Colonel George W. Goethals
Chief Engineer of the Panama Canal
The Americans continued the work in
progress by the French in the cut through the continental divide, commonly known as the
Culebra Cut, utilizing the French machinery until it could be replaced by more modern
appliances. This is the most formidable part of the enterprise on account of the
magnitude of the cutting and also because of the difficulties attending it, due to the
excessive rainfall and to the varying character of the materials encountered.
The efficient and economical working of the plant requires that provisions be made for the disposition of the large quantities of water that result from the rains. Whatever water is not carried off by the streams enters the cut, either through direct fallover the excavated area or by seepage into it. Proper drainage of the cut is therefore an ever-existing problem, and two distinct phases are presented, viz:
1. To keep out the water of the surrounding country.
2. To rid the excavated area of the water that collects in it.
A system of diversion channels accomplishes the first, and gravity drains and pumps solve the second. The canal line follows the Obispo River, which drains the area from the divide to the Chagres River. It has four principal tributaries, two from the east, the Masambi and the Sardinilla, and two from the west, the Mandinga and the Comacho. These are cared for by two diversion channels.
On the east side of the cut the Obispo diversion has been constructed almost parallel to the canal and carried through a depression in the hills so as to discharge into the Chagres about one mile above the point at which the canal line crosses the river.
To the west of the cut the Comacho diversion carries the waters from Culebra to the Chagres River through the old channel of the Obispo River. through a hill between Haut Obispo and Bas Obispo, which sharply deflects the river, the French had built a tunnel for diverting the flood waters, and this forms a part of the new diversion.
The canal follows the Rio Grande on the southern slope of the divide, and its waters are cared for by a diversion channel constructed by the French. They also constructed a dam across the valley, impounding the waters, and the resulting reservoir supplies the settlements from Culebra to and including Panama. During the wet season the diversion channel carries the overflow from the reservoir.
How the Shovels Work
The French so planned the excavation that after the removal of the peak of the divide and lesser summits they could work a number of excavators simultaneously at several points, so that a succession of benches resulted, lying one above the other, each with the natural surface as the point of beginning. By working in the direction of the length of the cut, the face of the bank gives the longest cutting possible, reduces the number of times the excavator must be hauled back, and secures a satisfactory drainage arrangement, since the cutting is carried up grade on either side of the summit.
The Americans have followed this same method,the only difference being in the character of machinery used. The width of the channel adopted by the French was 74 feet; the present plan is for a channel 300 feet at the bottom, so that the first work undertaken by the Americans was directed to securing the necessary widths for the upper reaches before attempting any increase in depth.
Whatever water entered from rains and seepage was drained from the summit of the cutting by gravity to the Rio Grande on the south and to the Chagres River on the north. As shovels in excess of those required for widening became available, they were put to work to secure increased depth, care being taken to maintain, as far as possible, free, easy, and rapid drainage. Shovels are started at either end and carried towards each other, cutting out at a new summit. These pioneer shovels on the next lower grade make the "pilot cuts," which constitute the new drains and to which water is led by laterals from various parts of the excavated area adjacent. The average grade or slope is about 36 feet per mile. The loading tracks for these shovels are on the level above.
When the "pilot cut" has progressed sufficiently far, its cut or trench becomes the loading track for a second shovel, which is started to widen out the cut already made by the pioneer, and so the work moves forward, the shovels, approaching the summit from either direction in echelon.
In 1904 the summit of the excavation was at Gold Hill and at reference 193 above sea-level. The summit at present is between Empire and Culebra and is at reference 106 above sea-level. the drainage to the south is still by gravity, through the old bed of the Rio Grande to the west of the Pedro Miguel locks. It is expected that before the next wet season the center culvert of the locks will be utilized.
On the north side conditions are now different. The reference of the low-water surface of the Chagres is 43 at the point of its intersection with the center line of the canal. The bottom of the completed canal is at reference 40. A dike separates the cut from the Chagres, but this is overtopped during the high floods. To get rid of the accumulated flood water, 24-inch pipes are laid through the dike, each with a suitable valve, and so arranged that all water above the pipes is carried into the Chagres by gravity after the subsidence of any flood.
