Digging a new manhole

[Donkmeister]

I need to have a new manhole to allow connection of a soilstack to serve a new bathroom. I've put the bathroom and the drainage on a Building Notice, however before I submit it and book a meeting with the inspector I was hoping to get a few things clear in my mind.

From conversations with neighbours, I know that the sewer is approx 6 feet below ground, and runs along the backs of the houses about 3m from the back of the house. It is of 1920s vintage, so clay.

As the sewer is shared between an unknown number of houses, I can't ask people to abstain from flushing whilst I connect into it. So, I need to take an approach of "get it built, THEN cut open the pipe and get the hell out of the pit".

Digging and moving of materials will be done by hand, however I have about 10 friends to help me do this who are happy to be paid in beer and barbecue.

Details I'm not clear on are:

1) Should I go for pre-cast concrete (1.2m circular?), or are these new plastic systems any good? Can they be added to an existing clay pipe system without too much risk of sewage leaking?
2) How do I set a slipper into the benching BEFORE cutting open the existing pipe?

 

[Hugh Jaleak]

Firstly elf an safety, any excavation over 1.2m deep needs shoring up, a collapse can be fatal. Also advise taking suitable precautions against Weil's disease, wear gloves etc.

Cutting into a 4 or 6" clay pipe, i'd go for a plastic chamber. Good site here for instructions and photo's: http://www.pavingexpert.com/drain15.htm

If you've a 9" pipe then it'll have to be a concrete or engineering brick chamber. If you form the base around existing pipe, then cut and remove top half to form a half channel. Half channel will be adequate to contain the flow. Fit connection, and bench to suit. Build chamber as required.

 

[Donkmeister]

Ah, so I chop the top off the existing pipe BEFORE I bench in the new connection?

If the pipe is of a size to use a plastic chamber, do I just try to time it for when people are unlikely to be using their toilets and work quickly? Or can I find an upstream manhole and bung the outlet with something on a rope so it can be pulled out (owner of upstream house permitting, of course).

Will be sure to shore up the excavation. In my head this should be easier with a rectangular hole than with the circular PCC I'd been looking at.

Thanks for the advice and the link.

 

[noseall]

A 1.8m deep I.C. is going to have to be large, maybe with step irons also.

A standard 450mm plastic fella will be too small.

Use a rubber drain bung whilst working on the drain, those upstream need not know.

Six foot deep drains are gonna take some digging by hand. The trench width at the top will need to be about 3 feet wide!

 

[Nige F]

Six foot deep drains are gonna take some digging by hand. !

` Tie a strong rope onto the bung ( behind the wing nut)- they have been known to fly out of your hand with the pressure of water built up behind - then immidiately bung the outflow of the manhole

 

[theoldun]

Briefly, in our opinion too much work for DIYer.
Inspection chamber and manholes two different animals.
Any drainage serving more than one property is classed as sewer, either adopted or un-adopted, If adopted your sewerage undertaker may wish to poke their nose in and agree, disagree and inspect.
450mm dia plastic IC used to be acceptable up to 1200 deep. Since the introduction of Working in Enclosed Spaces and Health and Safety man entry and non man entry IC and MH, without wishing to contradict me old mate nose 450mm dia IC are now acceptable up to 3000 deep providing cover reduced to 300mm. However you must run it pass your BI first.
Forget all about concrete sections, as to comply with regs for MH at 1800 deep the size of rings would need to be 1200 dia and rectangular sections would need to be 1200x1000, but as the latter is not a stock size you have to jump up to 1475x1000. Without a JCB or 5 tonne 360 on site rings or rectangular sections will be to heavy to manhandle into place. If we had to put a manhole in with no access for machines, we would run her up in class B engineering brickwork.
However if we were pricing this for you, we would ask BI if he will accept a non man entry IC out of 600x450 concrete manhole sections with non entry cover. Cut a 550mm long channel out of existing pipe, sods law you will hit a collar so cut very careful through collar. Instead of running connection down to invert level, lay connection about 600mm deep and connect to channel with pre formed slipper (we cut our own on site) and then a back drop. Cut rodding access in side of sections for back drop. Take to long to explain back drop, so Google external drainage back drop or tumbling bay and you will see what we mean. some back drops are inside manhole but you want it outside.
We cut our channels whilst pipe is live, but you could be unlucky and two baths and two WCs. are let go at the same time. If this happens you get slightly damp, so stick a bag with extended tube in the next manhole upstream.
Word of warning. Safety, safety, safety. We never dig down below 1200 with out all necessary planking, poling and bracing as required especially in previous dug ground.
Never work on live drains at that depth unless someone is on top to help you out if you get gases from pipe.
oldun
.

 

[Norcon]

We single face shutter large man holes like that with peri trio. Though not for sewers. Very quick ,safe and easy to do and any pipes are set against the shutter. Might be someone in the hole for 10 or 20 minutes at the most.
Just excavate and build the shutter above ground and swing it in and suspend it with strong backs, pour concrete with a tight slump and then cut open the pipe and bench the sides. Can be all finished in a single pour.
Then a 1.8m sq reinforced biscuit lid we form on site and finally the 10 ton rated 600mmx600mm cast lid.

