The site was classified a “Class D –Deep or Soft soil site” as defined in the New Zealand Standard NZS 1170.5:2004, with soils comprising Hinuera Sands and Silts up to 10m thick overlying Puketoka Clays. The upper sands and silts were deemed liquefiable and the new bridge structure needed suitable ground improvement measures incorporated into its foundation design. Deep-soil mixed columns were proposed by the Principal’s Geotechnical Designer, Earthtech, to mitigate the liquefaction potential beneath the new bridge abutments. DSM columns were specified to a depth of 11m, allowing for 1m penetration into the Puketoka Clays, and on an open grid arrangement, 900mm diameter, spaced on a 1900mm triangular grid to give a 20% area replacement ratio.
Working alongside and advising Earthtech, Geostabilization offered a design and construct alternative reducing the Area Replacement Ratioto 12%. However, for this project the cost savings of an optimised design were not realised, with NZTA’sinsistence on maintaining a 20% ARR. Column strengths were designed to be 3MPa, with strength verification testing through daily wet grab sampling and column core samples tested for UCS strengths.
Deep Soil Mixing blends a wet grout slurry in the ground and creates an homogenous column of soil and cement that has an increased compressive strength to the desired design requirements. For this Wairere Drive project the specified compressive strength was 3MPa and this was verified through the specified testing regime. There were 373 No. columns across both abutments, with 10 columns cored & representative samples tested at 2m centres. The age of the column core testing was a minimum 28 days from installation and extending up to 56 days.
Daily wet grab sampling was also undertaken with at least one 7 day and two 28 day tests undertaken. The table below summarises all the UCS test results for the project. A chart showing strength gains over time demonstrates the long term improvements provided by deep soil mix ground improvements.
Following a series of sub-tropical rainstorm events, Waiheke and the wider Auckland region suffered extensive landslip issues. Many roads in the region developed both underslips and overslips, severely affecting the flow of traffic.