Page 103 - Moving the Earth_ The Workbook of Excavation

SURVEYS AND MEASUREMENTS

SURVEYS AND MEASUREMENTS 2.53

to carry that load in material that weighs 3,000 pounds to the loose yard. Lighter materials can be
heaped, heavier materials should not fill the body completely. Bodies can be purchased in special
sizes for regular handling of material of very different weights.
Capacity Tests. When a capacity test is to be run, the bank is trimmed and measured and a num-
ber of approximately equal loads are taken and counted. The bank is then trimmed off and mea-
sured again, and the difference calculated in yards. Then
volume removed from bank
Truck load
number of truck loads taken
Such tests should be run on the largest practical volume for best accuracy. Minimum amount
would be about 200 yards. Counting of excavator bucket loads during the test will supply con-
tainer and output data for it also.
Scraper Bowl. Scraper bowls are irregular in shape and in height of rim. Manufacturers’ rating
on struck load may be accepted if known. If not, inside measurements are taken and capacity is
worked out, making allowance for curves and irregularities.
Scrapers are usually rated for heaps with 1-to-1 slopes, 3 times as steep as the S.A.E. standard.
Dirt may stand a little more steeply above a scraper because of the way it is crowded up from the
bottom, but there is no justification for assuming that it can stand on such a slope, either at the
moment of loading or during the shaking and vibration of a haul. Also, such slopes are never car-
ried up to peaks.
A good rule of thumb is to take the scraper’s maximum heap at one-half the manufacturer’s
rating. That is, a scraper rated at 20 yards struck and 28 yards heaped can be assumed to have a
heaped capacity of up to 24 loose yards, unless there are definite indications to the contrary. If
swell factor were .80, the heaped load would contain 19.2 bank yards. Extensive checking by the
U.S. Bureau of Public Roads indicates that scraper loads in bank yards seldom equal their rated
struck capacity, so this works out well enough.
Push loading compacts material, so for the same heap a scraper carries fewer bank yards if it
is loaded from the top.
Shovel Bucket. Shovel buckets are rated at struck measure, in yards and fractions of yards. Size
can be checked fairly accurately by measuring and multiplying length, width, and height, as in the
case of the truck body. Taper, curves, and extension of the lip can be used to increase the result
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to the nearest ⁄ 4 yard. Heaps, when digging conditions permit, range between one-eighth and one-
fourth of struck capacity.
Hoe buckets are rated and measured in the same manner. Heaps are usually small or absent.
Width is an important factor in loading efficiency. Deep narrow buckets may average loads less
than one-third of rated capacity.
The open front of the dragline bucket makes it difficult to measure accurately, as the capacity is
determined largely by the steepness of slope of the load. A chunky mud that can bulge over the teeth
and pile steeply over the top may have almost as much in the heap as there is in the struck load.
Clamshell buckets may be rated water level with the end plates, struck measure along the tops
of the sides, or heaped. Another rating that may be important is that of deck area—the length and
breadth of the space on a level floor occupied by the bucket when fully open. The same space is
required between braces in a trench.
Loader Bucket. Front-end loader buckets are rated at struck capacity across the sidewalls. If the
bucket can be tipped back 20 degrees or more at ground level, and digging conditions and tractor
stability permit, it can carry a very large heap, perhaps 50 percent of struck capacity. A bucket
that will not tip back seldom can take one-eighth more than struck capacity. However, in hard dig-
ging few buckets can pick up even a struck load, and tractor stability may not permit carrying an
overload. On the average these buckets are underloaded by loose measurement more often than
they are heaped.