Leaks: Why do pipes leak?

Pipes are watertight when they are laid in the ground. Although we may not notice it, the ground often moves when there is a very hard frost, an excess of water or when it dries out in the summer months.
This movement can cause pipes to pull apart at the joints or crack and consequently leak valuable water. In addition, the physio-chemical properties of some soils will sometimes speed corrosion, especially on iron pipes, to cause leaks.