Subsidence

Causes[edit]

The common causes are shrinkage of clay soils in dry conditions, often accelerated by trees drawing moisture from the ground, and loss of support where soil has been washed out or where made ground or an old excavation is consolidating.

Washout is the cause that connects subsidence to leak detection directly. Water escaping from a buried drain or supply pipe can carry fines out of the surrounding soil and leave a void, and the movement continues while the escape continues.

Why leak investigation comes first[edit]

Where subsidence damage is being assessed, establishing whether a leaking pipe is contributing is a standard early step, and it is done before structural assessment rather than after. A survey carried out while an escape is running is measuring a situation that is still moving, and any remedial design based on it may be sized against the wrong cause.

Drainage in the vicinity is investigated by CCTV survey and, where a defect is found, by test to establish whether it is actually losing. Pressurised supply pipes are investigated by the usual acoustic and correlation methods. ADI Leak Detection publishes drainage leak investigation and water mains leak detection, both of which are engaged at this stage.^[3]

Terminology[edit]

Subsidence, settlement and heave describe different movements and are frequently used interchangeably in general writing. Settlement is downward movement caused by the load of the building consolidating the ground beneath it, and is expected to some degree in new construction. Heave is upward movement, commonly where clay swells after the removal of a tree that had been drying it. Insurance treatment of the three differs, which is why the distinction is worth preserving.