007 - Techniques - Drainage Ditches
An essential part of overall house design
Water causes damage
Having a good drainage design and implementation on your plot is important, particularly if you live in a wet area such as Seattle.. Not only do you not want to be walking through mud all the time, but water in the wrong places can do lots of damage. Good drainage ditches in the surrounding land are necessary to prevent water getting into your basement.
Drainage ditch design
The drain rock needs to be surrounded with landscape fabric to stop the drain rock getting clogged up with soil.
Where to put them
As discussed here, you need to have a detailed plot map. This will show the position of the house. You will also know the slope of your land and of course you know water flows down hill. You should aim to shield your house all along the up hill side with a drainage ditch. Any water flowing down your land will be caught by the drainage ditch and diverted away to either side of the house.
You should put the ditch a decent distance from the house. Something like 20 feet away is about right. Note that you will also have a drainage system around the house footings, but the drainage ditch we're talking about here is your first line of defense. Where possible you want to stop the bulk of the water getting anywhere near your house site.
Note that a drainage ditch cannot magically make water flow up hill - all it can do is modify the path slightly so it diverts around your house rather than through your house.
How deep the ditch needs to be is a function of your particular plot and the geology of the ground. The first few feet below your feet tends to be fairly loose top soil and water flows through this well. Sometimes (as in my case) there is a fairly impervious layer of soil under this, about 4 feet down. In my area this is called the "Tokul" and it was put there during the ice age. The glaciers highly compressed it and that's what makes it fairly impervious to water. Water will flow along the top of the impervious layer. You want to dig the ditch down to the top of the impervious layer because that's were most of the water will be flowing.
Digging the trenches
You could dig the trenches by hand with a shovel, but it's not recommended. A much better bet is to hire an digger such as a trackhoe excavator or a backhoe.
Something like this can dig ditches at a fantastic rate. It will probably cost you $100 per hour to hire the digger and its operator, but it can easily dig 100 feet of deep trench in that time.
The width of the trench is going to be whatever the width of the bucket on the digger is. For a drainage ditch you only need about a 10" wide trench, but the digger may well have a 32" wide bucket. Having a wide trench potentially means a lot more expensive drainage rock is required to fill it, but using proper design of your drainage ditch (as described on this page) you can mitigate this.
As discussed, you should ask the digger operator to dig the trench down to the top of the impervious layer. Typically the color of the impervious layer is different from the soil above so you will see when you've got to the right depth. A locally based digger operator is likely to be well experienced with the soil in your area, so tap into that expertise. I used C&S Construction and Cal knew exactly what he was doing. I learnt a lot from him.
The trenches need to slope at a minimum of a quarter inch per foot so that the water will flow properly. If the land is sloping then the impervious layer will also slope to follow the lay of the land.
Avoid accidentally digging through utilities
Perforated drain pipe
You need to use the white pipe that comes in 10 foot lengths. This is far better than using the flexible black pipe because the black pipe has issues with proper flow due to the crinkles. In some areas the building inspector will not allow the use of the black flexible pipe.
The 10 foot straight sections have a connector molded on at one end.
Pipe bends and joints
When you need bends then use PVC corners, eg 90, 45, or 22,5 degree.
They don't overlap the joint as much as the mounded on connectors on the end on the end of the 10 foot straight sections, so it's best to glue them. The best glue to use is PL Premium construction adhesive .
Even when joining the 10 foot straight sections of perforated pipe, it's a good idea to squirt a bit of PL Premium construction adhesive in there to stop them coming apart. It's not essential as the joint overlaps are pretty big, but it doesn't hurt to overkill things. It also makes sure the holes stay at the bottom.
At the top you see the wide form used to put the drain rock around the drainage pipe. Just visible at the bottom is the narrow form used to form the drain rock up to grade level...
Finished ditch where all you can see at the top is a 6 inch wide line of stones...