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Building Project - Hazard Analysis Print

Technologically, the Underwater Park belongs to the more challenging constructions. We excavated 4m beneath the level of ground water, where very soft sand had to be stabilised with 2,000 tons of scree and gravel. Because soil weighs about twice as much as water, the load distribution can't really have changed much after flooding water 7m high.Also the walls inside the area have a massive weight, but the load is distributed thanks to the concrete slab, so that the approximately 16,000 tons correspond to a 1m layer of soil.We should be able to replace the weight of rest of the removed 3m of soil by 6m of water. The total loads should therefore not change much compared to the pre-construction situation.

The weight that rests on the liner is massive. The reef-pillars alone weigh more than 100 tons and the entire cave surely weights over 1.000 tons.

Nevertheless, a residual risk remains due to the inevitable dynamics of the excavation. By removing load from the excavation the ground has risen slightly. We stabilised that so far that our 23-ton wheel-loader wouldn't create any deformations any more, but the slab would still bob about 30cm depending on the pressure from the ground water. The theoretical worst case would be that the ground would break, such as has happened spontaneously some 20km away, when the soil load pushed into the soft under-layer. This has caused the creation of several hectares large lakes (such as Heiliges Meer). Since the geological conditions here are slightly different here, this is unlikely, albeit not impossible.

The sinking excavator illustrates how soft the ground is. Therefore we monitored earth movements with laser-levels and plummets.

We therefore decided to separate sealing from load distribution. If the water load only rested on the concrete slab, there would be a risk of it breaking and pushing down into the liner.

Therefore there are vents in several places so that the concrete slab essentially holds itself and the constructions on top of it.

The water load is irrelevant because the pressure above and below the concrete slab are the same. It is also conceivable that a hollow forms between concrete and liner because the pressure from the water is greater than the weight of the interior constructions.
The surface at the bottom, with a 50cm layer of gravel, should withstand the water pressure.

Remains the risk that the liner is or becomes leaky.
We reduced this risk by using two separate liners. Each seam was checked by another worker. In the unlikely case of leakage we installed several layers of fleece, each on its own being sufficient to stop a leak. The clay powder spread over it would be washed to the leak and stop the fleece up.In the unlikely case that he self-repair mechanism should be insufficient we installed pipes through which we can inject clay minerals between the lower and upper liner and the fleece, so as to stop up any leakage.
Both layers of liner were covered with clay powder that will be washed to the leak and stop it up in case of leakage.

The greatest hazard during the construction phase was posed by quickly rising ground water. Especially after heavy rains we had to pump several hundred cubic metres per day to prevent the pressure on the liner from becoming too great. Initially is wasn't a problem if the concrete slab would rise slightly on a surface of several thousand square metres. The system could thus retain hundreds of cubic metres. This changed, however, as the construction work proceeded. The weight of the interior constructions limited the area where the concrete slab could rise down to a few hundred square metres - correspondingly the retention capacity and warning period were reduced.
In the final stage the construction stood on 'razors edge' several times. Massive power outages in December 2005 made the situation even worse. Neighbouring areas of the Münsterland were without electricity for days. Here the power cuts were of short duration but they caused the evacuation system to collapse time and again.This caused several cracks to appear in the concrete, which were later concreted over. We wanted to limit water exchange, so that uncovered structural steel in the cracked areas would not corrode too much.
The cracks in the over-stressed concrete slab were covered again so as to only allow oxygen-free 'dead water' to come in contact with the structural steel.

Several problems were recognised and defused in time. Others couldn't be corrected anymore. For example, we could have substantially increased the safety reserves during the construction phase. The remaining risks will probably not cause any problems because the built-in safety reserves should kick in.

'One is always wiser afterwards...' but that is exactly the aim of such projects: You grow with the challenges. And from the customers' point of view it is desirable that new construction techniques be tested at the company rather than at the clients'.

Now, of course, the question arises about what to do with this newly acquired knowledge. We have had several inquiries by now - even from different continents.

We shall see...
The Underwater Park building site: In the foreground the second liner seal is being laid. In the background the upper fleece cover is already finished and structural steel is being installed.