Sewer connections: how are they formed?

Sewer connections: how are they formed?

Sewer connections may not initially seem like the most complex part of a construction build when you consider the entire process – however, if you were to get these important groundworks wrong, you could quickly end up with a big bill for putting any problems right. It’s therefore critical to ensure the water authority will adopt the connection (when you make your Section 106 application).

Also, although a basic sewer system might appear straightforward – with no mechanical moving parts – it’s still important to adhere to best practice for reasons of stability and functionality. The last thing you want is for a poorly-made junction to cause a blockage, and for raw sewage to get backed up into a property, or to have it overflow and contaminate nearby surface water drains. The first step is most likely going to be an asset search, to ascertain the size of the main that is being connected to.

Sewer connections 101

But what does best practice look like? Well, we thought we’d write a blog about the basics of how connecting to a mains sewer should be done to answer this very question.

There are technically four possible approaches:

  • Within an existing manhole
  • Within a new manhole, located over the public sewer main
  • Using a pre-formed junction, to match the mains sewer’s material
  • By core-drilling into the pipe and installing a proprietary drainage saddle

Connecting to a mains sewer through a manhole

It is sometimes possible to connect to a mains sewer through an existing manhole, but it’s not ideal. You will need to check whether the manhole can accommodate the new connection before you apply to the water authority (just call us at Premier Drainage Solutions if you need help, we can do a quick on-site visit and tell you if it’s feasible or not).

If the existing manhole is suitable for adding a new connection to a mains sewer, it’s important that the soffit levels of the new and existing pipes match up – also, the new pipes should never be plastic. If the existing manhole has a preformed invert, then the connection should have its invert formed, and be made good in granolithic concrete.

If the pipe levels don’t correspond, and it’s not possible to make the levels the same, then a backdrop should be constructed outside the manhole chamber (according to Sewers for Adoption 8th Edition specification). Backdrops within a manhole chamber rarely get passed through a Section 106 application – the only ones that might possibly be accepted are made from ductile iron, and you would need to provide detailed drawings and method statements on application, to be in with any chance of the express written approval required.

A new manhole, created over the public sewer main, is often a better option than trying to utilise an existing manhole. The size of the chamber should allow for maintenance tasks including jetting, surveying by CCTV, and physical entry into the channels, while a minimum 600mmx600mm opening should be provided.

As with an existing manhole, the new pipework must meet the main channel at soffit level, and connecting channels must be swept with matching main flow direction. ‘T’ junctions should not be used.

Pre-formed junctions for sewer connections

Sewer connections can be made with either an oblique or a square pre-formed junction.

Oblique junctions have a branch joining the main sewer pipe at a 45-degree angle, to make sure that effluent reaches the main at some speed, therefore avoiding backwash or disruption. It’s best practice to use oblique junctions on all connections if possible, but especially on a connection from a toilet or soil vent pipe.

Square drainage junctions, also known as curved square junctions, have a branch joining the main sewer pipe at 87.5 to 90 degrees. They’re best used for storm drain connections, dip pipes on septic tanks, chamber backdrops, and rodding points.

Whatever the type of pre-formed junction you’re using to make a new connection in the sewer system, it’s good practice to have a boundary manhole situated within a metre of it, providing rodding and jetting access, in case it’s ever required.

It used to be the case that the two sewer lines were connected with no consideration for future access, but these days specifications factor in this requirement – with the installation of inspection chambers wherever there is a change of direction, or at the point where a toilet line connects to the main sewer. This has also led to the development of products to suit demand, and you can now get pre-formed manhole bases, with built-in channel junctions entering at 45- and 90-degree angles.

Obviously, it’s possible to take this measure a little too far. If you install a chamber at every such point, should the property have numerous bathrooms, you could end up with lots of manhole covers dotted around the surrounding area. To keep the number of chambers to a minimum, you can often instead opt for roddable gullies or rodding access points on soil vent pipes – it’s best to check with Building Control at your local authority before the groundworks begin, if you’re undertaking a new-build project.

Using a drainage saddle to connect to the mains sewer system

Drainage saddles are the least favourable option. They were originally developed to make life easier for groundworkers and developers – essentially, they make a bodge-job feasible by ensuring it’s done with some control. They’re designed with either 45- or 90-degree angles, and an outer flange that stops the connection projecting into the outfall pipe.

We should note that as contractors who only meet the highest standards in our work, we always seek to apply the best sewer connections available – and strongly avoid ‘bodging’ or ‘fixing’ poor workmanship in favour of utilising the most suitable and reliable option for our customers.

To install drainage saddles, a template is used to ensure the correct-sized hole can be cut, using a disc grinder to carefully stitch-drill, or by core drilling (the better option). This has to be done extremely carefully and slowly, to preserve the integrity of the main sewer. Once the saddle is fitted the inside join is sealed with fast-setting mortar, and concrete is used to seal the pipe on the outside.

It’s actually just as easy to fit a full-sized junction on pipework that’s between 100mm and 225mm diameter, so saddles only offer an advantage on larger pipework where their use can avoid the expense of over-pumping a fast-flowing sewer during the works.

But – and it’s a big but – thanks to the development and use of drain camera inspections, we now know that the poor fitting of saddles can cause significant damage to sewer systems and, of course, even well-fitted saddles can affect the integrity of the outfall pipe, owing to increased pressure on the pipe wall (which has also been weakened by the works).  As a result, many water authorities have banned the use of drainage saddles.

At Premier Drainage Solutions, we’re experts in specifying the right sewer connection solutions to meet your budgetary requirements and, of course, legislative compliance. For free advice on your works, call 01268 950050 or send us an email to [email protected].

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