Excavated material is removed from the working chamber in buckets through small shafts with special airlocks near the top. The buckets are usually operated by hoisting engines outside the shaft, but sometimes compressed-air cylinders in the shaft are used for the purpose. In the caissons for the Brooklyn Bridge foundations, an open shaft extended through the caisson into a sump below the bottom of the Nvorking chamber, the sump being filled Nvith water to a height sufficient to balance the air pressure, the material being removed by dredging through this shaft, and thrown into the sump by the men in the working chamber.
The blow-out or sand-lift method may he used in the removal of sand or mud. It consists in blowing the material through a pipe by the use of the air pressure in the working chamber. An open pipe 4 or 5 inches in diameter leads upward from the working chamber Nvith a valve near its lower end. The material is heaped about the lower end of the pipe and the valve opened, thus blowing the mud and sand out through the pipe—a method that has been found quite satis factory in many instances. The pipe wears rapidly on account of the high velocity of the sand passing through it, and it is sometimes difficult to prevent fluctuations in the air pressure in the working chamber due to the amount of air suddenly withdrawn.
The mud pump is used for driving the sand and mud upward through a pipe by means of a stream of water under pressure. This method was first used by Mr. Eads in the caissons for the St. houis arch bridge. The suction of the pump is placed in a sump which is kept filled with water at the bottom of the working chamber, and the material to he removed is shoveled into the sump by the men.
208. Physiological Effects of Compressed Air.—The depths below water surface to which the pneumatic method may be employed is dependent upon the ability of men to work in compressed air. Ex perience has shown that under careful management, men in good physical condition may safely he subjected to an air pressure of about 45 or 50 pounds above atmospheric pressure, and work has been successfully carried out in several instances at maximum depths of 110 to 115 feet below water surface. Very careful attention to the physical condition of the men and to the methods used in entering and leaving the compressed air are necessary to prevent injurious results.
When the men enter the air lucks and the air pressure is gradually increased, a sensation of giddiness, with pain in the ears and oppres sive heat is felt. When equilibrium between the air pressures outside
and inside the body has been reached, a feeling of exhilaration results while breathing the more dense air. Labor in the compressed air is more exhausting than in the outside air, and is carried on in shorter shifts. As the pressure is reduced, on leaving the caisson, a sensation of intense cold is experienced, accompanied by an itching feeling under the skin. Warm clothing is necessary, and it is customary to serve hot coffee to the men as they leave the locks. These are the usual and normal sensations experienced by those working in compressed air. The effects are greater the first time the air is encountered, and the unpleasant sensations are gradually eliminated as experience teaches the proper method of meeting them.
Caisson disease is a malady which sometimes results from working in compressed air and develops severe pains in the joints, resembling rheumatism, causing the patient to double up, and is commonly known as the "bends." It is experienced only after returning to atmospheric pressure, and is sometimes relieved by returning to the compressed air and coming out again very slowly, medical locks being sometimes provided for this purpose. In many instances the patient is partially paralyzed, and when the attack is severe a long time may be required for recovery. In the most serious eases, congestion of the brain and sometimes death may result.
Much has been learned through experience concerning the methods of preventing and treating caisson disease since the pneumatic proc ess has been in use. In sinking the caissons of the St. Louis bridge, 119 eases of caisson disease developed and 14 deaths occurred. The better control in later work has largely eliminated this danger, but failure to exercise sufficient care, or unforeseen contingencies, still frequently cause trouble from this source.
The rate of decompression in coming out of the compressed air is a matter of importance, and the time allowed is not usually sufficient, according to the opinions of most medical authorities, to insure safety. The length of working shift should be reduced as the pressure increases The proper ventilation of the working chamber is of greater impor tance than for men working at atmospheric pressure, and arrange ments must be made for frequent changes of air.