FIRE UNDER A RIVER.

New Zealand Herald, Volume LXVII, Issue 20623, 23 July 1930, Page 11

FIRE UNDER A RIVER.

HUNDRED FEET BELOW WATER FLAMES IN BRIDGE AIR LOCK. MEN'S EXCITING EXPERIENCE. One hundred feet below the surface of Hie Brisbane .River recently, workmen in the, air lock of the new Grey Street Bridge found themselves face to face with—fire. Already they were braving the ceaseless dangers of working under high pressure. Now, with a blaze in the wooden props, there was only one thing to do. They made for the surface. for safety. The lock was flooded, and the fire extinguished, but not until after 10 o'clock at night had the fumes cleared sufficiently for work to be reI sumed.

The fire came when the excavations for the bridge had just been completed after months of difficult work. One gang, of six men, had just finished its shift and had entered the decompression chamber of the air lock, about 4.60 p.m. Iho relieving gang of six entered the compression chamber, and later climbed down the ladders toward the bed of the river. 'Hie men, however, had hardly reached bottom when they discovered a fire in the timber used for props in the workings. Smoke, too, was gradually filling the chamber. Tlieio was only one thing to do—climb to safety. The men made for the trapdoor leading to the decompression chamber, and hammered upon it. Luckily, the gang before them was still under decompression. Hastily, the air was forced into the chamber and full compression restored. Then the trapdoor was opened, and the waiting men admitted. After decompression they were able to go into the open. Meanwhile, the fire burned upon the river bed. The only way to combat it was to flood the cavity. This was done, but even with the flames out. work could not be resumed. Heavy fumes still pervaded the lock. The air cocks were opened and ventilation forced through, but though the air stream was going in

at the rate of 1000 cubic feet a minute, the dealing of the atmosphere was slow. Driven Back by the Fumes.

At eight o'clock men essayed to begin work, but were driven back by the fumes. -Again fresh air was forced through. Not until after 10 o'clock, however. did a gang get below and begin to clear away the debris. Timber, 15in. in diameter, they found burned right through. None of the men was injured in the fire and all set to work to clear up in preparation for filling in the core with concrete, which, because of the outbreak "I fire, was somewhat delayed. Nevertheless, the day was a memorable one for the contractor, Mr. M. 11. Hornibrook, and the consulting engineer, Mr. A. E. Harding Frew. The foundations were at last finished. The sinking of the cylinders bad been begun in January last, said Mr. Hornibrook, and had been finished at the rate of one a month. The last of the cylinders is a giant mass. In external diameter it varies from 28ft. at the base to 18ft. at the surface, and has a central cavity 15ft. in diameter. In this cavity the men work at, excavation. Each cylinder is built, as the excavation progresses. The base of it is laid on top of a temporary pier of sand enclosed by inter-locking sheath piles. Then the sand is gradually dug out, and as the base sinks I down, by its own weight, its top is built up again above water-level.

Method of Sinking. Ordinary dredgo excavation, however, lias sufficed only while the sand and the underlying bed -of liver mud was being penetrated; when rock was reached, new metliods had to be evolved. Here, ingenuity won out. The top of the cylinder was sealed airtight, and trapdoors built upon it, leading to two separate chambers. Air was pumped into the sealed cylinder until sufficient pressure was reached to force out the seepage of water from beneath the unfinished foundation. Into one of the surface chambers (he workmen entered. Gradually the air pressure about them was increased, and after 20 minutes, when they had become accustomed io an equivalent pressure, they entered through the trapdoor into the body of the cylinder. Shifts were limited to iwo, of one and a-half hours daily. Health necessitated that. A doctor also was placed permanently in attendance on the works, and an air-lock hospital established. At the end of each shift the workmen entered a decompression chamber where, during a wait of about 45 minutes, air pressure upon them was gradually reduced to normal. This method of cylinder sinking, entirely evolved in Brisbane, has gained notice overseas, and has been adopled for bridge work in America.