52 BUILD 131 August/September 2012 REMEDIATION FOUNDATION REPAIRS AND NEW BUILDS The residential stream of the Engineering Advisory Group set up after the Christchurch earthquakes has advice on what needs to be done to remediate damaged houses and the types of foundations required in new builds on earthquake-affected land. By Graeme Beattie, BRANZ Principal Engineer T he devastating earthquake sequence in Canterbury that began on 4 September 2010 has had a significant effect on a large number of house foundations in areas where soil liquefaction resulted from the seismic activity. Land near waterways vulnerable Much of Christchurch city and its immediate neighbouring settlements are situated on river deposits, beach dune sand, estuaries, lagoons and swamplands that have been drained over time. Before the construction of stopbanks and river realignment in the mid 19th century, the Waimakariri River regularly flooded the area now settled as Christchurch and Kaiapoi. The Avon and Heathcote Rivers serve as the major drainage channels to the estuary and the sea. Similarly, the Kaiapoi River passes through Kaiapoi. Many areas of liquefaction were near these waterways, and lateral spreading of the ground towards the rivers was also evident. Limited checks before 1978 Most house foundations are shallow constructions, extending little more than 300–400 mm into the ground. Foundation investigation requirements were limited until 1978 when the first NZS 3604 introduced requirements for checking the bearing capacity of the soil beneath a foundation prior to construction. Several observational criteria must be satisfied and a subsurface investigation carried out to define bearing capacity down to a depth of generally not more than 1.2 m below the underside of the proposed footings. Liquefaction caused most problems The major problem in Christchurch has been the occurrence of liquefaction in soil layers beneath the upper 1.2 m or so where the soil investigation was carried out. During the earthquake, the liquefied sand often made its way to the surface – usually through cracks that formed as the ground spread towards a waterway or through cracks that formed as the ground rippled. Although ground settlements from lique- faction occur anyway, where ejection of sand occurred, it caused additional ground surface settlement, often by uneven amounts. Concentrated loads from house foundations with heavy wall and roof linings have sometimes caused greater settlement around the perimeter of a house than inside the perimeter. Engineers are finding solutions The residential stream of the Engineering Advisory Group (EAG) to the Department of Building and Housing (now the Ministry of Business, Innovation and Employment – Building and Housing Group) that formed following the 22 February 2011 earthquake is formulating appropriate foundation solutions for new houses in Christchurch. It is also finding solutions for remediating existing foundations that have settled within limits that allow them to be successfully relevelled. An EAG subgroup deals with the geotechnical issues associated with poor subsurface conditions. Because the city extends over a wide area inland from the coast and covers a range of subsurface conditions (soil types, soil strengths and water table depth) and surface geometry (surface slopes and the presence or not of unsupported river banks), there is a range of future expected performance, predicted by: ❚ observing past performance ❚ investigating subsurface conditions ❚ assessing experienced ground-shaking levels. Subsequently, the government categorised the land in affected areas into two zones: ❚ The red zone – areas where further occupation of the land by residential houses will not be supported. ❚ The CERA green zone – areas where occupation may continue. 3 categories of land in green zone A range of future land performance is expected in the green zone, depending on location. This zone has been subdivided into three technical categories to reflect this expected performance: ❚ TC1 – future land damage from liquefaction is unlikely and ground settlements are expected to be within normally accepted tolerances. ❚ TC2 – minor to moderate land damage from liquefaction is possible in future large earthquakes. ❚ TC3 – significant land damage from lique- faction is possible in future large earthquakes. A house that has been deformed due to liquefaction of the ground beneath.
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52 BUILD 131 August/September 2012
REMEDIATION
Foundation repairs and new buildsthe residential stream of the engineering advisory Group set up after the Christchurch earthquakes has advice on what needs to be done to remediate damaged houses and the types of foundations required in new builds on earthquake-affected land.By Graeme Beattie, BRANZ Principal Engineer
t he devastating earthquake sequence in Canterbury that began on 4 September 2010 has had a significant effect on a large number of house foundations in
areas where soil liquefaction resulted from the seismic activity.
Land near waterways vulnerableMuch of Christchurch city and its immediate neighbouring settlements are situated on river deposits, beach dune sand, estuaries, lagoons and swamplands that have been drained over time. Before the construction of stopbanks and river realignment in the mid 19th century, the Waimakariri River regularly flooded the area now settled as Christchurch and Kaiapoi.
The Avon and Heathcote Rivers serve as the major drainage channels to the estuary and the sea. Similarly, the Kaiapoi River passes through Kaiapoi. Many areas of liquefaction were near these waterways, and lateral spreading of the ground towards the rivers was also evident.
Limited checks before 1978Most house foundations are shallow constructions, extending little more than 300–400 mm into the ground.
Foundation investigation requirements were limited until 1978 when the first NZS 3604 introduced requirements for checking the bearing capacity of the soil beneath a foundation prior to construction. Several observational criteria must be satisfied and a subsurface investigation carried out to define bearing capacity down to a depth of generally not more than 1.2 m below the underside of the proposed footings.
