This is a term to describe a build-up of material on the side of the drain pipe that will eventually restrict the flow of liquid and cause the drain to block.
Scale is often associated with kitchen waste such as fats that stick to the side of the drain. This is usually best dealt with through the use of a high pressure jet cleaning.
High pressure jetting must be carried out by skilled operatives to ensure that the very powerful water jets used do not damage the drains that are being cleaned.
This describes a problem where:
The ground might have shifted or settled pulling part of the drain with it, away from the other parts of the drain.
The drain has been physically broken, perhaps due to a large and heavy vehicle driving over it, compressing the soil and then the drain.
Roots from surrounding trees have grown alongside and then into the drains - perhaps as a source of water and nutrients. The roots have got bigger over the years, eventually filling or breaking the drains.
The drain is very old and over the years has crumbled.
Though you might not notice, these problems usually occur over a long period. Finally the drains are not able to adequately flow and clog up very rapidly.
In all of these cases it is necessary to replace all or part of the drains. The drains must be replaced to preserve the correct flow and the surrounding ground reinstated in such a way that the problem will not recur.