Hornsby Quarry - Hornsby Shire Council

Project Summary

In early 2001, Hornsby Shire Council took ownership of Hornsby Quarry following its closure by Boral, creating a need to decide what to do with the space. In 2014, the NSW State Government announced the construction of the M1 tunnel extension (NorthConnex) linking the M1 to the M2, and this provided Council with an opportunity to fill the quarry with clean material and create a recreational facility.

The Challenge

The overall challenge of what to do with a 3.5 million cubic metre hole in the ground was not insignificant. Many years ago, this could have been turned into a landfill or tip site, but with today’s environmental responsibilities this was obviously not an option.
A design solution had to be undertaken, and the volume of material required to fill the quarry excavation to a suitable level had to be investigated. There were many geotechnical, environmental and engineering issues involved - e.g. slope instability, preservation of the geological features, areas of sensitive vegetation, and providing suitable access to the finished surfaces.
Preliminary conceptual designs were undertaken, with the constraints taken into consideration and early volume calculations performed, but during the early design stages the volume of material likely to be provided from the tunnel excavation was not known. One of the biggest design problems was providing a finished design level that would not only make the area functional and retain some of the existing topography (too low and it would still look like a quarry hole and have little functionality, too high and the area would look like a billiard table) but also suit the volume of material likely to be provided from the tunnel excavation.
According to Roads and Maritime Services NSW (RMS), the construction of NorthConnex produced around 2.6 million cubic metres of spoil. Hornsby Quarry was one of six sites identified in the NorthConnex Environmental Impact Statement as having the necessary capacity to receive spoil material generated by the project.
Once the NorthConnex team was able to provide Council with the estimate of the volume from the tunnel, a considerable shortfall was found, so designs had to be reworked or additional material sourced from the surrounding area through possible reshaping of the quarry walls or lowering the Old Man’s Valley area.
Again, designs were reworked and volume calculations were undertaken to try and achieve a suitable balanced result.
The quarry was filled by NorthConnex during 2018 (see attached photo). The fill material was spread and compacted to a level finish. Another detailed survey of the site was undertaken, and with the use of 12d Model software, volumes were calculated to determine the total volume of fill that was imported into the site. The accuracy of this was very important as the funding agreement between Council, RMS, and NorthConnex relied upon the quantity of fill placed in the quarry (measured in cubic metres).

The Solution

Through using 12d Model, numerous designs and finished levels were able to be explored and tested to provide the most suitable and balanced solution. Information was able to be extracted and documented in reports to Council.

12d Model’s visualisations/flyovers/drive-throughs made communicating designs to non-technical people, and to Council, a breeze. These were prepared and presented Council at workshops to assist in explaining design options and give a better understanding of issues involved as well as showing them what the final design may look like.

The Result

A functional recreational facility of great benefit to the greater Hornsby community was created. Transport for NSW managed the beneficial reuse of more than one million cubic metres of spoil from NorthConnex at Hornsby Quarry - an environmentally sustainable use for the spoil. 12d Model was integral to many stages of this complicated process.