Dendrobium Ventilation Shaft - BHP Billiton

Toolijooa is undertaking post-construction rehabilitation works at the Dendrobium Ventilation Shafts 2 and 3 in the Cordeaux Catchment west of Wollongong. The vent shafts have been constructed to provide ventilation for the Dendrobium Mine and post-construction rehabilitation works are required to deliver an ecologically sustainable outcome within the locally affected area over the thirty year operational life of the shafts.

The site is located upon the Hawkesbury Sandstones of the Woronora Plateau, surrounded predominately by Exposed Sandstone Scribbly Gum Woodland on exposed ridges and slopes and Sandstone Gully Peppermint Forest within sheltered slopes and gullies. Tall Open Peppermint-Blum Gum Forest is also found within the site on Narrabeen Sandstone and Shale slopes that occur towards the edges of Lake Cordeaux.

Construction works removed native vegetation entirely from both the shaft site compound and access easement leading to it. Topsoils from the entire disturbed area were excavated and stockpiled by BHP during initial construction and respread eighteen months later following completion of physical landscaping and site stabilisation works. 

Toolijooa’s post-construction rehabilitation works have included:

  • viability trials of stockpiled topsoils to determine direct seeding and tubestock rates,
  • advising on and supervising respread of stockpiled soils,
  • collection of over 12kg of native plant seed from within the local catchment for site rehabilitation works,
  • direct seeding of the entire 4.56ha site to augment native plant regeneration from stockpiled topsoils,
  • propagation and installation of 13120 native species tubestock,
  • ongoing maintenance and monitoring works.

The testing of soil seed viability was particularly important during this project as the soil excavation and stockpiling process was considered sub-optimal. Topsoils and subsoils were heavily mixed, rather than being excavated and stockpiled separately, while it is considered that the eighteen month stockpiling period led to substantial losses of soil seed viability and diversity. Original soils also varied broadly across the site and different soil types were not kept completely separate. Stockpiled soils of obviously different colour and texture were sampled separately and germinated under two different watering regimes and smoke water treatment. Viability testing subsequently directed direct seeding and tubestock rates to be utilised at the site.

The species collected were derived from an analysis of species suitable for revegetation purposes from the local native vegetation types and are indicative of the overall aim of rapid rehabilitation of the site.

Native species germination rates have been good within the first year since spreading of stockpiled soils and direct seeding, with an average of approximately 25 seedlings and 8 species emerging per m2 .