Recourse must be had to pumping whatever water remains from the floods below this level, and such as may be collected by drainage from the south; for this purpose a sump has been dug to elevation 32 and pumps installed. It is not possible to estimate the quantity of water that will have to be handled, but three pumps are in place, each capable of discharging 12,000 gallons of water per minute.
Very Troublesome Slides
The greatest difficulty encountered in the excavation is due to slides and breaks, which cause large masses of material to slide or move into the excavated area, closing off the drainage, upsetting steam shovels, and tearing up the tracks. The term "slide" is applied to the movement of the overlying clay upon smooth, sloping surfaces of rock or other material harder than the clay.
"Breaks" occur at points where the underlying rock is of poor quality, intersected by vertical seams or seams sloping toward the canal, and which is unable to bear up the superimposed mass. Generally, the upper surface of the broken portion of the bank remains approximately horizontal, settling nearly vertically. The weight of the broken portion forces up and displaces laterally the material lying directly below it in the bottom or on the berms of the canal. As the material thus forced up is taken away the upper part gradually settles and moves toward the axis of the canal until the entire broken portion is removed.
The greatest slide is at Cucaracha, and gave trouble when the French first began cutting, in 1884, and still continues. Though at first confined to a length of 800 feet, measured along the line of excavation, the slide has extended to include the entire basin south of Gold Hill or for a length of about 3,000 feet. The original slide covered an area of about six acres, but the latest surveys show that it has extended to cover 47 acres.
No Danger From Slides After Canal
There are all told nine "slides" and "breaks" to be reckoned with, and there is nothing to do but to remove all the material embraced within their limits. As the cut is deepened these may be aggravated or others may develop. There is no method known to stop or to prevent them. Usually the first indication received, if there be a forewarning, is lifting or moving of a shovel and tracks.
The cut has therefore developed into the uncertain and experimental feature of the work and its completion will mark the date of finishing the canal. No apprehension is felt because of the slides after the completion of the work. They develop as the depth of the cut increases, and the banks slide or break because of the condition of unstable equilibrium that results from the cutting; when grade is reached equilibrium will be established, and the back pressure of the water will result in greater stability. Whatever slides occur subsequently will be relatively small, and the material can be easily handled by steam shovels on the berms that will be left and by dredges that will be available.
Some idea of the magnitude of the slides can be obtained from the fact that during the fiscal year 1909, of 14,325,876 cubic yards removed, 884,530 cubic yards or 6 per cent, were from slides. For the fiscal year 1910, of 14,921,750 cubic yards that were removed 2,649,000, or 18 per cent, were from slides or breaks that had previously existed or that had developed during the year.
The Material Is All Rock
Except for the slides, which are of earth, the material to be removed is rock, and requires blasting to enable the shovels to handle it expeditiously. The largest part of the drilling is done by churn or well drills, though tripod drills replace them where the others cannot be used to advantage. The drills are operated by compressed air, supplied by three compressor plants, which are connected together by a 10inch pipe line about five miles long, with 6-inch and 4-inch leads running into the cut. The drills operate in batteries of from 4- to 12; the holes, from 15 to 27 feet in depth, are spaced from 6 to 16 feet apart.
The explosive used is dynamite, 45 per cent to 60 per cent nitro-glycerine. Excessive moisture and water in the holes prevent the use of blasting powder. When the holes in any section are ready for blasting, they are "sprung" -- that is, four to six sticks of dynamite are lowered to the bottom and exploded -- thereby forming a chamber for the reception of the charge. The charges vary from 25 to 200 pounds, depending upon the local conditions; the tamping follows, and the explosion is effected by an electric current from one of the lighting plants.
