Retaining wall – cracks

Causes

Corrosion of reinforcing

Inadequate initial design/construction

Inadequate foundations – overloading of the soil supporting the wall

Lack of free-draining material behind wall

Wall at the end of its serviceable life

Tree roots

Overloading from soil/vehicles at the top of the wall

Build up of water behind retaining wall due to blocked or no drainage

Common

Listed under: Section, retaining walls, durability, corrosion

Cause

Corrosion of reinforcing

Repair

  • determine whether the wall is at risk of further damage – can it be repaired or does it need to be replaced? If necessary, engage a chartered professional (structural) engineer for advice

  • if the wall needs 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

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Cause

Inadequate initial design/construction

Repair

  • see the comments for the first repair above

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Cause

Inadequate foundations – overloading of the soil supporting the wall

Repair

  • see the comments for the first repair above

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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.

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Cause

Wall at the end of its serviceable life

Repair

  • the wall will need to be replaced. See the comments for the first repair above.

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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

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Cause

Overloading from soil/vehicles at the top of the 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

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Cause

Build up of water behind retaining wall due to blocked or no drainage

Repair

  • where there is drainage, ensure drainage slots/soil drains etc. are clear
  • where there is no drainage, 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.

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