How to install underground drainage
I was getting a bit fed up with digging, but the weather forecast suggested I had one more good week of weather before the rains came. I buckled down and set myself the target of getting the drains in, signed off and back filled before the site turned into a bad Glastonbury experience.
Several years back, the water board insatlled a vacuum sewer into our village. I had the foresight to pay for a connection at the time. I couldn't get the digger right down to the corner of my workshop by the pipe, so had to dig by hand in a confined space. It was not fun, but eventually I did locate the connection that had laid dormant all this time.
When the Building Control Officer came to inspect my footings, I showed him the drawing of my proposed drain layout. He ok'd it.
Ideally the pipe fall should be 1 in 40, though 1 in 80 can be acceptable in certain circumstances. Though I still had not done an accurate check of any levels, we both felt confident that the exposed connection looked deep enough to allow for the fall I needed.
I thought I would get Pete and the digger to dig as much of the drain run as possible before he dug the footings and had to leave site. I sprayed a yellow line, and, with a 300mm bucket, Pete dug as far as he could from the connection I had uncovered. I had not measured levels, but just asked Pete to try to get gradually shallower. Provided the bulk of the soil was out, I would clean up the trench by hand once the footings were in.
It was now a matter of digging with pick and spade. Despite the fact that Pete and the digger had done all he could, I still had about 18 Meters of trench to dig by hand.
Once the trench was dug out, I checked the levels. My staff is simply a piece of 2 by 2 onto which I make pencil marks. I bought a fence post level to ensure I held the staff vertically and, therefore, minimised errors.
Using the staff and laser, I checked both ends of the trench on its longest run. Over a distance of 23 meters I had a fall of about 600mm. This was a 1 in 38 fall. (23 / 0.6)
Having ensured I had dug 100mm below the exposed connection to allow for pea gravel under the pipe, I marked the staff. 1 in 38 fall is 79mm every 3 Meters (3000/38), so I then marked the staff at 79mm increments. I marked along the trench every 3 Meters and then worked my way along the trench with the staff at each 3M mark, noting where I was too deep or too shallow.
It was a long job to clean out the trenches and get an even fall. A good deal of time was wasted in repeatedly emptying grit and soil from my boots that fell in from the trench.
It was a hard job - this is the shallow end of the trench!
I didn't want to put in too much pea gravel and end up digging some of that out too, so I placed pea gravel to support the pipe every couple of meters and ensured the fall was right. I later worked more pea gravel around and under the pipe ensuring the pipe was well supported along its length.
I placed my lengths of pipe and inspection chambers in the trench, propped up to simulate the height it would be with 100mm of gravel under, and ran along the trench with the staff taking measurements from the crown of the pipe. It all seemed to be running nicely and evenly downhill. I just needed to straighten the trench slightly here and there.
Because of the sections I had shuttered off when laying the concrete footings, the pipes could run through at the correct levels. I required a very slight kink in one length, but it was only about 5 or 6 degrees (it looks more in the pic). I called the BCO. he turned up promptly, as always, and checked my levels and amount of fall on the pipes. The BCO was fine what I had done and gave me the ok to backfill the trench.
As my soil is not very stoney, I wanted to start covering the pipes with the soil. however, the BCO said I should makes sure I cover the crown of the pipe with pea gravel first. It took a few more barrow trips to get the gravel in to cover the pipes, but eventually it was done. At last I could fill the trenches with soil.
The rain arrived a couple of hours after laying the drains!