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Last updated: August 2012
Name of Applicant Group: Meridian Energy Ltd on behalf of Hunter Downs Irrigation Scheme
Contact Person: Brian Ellwood, Water Infrastructure Project Manager
Address: Meridian Energy, PO Box 2454, Christchurch
Telephone: 021 676052
CIF funding: $240,000 excl. GST
Proposed start date: 1 December 2010
Proposed finish date: 30 June 2012
Related website links:
HDI Final Report
HDI Preliminary Geotechnical Investigations Design Recommendations
HDI Preliminary Geotechnical Investigations Notes on Existing Canals
The Hunter Downs Irrigation scheme (HDI) is an irrigation proposal located between the Waitaki River and Timaru in South Canterbury in the South Island of New Zealand. The water source for the scheme is the Waitaki River. The river has a mean flow of approximately 360 m3/s. The scheme proposes to take water directly from an intake located on the Waitaki River and deliver that water to the command area via primary and secondary infrastructure which will comprise a series of canals, pipes and pump stations.
The scheme has the potential to irrigate up to 40,000 hectares (out of a total area of 60,000 hectares) from the Waitaki River stretching as far north as Otipua. It will provide opportunities for land use diversification, including horticulture, sheep, beef and dairy farming. The scheme area topography also allows for the potential generation of energy via in-canal hydro power units.
The specific project for which funding from the Community Irrigation fund is being used for is to advance the geotechnical understanding of the south Canterbury region within which the HDI scheme will be constructed.
To date most of the work relating to HDI project has focused on consenting the water take and use.
To advance the engineering design past pre feasibility level, detailed field investigations are required to assist in the design of canal earthworks. Currently there are no field related geotechnical reports which relate to irrigation scheme design in this region. This specific project will address the follow gaps in the information.
This project is important to understand the natural materials in the region and suitability of these materials for use in canal construction. The earthworks and canal structures form approximately 30 to 40% of the schemes costs. The understanding the materials that are likely to be encountered during construction will reduce construction risk premiums and scheme costs.
The Geotech information will specifically inform how the canal can be built, the canal geometry along with sources and suitability of natural materials for water proofing.
To meet the project objectives, a range of information sources and field test will be undertaken.
The pre-field work will include reviewing existing bore log data, regional geological maps and aerial photography.
The field work includes 60 test pits randomly across the area, two deep boreholes and surface mapping of geological features such as road cuttings, gully banks and rocky out crops.
During the February-July period laboratory testing and draft reports have been completed. With the completion of all field work the project is now in the reporting phase. International and local peer reviewers are providing comment on the reports and future design recommendations.
This study has produced four factual reports and one set of HDI specific design guidelines:
1. Hunter Downs Preliminary Geotechnical Investigations Factual Report. This summarises preliminary geotechnical investigations undertaken between May 2011 and July 2011, including the geological setting, field investigations such as test pits, drill hole, geological mapping and associated laboratory testing, and identifies geotechnical design issues that need to be addressed.
2. Hunter Downs Preliminary Geotechnical Investigations Trial Embankment #1 Factual Report. This describes the construction of a trial fill in clayey loess, including trials with lime stabilisation by addition of up to 5% hydrated lime.
3. Hunter Downs Preliminary Geotechnical Investigations Trial Embankment #2 Factual Report. This describes the construction of a trial fill in clayey loess, including trials with lime stabilisation by addition of up to 5% hydrated lime.
4. Hunter Downs Preliminary Geotechnical Investigations, Notes on Existing Canals.
5. Hunter Downs Preliminary Geotechnical Investigations Design Guidelines.
The outcomes and test data from the investigations will be used in future scheme designs and quantitative risk analysis.
During the December to February period two trial embankments have been completed in the field. With the completion of the embankment trials all field work is now complete and the project is now in the sample analysis and reporting phase.
The trails have been design to meet the following objectives
The trials have provided an opportunity to investigate the foundation and construction material issues identified from the test pit lab tests
For the period July to November samples collected during the testpits have been analysed and the results interpreted. The key finding is that the assessment of loess variability indicates that the upper approximately 5m of primary in situ loess is similar on all surfaces. Loess deposition mechanism (e.g. in situ, fan, colluvium) was found to be more influential on geotechnical characteristics than the surface the loess was on. Loess does not appear to be thicker on terrace tops than valley side slopes or valley floors within surfaces.
From these results two sites have been selected for trail embankment construction. Embankment planning and construction equipment tendering has been completed.
Geologists began work on the test pits in May and completed it in July with little disruption due to weather. Sixty test pits have been excavated and one bore hole with water level monitoring equipment has been completed. Lab test results are expected by mid August and these results will be used to inform the trail embankment locations, soil types that need to be investigated and the nature and behaviour of the materials as a lining material.
Meridian Energy and the South Canterbury Irrigation Trust will begin geotechnical investigations into the Waitaki’s proposed Hunter Downs Irrigation scheme to help determine the scheme’s engineering design, infrastructure layout, and cost. It will involve material sampling, testing and trial constructions of embankments to understand how the soils and gravels can be best used for construction.
A random series of about 60 test pits have been selected to ensure the engineers get details of the various soils and sub-soils across the region. We are working with a number of landowners so the engineers have access to the test pits and geologists can look at the geological features like test quarries, track cuttings or open faces on hillsides. The work will start in May and is expected to take 6 to 8 weeks depending on the weather. The test pits will be about one metre wide; five metres long and four to five metres deep. The sites were identified by geologists to give regional description of the surface geology in an area between the Waitaki River northward to Otipua. The geotechnical information and other factors, including ground topography, irrigation demand and the cost of canals versus pipes will also be considered when determining the final scheme design, layout and cost.