PURINGTON". 
DREDGING. 
185 
older boats were provided with short stackers and tail sluices, which 
resulted in the narrow distribution of all the tailings, but more par- 
ticularly the lines. Under these conditions the stern of the dredge 
was frequently aground, unless kept clear by the use of a sand pump. 
This difficulty has been remedied to a certain degree by the lengthen- 
ing of stackers and tail sluices, but will always be much more serious 
for the pivotal-spud dredge than for the headline dredge. The head- 
line dredge can use the maximum amount of tailing area. For exam- 
ple, in unusual cases, as in dredging a pay streak of defined boundaries, 
tailings can be deposited on barren ground outside of the cut. To 
accomplish the same result with a pivotal-spud dredge numerous 
changes of position would have to be made, an expedient likely to 
result in much loss of time and 
some loss of ground because of the 
difficulty of relocating the pivotal 
points. The wasting of unworked 
ground bj 7 the faulty discharge of 
tailings is well illustrated by fig. 
37. In this the distance from the 
pivotal point to the lower end of 
bucket ladder excavating at the 
maximum depth, called the digging 
length, is 90 feet. Here P x and 
P y represent the distances from 
the pivotal point to the point of 
discharge of the stacking ladder 
and are called "tailing lengths." 
Here the discharge is at points 
marked z and z' ', which are, respec- 
tively, only 18 feet and 5 feet from 
boundaries of cut, whereas they 
should be not less than 40 feet 
awa}^, if the dredge be digging in 
ground 20 feet deep. It also shows 
that in some methods of manipulation too long a stacker is detrimental. 
This illustration is taken from two dredges now in operation. 
In tig. 38 a method in general use is shown to prevent the covering 
with tailings of unexcavated material adjacent to cut, To bring this 
about it is necessary to move the dredge from P to P' as often as the 
> distances from C and C to x, while moving ahead in excavating arc 
A, B, C, and in the advancing of tailings in arc x, y, z, become so short 
as to make it difficult, as ,/•' approaches P, and x approaches F, to 
move to and dig in the other half of the cut. The changing of the 
! dredge from one half to the other half of the cut is generally made 
\ every few days and consumes considerable time. There is, moreover, 
Spud digging with tailings piled too 
near unworked ground. 
