drainage #2

Below ground drainage complete!

Finally the weather has held sufficiently for us to now complete our below ground drainage, which means we can install and test the pumping station, and in turn connect a (temporary) WC inside the building.

Every new house that connects to the mains drainage system should have a disconnecting chamber (either a manhole or an inspection chamber depending on the depth required) at the edge of it’s curtilage which connects the foul pipe work from the house via a section of gravity pipe work to the main sewer.

In our situation there is a common manhole just beyond our own private drive that connects to the main sewer under the street, but it is higher than our plot, and therefore quite a bit higher than the invert level of the drainage from our WC’s once they exit the building underground, so we need to install a pumping station underground which will collect the waste and pump it up to a disconnecting chamber at the edge of our plot. From there there will be a relatively short section of gravity pipework to the common manhole.

These first photos show the common manhole on the shared drive and the start of the trench being dug.... The black corrugated ducts coming out of the ground carry the mains electricity and water into the building under the foundation / slab.

The first task was to separate the Type 1 hardcore (that we had laid at the start of the ground works) down the drive and around the house from the soil below. In an ideal scenario the time to install all of the below ground drainage would have been at the start of the project, before any Type 1 was laid over the ground, however due to the size constraints of our site (it is very narrow with only one entry point for vehicles), and a few other logistical reasons, we delayed this installation as the pumping station sits directly in front of the building under the driveway; also we didn’t want to have heavy machinery (such as the concrete pump for the slab) sitting directly on top of it as it is only designed to take the load of a domestic vehicle. If it had been driven over by heavy machinery we could have damaged the station itself and affected the fall of the below ground pipe work without knowing that this had happened, so we decided it was safer to wait until we no longer needed any heavy duty vehicles to come to the site before carrying out this installation.

Our ground is basically clay so as you can see once water enters the excavation it is pretty hard to get it out!

Some materials being delivered (cement, ballast and pea gravel for pipe bedding / encasement) on to the street - unfortunately but rather typically the weather decided to change at this point!

During the excavation the hole filled with water from a broken below ground clay land drain so we had to bail it out; we had shored the sides using heavy duty scaffolding boards which held the sides in place to allow working in the hole. Once the water was removed we were ready to mix and pour the concrete bed for the pumping station itself.

We did the concrete mixing for this on site rather than getting ready-mix, partly because it is slightly cheaper and partly so that we could more readily control the quantities depending on the weather. Once the concrete plinth was poured and the pumping station lowered into position we could begin the process of back filling the excavation. The particular pumping station we are using here requires a concrete surround to resist the pressure of any ground and ground water, so we had to form shuttering for this. Once we had reached a point the ground water started to seep into the hole again at a surprising rate, so we decided to install a perforated land drain around the perimeter of the pumping station all the way out to join the land drain in the back garden that came from around the house.

The second photo below shows the land drain temporarily in position to check it was falling properly as it needs to be bedded in pea gravel to ensure it doesn’t get blocked with silt and dirt.

The next photo shows the exit point from the pumping station that is at high level & we used a temporary piece of duct (white in the photo) to protect the pre-assembled pipe fitting that comes with the station from dirt and concrete while we poured the concrete surround (and bailed out yet more water!).

The next shows the shuttering around the pumping station: we poured the concrete in 2 stages to ensure we could adequately tamp the pour (make sure there were no air pockets) and to allow us to back fill around it in stages to get out of the deeper excavation as quickly as possible.

The two corrugated black pipes to the left of bottom middle picture are the 50mm diameter ducts that contain the electrical cables running between the pumping station itself and the corresponding alarm box and consumer unit inside the dwelling, which provide power and signalling to the station’s submersible pump.

The fourth photo in this group shows the inspection chamber in the background that takes the below ground gravity pipe work from within the building, and which is connected to the pumping station via another section of gravity pipe work (buried under pea gravel by this point). You can also see the mains electricity cable: the black cable on the left with the two yellow taped points and the loose end of the mains water pipe, all of which enter the building via similar 50mm ducts.

