have a large plot of land but the ground is quite hard. Can I dig a shallower trench?
Yes, the ground loop coils can sometimes be laid so that the coils lie horizontally in the trench rather than vertically. This would need a wider but shallower trench depth to at least 1.2 metres. However, this is not as satisfactory as a deep trench in which the coils are vertical and you will need a special slinky configuration and probably longer trenches. Your system supplier should be able to advise you accordingly.
