Soft Soil Engineering On The Ballina Bypass


Hi, I’m Peter Colquhoun. Now road construction is a complicated business, a case in point is the Ballina Bypass. Here soft and saturated soils is added to
the complexity of the design and the construction. In
this instance the road will open to traffic but they’ll be ongoing intervention works to keep the bypass in tip top condition. I caught up with Bob Higgins, General Manager of the Pacific Highway to learn more Manager of the Pacific Highway to learn more. I’m with Bob Higgins, the General Manager of the entire Pacific Highway and we are looking down at the Ballina Bypass, and Bob it’s a beautiful part of the country and a very complex project because it
travels through a lot of flood plains. Bob: Yeah, when you look at the top end of the Ballina Bypass project, you can see we take our
environmental obligations very seriously, and this means that before we even start
work we do a lot of measurements, field investigations, to understand how water flows. Not only above the ground, but also below the ground. Peter: I just wanna go and build the road. Bob: No, we can’t do that, we can’t do that. So we have got to understand all that and then we actually put the design in, and we look at how we impact upon it, and it’s not only about the construction or the final when we open to traffic, we need to understand how it affects it during the construction process. Once we understand that, work out all how to mitigate it, the guys can then go out and build it. But it’s not only then saying “well just go and build it”. We have got to go through a process of monitoring during construction, if we’re causing impacts or that, we have then got to work out how to mitigate it, and then when we finally get it opened we don’t stop there we actually come back and then monitor to see that it’s done what we thought it would do. done what we thought it was gonna day so
that we can actually correct any issues but also from that learn from that for
future projects. Peter: Can we take a closer look? Bob: Absolutely. Peter: Well Bob it’s seems very wet and very damp to be building a major roads project. Bob: It is, but it’s not only rain..it’s..we’ve got a higher ground water table here that that we have had to deal with and alot of underlying soft soil. Peter: My obvious question is why wouldn’t we just build a big bridge all the way thru here? Bob: Well those are the sort of things we do look at, but we are talking about something like about six to seven kilometres of bridging over the flood plain. Peter: Well now you’ve made the decision to build through the flood plain based on all those other considerations and then you come into the contact with the soft soils and everything you have to deal with now. Bob: That’s when we bring all the expertise, in this particular case we’ve got Alliance. So we have got the RTA expertise, we’ve got the private sector expertise, how can we bring them all
together to come up with the best solution? Peter: Right, based on the community economics, now that’s always the next
question if it’s a soft soiled situation that
means it will have on going maintenance. Bob: Well it depends on what you do. On one hand we could spend over ten years here building this, just to make it a from an engineering point of view minimal maintenance from there on in the future. But that meant that we can’t provide a bypass at Ballina. Peter: For another 10 years. Bob: And that’s a.. a very significant benefit because that’s what the community of Ballina wants. So how do we bring that in? How do we look at the capital cost and how we then look at the performance,
the ultimate performance of this over time? So then we have got this other equation going on that we are looking at.. Peter: Wow…yep
Bob: and then you, you look at all the options and then you try and find the best fit that meets all those needs. Peter: OK, so that’s how we ended up
with these connecting A with B through this particular flood plain was the best case scenario.
Bob: That’s right, because it provides that optimal outcome for everyone involved,
Peter: Right Bob: including the community. Peter: Right, so Bob there is an enormous amount of stuff that we can’t see that is going on here, decisions apart from engineering?
Bob: That’s right, what to see out there now you only see part of the thing. There’s a lot of work that’s been done under the ground to help stabilize those soft soils so that we can actually build a road pavement on there. But even to get to that point, we’ve had project teams that have been working for months, looking at a range of options, doing trials on this, just seeing how it settles, and then from there we actually bring all that together with our experts, as a, as under the Alliance to then make a decision on which way to go forward. So it really is alot of work’s actually gone in to get it to this point. Ballina has got this uniqueness, cause what we have done is we’ve done is we’ve actually brought in innovative techniques about vacuum consolidation, deep soil mixing but we’ve also had
other techniques such as just normal embankment and pre-loading, surcharge, wick drains and that, where we have been using for many years we’ve actually looked at those, how can we adapt them further to meet these needs. Voiceover: What you are seeing to here is a process known as vacuum consolidation to stabilize the underlying soft soils. This technique involves inserting special pipes into the underlying soft ground to the required depth.
These pipes are connected to large vacuum pumps. The ground is covered with a heavy airtight membrane to seal it and the the pumps are then used to create a vacuum which helps remove the water and air from the soft soils. What you are seeing here is a process known as Deep Soil Mixing, where we
strengthen the underlying soft soils by adding cement or lime. A large crane with a hollow auger shaft
attached to a head is extended into the grand to the
required depth. As it extends it mixes the soil, when the
head reaches the required depth the binder (cement or lime) is injected. As the auger head is raised, cement or lime is mixed into the soil. Over time the soil hardens and becomes a rigid column which is then able to support loads. What you are seeing here are wick drains being inserted into the soft soil to make it easier to extract the water. The wick is a tube of plastic covered
with a geo fabric to filter the water from the soil. The wick is driven down to the required depth in the soft soil, a surcharge on top of the wicks in the soft soil forces the water out of the soil via the wicks. What you are seeing here are stone columns being built in the soft soils. A vibrating probe sends the tube to the bottom of the soft soil, small stones are then pumped through the
end of the tube as it’s withdrawn. The process involves pushing the stones down to form a column which is vibrated and compacted all the
way to the surface. These columns can then support the loads
on surface. Peter: So building through flood plains, as
the Ballina Bypass is doing, is so many things that, that people don’t even think about let alone appreciate.
Bob: They bring a lot of complexities and a lot of challenges to the project teams. That is why we bring a whole range of experts in, who bring their own specialist knowledge, and then we’ve got our project managers that bring it all together and then find a way through. But importantly it’s just not about going out there and building it’s about monitoring, seeing how we’re
going, adjust to modify the process so that our final outcome is the most optimal that we can provide to the community. Peter: So you’ve thought it all the way through. From the frogs all the way through to the final traffic. Bob: So the frogs are but one and then we have the human per residents, and the engineering issues they’re all going to
be brought together for these projects. Peter: Steve Summerell is a Geo technical expert from the RTA, now Steve this is what we call controlled
settlement? Steve: Yes, its we’re trying to control it and manage it, so it avoids having a really high
initial construction cost to build it all out but still delivers the benefits to the
road users. So we are building the embankment just high enough to give us the flood immunity we need so that the road keeps open. This means we put the embankment there and over the construction period of
about 2-3 years it will tend to settle between about 300 mills
and 600 millimetres will then open the road to traffic, after
about twelve months will put a final asphalt layer on it of about
45 millimetres to make sure the ride is smooth, and the road’s functional for the
road user. That embankment will continue to settle over the next twenty to forty years, and every five to ten years we will have to
come back and do a correction course of pavement from 50 to 200 millimeters thick just to reinstate our flood immunity and get our ride right. Peter: Well Bob you’ve used all your tricks haven’t you here at the Ballina Bypass? Bob: Yeah and it’s a team effort to get it to this point.

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