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 are installing a half-perforated pipe such ass 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?
The water surchanrges 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 drip or seep into the perforations, this isn’t the case. The pipe should be surrounded by gravel, and the surface water falling from above will pass through the gaps in the gravel and 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.