Retaining wall bowing, bulging, tilting


Listed under: Section, retaining walls

Cause

Inadequate design/foundations/construction

Repair

  • the wall is likely to need replacing. Check whether work requires a building consent. Building or repair work on a wall retaining less than 1.5 metres of ground and not supporting a surcharge does not require a building consent. (A surcharge is an additional load on the land above the wall, such as a car park or driveway.)

  • if the wall is taller than 1.5 m or there is a surcharge, then building consent is required. When a building consent is required, the wall must be designed by a registered engineer

  • some proprietary retaining wall systems (particularly crib walls) have pre-engineered design tables that may be used without the need to engage an engineer

  • BRANZ recommends that advice is sought from a chartered professional (structural) engineer for all walls over 1 m high. In some circumstances (such as close to buildings) lower walls may also benefit from engineering advice

  • where there is a suspicion of ground movement, getting advice from a geotechnical engineer is recommended

  • even where consent is not required the wall must still comply with the Building Code, including clauses B1 Structure, B2 Durability and F4 Safety from falling

  • in almost all circumstances, good piped drainage behind the wall is essential to dispose of water – a build-up of water behind the wall is the principal cause of retaining wall failure

  • for more guidance see the Build magazine articles online here and here

Cause

Overloading from soil/vehicles at top of wall

Repair

  • see the comments for the repair above. Where the retaining wall is supporting a surcharge – an additional load on the land above the wall – rebuilding the wall will require building consent, and the design must be by a registered engineer

Cause

Pressure from soil/build up of water behind retaining wall due to blocked or no drainage

Repair

  • ensure drainage slots and/or soil drains are clear
  • check that there are no leaking drains above or behind the wall
  • if necessary, consult a chartered professional (structural) engineer to formulate repair or replacement options. For lateral ground movement contact a chartered professional (geotechnical) engineer to advise on how to remedy

Cause

Lack of free-draining material behind wall

Repair

  • see the comments for the first repair above. Where damage to the wall is only slight and the wall is not too high, digging out the ground behind the wall and installing appropriate drainage, backfilling with free draining material, may be possible. Where the wall is more seriously damaged, replacement may be necessary.

Cause

Incorrect wall slope

Repair

  • replace retaining wall – consult a chartered professional (structural) engineer to formulate repair or replacement options. For lateral ground movement contact a chartered professional (geotechnical) engineer to advise on how to remedy

Cause

Tree roots

Repair

  • see the comments for the first repair above. The tree roots will certainly need to be removed, and to avoid future damage the tree itself should be removed

Cause

Ground in front of retaining wall has been excavated/undermined

Repair

  • •see the comments for the first repair above. Professional engineering advice is recommended