calcs & soil state have a 200 or 250mm slab and say 150mm insulation on 50mm of sand and 100mm MOT. That might be 500mm depth. 150mm EPS insulation gives you 0.25 U-value which, I believe, is min regs for an existing building. So, that saves about 0.5m from your calcs.

 

Do you have scope to put the extension roof on exactly the same level as the existing one?

[Richard Anthony Johnson]

permeable CLAY is an oxymoron. Clay contains sulphates. Sulphates and concrete are not friends. 1.5 meters deep will require retaining walls to prevent side wall colapse.  These will need to be tied into a raft or very large concrete base. Raft would be better as you can tank it externally with a polythene, run up the outer walls.  Concrete throughout will need to be a minimum C35/ 45 grade.