The Earth And Rock From the Culebra Cut
Are Used For The Breakwaters And Embankments
Through the blasted area the steam shovel cuts its way, averaging 34 feet wide at the bottom and 50 feet at the top for the "pilot cuts," which are 8 to 12 feet deep. The widening cuts are about 26 feet wide and from 15 to 24 feet deep.
The best results are secured with the 95-ton shovels, though the 45-ton and 70-ton shovels are also used. the 95-ton shovels have dippers of four and five yard capacities, the former removing rocks containing as much as six cubic yards.
When the rocks are too large to be lifted by the shovel, they are "dobie" blasted, and thus broken to sizes convenient for the dipper. This is done by placing three or more sticks of dynamite on the rock, covering them with mud, and igniting by means of a slow match.
The shovels load the material on dirt trains, consisting of 20 flat cars and from 25 to 35 steel side dumps to a train. The available dump grounds in the vicinity of the cut were utilized to their fullest capacity by the French and during the earlier periods of American control, so that longer hauls are necessary.
The new line of the Panama Railroad, being above the lake level, requires many heavy embankments, which offer suitable places for depositing material. The breakwater from Balboa to Naos Island offers a dump, though requiring an average haul of 11 miles.
As difficulty is experienced in extending this breakwater, additional dump tracks are provided at Balboa, so as not to delay the trains, and land at the inner end of the breakwater is being reclaimed; already 253 acres have been filled in. The interior swamps in the vicinity of Ancon are also to be filled. From 16 to 22 trains of material are sent daily to Gatun, an average haul of 25 miles, and used in building up the toes of the dam or for large rock to place in the concrete. The remainder of the excavated material is wasted on extensive dumps at Miraflores. A large dump ground at Tabernilla was used, but is abandoned for the present.
The Lidgerwood flats are dumped at Miraflores, Balboa, and on the relocated line of the Panama Railroad, where special equipment is kept to handle them, consisting of plows, unloaders, spreaders, and track-shifters. the plow is attached to one end of the train, and the unloader, consisting of a steam-driven drum on which is wound the cable, at the other end. To stretch the cable, the train passes between two uprights to which the cable is attached temporarily, and by moving the train the cable is drawn from the drum to the plow to which the end of the cable is attached. Winding the cable on the drum draws the plow the length of the train, removing the load.
After the material is plowed off, the spreader performs its functions, and, when no longer capable of throwing the material beyond the edge of the dump, the track is shifted by a device patented by W. G. Bierd, formerly general manager of the Panama Railroad, which raises by one motion the track with the ties so as to clear the ground, and by another motion pulls it sidewise. The usual throw is two and a half to three feet, though, if the rails will permit, the track can be thrown as much as nine feet in one throw.
The steel dump-cars require no special or extra appliance for their operation, and can be dumped as easily on curved as on straight track. They also dump to either side.
Work has been in progress on the Culebra Cut since 1880, and during the French control 18,646,000 cubic yards were removed. Between Gatun and Bas Obispo, the northern end of Culebra Cut, the French excavation which is useful to the present project amounted to 2,201,000 cubic yards, or a total in the Central Division* of over 20,000,000 cubic yards. the total estimated amount of material to be excavated from May 4, 1904, in this division was 97,125,018 cubic yards, of which, up to January 1, 1911, 67,692,855 cubic yards have been removed, or 69.7 per cent. It is expected that all the excavation in the lake section will finished by July 1, 1911.
Some idea of the magnitude of the operations may be formed from the fact that this division has within its jurisdiction over 200 miles of 5-foot-gauge track laid, about 55 miles of which are within the side slopes of the Culebra Cut alone.
*The Department of Construction and Engineering is divided into three construction divisions. The Atlantic Division embraces the engineering construction from deep water in the Caribbean Sea to include the Gatun locks and dam; the Central Division extends from Gatun to Pedro Miguel, and the Pacific division from Pedro Miguel to deep water in the Pacific Ocean.
An excerpt from an
article in the National Geographic Magazine, February, 1911
May 2, 1999
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