Maintaining My Home

peat is compressed dead vegetation and typically occurs in low-lying areas. Although the ground surface can appear stable, peat may be present below in a deep layer. The extent/depth can be determined by drilling bore holes

for very minor problems, fill shallow slumping with soil

where the problem is more serious, or is affecting a building or potential building site, consult a chartered professional (geotechnical) engineer for advice

for very minor problems, fill shallow slumping with soil

where the problem is more serious, or is affecting a building or potential building site, consult a chartered professional (geotechnical) engineer for advice

fill that has been placed in residential areas in recent decades must comply with NZS 4431:1989 Code of practice for earth fill for residential development. (An updated version of this standard was published in 2022: NZS 4431:2022 Engineered fill construction for lightweight structures. The 1989 standard currently remains the one deemed to comply with the Building Code) 

where the land is intended as a building site, tests must be carried out to determine the bearing capacity of the fill. In many cases fill will not meet the required bearing capacity, so foundations must pass through the fill to solid bearing below it

where there is drainage, ensure drainage slots/soil drains etc. are clear

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

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

For advice about ground movement, consult a registered geotechnical engineer

even where consent is not required the new 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