Broadscale Subdivision Development
Mango Hill, Moreton Bay.
The project involved a broadscale residential subdivision with a large lake and associated civil infrastructure.
Core Consultants were engaged as specialist geotechnical and acid sulfate soils consultant to assist in design and construction of the later stages of the development across the lower lying estuarine areas of the site.
Issues requiring specialist geotechnical and ASS assessment included:
- Foundation design and stability assessment for large arch structure
- Detailed site investigation to identify the extent of soft clay preload and surcharge design of filling across the estuarine area with varying soft clay layer thickness up to 7m
- Lake edge stability assessment including design of geofabric reinforcement to obtain adequate factors of safety
- Preparation of site ASS Management Plan
- Verification testing of lime treated material to ensure neutralisation of potential and actual ASS and acidic soils
- Groundwater and surface water quality testing.
- During subgrade preparation of the arch bridge approach embankments, it become apparent that a paleo channel was present under the site. The ancient channel had been infilled with unconsolidated soft clay, an issue from both a geotechnical and acid sulfate soils viewpoint.
- Prior to bulk filling in the estuarine area Core proposed further delineation of the subsurface conditions across estuarine flats with targeted electronic friction cone testing (CPT) combined with seismic survey (MASW). The advantage of this geophysical technique is that it provides information between discrete testing locations (boreholes and test pits) thus reducing geotechnical risk.
- The batter edges of filling in the estuarine area could be susceptible to instability so slope stability analysis was undertaken. Core designed basal reinforcement with geofabric layers to ensure adequate factors of safety were achieved for the design batter slopes whilst filling occurred over the soft clay channel.
- Plate load testing was carried out on embankment fill material at the arch bridge approach to assess allowable bearing capacity of the material. Core’s analysis showed that high level footings for the spandrel retaining walls would be suitable and saved on expensive deeper pedestal or piled footings.
- Prior to and during earthworks activities, Core carried out surface water and groundwater monitoring to assess water quality by installing groundwater monitoring standpipes at critical upstream and downstream locations. Samples were regularly analysed for a suite of water quality parameters to monitoring earthworks impacts.
- Once the extent and thickness of soft clay had been further delineated, preload and surcharge design was carried out. Expected consolidation durations which were expected to be well over 12 months were accelerated to approximately 6 months by the installation of vertical wick dains.
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