(_To face page 80_.)

Screw piles may be used if the ground is suitable, but, if it
is boulder clay or similar material, the best results will
probably be obtained by employing rolled steel joists as piles.




CHAPTER XI.

THE ACTION OF SEA WATER ON CEMENT.


Questions are frequently raised in connection with sea-coast
works as to whether any deleterious effect will result from
using sea-water for mixing the concrete or from using sand and
shingle off the beach; and, further, whether the concrete,
after it is mixed, will withstand the action of the elements,
exposed, as it will be, to air and sea-water, rain, hot sun,
and frosts.

Some concrete structures have failed by decay of the material,
principally between high and low water mark, and in order to
ascertain the probable causes and to learn the precautions
which it is necessary to take, some elaborate experiments have
been carried out.

To appreciate the chemical actions which may occur, it will be