Post foundation near drain

[js.b1]

I will be bolting a wooden gate post to an inverted T-piece of galvanised steel, itself bolted via M12 stainless studwork to a lump of concrete. The post top is braced in all directions - to a house wall via a rigid cross-beam, to the ground either side by stainless wire rope.

So the lump of concrete need be quite minimal re just providing location. The inverted T-piece lifts the wood off floor level and also permits easy removal/replacement if ever necessary.

A drain passes the post, diagonally about 6in at its closest point and about 12in deep to the top (not the invert). It is a normal brown glazed drain, I have just exposed the top edge to get location & vector info, it is buried direct in clay (no 10mm pea gravel).

Am I right in recalling a rule on "foundations" if there is a drain nearby you go below its invert level due to load spreading at a 45- degree angle?

I was planning on a simple 6in thick slab. The gate hinges away from the drain both when closed and fully open. Moving the gate would be quite messy (scouring the walls of past messy painters for one thing). The gate is wood 1.4m wide, 1.8m high, not 4in box steel.

[Roger Mills]

Can you not drive some steel into the ground either side of the pipe, and build a steel bridge over the pipe - and attach the post to that?

[js.b1]

You mean lintel it, yes I could. I would just prefer to keep any foundation from spanning it if possible.

I could use an offset foundation.

[Roger Mills]

Whose drain is it? Is it part of a publicly adopted foul or storm drain, or a private one just serving your property? If public, then technically you need to comply with the relevant authority's (probably water board's) requirements for building over drains - which you mentioned in your first post. If it's a private drain, you can get away with more - but it's then down to you if anything nasty happens to it.

Personally, I'd consider what was *reasonable* - rather than what is strictly required - and just *do* it without asking/telling anyone. The consequences are unlikely to be too dire.

My suggestion of using a steel 'bridge' was based on keeping all load away from the pipe without actually having to excavate and build a concrete structure around it. You could pile drive the underground uprights.

[Tim Watts]

js.b1 ( [e-mail]) wibbled on Saturday 29 January 2011 16:59:

There would be some turning moment on the post which may dry to twist the foundation into the pipe causing sideways pressure.

If the pipe runs outside of the gate opening, I would either:

a) Auger (post auger) a hole as deep as you can (3-4 foot perhaps) past the drain and as clear of it as possible (6" you say?) - and fill that with concrete. As long as there is plenty of earth between the post and pipe and there is no way the post could reasonably move to touch the pipe, I don;t see that could ever cause much pressure on the pipe.

b) Another solution would be to use an offset foundation - so just enough exists on the pipe side to set the support into, with most of the mass of the concrete to the side away from the pipe.

If the drain is not running under the gate but to the side of the opening, this ought to provide enough counterbalance - or in the case of a), if the foundation is sufficiently deep, the part on the level with the pipe will tend to be pulled away from the pipe, and the part that's being levered towards the pipe would be well underneath it.

If the pipe runs under the gate opening, the reverse will be true so would need a different approach.

You can always make the block longer in the direction of the drain to dissipate any load.

I don;t think the vertical load of the gate is going to cause any problems as long as the concrete block ends below pipe level, but the sideways forces would be a concern.

Can you do an ascii art diagram of the pipe relative to the gate?

How heavy and wide is this gate?

[js.b1]

Own soil pipe, private, on own land.

That idea attracted me, I recall a grand designs where they drove in rods with fixings on top. There is I believe a hammer-in "micro-pile" of about 12-24" length with an M12 or M16 nut on the top.

Unfortunately I need to excavate to be certain the supply cable and water supply are not (by sods law) in the same area. I will find out tomorrow with my plastic trowel.

[js.b1]

The gate has a cross-bar to the house wall - 100x70mm, that should stop the post rotating much?

I too was thinking off an offset foundation - basically a post 12" away from the drain, ending below the drain, with a mesh reinforced cap onto which the inverted-t is fixed by studs.

Viewed from above (/ =3D pipe location & vector, o =3D gate post) / o

Viewed with gate closed from above / o ! ! ! H (H =3D House Wall)

Viewed with gate open from above / o-------

The gate is 1.4m wide and 2m high. It comprises 7 horizontal 1400x150x19mm planks, 2 vertical

2000x150x19mm planks, the wood is a mix of mostly pine & larch, diagonal is sapele, verticals are iroko. Hinges are 14" standard gate hinges.

