Auckland Inner City Reserve Rehabilitation
Rehabilitation of the Arch Hill Reserve by the EnviroWaste Contaminated Site Remediation Group
EnviroWaste Services Ltd was engaged by the Auckland City Council to carry out the remediation and initial rehabilitation of the Arch Hill Reserve walkway in Grey Lynn in May 2008.
The remediation works were required in order to meet the human health guidelines for exposure to Asbestos Containing Materials (ACM) that were present on the slope most likely as a result of historic illegal dumping. The ACM was identified by parks staff adjacent to a public walkway where landscape planting was to be installed as part of programmed improvements to the area. The walkway is used extensively by local residents to access Great North Road and the Grey Lynn shopping precinct from surrounding residential streets.
The rehabilitation contract was completed by EnviroWaste within the anticipated 15 working days, not withstanding a significant increase in the volume of soil that was removed to allow for the anchor trenches, and an overall increase in the barrier area of almost 30%. No complaints relating to the Contract works were received from the public which was regarded as exceptional given the sensitivity of some of the stakeholders to the proposed works.
The installation of complex engineered barrier systems by EnviroWaste is a capability that has applications in many different remediation projects, including closed landfills, hazardous waste containment facilities, as well as to protect sites where minimal soil disturbance is required.
Auckland City Council's Urban Soils Review Project Manager Marcus Herrmann said, "EnviroWaste Services Ltd performed exceptionally well in carrying out site remediation works in a residential neighbourhood, in terms of ensuring worker and public safety, adhering to the remedial design provided, carrying out effective works under difficult site conditions, and achieving all project goals within the specified timeframe."
Auckland City Council engaged Tonkin and Taylor Ltd as Environmental Engineer to design a methodology for the removal of the visible ACM and the installation of a barrier system to mitigate against potential release of ACM from the slope in future. Due to the slope angle of approximately 45 degrees it was judged to be too difficult to attempt anything but limited mechanical excavation. The barrier system addressed the restrictions imposed by slope angle, slope stability and human health concerns relating to soil disturbance. Contract supervision on behalf of Auckland City Council was carried out by Beca Carter Hollings & Ferner Ltd as Engineer to the Contract. EnviroWaste staff worked closely with both Consultants to ensure that the final design was able to be safely constructed within the client's time frame and with minimum of disruption to the community.
The constraints imposed on the designer and contractor by Auckland City Council mainly related to the requirements to minimise soil disturbance that could potentially release Asbestos fibres, and to minimise adverse effects on the local community from the remediation works. The Programme was limited to 15 working days and working hours restricted to 9am - 3pm Monday to Friday. The pedestrian footpath was to be left in a clean and tidy state for use by numerous school age residents who would otherwise have to walk an additional 5km via great North Road to reach their homes in nearby Kingsland.
The barrier system design required installation of an anchored Geofabric liner and Geocell mattress over approximately 420m2 of the upper slope, once the surface had been cleared of ACM by hand picking. Three rounds of hand picking recovered a small amount of loose fibrolite materials that were disposed to Hampton Downs North Waikato Regional Landfill at Meremere. Once hand picking was complete a small excavator and two bobcats were mobilised to site to prepare the anchor trenches and remove the soil to waiting trucks. The use of small earthmoving equipment with low ground pressure meant that construction and removal of expensive temporary haul roads was unnecessary.
The removal of approximately 100m3 of soil to landfill from the anchor trenches was carried out progressively to allow the Geofabric liner to be installed over exposed soils. This minimised the potential for fibre release during the works. During this phase of work approximately 300 planter holes were hand excavated for later installation of suitable plant species by Auckland City Parks staff once the barrier was completed. The requirement to provide for future planting was identified as a constraint during the design phase that would have compromised the barrier system if not addressed up front.
Once the trenches were completed a pair of 100mm DIA polypropylene deadman anchors were installed to secure the Geocell mattress to the slope. The anchor trenches extended around the perimeter of the remediation area, in addition to bisecting the slope at mid height. The Geocell panels were available from Maccaferri in 6m lengths which therefore required an intermediate anchor trench mid slope. Once the Geocell was in place, 300 biodegradable planter bags were installed in the prepared holes.
To complete the barrier system the 200mm honeycomb style Geocell voids were filled with a 60/40 drainage aggregate. It was identified during the design phase that the Geocell could not withstand traffic by excavators or bobcats until it was filled, and that even foot traffic could damage the cells. Therefore the only suitable method of infilling the cells was to use a 100 tonne mobile crane located above the slope in the Grey Lynn Library car park. The crane was fitted with a concrete bucket to place the aggregate accurately in the Geocell. Approximately 50m3 of aggregate was placed using this method in the 8 hour window allowed by the client for this one off activity.