Some Engineering Problems Of The Panama Canal In Their Relation To Geology And Topography

HEATING OF LOCAL AREAS OF GROUND. io to dull-reddish color. The first hypothesis entertained, in looking toward a solution of the heating, was that possibly the heavy blasting had furnished heat enough to break down the calcium carbonate present to the oxide form, and that ground water and atmospheric moisture reacted on this to slake it and thus probably generate sufficient heat to start the oxidation of the carbonaceous material. This hypothesis was, how- ever, rendered untenable by three lines of evidence: (1) The heating was much more local than the calcium carbonate and the carbon- aceous matter. (2) The heating bore no definite relation to the lime and carbon content of par- ticular beds. (3) Col. Gaillard has observed that in some instances the heating began in the holes some time after they had been drilled, but before the ground had been blasted at all. PYRITE AS A CAUSE OF HEATING. Another line of inquiry was suggested by finding a small amount of pyrite in some of the beds which were heating. It was suspected that this, through its oxidation, was a factor in furnishing the initial heat of the action. In April, 1911, samples of the beds then heating were sent to the chemical laboratory of the United States Geological Survey with request to make qualitative tests for sulphur and other products that might serve, through oxidation, as the mainspring of the action. These tests revealed the presence of sulphuric acid to the amount of 1.92 per cent, also minute crystals of gypsum. This was in confirmation of the hypothesis that pointed to the pyrite present as the substance acted on by atmospheric oxygen to develop the initial heat. NOTEWORTHY EXAMPLE OF HEATING. The most aggravated case of heating so far noted is now going on in Culebra Cut about 350 yards north of the foot of the stairs at the observation tower near Culebra station. The mass of heated ground here is about 500 feet long by 20 feet wide, and the action reaches a depth of perhaps 15 or 20 feet. Blue smoke, which contains a hirfi percentage of sulphur dioxide, issues from vents in the mass, and fragments of wood inserted in these are readily charred and consumed. A small amount of steam may also be detected emanating from local moist spots, but this is mainly due to the vapor- ization of ground water. In the investigation of this heated mass samples were taken, and these were tested qualitively for sulphuric acid and for sulphates of calcium, aluminum, and magnesium. The tests were made by Mr. Jacob, of the hospital laboratory staff at Ancon, and they revealed the presence of all of the above substances, both in the shale and as the white coating on the moist spots and steam vents of the mass. The yellow deposit near the larger vents is sulphur. Sulphuric acid especi- ally was shown to be present in considerable quantity. The origin of the sulphuric acid here was at first a puzzle, because the examination of many samples, with the naked eye and with the microscope, failed to reveal the presence of pyrite. Finally samples of 8 to 10 pounds were taken, ground with water in a large mortar for some minutes, and then concentrated to a few ounces by washing or “panning.” This concentrate showed a high content of pyrite, much of which was so fine that it could scarcely be seen with the naked eye. Under the microscope very small crystals of pyrite were noted; also considerable magnetite, present as black sand, and some subangular to fairly rounded grains of quartz. The mainspring of the action here, then, as in the other instances observed, has undoùbtedly been the oxidation of the pyrite. The reasons why this oxidation has been so rapid and effective seem to be as follows: (b) The finely divided, almost microscopic, character of the pyrite gives maximum surface exposure to atmospheric agencies and greatly promotes oxidation.