Depth of trench foundations?

[Moonshine]

This has got my goat, no where in my SE information pack is there information on the minimum depth of the trench fill foundations, only that they need to be 600mm wide, i am after the green demission below

 

 

Is this not information that should be provided by the SE and confirmed with the BCO via site inspection prior to pour?

 

The ground is good and has a safe allowable bearing pressure of 200kN/m²

 

Any ballpark ideas, 50mm, 500mm, 5000mm!

Edited July 16, 2021 by Moonshine

[markc]

rough scaling off would imply 500mm

[Moonshine]

12 minutes ago, markc said:

rough scaling off would imply 500mm

 

These are drawn at 450mm, but are only indicative. 

[Ronan 1]

Hey Moonshine, he will need to give you the minimum as it's hard to say without knowing the house size and construction and your ground type.

 

Typical minimum depths are usually between 300mm-500mm BUT you'd better ask the SE

[saveasteading]

So the number is missing. A minor thing. Ask.

If you ask politely it will probably be confirmed as 450 or 500, and 1m below ground.

 

There is a '2000' adjacent, but out of context I don't know what  for.

[Moonshine]

Just now, Ronan 1 said:

BUT you'd better ask the SE

 

I will be, but can't get hold of him! ?

[BenP]

Standard note would be as follows:

 

"All foundations to be a minimum of 1000mm below ground level, exact depth to be agreed on site with Building Control Officer to suit site conditions." 

 

Usually bco will confirm on site based off the nhbc foundation depth calculator. You may hit chalk etc and not require as much, or it may be very clay-y and have trees nearby.... 

 

Ben 

[saveasteading]

The drawing is to scale, so is prob fine to proceed at that, until further notice.

 

For your comfort, drop him an email that he has omitted that dimension, scales x and you are working to that, and please advise urgently if otherwise.

[crispy_wafer]

Yeah, I'd expect something on your buildings regs drawings to be honest, but even that could be a standard figure rather than specifically tailored to your site.  Dont stress give them a call Monday morning, or give building control a quick ring, they'll give you a typical number where load bearing strata may be found in your area, then once you think you are ready they'll be out to investigate and give a green light before concrete pour.

[Temp]

There are two things here...

 

The bottom of the trench should normally be 750 or 1m down..

https://www.homebuilding.co.uk/advice/foundation-systems-and-soil-types#:~:text=Strip%2C trench fill or pad,to 3m may be necessary.

 

Strip, trench fill or pad foundations must be cast at a minimum depth of 750mm in low plasticity clays, 900mm depth in medium, and British Standard 8004 recommends a minimum depth of 1m for foundations in the highest risk areas. If there are, or were, trees nearby, depths of up to 3m may be necessary. The depth necessary will depend on the type of tree, as species have different water demands, with tall broad-leafed species such as poplars having the greatest impact.

 

Then there is the thickness of the concrete...

 

Many people go for what are called Trench Fill foundations. This does not mean fully filling the trench. With trench fill foundations the services go through the concrete rather than block walls above so the concrete must be at least 500mm thick or risk cracking where pipes go through. In practice the trenches are filled with enough concrete to get to a convenient multiple of bricks and block courses below DPC which is 150mm above ground level. It is that and/or the 500mm minimum that sets the thickness of the concrete.

 

If you go for traditional strip with services  through the block walls you can go thinner than 500mm. The normal range is 150mm to 500mm but this needs input from your SE.

 

[Moonshine]

Well I couldn't get a concrete (pun intended) answer from the S.E and time is ticking on, so decided to dig 500mm deep trenches which are at least 1m below final ground level. Also deep enough not to sucharge retaining walls as per SE spec.

 

All by hand (there was suppose to be a digger on site to do it but that is another story)!!

 

It took all yesterday with help from my brother in law in the morning. Off camera to bottom left is a 8m run of trench 

 

I am going to get the bco on site mon/tues and hopefully poured Thurs if its o.k (praying for little rain),  may just fill the whole depth.

 

 

Edited August 1, 2021 by Moonshine

[Moonshine]

BCO has been today, and is happy (the first thing he said when he came to site was 'that looks solid').

Conctrete booked for Thursday, woohoo.