Abstract
Watercare Services Limited (WSL) operate a very large municipal wastewater treatment plant at Mangere, Auckland. A by-product of the treatment process is the creation of a stabilised biosolid. Through a review of operations an opportunity was identified to rehabilitate a retired quarry on the adjacent Puketutu Island site, both improving the Islands environment and reducing the cost and transport burden to take this material to commercial landfill. The new facility, referred to as the Puketutu Island Rehabilitation Project, is a modern fully lined monofill facility located in a central volcanic cone that has been historically quarried out and partially backfilled.
CH2M Beca Ltd. were engaged to prepare the detailed design and construction management of the first two phases of the facility and this paper presents some of the challenges faced in preparing those designs and the subsequent construction.
The nature of this past site use created some design challenges for construction within the consented footprint. This included backfilling of large ponds, rock blasting, a large landslip, and known and unknown wet bin locations. The facility was designed to a similar standard as a landfill, with full liner, leachate collection and biosiolds affected stormwater capture.
A 3D geological model was created to better understand the complex underlying geology. Some interesting aspects include the design of the embankments using the variable materials on site and underlain by existing uncontrolled fill and wet bins to achieve stability under a 1/2500 AEP earthquake, confirming liner integrity, staging of the project while maintaining a fully operational facility, and construction of internal access roads to allow the biosolids to be placed in cells by the tractor/trailer units. Additionally, it was necessary for the construction to be able to take place year round to meet the project timeframes.
1 INTRODUCTION
Puketutu Island is located in the Manukau Harbour in Auckland, New Zealand. It is approximately 195 hectares in total and has a central volcanic cone with associated ash deposits (tuff) and lava flows (basalt) around the island perimeter. Within the footprint of the Puketutu Island Rehabilitation Project Monofill Facility, extensive quarrying and clean fill operations have occurred over the past five decades.
The project involves rehabilitating part of the quarried areas of the island with biosolids to create a landform that is consistent with the remaining geology of the island. The landform will comprise an outer embankment within which the biosolids will be placed in order to create an elevated central landform. The Island will eventually become a regional park, with parts of the island being opened to the public as soon as practicable.
The main objectives of this rehabilitation project, which form the basis from which the design philosophy and rehabilitation project have been developed, included providing a sustainable and economic use of biosolids from the population served by WSL, incorporating design features that manage any potential impacts on the environment and be considerate of any cultural or social implications.
The first two phases (referred to as Phase 1, but split into two sub phases of 1A, 1B and 1C – see Figure 1) of design, discussed in this paper, have been peer reviewed and approved by Auckland Council to ensure that the key elements of the consented concept design have been maintained and equivalence proven when changes to the concept have been implemented.
The resource consent process was completed prior to Beca’s involvement; therefore any changes to the consented design needed to go through a thorough peer review process to prove equivalence and gain acceptance.
Figure 1: Phase 1 Layout
2 DESIGN DETAILS
The core geotechnical analyses completed for the design of the facility included slope stability analyses (for short term, long term, seismic and during construction), settlement analyses, liner design and groundwater modelling.
Figure 2 shows a typical leapfrog geological cross section; running approximately E-W across the site.
