MEMORANDUM BY MR. JOHN F. WALLACE ACCOMPANYING DIAGRAMS ILLUSTRATING TENTATIVE METHOD OF CULEBRA EXCAVATION.


 Herewith are seven diagrams illustrating a tentative method of working the mile in which the maximum excavation exists at Culebra:
 Diagram 1 is an average maximum cross section and shows in numbers and colors the different phases of steam-shovel progress.
 Diagram 2 shows the "A" and "B" sections at kilometer 54.74 on the line of the maximum cross section above elevation 185 with the different phases in colors and numbers.
 Diagram 3 is a progress diagram showing the relative positions, horizontally, of the different steam shovels which may be installed at elevation 185 and below. It also shows the time of entering and the completion of each phase of the work.
 These diagrams are not put forward as an exact plan to be followed, but simply to illustrate the process of devising such a workable plan as the engineer in charge may desire, and should be regarded only as a tentative basic study.
 The sections shown on diagram 2, and also that part of diagram 1 above elevation 185, as indicated by the letter "C," maintain the form shown on the exhibits only for a portion of the section. That part, however, of the average maximum section (diagram 1) below elevation 185, will remain throughout the entire length of the mile section substantially as shown by the exhibit.
 The phases above elevation 185, being masses "A," "B," and "C diagramss 1 and 2), can and should be removed before the excavation of the central portion of the average maximum section below elevation 185, as they progress transversely to such an extent as to cause no interference, and they have therefore been given a separate system of numbers.
 In accordance with this scheme it is presumed that the sections designated by the same letter or number should be removed approximately simultaneously on each half-mile length, and that the phases designated by the low numbers should of course be removed before commencing on the sections designated by the higher numbers.
 It will probably not be found necessary to excavate back to a 1 to 1 slope, as shown on the diagram (diagram 2), for that section west of the center line and above elevation 185. This section is intended to show the maximum amount which, in the opinion of the writer, will require removal.
 It is presumed that rock at the greatest depth of the central excavation will stand at a much steeper slope than shown in the diagrams. If this is the case, new slopes which will diminish the amount of excavation may be introduced at such points as the engineer in charge may deem advisable.
 In the memorandum of quantities which has been furnished the writer, he notes that on the assumption of a bottom width of 200 feet at an elevation of 40 feet below sea level, with berms on each side of 50 feet and side slopes of an average of 1 to I from that point to the top of the excavation, there exists approximately 1,200,000 cubic yards of clay above an average elevation of 290 feet above sea level. This is the clay formation that heretofore furnished the material for slides on the east side of the excavation and north of the maximum cutting at Culebra, which the writer commenced to remove during the dry season of 1904-5 while he occupied the position of chief engineer. The remainder of this material can be removed during the next two dry seasons at least, completing this portion of the work by April 1, 1907, without interference with the general scheme of excavation and far enough in advance thereof to remove any danger of slides from this class of material.
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