Case Studies, Environmental News, Features, Geology, Geology Talk, Industry News, Recycling, Regulation, Regulation News

Black Clay Pit: From sinkhole to community asset

Penrith Lakes

 

Once an integral foundation of the Sydney construction market, the Penrith Lakes area – and in particular a 24ha clay pit within it – has been transformed into a centre for recreation, commerce, education and conservation in the past two decades. 

It was once considered the largest sand and gravel quarry in the Southern Hemisphere, at one point supplying about 90 per cent of the local Sydney market’s needs. 

After quarrying commenced at the Penrith Lakes in the 1880s, more than 160 million tonnes of construction materials were excavated from the site. That is enough to fill the Melbourne Cricket Ground, Australia’s largest sports stadium, 52 times. 

As quarrying at Penrith Lakes neared the end of its life before ceasing in 2015, the site underwent one of the world’s largest and most innovative quarry rehabilitation projects. 

First conceptualised by the Penrith Lakes Development Corporation (PLDC) in the 1980s, the vision was to transform the 2000-hectare area on the banks of Sydney’s Nepean River into a system of interconnected lakes, wetlands, native wildlife havens and parks. 

The first stage of Penrith Lakes to be rehabilitated was the Regatta Lakes area. It was showcased to the world when it hosted the rowing, canoeing and kayaking events for the Sydney 2000 Olympic Games. 

With the restoration program nearing completion, Penrith Lakes is set to become one of Sydney’s most popular recreation, education and conservation destinations. 

Featuring native flora and fauna unique to the area at the foot of the Blue Mountains World Heritage Area, Penrith Lakes includes parks, bushwalking trails, and areas for fishing, boating and swimming. The waterfront along Penrith Lakes also features Western Sydney’s largest recreational sandy beach, about the same size as the city’s iconic Bondi Beach. 

Some remediated areas at Penrith Lakes have also been prepared for future commercial and tourism use. 

The lengthy rehabilitation process included some significant engineering challenges along the way. Overcoming these hurdles resulted in the project setting new world-class benchmarks in quarry rehabilitation and end use.

One of the major obstacles encountered during the project was remediating the 24-hectare area known as the Black Clay Pit to an urban land standard. Nicknamed “the sinkhole”, the Black Clay Pit posed numerous challenges for the Penrith Lakes rehabilitation team to solve.

DRAINING AND CAPPING

Roger Moona, who has been working as an operation manager on-site since 1983, recalled that the clay was mostly used as “the plug (clay core) for Penrith Lakes between the Nepean River and the major lakes systems bordering the river on the west”. 

“As the clay was removed, not much else happened in the pit, so sediment and of course water also started to find its way in there,” he said. “This wasn’t such a bad thing until the time came for rehabilitation.” 

The first issue was that 75 per cent of the Black Clay Pit was inaccessible before the rehabilitation and preparatory works commenced. It was also full of sediment and about 20 per cent water. This was the first challenge: how to drain it and then access the pit to fill it up. 

Will Myles, also an operations manager for Penrith Lakes, said the drainage was one of “the most difficult and technical parts of the operations because it had a lot of different types of material in the bottom of it”.

Preparation works commenced in March 2015 and included draining and pumping the water out of the pit and placing a 1.25-metre thick sandstone capping across 3.5ha of untraffickable silt material (placed in two lifts of 0.75m and 0.5m). It also included cut/fill works and relocation of the historical quarry stockpiles and unsuitable material within the pit.

“Once we were able to drain it, we used sandstone to build a platform, allowing machinery in there to commence its consolidation and fill,” Moona said.

Although the sandstone platform was a success, it didn’t stop the tidal waves of clay and mud that came across while working on it. Oversized material was used in particularly difficult areas to ensure the capping layer and platform was achieved. 

“It really was a bit of creative skill and patching rock together to build the platform while we still had water in there,” Moona said. “Once we’d set up a permanent pump and the platform finalised, it was all systems go.”

The key to success was sealing off the floor and draining the water before starting the fill. Lots of little holes created issues. In the end the team had to excavate down about seven metres to retrofit any issues that had happened in the mining phase. This was a solid foundation to work from. 

“Once all the foundation work of capping, building the platform and draining the water from the pit had happened, the filling process has been fairly smooth sailing, but still no small feat,” Moona said. 

An excavator digs deep to remove clay and mud.

FILLING AND COMPACTING

The pit then required four million tonnes of virgin extracted natural material (VENM) – also known as “clean fill” – to be filled to its final landform levels. The depth of fill to complete the landform was up to 15m in some areas. 

Fortunately, during this time the New South Wales Government was rolling out several major infrastructure projects, including the WestConnex motorway, which had the exact amount of fill needed for Penrith Lakes. So, it was the tunnel VENM from the WestConnex project that provided most of the fill for the Black Clay Pit.

Once the fill supply was secured, every delivery required an additional environmental control before it was placed. As it was being delivered into the pit it was compacted into 300 millimetre layers, in accordance with the earthworks specification. 

“Obviously it was paramount to get the geotechnical engineering design and specification just right because we eventually want to be able to build on it, and the extraction process that was originally undertaken was substantial,” Arthur Ashburn, project manager for Penrith Lakes rehabilitation, said. 

Geotechnical data has been gathered, including material specifications, density testing every 500 cubic metres of fill, recording/surveying of test co-ordinates and reduced level (RL), as well as daily and monthly reporting. 

The VENM import project for the whole lakes scheme commenced in 2014 at a rate of three million tonnes per year. A substantial amount went to the Black Clay Pit remediation, which was completed in 2019. The Penrith Lakes engineering team included the geotechnical consultants Pells Sullivan Meynink (PSM) and was specifically developed to ensure the rehabilitation program was completed on time, to budget and more importantly with no safety or environmental issues. 

“We are proud to say that we had no safety or environmental issues to rehabilitate the Black Clay Pit,” Ashburn said. “It was a project that demanded a lot of planning and resources even just to enable access, but we’ve managed to get it right and the outcome speaks for itself.” 

Where there was once sand, clay and mud, some parts of Black Clay Pit today are lush, level grassland.

FEEDBACK FROM THE MAJORS

Boral, Hanson and Holcim (and their forebears) historically managed quarries in the Penrith Lakes region. The CEOs of the three companies have been complimentary of the PLDC’s work in the past three decades to return the land as a community asset to the Western Sydney region.

“The completion of the rehabilitation fulfills the four-decade vision by the owners of the Penrith Lakes Development Corporation to transform the site into Western Sydney’s premier parkland, conservation and waterfront precinct,” Boral Australia president and CEO Wayne Manners said. 

“The Penrith Lakes Scheme is one of the most ambitious and innovative quarry rehabilitation projects ever undertaken in the world,” Holcim Australia & New Zealand’s CEO George Agriogiannis said. “It is destined to become Western Sydney’s landmark recreational and lifestyle destination.”

“The Penrith Lakes Scheme is one Australia’s iconic engineering projects,” Hanson Australia’s CEO Phil Schacht declared. “After being Sydney’s main source for sand and gravel for more than a century, it is now playing the important role as a major flood mitigation zone and habitat for native animals and plants.”

Source: Penrith Lakes Development Corporation

©2019 All Rights Reserved. Quarry Magazine is a registered trademark of Prime Creative Media.