Document 51: "Handling Earth by Belts"
W. F. Way, “Handling Earth by Belts,” Engineering News-Record 105:21 (27 November 1930): 838-40.
Handling Earth by Belts
Experience in Denny Hill Cut, Seattle, Wash.,
Shows That Clean Discharge Results If
Conveyors and Material Are Wet
By W. F. Way
Construction Engineer, Seattle, Wash.
Absence of noise and nuisance are the outstanding features of the Denny Hill regrade project in Seattle, Wash., which involves taking 4,200,000 cu.yd. of material from the center of the city and moving it across seven main thoroughfares without any delay to traffic. The smooth quietness of the operation as a whole is in remarkable contrast to such nerve-racking noises as blasting, steam-shovel exhausts, the screeching of muck-car wheels on curves and the puffing of steam locomotives. There has been no blasting on the Denny Hill job, the electric shovels operate very quietly, and no muck-cars or trucks are used ; the spoil is moved on motor-operated belts which slip along on ball-bearing rollers in almost complete silence. The results of an analytical study of methods and progress on this project are given in the following.
The present Denny Hill operation is in the final stage of a major regrading project whose first part was completed some twenty years ago. . . .
Material—The question of whether to use explosives to loosen up the ground for the shovels was given careful consideration. It was believed from experience with other excavation near by that this area might be underlain by a stratum of quicksand which would readily transmit to nearby structures any vibrations resulting from deep shooting. The large fragments resulting from deep shooting would be difficult to pass through the grizzlies, and on the whole there was some doubt about the fundamental question of just how much benefit there would be in blasting. It was decided, therefore, not to use powder in connection with the shovel work.
The material handled consists of glacial detritus in which there are frequent variations. Hardpan, cemented sand and gravel, blue clay, loam and sand have been encountered. The major portion to date has been hardpan and blue clay. Where the formation is cemented sand and grave the digging has been very difficult for the shovels, as witness the fact that the shovel teeth have had to be renewed every four to five hours. This is in soil the moisture content of which ranges from 5 to 12 per cent and which gains about 45 per cent bulk in going from bank to loose measurement.
Effect of Weather—Contrary to expectations, rainy weather has had little effect upon the movement of these materials over the belts. In fact, the material leaves the belt better when wet than when dry; even wet yellow clay, the stickiest of all soils encountered, will go over the belt and leave it clean, although this same material when moist will so completely choke up the shovel dipper that it is necessary frequently to clean the dipper with hand shovels.
The point in the earth-moving system where the greatest tendency to choke occurs is the main loading chute. Experience on the Denny Hill job definitely has indicated that chutes for conveying the material even for short distances are undesirable. The large chute carrying the material from the portable conveyors to the loading end of the main conveyor is steel-lined and is set on a slope of 60 deg. With the horizontal. Soil pounding down on this smooth surface builds up to such an extent that it keeps one workman busy clearing the chute. When the soil is wet and sticky it is particularly hard to keep moving on this chute. The only other chute on the job is the one between the end of the belt line and the scow. This is very steep, but even so, it has given some trouble. Bunkers for storing the material even for a short period would be very troublesome. The material must be kept moving.
A very notable result of rainy weather has been the longer time required to shift the portable conveyor units. In fair weather a 250-ft. Conveyor section is shifted easily in one hour or less, while on a muddy field with the same crew, this time is lengthened to two hours solely by the reduced efficiency of the men when required to work in the rain on a slippery muddy surface. . . .
Each shovel crew consists of a shovel runner and a ground man. All portable conveyors are moved by a crew of fifteen men with the assistance of a tractor and the tractor crew of five men. These twenty men do all the shifting for the three portable conveyor systems. The work is carried on at a rate of three eight-hour shifts per week, six days per week. One operator is assigned to each 250-ft. Portable conveyor section, thus making up a grand total of 33 men per shift or 99 men for the three daily shifts. In addition to these men there is a mechanical crew of six men which works only during the day shift.