Silica is silicon dioxide, a naturally occurring and widely abundant mineral that forms the major component of most rocks and soils. Crystalline silica dust particles can penetrate deep into the lungs and cause illness and disease.

In this broadcast, Christian D’Hondt discusses how he and his team ‘stay ahead of the game’ by applying their learnings about controlling silica exposure from past projects, to each new project they take on.

Who is this seminar for?

This seminar is useful for PCBUs and WHS consultants or advisors in the construction and infrastructure sector.

Recommended viewing

Representatives from the same Sydney Metro project discuss their proactive measures to address silica dust in The role of the client in the WHS landscape and Shifting perceptions and outcomes in occupational health and wellbeing

About the presenter

Christian D’Hondt is a Construction Manager with John Holland CPB Ghella Joint Venture, Sydney Metro City and Southwest.

Additional resources


Silica series: Staying ahead of the game – Controlling silica exposure in tunnel construction

Presenter: Christian D’Hondt, Construction Manager, Sydney Metro Tunnel and Station Excavation Contract, JHCPBG Joint Venture

Virtual Seminar Series - Transcript

I'm Construction Manager for our joint venture for the Sydney Metro tunnels and stations excavation project.

We're constructing twin tunnels between Chatswood at the north and Marrickville at the south. For the next section of the Sydney Metro. So, twin 15 and a half kilometre tunnels, stretching, as we said from Chatswood to Sydney, well Marrickville in the south there, passing beneath Sydney Harbour and the CBD. We excavate dive structures at Chatswood and Marrickville to join into the existing network. We have six underground stations, or six stations, three of those underground at Vic Cross, Martin Place and Pitt Street. And open stations at Barangaroo, Waterloo and Crows Nest with a cross over cavern at Barangaroo.

We're also setting up the precast facility at Marrickville for the fabrication of 99,000 precast segments. And we also novated the demolition contract for all the buildings on the station, along the alignment.

So our bored tunnels on the Sydney Metro project we're utilising tunnel boring machines for those mainline tunnels. We launched to TBMs from Chatswood, they'll excavate 6.2 kilometres through to Blues Point where we have a temporary retrieval shaft. They're hard rock , double shield TBM, and they'll be passing through about 90% sandstone, and about 10% shale, just at the commencement of their drive.

We have a slurry TBM for the under harbour drive so that passes from Barangaroo through to that temporary retrieval shaft at Blues Point. It'll do that crossing twice, recovered from Blues point and taken back. It's about 80% sandstone with about 140 metres in soft grounds made up of clays and sediments. Remaining two TBMs come from Marrickville in the south through to Barangaroo again double shield hard rock machines. They've got just over eight kilometres to do each. And again, similar amount of 90% sandstone, a little bit of shale down at the Marrickville end.

I guess we're conscious of the different rock types and the different silica contents that those different rock types have. I guess the key sources of silica that we see in tunnelling and in this project firstly in the host rock disturbance, two real sources of that in the works that we're doing. Tunnel boring excavation and also in our cross passage mine tunnel excavation. The other significant source of silica is from other operations occurring behind the face, so all of that spoil transported out of the tunnel using conveyor belts, so that's a dust generating operation. And also our vehicle movements within those tunnels.

I mentioned also that we're doing the precast yards, so, all of our tunnels align with the pre cast concrete element. So we do have repairs to those segments, so the exposure during the drilling and grinding for those repair operations is another key source of silica.

I've worked in tunnelling in Sydney for coming up to my twentieth year, this year. So a fair bit of exposure to silica. I do go the Dust Board for my regular check-ups and fortunately, unlike some of my colleagues, I am clear. But, I guess, you know, we're continuing to work off of what we've done in the past. And our statement is 'staying ahead of the game'. And I guess that's really testament to what we're trying to do, and learning off what we've done in the past. At Northwest Metro we had pretty good levels of compliance, but we know there's always areas for improvement.

And, I guess, today I just wanted to focus on four key areas that we've identified. There's always many, many more, but the four key areas we saw, and I wanted to share with you, of how we're trying to up that game. So those four areas: TBM ventilation, cross passage ventilation, some of the modifications we've made to plant, and some of our concrete dust extraction ideas.

So this is a schematic of a tunnel boring machine, at the left end of the screen there, is the front of the machine, so that's the cutter head. That first 12 or 13 metres is what we call the shields. Within that section there is where we do the excavation and we also build that precast lining.

Trailing on behind that we have the backup, they're generally around 150 metres long, and that's where we house all the logistics of the machine. And it's also where most of our workforce are stationed. And I'll point out some of the stations there because you'll see some of the significance in the next couple of slides. At the front we have the ring builders, we have an operator in a cabin. Next behind that is our grouters. Other people on the machine segment handers unloading those segments. We have service extensions towards the rear of the machine and we have deliveries in and out of that TBM.

At Northwest Rail the ventilation setup there, fairly typical, fresh air supply from the surface into the rear of the machine. We use a flexible vent bag to do that. So we're blowing fresh air from the surface that would then force under pressure and flow back out of the tunnel. Within the TBM itself we allow that air to flow through the machine so direction of airflow is from the rear of the machine. With some additional fan assistance, but essentially airflow forward through the machine to a dust extraction point at the front of the machine. We collected the dust from the cutter head and any other dust that was created during the operations of the machine at the front of the machine there.

