Underpinning - max bay size & support

[guitarbonz]

To achieve the necessary depth for underfloor heating & insulation we have to remove a 4 inch concrete slab and replace it.......sounds simple until you discover as we have that all internal walls of this late '60s house are built straight off the slab.

Structural engineer describes this as "outrageous" and i agree...however apart from very slight movement/slope to floor (millimetres) the house remains standing and so do the walls built directly off the slab. One of the (4" thick) walls rises to 1st floor and does not support anything. Two of the (4" thick) walls are load bearing carrying 1st floor joists/floors and then rise to loft level, they are not supporting anything in the loft.

I know from excavations outside that the house foundations are at depth of 1.2M onto sand and gravel, building inspector has been happy with sand and gravel as a base for extensions that I have built.

Regarding underpinning, the two load bearing walls are 2.2M long and run parallel to each other, the non-load bearing wall is approx 2M long and joins each of the load bearers at right angles. (The external cavity wall makes up the box of a room with these three walls..fortunately the cavity wall has a solid foundation.)

I plan to work from one side of the wall, disk cut the existing slab close to the wall and then working a small section (bay) at a time, dig one bay at a time 1.2M deep, shuttering as necessary and concreting up close to the base of the existing slab. Once new foundation has cured we shall dry pack so that the section of wall is completely supported on new concrete foundation. When all bays are complete I shall backfill remaining trench, compact well and then replace the slab.

Does anyone have any advice/experience as to the maximum width of each bay and whether needle or strongboy support is necessary above a certain width of bay? Also any advice as to the width of the new foundation would be helpful, would 300mm be sufficient? (Recognising that i would need to dig at least a 600 wide hole for access & to set up shuttering etc)

Many thanks in anticipation of your replies.

Phil G.

 

[theoldun]

Phil, Back in the late 60s early 70s, as near as I can remember N.H.B.C. brought out the 600mm fill regulation, whereby you could not fill and concrete an oversite if the fill was in excess of 600mm.
To overcome this we used to dig and concrete all founds normal manner including internal walls, Take external brick skin up to d.p.c. and leave internal skin and internal walls 2 course down, wedge shutter board in cavity 150mm high, fill the oversite up with whatever you could get hold of, usually excavated sub soil and wack down level with top of brickwork 150mm below d.p.c.
Lay polythene dpc in normal manner, throw A 193 steel mesh in lapping over external inner skin and partition walls and pour 150mm concrete, giving you a reinforced 150mm concrete slab. Any room with a span over 3 metres, then we would tie in to the mesh a 18mm steel rod every 400mm to give a bit more meat. This was fully acceptable in those days both by NHBC and BC.
I am 99% sure that this is what you have got and any decent structural engineer should have advised you of this possibility.
Now if I am correct this means that your oversite is also sitting on the internal skin of your external cavity wall. If this is the case, then you are up s**t creek without a paddle.
Would suggest that you blow out two or three inspection holes to determine what I have said. Once you know exactly what the construction is then come back and will try to help you out.
old un.

 

[JohnD]

To achieve the necessary depth for underfloor heating & insulation ................

I suggest you don't bother.

 

[guitarbonz]

Would suggest that you blow out two or three inspection holes to determine what I have said. Once you know exactly what the construction is then come back and will try to help you out.
old un.

Hi Oldun,

thank you for your reply and for your interesting comments, i have already made two inspection holes and fortunately? (or not) what you describe is not the case. There are no foundations under the internal walls, they are built straight off the 4" concrete slab which is resting on clinker. The slab is not resting on or integral to the inner leaf. There is no steel reinforcement. After 42 years with only slight settlement i suspect that these walls aren't going anywhere. I am just concerned that if i dig bays any wider than (a tight!) 500mm the walls may decide to prove me wrong.

cheers

Phil

 

[Static]

What is your SE suggesting on this? Or have you even asked?

 

[guitarbonz]

SE is suggesting underpinning.....or leaving the slab in place and advising my missus that UFH isn't an option....which I am working up to!

I haven't asked the SE the question about max bay width because I had hoped that a helpful builder might have an opinion, given that SE's do the theory whilst builders make it happen and may have practical experience. (No disrepect to SE's intended).

 

[Static]

The SE will normally specify the bay width for underpinning as well as doing a sequence of works..

Most would shy away from any underpinning works as it becomes sticky when insurers learn of the works..

If it is not absolutely required then you should avoid it..

 

[theoldun]

Phil, Sorry I was wrong. From description, it sounded a definite meshed slab.
If determined to go down route of ufh, would consider acrow ceilings and long needles to wall and rattle partitions over, machine cut 300mm wide trench through slab excavate founds, etc and rebuild partitions. You say you only have 6.4 lin metres say 16ms2. Of the top of me head, to rebuild partitions, say£800.00 to £900.00 to block, set and skim.
old un.

 

[guitarbonz]

No need to apologise Oldun; your helpful comments and interest in my problem is much appreciated, as are those of the other respondents. Many thanks Phil G.