18 July 2012 - Canterbury developers working to bring approximately 6600 greenfield sections on to the market during the next two years are making substantial investments to ensure the land complies with the Department of Building and Housing’s new technical categories, particularly TC1 and TC2.
About 3170 of the new sections will be in Christchurch city.
Dr Jan Kupec, Technical Director, Ground Engineering, with Aurecon Group in Christchurch, is involved with different types of remedial work on several of the new subdivisions.
TC1, the more favoured of the two categories, means land on which future damage from liquefaction is unlikely and where standard foundations for concrete slabs or timber floors can be used. TC2 is land that could suffer minor to moderate land damage from liquefaction and where enhanced slab foundation or piles are required.
“Developers can really only bring TC1 and TC2 land to market, but to get it to this stage can take a great deal of effort and expense. Achieving TC1 status is the main aim, but achieving a high standard of remediation on TC2 land is also quite acceptable.
“Sometimes the ground conditions will limit the engineering options, which means you may only be able to get the land back from TC3 to TC2. Cost can be a key factor,” Dr Kupec said.
"Basically the developers are looking to mitigate against liquefaction and lateral spread and there are a variety of techniques, some that can cover both problems, and others that are applicable to just one of them. “
The different techniques being used or considered around Christchurch, include:
Dewatering - liquefaction: This permanently lowers the ground water table by installing drains that will help suppress liquefaction as dry soils do not liquefy. This can be difficult to achieve as it needs to occur across all of the site and to be maintained for the life of the subdivision. It may be applicable for borderline TC3 sites but is entirely dependent on available drainage fall across the site.
Ground Densification – lateral spread and liquefaction: Ground densification involves densifying and improving the soil strength so it is less susceptible to liquefaction. Some of the methods available include stone columns, vibro-flotation or an impact roller.
The impact roller – an oversized compactor with an out-of-shape roller – will pack the soil particles more closely together to prevent liquefaction. Depending on the soil type, the compactor needs to run over a site between 20 and 40 times to improve the ground conditions and improve the technical category. It is not suitable on all sites but may be useful where the underlying ground conditions are sands. Stone columns are another option and these can penetrate to depths over 15 metres. At present this is more suited to larger lots, but will become more economically viable for smaller developments.
Grouting - liquefaction: Typically not for a greenfields subdivision, but more for existing built up areas of high land values. Cement-based grout is injected into the loose soils and forms a homogeneous grout bulb that displaces, densifies and thus strengthens the surrounding soil. Alternatively jet grouting which involves mixing the soil with cement grout at high pressure to form a grouted column. Originally developed as a remedial measure for building settlement control, the technique has evolved to treat a wide range of sub-surface conditions including rubble and poorly placed sill, loosened or collapsible soils, sinkholes and liquefiable soils. While it can be expensive, it can be used in tight spaces.
Reinforced Embankment – lateral spread: This involves forming a reinforced embankment adjacent to a ‘free edge’ to minimise lateral spreading and is particularly appropriate if liquefiable soils are present in the upper soil profile. Loose material is stripped from the site and compacted. Gravel hardfill is re-introduced, interspaced between polymer sheets, known as a geogrid. The geogrid confines the gravel, which interlocks with the geogrid and gives the soil the ability to resist tension. This binds the entire raft of land meaning that it is unlikely to move as a whole and hence buildings and land would be protected.
Buried walls - lateral spread: These are ideal when placed along edges of the many streams that flow through Christchurch, and also along large culverts and drains. An auger is used to drill a series of holes, which are then filled with concrete and possibly steel reinforcing. An alternate is to drive a series of flat profile steel sheets (sheetpiles) into the ground, which lock together and form a wall. Both these options become economic when considering large areas affected by deep liquefiable soils.