Barriers Against Lava — MACDONALD 
275 
Fig. 9- A, Diversion barrier consisting of a series of 
short segments set en echelon. B, Manner in which one 
segment might be extended to catch a spill-over from 
the previous segment. 
indicates that this is not an impossibility. It is, 
however, less desirable than the construction 
of a barrier well in advance of the need, be- 
cause work done under such urgent circum- 
stances is likely to be less well done. Time 
may not be sufficient to finish the job prop- 
erly, and it is possible that a flow such as that 
of 1859 might descend the mountain so 
rapidly that the barrier could not be built at 
all. At any rate, if such emergency construction 
is to be relied upon, plans should be carefully 
prepared and a route for the barrier chosen, 
so that work can be started without delay or 
uncertainty when the need arises, and the bar- 
rier can be placed properly for maximum effi- 
ciency. Construction of the barrier should be 
started at its upper end, at a point determined 
by the course being followed by the flow, and 
work should progress down slope ahead of 
the flow. 
Also, it has been suggested that a network 
of roads spaced about a mile apart in the area 
of proposed barrier construction be prepared 
in advance and kept clear and trafficable; but 
such a network appears unnecessary. Instead, 
a truck trail might be opened by bulldozers 
along the route of the barrier itself, both to 
serve as an access route and to mark clearly 
the line along which the barrier should be 
built when the emergency arises. Even if con- 
struction of the barrier is deferred, the route it 
is to follow should be marked as soon as pos- 
sible so that under emergency conditions con- 
struction can proceed with minimum delay 
and along the correct line. 
Present Views on Construction Methods. De- 
velopments in construction machinery, and 
experience with barrier construction and op- 
eration during the 1955 eruption, have made 
it apparent that barriers can be built much 
faster and cheaper than previously believed. 
Actually, the barrier need consist only of an 
elongated heap of rubble, obtained locally 
and pushed into place by bulldozers. Rock 
fragments should predominate, but soil and 
plant debris, even large tree trunks, may be 
incorporated. The use of excessive amounts of 
vegetable materials probably should be avoid- 
ed in a barrier built in advance of the erup- 
tion, because such material will eventually rot 
away and allow the heap of rubble to slump, 
possibly requiring repair of the barrier. 
There is no need of maintaining side slopes 
of 45°, or of careful dressing or smoothing of 
the slopes. Loose material pushed up to the 
required height will settle into equilibrium 
slopes probably between 30° and 40° from the 
horizontal. Such slopes are wholly saitsfactory 
so far as performance of the barrier is con- 
cerned. For convenience in construction, it 
may be desirable to build the uphill slope 
somewhat flatter, so that the bulldozers can 
convey their loads to the top of the barrier 
more easily. 
The material for construction should be 
obtained entirely on the upslope side of the 
barrier. This has the advantage of somewhat 
deepening the channel created on the uphill 
side, for any given height of wall; and just as 
important, of clearing a wide swath (at least 
