The Mon Kok area of Hong Kong is reputedly one of the most densely populated areas of the world. To put it in perspective Kowloon has over 2.1 million people packed in to only 46 square kilometres. That’s over 46000 people per square kilometre compared with Sydney at only 1700 per square kilometre. Hong Kong also has a huge annual rainfall on average being 2215mm. When the rain comes in the wet season it buckets down and can cause major flooding right in the middle of this huge population base.
Recognising this problem the Hong Kong Drainage Service Department (DSD) designed the Kai Tak drainage transfer tunnel to collect storm water and divert it out to sea. The tunnel is 5170mm in diameter and some 6.5 Kilometres long.
The problems of constructing a tunnel of this type are many. The first is that a lot of the ground is "soft ground". That means it has to be supported while tunneling or it will cave in. The second problem is that this tunnel is below the water table and therefore has the potential to flood the tunnel. Tunneling in hard rock is, by comparison , a breeze. Just put in a road header and watch it come out the other side but soft ground below the water table needs a very specalised , slurry type, tunnel boring machine (TBM). This type of TBM has a rotating cutter head chamber that is pressurised with Bentonite which is like a thin sticky mud. The Bentonite is injected into the cutter head chamber at a pressure equivalent to the ground water pressure and therefore prevents the ingress of water and at the same time helps to stabalise the ground and minimise the tendency for the ground to collapse. As the TBM progresses it lines the tunnel with concrete segments and grouts behind the sections leaving a watertight and completely finished tunnel in its wake.
Stage two of the Kai Tak transfer scheme contract was awarded to the China Harbour Transfield Joint Venture in March 2001 for a reported HK$382000000. Some of the Transfield team had just finished the Sydney Airport Rail Link Tunnel and because they were happy with the services that Descend had provided on that project we were again invited to tender for the contract of preparing and presenting the training package for hyperbaric workers and lock operators. Descend won the contract in June 2002.
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From then it was all go, we designed both courses, wrote two new manuals, prepared two new PowerPoint presentations, designed the exams and certificates all with in about six weeks. At the end of July we presented our first workers Hyperbaric Training Course in Hong Kong. What a culture shock. We were teaching in a converted freight container and most of the students were either Nepalese or Chinese for which English was very much a second language. I learnt very quickly to delete all unnecessary words and simplify all new concepts. Courses2, 3, & 4 followed in close succession and by the end of August the workers were ready for their first manned intervention. The only problem was that we didn’t have the TBM (Tunnel Boring Machine), in the ground. It was in pieces in the storage yard. The logistics of actually starting to tunnel are mind blowing. When we did the Sydney job they just dug a huge 100 metre long trench that was 25 metres deep and assembled the TBM and all of its carriages (we call them gantries) in the ground and started to dig. In H.K almost all usable land has a high rise building on it so a trench big enough to assemble our TBM, christened the Phoenix, was out of the question. The biggest piece of unused ground in our location only allowed an entry shaft of about 10 metres in diameter. The solution was to put the TBM into the shaft one piece at a time and as it tunnels into the ground keep adding gantries until its all underground. That sounds simple however as each piece goes into the ground the gantries remaining on the surface have to have all of their services re-connected down the shaft – that includes, power, water, bentonite delivery and return, lines, tunnel ventilation - the works.
The next problem occurred when the cutter head finally started to cut. It was found that the ground treatment was not satisfactory and as they cut into the ground the ground water started to run into the shaft. A huge drill was brought in to drill a series of holes in front of the cutter head. The holes were then filled with grout effectively creating a concrete wall in front of the TBM. The TBM then started to cut through concrete and once the first of the tunnel segments were fitted and grouted the leaking ceased. Each tunnel ring is concrete 1200mm long and 250mm thick and made up of six shaped pieces. Each completed ring weighs over 11 tonnes. Behind each ring is about 2.6 cubic metres of grout to fill the space between the ring and the ground and to seal out ground water. Once the rings are in place the phoenix uses sixteen hydraulic thrust cylinders to produce over 1800 tonnes of thrust to force the TBM into the ground and continue cutting at a rate of 10 to 60mm per minute depending on ground condition, cutter and pick condition and slurry density.
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Wear rates on the cutter and picks depend on the ground condition and periodically the cutter head must be stopped and a team of Hyperbaric Workers enter the pressurised cutter head via a recompression chamber and their job is to clean and inspect the cutter head and change the worn tools as necessary.
The maximum pressure expected on this project is 2.3 bars (gauge) which is equivalent to 23 metres of water pressure. Divers call this having a dive or taking a dip but tunnel workers call this a manned intervention.
Our job of course is to train the workers on how to enter and leave the pressure environment safely and to train the man lock operators. (Recompression Chamber Operator) on safe lock operation. This includes a lot of normal diver theory including, basic physiology and physics and diving maladies such as decompression illness and squeeze. Of course decompression tables, regulations and fire safety are a compulsory part of the lock operators course.
Our most recent lock operators course was in March, 2003. At that stage this was the fifth trip to Hong Kong. The tunnel was progressing well and there had been minimal intervention and no hyperbaric problems, however the job was about to enter a much harder type of ground which will require more inspections of the tools so the management were anticipating a requirement for more Lock Operators.
We have now completed three workers courses, two Lock operators’ courses, one first aid course and one Dan oxygen management course.
The process has been very demanding but we have found it both educational and enjoyable and we hope will position our company nicely for further tunneling projects.
