Huntly, Northern Waikato, New Zealand
Background
Awaroa Stream currently runs through the Rotowaro Open Cast coal mine in Huntly, which is situated approximately 1hr south of Auckland.
The existing mine has been in operation since 1958 and is currently the second largest coal mine in New Zealand. The majority of the coal is transported to local power stations servicing the North Island's industrial and home heating markets
Proposed future mining works at the site require the existing stream to be diverted to allow coal recovery to continue. Current plans propose two stages of diversion for the stream: relocation to the upper cut slope of the western pit and then centralised over the central section of mine supported by a 50m to 60m high earth-fill embankment.
Scope
As part of the on-going mining works, CMW have provided geotechnical advice relating to the proposed cut slope stability for the temporary channel alignment; and undertaken an initial assessment of the potential settlements for the proposed 60m high earth-fill embankment. Analysis for the embankment has included a review of existing settlement monitoring data for a near-by fill area; established of long term settlement rates and review of published data to qualify the potential collapse settlement issue found in large embankment / dam type structures.
Findings/Challenges
Our literature review indicated that collapse settlements when the final formation becomes inundated with water could be significant.
Only 4m of fall can be tolerated across the length of the permanent channel which will be difficult to achieve with the high predicted collapse settlements.
Further investigation and testing in required to quantify the potential collapse settlement or a leveed embankment could be constructed to prevent over topping of the main stream channel.
A layer of weak marine tertiary mudstone and quaternary solid are also of concern in the major cut slope and further soil testing is required to further assess the risk of potential collapse.
Current placement of proposed fill materials.
View of open cast mine floor where the proposed embankment will be constructed.
Soil units and tertiary mudstones forming temporary diversion cut slopes.