Liquefaction caused most problemsThe major problem in Christchurch has been the occurrence of liquefaction in soil layers
beneath the upper 1.2 m or so where the soil investigation was carried out.
During the earthquake, the liquefied sand often made its way to the surface – usually through cracks that formed as the ground spread towards a waterway or through cracks that formed as the ground rippled.
Although ground settlements from liquefac tion occur anyway, where ejection of sand occurred, it caused additional ground surface settlement, often by uneven amounts. Concentrated loads from house foundations with heavy wall and roof linings have sometimes caused greater settlement around the perimeter of a house than inside the perimeter.
Engineers are finding solutionsThe residential stream of the Engineering Advisory Group (EAG) to the Department of Building and Housing (now the Ministry of Business, Innovation and Employment –
Building and Housing Group) that formed following the 22 February 2011 earthquake is formulating appropriate foundation solutions for new houses in Christchurch. It is also finding solutions for remediating existing foundations that have settled within limits that allow them to be successfully relevelled. An EAG subgroup deals with the geotechnical issues associated with poor subsurface conditions.
Because the city extends over a wide area inland from the coast and covers a range of subsurface conditions (soil types, soil strengths and water table depth) and surface geometry (surface slopes and the presence or not of unsupported river banks), there is a range of future expected performance, predicted by: ❚ observing past performance ❚ investigating subsurface conditions ❚ assessing experienced groundshaking levels.
Subsequently, the government categorised the land in affected areas into two zones: ❚ The red zone – areas where further occupation of the land by residential houses will not be supported.
❚ The CERA green zone – areas where occupation may continue.
3 categories of land in green zoneA range of future land performance is expected in the green zone, depending on location. This zone has been subdivided into three technical categories to reflect this expected performance: ❚ TC1 – future land damage from liquefaction is unlikely and ground settlements are expected to be within normally accepted tolerances.
❚ TC2 – minor to moderate land damage from liquefaction is possible in future large earthquakes.
❚ TC3 – significant land damage from lique faction is possible in future large earthquakes.
A house that has been deformed due to liquefaction of the ground beneath.
BUILD 131 August/September 2012 53
Subdivision of the green zone has been made on an areawide basis, and it is likely that, after more intense investigation, individual sites within each category – particularly in TC3 – will be found to be suitable for a different category.
Types of foundationsHouse foundation types in Christchurch generally fall into three main types, defined by the EAG as: ❚ type A – allpiled foundation ❚ type B – a perimeter concrete foundation with piles inside the perimeter
❚ type C – a concrete slabongrade foundation.Type A foundations are relatively rare in Christchurch, type B are very common in houses built before 1980 and type C are more common in houses built since 1980.
Remediation options are generally possible for all three types, but the suitability of these depends on the technical category that the house is located in.
Requirements for new foundations and remediationIN TC1In TC1, NZS 3604:2011 foundations are considered suitable in new houses when ductile reinforcing steel has been included in the foundation slab, as detailed in the amended DBH citation of NZS 3604. A shallow subsurface investigation will be required to confirm the bearing capacity of the foundation, and the EAG recommends this investigation extends to 3–4 m where practical.
Settlement of existing house foundations in TC1 are likely to be very minor and may have occurred before the earthquake sequence, so the houses are still liveable and will not need any relevelling.
However, if relevelling is required, a shallow subsurface investigation will be necessary to determine the bearing capacity of the soil.
IN TC2New houses constructed in TC2 are expected to be built on one of several foundation types that have been developed to ensure that the foundation slab: ❚ will not split apart if the ground spreads beneath it
❚ will span over a potential loss of support
without unacceptable curvature of the slab ❚ will be easily relevellable after future earthquakes.
NZS 3604:2011 timber floor construction is also considered acceptable for TC2 sites because these floors have easy access for carrying out any future relevelling work.
A shallow subsurface investigation is also required for new construction on TC2 sites.If a relevel of an existing foundation in TC2 is necessary, then at least a shallow subsurface investigation will be required.
IN TC3Before new houses may be built in TC3 and existing house foundations are relevelled if this is required, some form of deep geotechnical investigation will generally be required to determine an appropriate course of action – including whether or not the site could betreated as TC2.
The investigation will depend on the option chosen for the rebuild or the foundation type being relevelled.
The three types of foundation rebuild options in the EAG interim guidance are: ❚ deep piles ❚ site ground improvement ❚ surface structures with shallow foundations.
Each type has dwelling and land constraints that will influence its suitability for the particular site. For example, while deep piles will provide negligible settlement in small and larger earthquakes, they will not be suitable in situations where major or severe global lateral movement of the land is likely.
There are several factors to consider when relevelling and repairing TC3 foundations. These include the extent of foundation damage sustained, the observed performance of the ground around the house and the weights of the cladding materials and their apparent effect on the house settlement behaviour.
It should be noted that the overall process of selecting and documenting foundation systems and details for houses in TC3 is a specific engineering design process that requires input from a Chartered Professional Engineer.
For further information on house foundation remediation and rebuilding, see www.dbh.govt.nz/canterbury-earthquake-technical-guidance.
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