Abstract
The Anzac Cliffs project involves two main elements: reshaping of the steep cliff, and realignment of the Manawatu River away from the cliff. The realignment of the river below Anzac Cliff was the last major development of the Lower Manawatu Scheme, City Reach Project. The project included the realignment of a 460m section of the Manawatu River opposite Anzac Park. The reshaping works were undertaken to stabilise the 50m high Anzac Cliffs adjacent to the river. The remedial works addressed the ongoing erosion of the unstable cliff by the Manawatu River.
Hazards associated with the project include direct river erosion undermining the lower cliff, and ongoing instability of cliff-face materials. The construction of foundations for a fill buttress within the active river channel and below the unstable cliff provided significant challenges, particularly due to the presence of liquefiable materials in its formation.
This paper provides a review of hazards associated with the project; sets out geotechnical investigations and analysis undertaken; and describes the development of the detailed design for the cliff stabilisation works. A review of construction and monitoring techniques for both the foundation treatment and compaction of granular and earth fill materials is presented, together with an outline of the management of numerous considerable technical and safety risks.
1 INTRODUCTION
The proposed Anzac Cliff residential subdivision is sited above the southern true left bank of the Manawatu River, some 3.5km south-east of Palmerston North City Centre (Figure 1). The north-eastern portion of the subdivision was bound by an approximately 50m high cliff that was subject to large-scale ongoing river erosion. The rapid erosion of this section of the cliff became a concern to various stakeholders with regard to the additional sediment load it added to the river, and the potential adverse effects that its altered course would have on the existing river protection works on the opposite northern bank. In April 2006 works to realign this section of the river back to its 1992 course were incorporated into the City Reach Project, a major works programme to upgrade the lower Manawatu River Scheme that protects Palmerston North from flooding.
Due to the unstable nature of the land along the site’s north-eastern boundary, under the Palmerston North City District Plan, any building development within this portion of the proposed subdivision was a prohibited activity. It was identified that the proposed realignment of the river allowed for engineering works to be undertaken that would stabilise the land, making more of the site suitable for residential development. This led to a Plan Change that became operative in November 2011.
