ROCK DRILLS AT WORK IN THE CUT.
All that part of the section below this line (+160) remained to be excavated.
Most of the rock above this line has been removed, but not all, for the final width is not, of course, reached at any level until the central portion has been excavated below that level.
The level of the original rock line shown in this section was +275, i.e., 235 above canal bottom, so that the photographs show excavation of 115 feet of rock. There was, however, soil above the hard basaltic rock, of varying thickness—removed to the slope 1:2 as shown on the section. The highest original surface of the soil on the centre line of the Canal (between Golden Hill and Silver Hill at Culebra) was +312 feet,[17] so that the photographs in which Golden Hill appears show a total excavation of 152 feet along the centre line. As this line passed along a saddle between the two hills, the original surface at the sides was considerably higher, so that the total height shown in the photographs from the bottom of the cut to the highest berm, or ledge, on Golden Hill is considerably more than 152 feet.
[17] The profile at end of volume shows the stage of excavation when the height here had been reduced to +210.
The bottom of the Canal will be 272 feet below the original saddle, and its depth below this berm, which is seen on the photograph, is considerably more. Thus will the gorge appear when the excavation is finished and before the water is allowed to flow in. When full, the surface of the water will be 227 feet below the original saddle, and the passenger on a vessel will gaze upon the scarped banks of a somewhat greater height than this.
For a tide-level canal, not only would the depth be 85 feet greater, but, as the slope could not be made steeper, the width of the whole cutting would be correspondingly increased.
With reference to the slope of the sides, it is important to note that it has not been found practicable to adhere always to the proposed section, which has to be made flatter, thus considerably increasing the amount of excavation required. The behaviour of living rock is not susceptible of the precise specification which can be applied to quarried stone on the one hand or loose gravel on the other. Mechanically it is complex, both on account of its structure and of the rôle which water plays in its economy. In the case of the Culebra rock, the volcanic dykes by which it is traversed have altered the nature of the rock in their vicinity, and the part played by water is considerable, owing to the wetness of the climate. Moreover, the rock does not remain wholly unchanged when exposed to air, but deteriorates by "weathering," a chemical and physical process which proceeds much faster in an equatorial climate than in the temperate zones. The climate, however, has a compensating action, in so far as the rapid growth of vegetation soon clothes and protects the scarped slopes, thus acting as a "revetment."