From: brendan@otc.otca.oz (Brendan Jones)
Newsgroups: aus.followup
Subject: Re: Leaking Tunnel - some questions
Date: 30 Jul 92 23:32:05 GMT

in article <annick.712380399@cortex.physiol.su.oz.au>, annick@cortex.physiol.su.oz.au (Annick Ansselin) says:
>>the concrete will quickly have its salinity neutralised by the natural
>>alkalinity of the concrete, and the water presence will become benign
> That doesn't make sense to me. An acid + alkali gives salt + water. How do
> you neutralize a salt solution with an alkali?

Well, I'm not a concrete expert nor a chemist (just an Elec Eng!), I'm just
paraphrasing what I've read in various engineering reports.  I understand the
problem is not so much the presence of salt per se, but whether the salt is
continually washed through, which would suggest the presence of oxygen, or
whether it just stagnates when the cracks are repaired.

I understand If the leaking can be stopped internally, then a chemical balance
of some kind will quickly establish itself inside the concrete, leading to a 
benign environment.  If there is a continual flow of salt water, then there
may be corrosion and weakening of the concrete.

Basically, concrete totally immersed in relatively still water experiences a
much more benign environment that, say, a concrete building exposed to sun,
rain, wind, and changing temperatures.  I think that is the essence of why
stopping the leaks internally should prevent long term problems.

> Unless you were able to soak the entire thickness of the concrete from the
> outside in (is it 10 feet thick), I can't see how injecting the cracks from
> the inside is going to do a lot of good in a few years.

Providing the leaks are stopped (and that would imply that the injection of
the sealant does not cause further cracking) then the suggestion seems to be
that the concrete environment will become stable.

That's not to say there won't be other problems, say due to vibration.  But
there has always been an annual maintenance provision for continued operation
of the tunnel.

Think of it this way.  The Sydney Harbour Bridge is rusting.  Always has,
always will do.  A maintenance crew of about 20 traverse the bridge day in and
day out, painting and removing rust, and identifying any areas of particular
degradation or weakening.  That doesn't mean the bridge is dangerous, is in
danger of falling down, or worthy of an inquiry.

All structures need some maintenance.  The fact that the tunnel will should be
a surprise to no-one, especially when it's described in some detail in the
1986 tunnel plans.
 
> Why can't a new membrane being installed on the outside of the tunnel?

The membrane over the transition segment probably could be replaced, but it
would require the area under the Opera House Forecourt to be 'de-watered'
again, and this would require a new coffer dam (similar to the one removed
near the Opera House at the beginning of the year).  This would be very
expensive and time consuming.

I suppose that if the membrane can either be patched by divers, or leaking
stopped by satisfactorily sealing the tunnel internally, and you meet your 100
year lifespan, then that's all that's needed.

There's little point in replacing the membrane to make that section last 200
years when the tunnel has been built to a 100 year spec.  That's engineering
reality I'm afraid.

I don't wish to over-emphasise the point, but the whole tunnel is not leaking,
just the ceiling of the 30 m transition section between Immersed Tube #8 (the
Opera House End) and the south shore driven tunnels.

>>As the Sydney Harbour Tunnel is the 73rd immersed tube tunnel to be built
> Does this mean the problems have been really solved - permanently, or just
> contained?

I'd venture to say contained.  All mechanical structures suffer deterioration
and require maintenance.  The types of things that can occur are well known,
and there are technologies and procedures available to rectify problems if and
when they occur.

From what I've read about various construction projects, this tunnel has been
incredibly well built.  There are many other structures around (that haven't
fallen down) that have had construction problems of a far more serious nature
repaired or contained.

There's a story about the size of the bolts in the steel lattice supports to
the Sydney Aquarium that I won't go in to...  There's a story about a 30 level
office tower in Brisbane that started to lean when an excavation next door
released geosyncline pressures.  How can you straighten a skyscraper?  You
can't, but you can prevent it from tilting further.  There's many others.

I suppose the novelty of Australia's first long road tunnel combined with the
contoversy surrounding the initiation of the project has led to these events
gaining an unusually high exposure.

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