 

[geraint]

Briefly, in our opinion too much work for DIYer.
Inspection chamber and manholes two different animals.
Any drainage serving more than one property is classed as sewer, either adopted or un-adopted, If adopted your sewerage undertaker may wish to poke their nose in and agree, disagree and inspect.
450mm dia plastic IC used to be acceptable up to 1200 deep. Since the introduction of Working in Enclosed Spaces and Health and Safety man entry and non man entry IC and MH, without wishing to contradict me old mate nose 450mm dia IC are now acceptable up to 3000 deep providing cover reduced to 300mm. However you must run it pass your BI first.
Forget all about concrete sections, as to comply with regs for MH at 1800 deep the size of rings would need to be 1200 dia and rectangular sections would need to be 1200x1000, but as the latter is not a stock size you have to jump up to 1475x1000. Without a JCB or 5 tonne 360 on site rings or rectangular sections will be to heavy to manhandle into place. If we had to put a manhole in with no access for machines, we would run her up in class B engineering brickwork.
However if we were pricing this for you, we would ask BI if he will accept a non man entry IC out of 600x450 concrete manhole sections with non entry cover. Cut a 550mm long channel out of existing pipe, s***s law you will hit a collar so cut very careful through collar. Instead of running connection down to invert level, lay connection about 600mm deep and connect to channel with pre formed slipper (we cut our own on site) and then a back drop. Cut rodding access in side of sections for back drop. Take to long to explain back drop, so Google external drainage back drop or tumbling bay and you will see what we mean. some back drops are inside manhole but you want it outside.
We cut our channels whilst pipe is live, but you could be unlucky and two baths and two WCs. are let go at the same time. If this happens you get slightly damp, so stick a bag with extended tube in the next manhole upstream.
Word of warning. Safety, safety, safety. We never dig down below 1200 with out all necessary planking, poling and bracing as required especially in previous dug ground.
Never work on live drains at that depth unless someone is on top to help you out if you get gases from pipe.
oldun
.

the first sentence is the most important....

 

[Donkmeister]

I think I will heed the advice given here and understand my limitations! A neighbour tells me his was about 1500 quid some years ago, so I think I'll go with the option of a macerator. I was hoping to avoid that, but the grand I save in the shorter term will come in handy.

Will buy a commercial grade one to avoid the usual killer of macerators, I.e a guest unknowingly dropping sanitary items into it.

 

[Hugh Jaleak]

Where is the existing sewer serving your property in relation to the proposed new stack position? There must be an existing toilet, its highly unlikely the drain from that will be 6' deep.... Is it possible to couple into that pipe?

 

[Donkmeister]

Where is the existing sewer serving your property in relation to the proposed new stack position? There must be an existing toilet, its highly unlikely the drain from that will be 6' deep.... Is it possible to couple into that pipe?

Interesting idea... I've got a "down through the floor clay pipe" toilet... according to a neighbour who has replaced his toilet, this pipe also drops down a significant distance, possibly right down to sewer invert level (i.e. 6 feet down). But... the connection would be indoors.

Two possibilities spring to mind: I could either

1) Have the stack pierce the wall to run outside once it is out of the ground (is this "legal"?)
2) Have the stack remain indoors by either fitting an AAV or piercing the roof.

There are also two hopper/gullies (with trap) into which the sink, basin, bath, shower etc drain into. They are both underneath windows, so I couldn't run a stack straight down... this would effectively mean 2 right angle bends between stack and sewer. I could put rodding points into the stack, would that make such an arrangement compliant with the regs?

Can I add a T-piece onto the pipe below ground, below the current gully connection, then connect the stack to this? I have a combined foul/rain system, by the way.

 

[Donkmeister]

This is the sort of thing I mean:

Untitled

• Donkmeister
• 19 May 2011
• 1

Can I add a T-piece/swept junction below the hopper connection? The hopper definitely has a trap as I can see the water level a few inches below the grating.

There are no manholes on my property, however a neighbour believes I may have one below the foot-thick concrete raft that the previous owners built for the shed

This would be where my existing toilet connects to the sewer.

 

[Hugh Jaleak]

Be very surprised if all the pipes are 6' deep! Nobody does that amount of digging, even with a machine, unless absolutely necessary! Finding that manhole might be a good point to start, it may be under concrete (muppets ), but one day you may need to get to it!

It may be that the pipes run at an angle from the surface to the sewer. If this does prove to be the case then you may be able to locate a suitable point to connect nearer ground level. (My house is 1896 vintage, the drains from the original W.C. and gulley ran almost at 45deg down to the sewer.) I dug back to suitable point, cut the pipe, fitted a coupling, and then a suitable bend to bring the pipe to (almost) horizontal. Immediately after the bend fitted a small chamber, then laid new drain from there to new gulley and stack postions. Indoor connection unlikely to be feasible (or legal!), you'd need the stack to rise from the drain, bends in wet section are not allowed!

I'd look at the setup of the drainage in the neighbours properties. If all the houses were built at the same time, sharing the sewer then layout will be pretty similar, may give you some clues! If you feel energetic, dig out from a gulley, expose the pipework and see where it goes. Check with B.C.O. though before making any connections.