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to be approximate. Because of the relatively small extent of the site, significant stretching is not anticipated.
5.2.6 Discussion of Results
The predicted level of settlement, the calculated LSN value and the relatively modest expected stretching under seismic loading places this site on the border line between TC2 and TC3 category as defined by the guidelines for the Canterbury re-build5 . A light weight house with a suspended timber floor is recommended. This shall be founded on a geogrid reinforced granular raft, as indicated on Figure 4 in Appendix A. This is discussed further below.
5.3 Site Access & Platform Level
Site access is to be from Myrtle Drive. To achieve this, significant earthworks will be required to create an access road. A potential access road alignment and turning area (developed by Vitruvius Consultants) and a house platform is indicated on Figure 3. The proposed access road has a gradient of 1V:5H and assumes a 1V:1H side batters. Specifically designed soil reinforcing using geogrids will be required to achieve this side batter. Undercutting of approximately 1.2m of topsoil/fill will be required at the access road and replacement by compacted imported hardfill. Dewatering and placement of Terratex 180N geotextile (or similar approved) will be required.
A platform level of RL 3m (Moturiki Datum) has been adopted. This level has been adopted to level the proposed building area and to limit the amount of fill (and thus reduce the post seismic settlement). The predicted flood level for the site is not currently known. To achieve the required turning area and to address any future flooding issues, a lightweight house on poles is proposed. The ground level is to be left open and should not be a habitable space. This proposal is subject to district council approval. Significant further filling of the site (i.e. above RL3m) is unlikely to be acceptable.
All earthworks are to be carried out under the direction of a geotechnical engineer using approved fill material.
5.4 Foundations
The ground conditions do not comply with the requirements for ‘Good Ground’ as defined by NZS 3604:2011 ‘Code of Practice for Timber Framed Housing’. For this reason, and because of the potential for settlement and lateral spreading due to liquefaction, special measures will be required to enable the buildings to be safely constructed. A geogrid reinforced granular raft is proposed. A detail is shown on Figure 4 in Appendix A. It should be appreciated that the proposed detail is indicative only and will require significant development to address issues such as localised retaining, suitable surfacing details, stormwater runoff etc. The proposals shall be developed in conjunction with a geotechnical engineer.
Approved imported lightweight granular pumice fill shall be used. 200 mm of imported GAP 65 shall be placed at the base of the raft on Terratex 180N Geotextile, with two layers of E’Grid 4040 geogrid (or similar approved). The base of the excavation shall be examined by a geotechnical engineer. Undercutting of unsuitable soils and replacement by compacted fill will be required. For the undercutting, dewatering is likely to be required and a geotextile shall be placed at the base of the excavation.
5 Guidance for repairing and rebuilding foundations in Technical Category 3; MBIE (April 2015).
6c Myrtle Drive, Omokoroa O’Brien Geotech Ltd Project 533 February 2017
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