So this graph here is the exposure results. We did a lot of, like most people these days, we're doing a lot of monitoring of our work groups. And you can see here this really again lines up from the rear of the machine on the right hand side to the front of the machine on the left hand side of the page there. The red dotted line is that exposure standard. You can see towards the rear of the machine we're generally pretty good, as we get into that ring builder we're just exceeding. The operator being in the cab, again's in a pretty good place. And our ring builders at the front there are exceeding that limit. Those levels compared to historically, are very good levels of compliance. But obviously we were still very reliant upon our personal respiratory protection to ensure that those workers were, their exposure was at safe levels.

For Sydney Metro we're fortunate enough to buy five new tunnel boring machines, and have been able to incorporate some of the lessons from the last project into the specifications of those machines. Some of the key changes we made in those new TBMs, still reliant on our fresh air supply from the surface by vent bag. But the changes we made on the TBMs, dust extraction we're actually doing that at two locations so we've got dust extraction at the cutter head with a dedicated dust extractor. And we also have a second dust extractor at the conveyor transfer point. Though the two key areas that we identified that were generating dust and we're able to more efficiently extract from those two points with two stand alone dust extractors rather than ducting between them.

We've also reversed the airflow through the TBM. So we now will have forced ventilation from that incoming supply from the surface and force that ventilation through the machine and our airflow will now flow back through the TBM. We do believe these changes will reduce our exposure levels.

Between the two tunnels we construct cross passages. There's 57 cross passages across this project, 49 standard and eight with sumps. Those cross passages form the permanent fire and life safety egress cross passages in operational mode. Those cross passages we will construct in parallel with our tunnelling operations. So behind the TBM and they’ll involve what we can see a little bit in the picture there, we'll open up the segment lining ring, excavate the host rock and construct an in situ lining. Our method at Northwest Rail, a fairly commonly used method. Small hydraulic rock breaker on an excavator. We have a mobile platform there to give us access to do the works. And we have a dust extractor to collect that dust from those works. This photo here you can see is just at the commencement of one of those headings and as many of you will know, commencement of headings always forms challenges for us to control our ventilation. Once that heading's established and we get in a little bit we can get that vent can right to the face and our dust collection works a whole lot better.

So that was a challenge for us at Northwest Rail we did use different methods to try and contain that initial heading, small brattices put in place, again a reliance on the P2 masks and a key awareness of who was out by. Obviously the air flow is coming back from the TBM running up the tunnel. So any dust that we are not able to control is, flows back up the tunnel. Just point out in that photo, the yellow vent bag at the top there is all squeezed up in this photo. This photo was taken in a tunnel where we'd actually broken through to the next station box so we were just blowing fresh air through the tunnel and no longer relying on that vent bag.

What we've been investigating for Sydney Metro is the use of something called a vortex air curtain. That challenge, as I mentioned, we're trying to excavate those cross passages while we have tunnelling operations in front of us, so we need to be able to drive past on a very regular basis that cross passage excavation. To try to build a permanent brattice or some sort of wall to control that dust is always a challenge. What we're investigating here is the use of that compressed air to create a curtain that will essentially shroud that dust generation to allow the dust extractor remove it rather than getting blown back out of the tunnel. And still allow vehicles and others to pass by that work location. It's a system yet to be proven, but we've looked at it in similar applications and have a fairly high level of confidence that it will work for us.

Getting into some of the more simpler controls, for the control of dust. Tunnel vehicle movements in the tunnel are definitely dust creators. We do try and maximize our housekeeping to try and keep that material on the belt but, invariably, from time to time material does fall off as those vehicles travel up and down the tunnel. Just like on the surface, they can stir the dust up. Some of the things we've put in place there, we do have our housekeeping, we do have wetting down of the floors. But a quite a simple but effective one is just the direction of the exhaust from the tunnel vehicles. On the right hand side there you can see one of our segment carriers. On the left is the back up of a TBM as it's passing through underground cavern. It's a Castle Hill crossover cavern. Just by pointing the exhaust up of those vehicles, the dust isn't created from the movement of the vehicles, they're traveling fairly slow, sort of 10 kilometres an hour. But that airflow coming out of exhaust pointing down to the ground we found created a lot of dust. Quite a simple thing to do. But in a bespoke vehicle there we need to do that back with the manufacturer and best done prior to fabrication.

And then finally into the precast production. As I said we've got 99,000 segments to make for this project. All down in the bespoke plant at Marrickville that we've just set up, and started production last week. During that production we will get some damage to segments, some defects to segments that do require repairs and likewise when we install those segments in the tunnel we will have damage to those segments requiring repairs. Generally the repair of that segments is done through drilling and grinding. We do create dust during those operations and we're looking at two ways of controlling that exposure.

In the factory we're looking at a segregated area with a dust extraction device, so really trying to do that separately from the rest of our activities. And also extraction at source. A lot of our drills and the like these days you can get with the dust extraction trying to extract that at source rather than letting that become part of the larger atmosphere. Likewise in tunnel repairs that extraction at source is what we're striving to achieve.

It all comes down to if we can reduce those exposure levels we reduce that reliance on the personal respiratory protection which is a higher level of control. And as always trying to administer and get people to buy in to the controls. If we've got the levels, the exposure at a level we know and we can control then we eliminate that issue.

Thank you.


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