The post is braced in 4 directions. ....o.... (... =3D stainless steel wire rope to ground anchor) X X X (X =3D 100x70mm cross-beam) X H (H =3D House wall)

The left steel wire rope handles tilt when the gate is open. The 100x70mm cross-beam handles sag when the gate is closed.

Hope my ASCII works :-)

[js.b1]

Update:

Soil pipe runs 45-48 degrees. Gate weight is 39.1kg. Gate width is 131cm.

So the 6" post-to-pipe becomes 12-14" when you move 6" closer to the house (H on the ASCII art).

[Tim Watts]

js.b1 ( [e-mail]) wibbled on Saturday 29 January 2011 21:38:

OK - it is a heavy gate - but *all* the tendency to inflict sideways pressure is away from the drain pipe as long as the foundation goes a couple of feet below the pipe - applies to both open and close positions.

I think a 3' (or a little more) deep hole, offset away from the pipe as much as practical should be quite sufficient.

One indisputable fact is then the vertical component of the load (weight of gate + post) will be bourne on soil 18" under the pipe so no chance of pressure on the pipe.

Also, for a 3' deep foundation, a foot wide or whatever is practical to dig, assuming the foundation tries to pivot about its midpoint, the bottom 1.5' will press towards the pipe under load and the top 1.5' will try to pull away.

In this case adding a support over and behind the pipe could make it worse as there will then be a pulling force trying to drag the pipe sideways.

There will be a small load on the pipe due to the 45 degree load spread rule, but that would seem to be a very small pressure all things considered.

I can see no way such an arrangement could ever put more than trivial pressure on the pipe. Does that make sense (its your pipe - so assume I'm wrong and question everything)?

Last time I had anything to do with a gate that size, my mate did an 18" or so foundation and it wasn't quite enough as the post did tilt under load. So another reason to go deep.

Cheers

Tim

[Tim Watts]

js.b1 ( [e-mail]) wibbled on Saturday 29 January 2011 21:55:

Oh - is that all? I was imagining a much heavier gate.

I don't see a problem at all there, with a 3' deep block - in normal circumstances that would be overkill, but given your pipe, it is probably a wise precaution.

At least the gate post will never lean :)

Even better.

[js.b1]

Spanning the pipe worried me. House foundations commonly use lintels to span pipes, but foundations tend to be "bulk" and "box shaped" so there is no tilt and merely pressure 45-degrees out from the bottom of the foundations. Wall foundations are not a box and can suffer tilt, but rely on their longitudinal "bulk". Post foundations have "not much" in their favour.

Offset sounds good.

Would the ideal be a tripod of three small separate foundations, linked by a mesh re-inforced cap? That might actually be needed if I find anything else in the area.

The area has seen daily 1t-1.6t car traffic for 50yrs (garage), the existing concrete is so foundation and it wasn't quite enough as the post did tilt under load.= So

Being able to do a rigid cross-beam to the house (H) makes a huge difference. The design of the gate being "planks" makes the cross-beam fit in so there is no visual downside and the mechanical bracing is simply fantastic particularly re tilt.

[Tim Watts]

js.b1 ( [e-mail]) wibbled on Saturday 29 January 2011 22:38:

I didn't quite get this the first time,but now you've explained it again, I do.

OK - now you have zero turning moment in the direction of the house but you will have the original amount when the gate is open at 90deg.

Cross linking piles would work if you need to clear other obstructions.

One method which isn't that odd would be to bang scaffold pole into the ground if you think you could drive it 3' down with a sledge hammer. Do this

3 times in a triangle, rebar the tops together and pour a concrete cap covering perhaps 6" of the pole tops. That would be fantastically strong for as long s the steel lasts, which should be quite a long time.

The poles are thin, but they are going into undistubered ground and there are 3 working together.

[js.b1]

It would.

Will have an explore tomorrow, until I know exactly what is there it is "scenario planning".

[Tim Watts]

js.b1 ( [e-mail]) wibbled on Saturday 29 January 2011 23:31:

Could add - the poles do not have to be vertical. If 2 are driven splayed outwards, one towards the house and the other towards the gate's open position, which means both will run away from the drain, when it's concreted together, that MF is not going to lean if an elephant sits on the gate.

Just an idea... I wonder if splaying the 3rd away from the drain might mean that sinking them 2-2.5' would be enough if the ground is tough? The load of the gate would be spread effectively over quite a large area.