Katikati, New Zealand
Background
Highfields Crossing is a residential subdivision located on the northern edge of Katikati, within the Bay of Plenty, New Zealand. It comprises 61 medium to high density residential allotments across a landform that has been raised several metres in elevation above very soft, geologically young and normally consolidated estuarine muds by the placement of engineered filling. The filling was placed to provide a structural raft for support of footings and services and to raise the landform above the local flood level.
Previous stages of the development showed that these subsoils were sensitive to changes in surcharge conditions, which can result in high consolidation and post-construction creep settlements which can be problematic if not appropriately allowed for in geotechnical design.
Scope
CMW carried out a review of existing geotechnical investigation data, which was then supplemented with addition field investigations to refine the geological model – primarily to better define variations in the depth and lateral variability of the very soft subsoils. This information along with the proposed design profile and details of the future land use was used during detailed geotechnical design to calculate estimates of consolidation settlement under proposed fill and preload (temporary surcharging) and to ensure post-construction creep magnitudes were to remain within acceptable limits.
Review of survey monitoring information was recorded on a regular basis whilst the filling was placed and compacted which allowed us to calibrate the estimated magnitudes and duration of settlement against the initial estimates and so advise with confidence when the preload could be removed.
Findings/Challenges
Particular challenges for the project included:
- The presence of several large pre-existing stockpiles of filling material making estimations of consolidation across the site difficult.
- A lack of reliable pre-existing settlement data from previous stages that had been completed under the direction of other Consultancies.
Regular site visits during the earthworks phase provided opportunities for CMW to inspect ground conditions, fill placement and compaction and the staging of the works. This, combined with review of the settlement data meant the developer could better meet the demands of the construction schedule whilst reducing costs and delays during preloading.