Tunnelling
1. Damaged borehole casing
It was a very long tunnel (50 miles), being dug by drill-and-blast, with access from each end and seven access shafts, divided into three Contracts - the middle one had ground cover of over a thousand feet, while the ground cover for the other two Contracts was between 200 and 600 ft, so here, 3 ft dia ventilation shafts and 6" dia cased power supply boreholes were sunk at regular intervals.
At one such borehole, it wasn't found when the tunnel passed - it had diverged from the vertical.
A sliping blast was made to intercept the borehole, whose presence was located by filling it with water and observing seepage.
Unfortunately, the blast had squashed the borehole casing flat, so the cable could not go through.
A tunnel fitter then climbed up into the hole, to cut the casing with a "gas axe" (oxy-acetyline torch).
Unfortunately, he was overcome by the trapped fumes, and slid gently down the ladder unconscious. He wasn't hurt.
2. Lifting chains
It was a Victoria Line working site, in the early 1960s; the shaft had been sunk, and the adit was being dug, at the end of which a chamber would be dug for the digger shield to pass through later. The men were currently grouting the last few rings of lining that they had built.
I was sitting in my site cabin one lunch time, when there was an exposion, and clouds of smoke came pouring out of the shaft. I ran across to find out what had happened. At that moment the bansman appeared through the smoke, riding (as they sometimes did then) on the hook of the crane, covered in grey ash. He alighted, and I asked him to explain.
He had loaded a ton of cement onto a flat car, and had then hitched it onto the crane with a pair of chains ("brothers"; but he had only put on the two chains, one at opposing corner. Up it went, and over the shaft - and at that point it tipped, and the ton of cement slid off the car and down the shaft. The pit-bottom man saw what was happening, and fled inside the tunnel. No-one was hurt - though the inhalation of cement dust can't have done them much good.
3. Road collapse
An urban metro project. The Contractor was driving the tunnel through rock by drill-and-blast. The site investigation before contract had identified where the tunnel would emerge from rock into soft ground. Some metres before this interface, the Contract required that an enlarged chamber be excavated to accommodate an air lock, for in the soft ground compressed air would be used to support the ground.
But the Contractor chose to go "just one more round", and the final blast punched through the rock into the soft ground, which then flowed into the tunnel, causing the road above to collapse - this happened to be one of the busiest streets in the city.
Luckily, it happened very very early on a Saturday morning (night shift), and so it was possible to block off the road before anyone drove into it.
Curiously, the first thing they had to do was to obtain official permission to block off the road ! To "close" the road to traffic. The usual permit is issued when a utlity company wanst to dig a trench, so the actual permit is called a "Road Opening Permit". rather than a "Road Closing Permit".
The Client's senior engineer in charge could not be found. He had come home the night before "the worse for wear" and his wife had refused to let him in. He was eventualy discovered fast asleep on the floor behind his desk.
4. Water table is not what you think
See Item 2 on the "Site Investigation" page.
See also here.
5. Water down a shaft.
The Lochaber scheme collects water from three dams into a tunnel that then feeds a hydro-electric power station to provide power to run an aluminium smelter.
The tunnel also captures water from the streams that it crosses, by way of narrow deep shafts. Each shaft has a concrete structure at the top, with a solid concrete roof, and a grill to prevent anything but water entering the shaft. Correction - water mixed with air.
It was soon discovered that th entrained air tended to form air bubbles, that grew in size until the water flow was stopped. As the depth increased, so the pressure in the air bubble increased, until the bubble would force its way up the chaft, and hit the roof with such force that the concrete roof was broken off. and displaced. As a result, the flow os water down the shaft had to be restricted, which meant an expensive wastage of water.
Eventually a solution was found by diverting the flow so that it entered the shaft circumferentially, forming a vortex, with the air escaping up the centre of the shaft, while the water travelled down spirally against the shaft wall.
