New project for work

[MikeJhn]

If you do use softwood beams I would give the ends that are going into the ground
several coats of Bitumen up to ground level , It will help stop rotting.

Not so, any timber set into earth will rot at ground level, therefore protection needs to extend above that level, heat shrink wrap can be obtained to do this or just simply bitumen, but this is prone to damage when strimming etc:

 

[Stigmorgan]

Would 50mm not be too much into a 100mm (probably actually 90mm) sleeper?
The area will be woodchipped so use of a strimmer would be minimal and by myself so that wouldn't worry me too much, I plan to paint with bitumen then use a heat gun to get it soaking into the surface as much as I can.

 

[Trextr7monkey]

30mm doesn't sound enough to me, deeper and tight joints will help resist racking. Timberlock or other long screws will help with that too of course driven in from below at an angle, but the more depth the better really.

Think I did mine about 40mm or 50mm, as deep as I could easily manage with the tools I had and without compromising the depth in ground and finished height

We cut the trench on a radial arm saw so it was a snug fit then put 9 or 10” coach screws straight down a drilled hole then capped with a wooden plug. We put about 10 benches in place all well tested by the enthusiastic supporters, nothing has moved yet !!

 

[Phill05]

Would 50mm not be too much into a 100mm (probably actually 90mm) sleeper?
The area will be woodchipped so use of a strimmer would be minimal and by myself so that wouldn't worry me too much, I plan to paint with bitumen then use a heat gun to get it soaking into the surface as much as I can.

Stig, we used to burn the leg's that would be in the ground then soak the ends for a week in old engine oil and before they were dropped in the hole a good dose of bitumen.
Fencing posts also had the same they last for years and years.

 

[Stigmorgan]

Stig, we used to burn the leg's that would be in the ground then soak the ends for a week in old engine oil and before they were dropped in the hole a good dose of bitumen.
Fencing posts also had the same they last for years and years.

I won't have time to leave things to soak, I will only have the half term week to get it done as well as letting contractors into the building and keeping an eye on holiday club.

 

[Jameshow]

How about canting the legs outwards as this will lock the legs into the top and also into the ground say 10-15°

 

[Stigmorgan]

How about canting the legs outwards as this will lock the legs into the top and also into the ground say 10-15°

As in kicking the bottoms out or twisting on the vertical axis?

 

[Jameshow]

As in kicking the bottoms out or twisting on the vertical axis?

Kicking the bottoms out?

 

[Stigmorgan]

Kicking the bottoms out?

Diagonal / rather than vertical I

 

[accipiter]

I won't have time to leave things to soak, I will only have the half term week to get it done as well as letting contractors into the building and keeping an eye on holiday club.

With the mention of 'burning' - a Japanese wood treatment method you may/may not know of - it may be worth considering as it can be used on oak and other timbers. This may be of interest to you
https://www.instructables.com/How-to-Burn-Stain-Wood-Aka-Shou-Sugi-Ban/