The surrounding earth caved in slightly overnight on top of the concrete surround around the pumping station due to some unanticipated rain! Finally it was time to back-fill the chamber.

The next stage in the process was to excavate a narrow trench (300mm) up the driveway from the pumping station to the position of the new disconnecting chamber within our plot, and on to the existing common manhole at the top of the drive. The change in direction next to the large strainer fence post is where our disconnecting chamber is positioned, and where the 50mm diameter pipe that comes from the pumping station transitions to the 110m gravity pipework.

Once we exposed the existing manhole at the top of the drive we discovered that it was no longer fit for purpose and had to be replaced.

As you can see from these photos the brickwork didn’t properly support the cover on one side, and there were substantial cracks in the brickwork, which could have meant leaking of foul sewage into the surrounding ground over time. We therefore had to replace this before we could continue further.

The lid is a D400 grade of lid, which is suitable for vehicles up to 40 tons in weight (often used on a road) so we decided to use a plastic inspection chamber (as the base is less than 1m below the ground) encased in concrete and to bed the existing heavy duty lid on a concrete surround above this to spread the load.

The photos above also show the existing single rodding point in clay to the manhole from the main house that used to own the plot that we purchased and this was at the base of the manhole which had simply been constructed around this rodding point. (Adjacent to our own there is a second plot, which will also be connecting into this manhole, so the new inspection chamber will facilitate a simpler connection for this plot to the common gravity fed pipe work to the street.)

The next pair of photos show the new inspection chamber in position with the sections of gravity pipe work between our disconnecting chamber and it.

This run of pipe work is to be encased in concrete as it is underneath the common section of driveway and is at a relatively shallow depth. At each joint there is a non-compressible board (polystyrene insulation), which alleviates any strain on the pipe connection points from vehicular traffic above.

The first photo above shows the three branches into the inspection chamber: the re-connection of the existing drain, a new stub branch for the adjacent plot, and our new connection. Each pipe must be connected to the inspection chamber via a “rocker”, which is a short section of pipe between 300 & 600mm long, that allows some flexibility in the connections and consequently prevents leakage.

At the other end of the this gravity pipework is the disconnecting chamber with the flexible reducer that steps up from the 50mm pipe from the pumping station to the standard 110mm connection, shown below. Then we checked for level before concrete pouring begins¦ These chambers have a pre-formed base already on a fall which means it is essential to place the spirit level on the top rim and not down in the bottom!

The first photo below is looking back from the common inspection chamber to our plot.

The second shows the start of the concrete encasement. We temporarily used some spare bricks to support the polystyrene whilst the concrete is poured on one side, and before some can be placed on the other side. The third shows one section of the pipe now fully encased.

The chamber and pipe were encased separately only because they were done on different days. The disconnecting chamber fully encased and the 50mm pipe work embedded in pea gravel connecting to it.

Marking the pipe work on top of the concrete surround with marker tape allows for it’s discovery before damage in the event of future excavations. Generally all below ground pipes and cables should be marked with the appropriate tape a minimum of 200mm above to facilitate their discovery before damage during future excavations.

The adjacent plot’s mains electricity cable crossed the foul drain so we placed it in some duct to protect it and placed electrical marking tape 200mm above it.

Further down the drive the 50mm diameter pipe from the pumping station is sufficiently far below ground that it can be bedded and then surrounded by pea gravel, rather than encased in concrete. It is placed under the centre of our drive, which should also help to protect it from vehicular traffic.

The second photo shows the string line used to make sure the base of the trench was on an even gradient from end to end; the third shows the pipe in position being covered by min. 150mm pea gravel before the ground could be placed back in the trench.

Finally, these shots show the pumping station concrete haunch which beds the frame for the lid in position... and a view into the pumping station from above whilst we emptied it and cleaned it out with a submersible pump prior to final connection.