Drainage is one of the most critical things that is so often left out of garden construction. This can often be for two reason, either the contractor has not the necessary knowledge or design on where why and how to correctly install the drainage layout, or secondly it is cut out of a budget as deemed not critical
Nothing could be further from the truth. In simple terms, when soil gets wet it expands. When it dries out the gaps in the expanded soil fill in with particles of dirt, which in turn expand when they get wet again.
This in effect, creates a form of a hydraulic ram, that unless the pressure of the expanding soil due to the increased moisture content is taken away it can literally push over walls.
The other side of poor drainage is that the plants suffer particularly in clay soil. This is where the plant is planted in a clay hole that fills with water and the plant literally drowns. The placement of the agriculture drain is critical so you do not take away too much water and the plant suffers from stress of too little water or the placement is not correct and too little water is taken away
Either way do not underestimate the impact of well-designed drainage, an integral part of the total successful landscape