It is believed that the difference between the progress to be expected by the two methods employed on the Wachusett and Roosevelt dams, however startling it may appear at first sight, is correctly represented by the figures in the foregoing table. Under whatever method the masonry is built there are a variety of changing conditions between the start and finish of a masonry dam that affect very materially the number of cu. yd. it is possible for a derrick to build per hour.
In the bottom of the foundation, when starting the masonry, there is some slight delay on account of some final refinements in cleaning and preparing the rock which must be performed coincident with actual masonry construction.
The delay due to the handling of the water and proper insertion of grout pipes as described in the chapter on starting the masonry.
The plant is new and untried, some details of the method have yet to be worked out, and the men in all the various capacities even if experienced and able cannot at once take the stride that should develop after a period of team work.
Adjacent blasting or other operations connected with the excavation and preparation of the remainder of the foundation may have an effect on the efficiency of the derricks on masonry. Most certainly will this be the case if removal of material is allowed to encroach unduly on the time of the plant engaged in supplying the masonry derricks.
Exigencies in the way of limited space available may make it impossible so to place the derricks that they will work to best advantage and without interfering with each other.
The maximum of efficiency is reached when the bottom is entirely covered and brought up to such a height as to be above water or any delay from handling of water; when the work of preparing the foundations at each end has been reduced to a minimum or so prosecuted as not to be any interference; when an intelligent plan can be developed for placing the derricks properly, and an intelligent system can be adopted for properly coordinating their necessary changes of position; and lastly when the force has had its training in team work, and any defects in plant operation have been corrected. This maximum efficiency will be maintained without noticeable diminution up to such an elevation that the dam is about 60 ft. in width; at this point two causes (which indeed had begun to operate before) now begin noticeably to reduce yardage and to do so more seriously with every step to the top of the dam.
The face work is a larger percentage of the total work, and as it requires more time per cu. yd. than the interior work, it has a retarding effect on the total yardage of the derrick. The retardation increases up to the top of the dam, and especially on arriving at any special work designed for architectural effect such as pilasters, cornice and coping courses, parapet walls, etc.
As the area on which the derrick has to work has been cut down, the yardage is affected. A derrick at say 120 ft. width of wall can work in practically three-quarters of a circle of radius equal to the length of the boom. This may be cut down greatly without seriously affecting the efficiency but at about 50 ft. width it becomes necessary to turn the derrick so that one side of the A-frame coincides with the upstream face of the masonry, and the area available is about one-quarter of a circle. Still higher, the end of the other side of the A-frame must be supported outside of the masonry lines, and the reduced area under the boom must still afford room for temporary storage of materials until the wall gets so thin that landing stages have to be provided outside on the downstream face. (See Plate VIII, Fig. B.) Handling to and from such stages is a process conducive to very circumspect movements. The men are either in each other's way, or else the force is reduced somewhat, and in such circumstances all operations are conducted rather gingerly.
In addition to the above general conditions, affecting the efficiency of all the derricks at various stages of the construction of the dam, are the following particular conditions which affect somewhat the efficiency of a derrick almost from day to day: Whenever in the progress of the work a derrick must be moved, it is almost invariably the case that it is moved from a depression to a pinnacle, namely, from a place where it has been building above its level to a place where it must begin building below its level. Assume that a derrick has just been moved to a new setting, it must begin building upon a level 12 ft. to 15 ft. below the level upon which it sets. The masonry in the bottom must be thoroughly cleaned as must also the masonry on the racks as the depression is filled. When the landing and storage space are upon a level with the derrick, it is necessary to climb up and down the rack to handle the loads. The foreman may have to change position in order that his signals may be visible to the engineer, and they may even have to be relayed. The engineer often cannot see the load being handled or the stone being set. Again, because of the longer fall line the load does not follow the boom quite as readily and none of the motions are as precise. While these defects are slight individually, their combination has a bad effect on the output of the derrick. As a partial offset there is the fact that in filling a depression less face work is necessary. This means that while the work on the up- and downstream faces of the dam may be the same, the remaining two sides are against masonry already in, which is easier than building a rack.
The maximum output of the derrick is at the time when it is working between say three courses below and one course above the level upon which it is setting. Above this level certain causes operate to reduce efficiency; i.e., reduction of working area, climbing up and down the rack and possible interference with signals. Of course the fall line is shorter, and the engineer can nearly always see the load. There is a delay from the fact that there is more face work, i.e., referring to the transverse faces or racks which in this operation must be built, while in the previous one they were built against. While the rack is far from being face work in the sense of the up- or downstream faces of the dam, it still involves some selection of the rubble stone and considerable attention to their placing so that the rack may present a proper bond to the subsequent masonry. All the vertical joints between the rack stones must also be carefully filled and smoothed up on the face as it is to be left for some time and must be left with a workman-like appearance.
The foregoing are the unavoidable causes which affect output. The avoidable causes are: capacity of remainder of plant, which should be large enough to keep the derricks on the masonry fully supplied; amount of work trimming and cleaning the stone, which should be done in the quarry as far as possible; conduct of adjacent work such as excavation for foundations, etc.