Until relatively recently, the method of constructing
drainage around domestic buildings had changed very little since the Roman
era. It involved the connection of short lengths of glazed stoneware pipes
using organic seals covered with lime or cement. Because out of sight is
very often out of mind, deterioration of the joints can lead to leaks.
The presence of leaking drains and the improper discharge of rainwater is
the second most common cause of subsidence related damage to property.
The degree of damage can vary enormously and will depend upon the
geophysical property of the ground upon which the house is sited.
The most common occurrence is usually on sandy or silty ground when the
water from the leak washes out the fine particles beneath the foundation
causing voids which results in collapse or subsidence.
Alternatively the continual leaking softens the ground in the area around
the pipe, usually close to the house, and renders the ground incapable of
supporting the weight of the house resulting in subsidence.
Although the age of the drain is usually indicative of susceptibility to
leaking and that more modern methods using flexible joints are better it
is by no means certain that leaks are likely ... broken pipes or bad drain
laying can be the cause.
Testing for leaks in drainage or proof of watertight operation is
advisable if the pattern of superstructure damage suggests that leaks are
occurring. This can either be done by applying a suitable pressure test or
by observation from close circuit t.v. inspection.