Replacing A Manhole Cover

[Flyfisher]

Nice sunny day, not too hot, ideal for finally getting around to replacing a broken manhole cover and raising it to be more or less level with the grass instead of about 5 inches below it.

Unfortunately, after clearing away the grass and earth, I've discovered that the actual inspection chamber is larger than the actual cover. It's not easy to see from the photo, but the chamber is about 31x17 inches but the existing cover is 24x18. Width-ways, this is OK, although it means one side of the cover is not supported (which might be why it broke when I drove over it!), but it leaves a gap which appears to have been covered with cement. I suspect the cement was supported during setting by a roofing slate or (as I've found on a previous manhole) a piece of asbestos cement sheeting.

However it has been done, I'm concerned that it won't be strong enough to support anything driving over it so my question (at last, I hear you say) is what would be the best and easiest thing to cover the roughly 8x17 inch gap and [properly support the new manhole cover frame on all four sides?

I'll be supporting the new frame on a course of engineering bricks to raise the new cover to the level of the grass, but I don't want to cover the gap with anything too thick that would mean there is an insufficient layer of earth for the grass to grow. So, something about 1-1.5 inches would be idea. I'm concerned that a paving slab would not be strong enough, but I'm wondering about a concrete gravel board as used for fences - are these steel-reinforced?

I've got some very substantial 2-inch angle-iron but have no idea how long this would last before rusting away.

Any other ideas?

 

[skier-hughes]

I suppose you can't buy a manhole cover the right size?
Your idea of a gravel board or similar sounds like a reasonable compromise, I doubt they are reinforced.
Could you make your own reinforced concrete slab in-situ?

 

[Flyfisher]

I've already got the new covers. I picked them up at a local auction. They are actually slightly larger then the originals, which doesn't really matter, but I didn't realise the originals were not actually the right size until today.

If I put the new frame in the middle of the hole there's about 4 inches at each end so I'm now thinking that a 1.5 inch concrete paving slab would probably be strong enough for such a small gap. Plus the fact that I've just found some suitable pieces in some nasty garden retaining walls that are going to be removed at some stage, so I can recycle rather than skip them.

 

[Feltwell]

Concrete lintel from your local builders merchants? They're not very expensive and should easily be strong enough. Use instead of your course of engineering bricks perhaps?

 

[Flyfisher]

Now why didn't I think of that?! Obvious choice now you've pointed it out. I'll pop into Jewsons tomorrow and see what they've got.

I've since dug out the second (of three) manholes and found three different frames layered on top of each other. It almost looks as if they were added to maintain the top cover to be level with the grass - why does the ground level rise over the years?

 

[MdB]

Now why didn't I think of that?! Obvious choice now you've pointed it out. I'll pop into Jewsons tomorrow and see what they've got.

Go to Bradfords - they are half the price - or at least they were when I bought a couple last year.

 

[Flyfisher]

I've never heard of them, so I just checked and their 26 branches are all in the south west, so not too convenient for Norfolk. :wink:

 

[Penners]

Like Feltwell, my first thought was a concrete lintel. However, if I understood your requirements correctly, I think you want something only 1 to 1.5 in. thick. I'm not sure that you'll find a lintel that thin.

I don't think I would trust a paving slab, or any form of unreinforced concrete that thin. Maybe Graham's idea is best - cast something yourself with plenty of reinforcing rods in it.

 

[Flyfisher]

I was thinking of something fairly thin because I wanted to raise the manhole frame using engineering bricks, so a concrete lintel may not be too thick if I omit the bricks.

The following photo shows the arrangement I was proposing before the lintel idea. I'll check out lintels tomorrow, assuming I can get some fairly short ones - I don't really have anything suitable to cut them with.

 

[Feltwell]

I'll check out lintels tomorrow, assuming I can get some fairly short ones - I don't really have anything suitable to cut them with.

They're only a few quid normally - you can get ones as short as 600mm / 2' from memory, although 900mm / 3' is more common for use in internal doorways.

http://www.diy.com/diy/jsp/bq/nav.jsp?isSearch=true&fh_search=lintel

Have you not got a little 4 1/2" angle grinder? That would be fine for cutting them with, if it didn't go all the way through even if cut from both sides then a brick bolster would easily finish the cut off. If you haven't got a grinder I'd use it as an excuse to get yourself one, mine has been useful for all kinds of jobs - in fact I'm on my second, I eventually wore the first one out! Cheap is fine, my first cheapie one at £15 was better than the more expensive one I eventually replaced it with.

A diamond blade lasts for ages - worth getting if you're likely to be cutting much in the way of bricks / slabs etc.

 

[MdB]

Having a little grinder is well worth it. Diamond blades are £4 in my local store and I've yet to wear one out. Cutting steel rather than concrete melts the diamonds I believe (something like that anyway) so it may be worth swapping to a metal disc to cut through the reinforcing.

 

[Flyfisher]

Yes, I've got a 4.5-inch angle grinder and various discs but I assumed the reinforcing bars would wreck a diamond disc and it wouldn't be easy (impossible) to cut right around the multiple reinforcing bars to expose them to a metal-cutting disc, which would be wrecked by any uncut concrete.

Having said all that, a 900mm lintel will probably be OK anyway - it'll only protrude 150mm at each end of the frame and will be covered with earth/grass anyway.

I'll see what I can find tomorrow.

 

[Feltwell]

There's not that much reinforcement in a concrete lintel. I've cut through them with a cheap diamond blade before without any problems - it probably doesn't improve the life of the blade, but if taken steady they seem to manage OK.

 

[Flyfisher]

The lintel idea is looking good. Jewson had 600mm ones so no cutting required and I'm feeling a lot better about having some reinforcing in the concrete, even though the gap isn't that large. Another half day playing with the concrete mixer should have the job done.