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

64 ENGINEERING PROBLEMS OF PANAMA CANAL. with oxidation and they will crumble a little wherever ground water can loach out their soluble salts. As time goes on, there will also be some slight adjustive movements, created by the new conditions of rock pressure. All of these movements will be different, as the character of the rock changes from place to place. The erosion from wash of steamers may probably be best handled by allowing it to take its course and by dredging out the material annually accumulating from that cause. SOME SLIDES OUTSIDE OF THE CANAL ZONE. SLIDE NEAR PORTLAND. OREG. Ono of the most notable slides outside of the Canal Zone of which the writer lias knowledge is that at Portland, Oreg., described by Clarke.« Clarke’s paper sets forth the difficulties of the Portland water department with sliding ground on the face of a clay slope of considerable extent. It seems that two reservoirs wore built on this slope and were lined with concrete. Cracks in the concrete at first puzzled the engineers, but were finally referred to slow, sliding movements of a large area of ground. Many boro holes and several shafts were sunk to ascertain the cause and nature of the motion. Prom these it was found that an enormous mass of earthy material 50 to 110 feet thick was slowly creeping down the hill. The mass was moving on certain moist, slippery planes, or shear zones, close to bedrock, or else on slightly sloping beds of dense slippery clay, which was somewhat impermeable to ground water. When these sliding planes were intersected by boro holes and shafts they gave ofl largo quantities of ground water. This showed that the ground water was checked in its descent by the more dense material, and that it then tended to flow over the surface of the relatively imper- meable layers, thus creating a slippery, somewhat weakened zone along which sliding developed. The rate of the motion was exceed- ingly slow, and it varied directly as the rainfall, as is shown in the following table: % Relation of slide near Portland, Oreg., to rainfall b 'rime of observation. Total rainfall. Total movement. Average maximum movement per month. Minimum. Maximum. December, 1895, to May, 1896. June, 1896, to November, 1896. December, 1890, to May, 1897.. June, 1897, to November, 1897. December, 1897, to May, 1898.. ..................... June, 1898, to November, 1898. Inches. 27.58 18.08 21.74 18.69 25.33 13.12 .03 . 05 .00 Feet. 1.09 .25 Feet. 1.30 .45 .84 .11 .15 .03 Feet. 0.22 .08 .15 .02 .03 .01 “ (;1,i'rk0’D-D-’A Phenomenil1 landslide: Trans. Ann. Soc. Civ. Eng., vol. 53, December, 1904, pp. slid ! h*3 rea^nSs rePresent M,e minimum average movement at 8 different points along the center of the