Mechanics: The Science of Machinery

by A. Russell Bond

THE CAISSON DISEASE

Compression is rather annoying to a man who has not experienced the sensation, but the principal danger comes in decompression, particularly after a person has been in the working chamber for a long time. When breathing compressed air much larger volumes of oxygen are taken into the lungs at each breath than in the ordinary atmosphere, and one feels decidedly exhilarated by this unusual supply of stimulating oxygen. But with the oxygen large volumes of nitrogen are taken into the system as well. The nitrogen permeates the blood while the oxygen is consumed and exhaled in the form of carbon dioxide. The longer a man is exposed to the pressure the more nitrogen does he absorb. When the pressure is released suddenly the nitrogen begins to froth, just as a bottle of soda water does when the stopper is removed. The nitrogen bubbles in the small veins stop the circulation and produce the dreaded “caisson disease” which makes itself felt in the form of severe cramps and excruciating pains. Not infrequently this disease ends in death. By reducing the pressure gradually the nitrogen is enabled to pass off completely without bubbling or frothing, just as it is possible to let out the gas from a soda bottle without frothing by permitting it to escape through a pin hole in the stopper. The surest cure for a victim of the caisson disease is to put him immediately into what is known as a “hospital tank” and build up the pressure in the tank equal to that which he has just been subjected to in the caisson, after which the air in the tank is let out so slowly that there is plenty of time for the nitrogen to pass off without forming bubbles.

A pressure of forty-five pounds per square inch above that of the atmosphere is considered a severe pressure for excavation work. But work has been carried on in pressures up to fifty-two pounds, corresponding to a depth of 120 feet below water level.

Now that the cause of the caisson disease is understood formulas have been worked out to insure proper decompression. On one occasion a diver descended 306 feet into the ocean where the water pressure was 133 pounds on every square inch of his body. This meant that the air which was pumped down to him had to be compressed to the same pressure. The diver actually remained at the bottom only a very short time, but it took two hours and three-quarters to bring him to the surface. He came up half the distance very quickly and then had to rest on the bottom rung of a Jacob’s ladder. The rungs on this ladder were ten feet apart and he was instructed to rest on each rung a certain specified time. When he was ten feet below the surface he had to wait three-quarters of an hour for the last trace of nitrogen absorbed by his blood to pass off.

BORING TUNNELS THROUGH RIVER BEDS

It is comparatively simple to sink a vertical shaft into water-bearing soil, but a horizontal shaft involves serious difficulties. The action of a diving bell is easily illustrated by inverting a tumbler and pressing it down into a basin of water. The air trapped in the tumbler will keep the upper part of the glass dry, and by inserting a tube in the tumbler it is possible to fill the tumbler so full of compressed air as to drive out all the water. This is virtually what is done in the caisson; but when excavating a horizontal bore, the caisson must be turned on its side. Turn the tumbler on its side and it is impossible to keep the water out of it, no matter how much air we may blow into it. The reason for this is that the pressure on the open end of the tumbler is not uniform. At the bottom, where the water is deeper, it will be greater than at the top. If air is pumped in to equalize the water pressure at the upper edge of the glass, it will not prevent water from flowing in at the bottom; and if it be equal to that at the bottom, the water pressure at the top cannot hold the air in and keep it from pouring out.

FIG. 44.—SECTIONAL VIEW OF A TUNNEL SHIELD

Fortunately most of the soil through which a subaqueous tunnel is driven is not very fluid. It is either sticky, as in clay, or sluggish enough to prevent the water from flowing in rapidly. If there is enough cover of silt or earth above the tunnel bore, it will help to hold the air in the tunnel. When the bore comes very close to the surface of the bed of the stream that is being tunneled, loads of clay are dumped along the line of the tunnel to provide the requisite cover. In tunnel boring a shield is used which is the equivalent of the caisson in vertical boring. The shield is a cylindrical box with a diaphragm across it corresponding to the deck of the caisson. (See Figure 44.) In front of the diaphragm there is a small working chamber which is protected above by an extension of the shield known as an apron. In the diaphragm there are a number of doors at different levels, which may be closed in case of danger. If work is proceeding near the top of the shield, the upper doors are opened and the pressure is regulated to equal the water pressure at that level. If the work is carried on near the bottom of the shield, the upper doors are closed and only the lower doors are open, and the pressure is increased to equal the water pressure at that point. Sometimes the material is of such a nature that the men can safely pass out of the doors into the working chamber outside, but more often it is possible to work only within a limited area immediately in front of the doors.

When the material is very soft it is often unnecessary to do any actual excavation by hand in front of the diaphragm. The shield is merely pushed forward through the mud or silt and the doors are opened to let the material flow in through them. Workmen dig out this mud and it is hauled out of the tunnel. Whenever bowlders are encountered it is necessary for the men to work outside of the diaphragm to chip away the rocks with compressed-air drills, or else bore them and blast them with small charges of dynamite.