The Anatomy of Bridgework

by William Henry Thorpe

Cast iron does, however, behave somewhat badly in sea-water, the metal sometimes losing its original character, and becoming in time quite soft; though, if not worn away, as by the attrition of shingle, maintaining its original bulk.

Of some forty-five cast-iron piles belonging to various structures, examined whilst engaged upon sea-pier work for Mr. St. George-Moore, though the author found somewhat diverse results, in no case did there appear to be any general softening of the whole thickness, but a distinct change for some definite distance inwards, generally to be decided without difficulty, beyond which the metal appeared to retain its original character. In all cases any material depth of softening was found close to the ground, this depth rapidly decreasing higher up, till, at a height of 5 feet, but little if any softening could be detected. At 2 feet above ground the softening was frequently but one-quarter of that at ground level. There was, too, often a considerable difference in the behaviour of different piles in the same structure under similar conditions; one pile being found to have only one-fourth part of the softening noticed in others, or possibly none at all. For six different structures the amount of softening near ground level, of about twenty-five piles examined, was as given in the table on the next page.

The greatest depth of softening found (see No. 2) was ⁹⁄₁₆ inch, 1 foot above ground, in a pile thirty-six years old. The decayed material when removed was of a soft, greasy consistency, perfectly black, which a few hours later was found to have changed to a dry yellow powder, by the rapid absorption, it may be supposed, of atmospheric oxygen. It is apparent, therefore, from this example that deterioration may proceed to a considerable depth; but it should be observed that other piles of the set showed softening at ground level of ¹⁄₈ inch only.

Softening of Cast-Iron Piles in Sea-Water.

No.	Age.		Maximum Softening.		Maximum Rate of Softening.						Mean Rate of Softening.					Quality of Metal.	Materials Entered by Piles.
1	17	years	5⁄16	in.	1⁄8	in.	in	7		years	1⁄8	in.	in	15	years	Soft	Extremely soft sandstone.
2	36	„	9⁄16	„	1⁄8	„	in	8	1⁄2	„	No result					„	Rubble mound.
3	32	„	3⁄8	„	1⁄8	„	in	11		„	1⁄8	in.	in	15	years	Moderately hard	Fine sand.
4	38	„	1⁄10	„	1⁄8	„	in	47		„	1⁄8	„	in	140	„	Hard	Extremely hard rock.
5	17	„	Small		Negligible											(?)	Sand and Shingle.
6	14	„	Negligible		Ditto											(?)	Sand.

The least rate of softening noticed, apart from those structures of a more recent date, in two of which it was very slight, occurred in a pier thirty-eight years old (No. 4), where, of three piles tested, two were quite hard, and the third softened ¹⁄₁₀ inch only.

Whatever may be the precise cause of the change, it does not appear to be affected by the period or percentage of immersion during the rise and fall of tides.

Fig. 59.