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

SLIDES. 57 unyielding buttresses against slides in a section that would otherwise be most susceptible to them. Of course rock loosened by jointing, due in part to weathering and in part to former heavy blasting, from time to time sloughed off from these hard rock masses, but on the whole they have stood solid and immovable. YARDAGE OF THE CHIEF SLIDES CAUSED BY STRUCTURAL BREAKS. Th© chief slides due to structural breaks have taken place in that part of Culebra Cut lying between Gold Hill and Empire Bridge. On the west side in this section of the cut sliding began in October, 1907, and extended so that in all over 70 acres of material moved or was seriously cracked. This movement necessitated tho removal of over 11,000,000 cubic yards moro than was comprised in tho first estimates. This amount, of course, included the material removed from tlio upper part of the slopes in order to make them less steep and more stable. Sliding began on the east side in this section of the cut about January, 1907. In all some 55 acres of land surface has been in motion here since then, adding close to 8,000,000 cubic yards to tho first estimates. The relieving process on this part of Culebra Cut is at this dato (January, 1914) not yet quite complete, and a considerable amount still remains to be moved before perfect stability of slopes will prevail. Tho structural-break type of slide is therefore responsible for the movement of about 19,000,000 cubic yards of material over and above that included in the first estimates. PREVIOUS STATEMENT REGARDING STABILITY OF HIGHEST PARTS OF CULEBRA CUT. Before the subject of the structural-break typo of slide is left, it might be well to set forth the salient features of a memorandum re- garding the stability of tlie places of maximum height ol slope in the Culebra Cut, especially at Gold Hill and Contractors Hill. The mem- orandum was prepared by the author in tlio fall of 1911 in response to a more or less current opinion, to the effect that these hills would, because of their groat height and steepness, deform their bases and crush down into Culebra Cut as great structural-break slides. An examination of the geologic conditions showed that such would not bo the case, as mentioned in the excerpts that follow. Three sections are shown on Plate XV, namely: A-B, Gold Hill, through Contrac- tors Hill, to the lava slopes beyond the Panama Railroad; A-C, Gold Hill to Mount Zion Reservoir; and D-E, from a point northwest of Col. Hodges’s house to Contrac- tors Hill. These sections are drawn to depths of about 200 feet below sea level. The rocks shown at that depth are known to occur there from the fact that they outcrop at certain angles on both sides of Gold and Contractors Hills, and a projection of both angles and of the probable curvature of the beds locates them at about the depth indicated. That stronger limy sandstone rocks occur below the green clays in the vicinity of Contractors Hill is certain; that they occur at the exact depth shown on the cross section is not certain, for they may be, say, 25 feet higher or lower than the