Both land drain and twinwall are available with or without perforations, and these perforations are what allows the water to enter the pipe and drain the ground surrounding the pipe. If you choose a perforated pipe for your project you are likely looking to drain surface water from soil or grassy area.
Depending on what size pipe you go for, the number of perforations and where they are positioned on the pipe may vary.
If you have 60mm land drainage pipe there will be 2 perforations per dwell (or indent in corrugations). The bigger the diameter you go the more perforations you will get so the orientation of the holes becomes less of a problem. By the time you reach 160mm land drainage you will have 6 holes per dwell which should be evenly distributed around the pipe. This is often referred to as a slotted drain.
If you're installing a half-perforated pipe, such as half perforated twinwall, this is where the problem comes into question; whether the perforations are best facing upwards or downwards?
Contrary to popular opinion, the most effective method is to have the holes facing down.
Why do the perforations go downwards?
Water seeps from the bottom of the trench upwards into the pipe.
Although you would think that having the holes at the top would mean the water could fall into the perforations, as it flows through the ground, this doesn't offer the best performance.
The pipe should be surrounded by gravel in a trench where water moves through the ground, passes through the gaps in the gravel, and starts to fill the trench surrounding the pipe. As this happens the water will percolate into the pipe through the holes and flow out of the pipe when its installed with a fall.
If the pipe is placed upside down, with perforations towards the top of the pipe, it's not like it wouldn't work — the pipe performs better and transports water quicker if they are pointing down.
