Art. XI.—A Description of the Foundation of the Lighthouse in the Ponui Passage.
[Read before the Auckland Institute, 31st July, 1871].
The sandbank known to those engaged in the Thames traffic, and to all who have journeyed there during the last four years as the “Sandspit,” is a well defined feature in the route, narrowing as it does the navigable channel at Ponui, to a passage one-tenth of a mile wide.
At extreme low water it dries to within about forty fathoms of its extremity; the water thence abruptly deepens from six feet to fifteen feet in
the channel. In the greater part of its length it is a narrow ridge stretching three-quarters of a mile in a direction E.N.E. from Pahiki; thence it curves backwards W. by N. for about three-eighths of a mile, assuming a hook-shape. On a flattish table bank in the bend of this hook, the lighthouse has been erected, the depth of water at low springs being five feet, and at high water sixteen feet.
At the request of the late Colonial Marine Engineer, the structure of the sandspit was ascertained—about two years ago—by Captain Burgess, Chief Harbour-master, Auckland, from borings made on the site of the proposed works. It was found to consist of loose sand and shells, with shingle, overlying at a depth of seven to fourteen feet a hard crust, the exact thickness of which was undetermined.
Below this was a homogeneous sub-stratum of soft muddy and sandy clay; a very hard, but not rocky bottom was reached at a depth of thirty-one feet from the surface of the sand.
At the time of Mr. Balfour's death, he was understood to have made some progress with a design for this lighthouse; however, no sketch or trace of such was found, and the writer was intrusted by his Honour the Superintendent with the design and erection of the work.
In determining the nature of the foundations, the question was a narrow one. Screw piles offer by far the most advantageous method of supporting heavy weights on sub-marine sandbanks. But the details of form of screw, area, and pitch of thread, etc., have to be adapted, as well as can be, to the nature of the material in each case; not only with a view to the efficient support of the structure, but also to the screwing, and the amount of torsion necessary to be borne by a long pile (in this case forty feet) working down through stiff material. It is unnecessary to describe in full the calculations leading to the form and elements of the screw pile adopted; it may be sufficient to remark, that for the sake of permanency, cast iron was used for the complete pile (excepting the joint bolts); the stem is ten inches outside and eight inches inside diameter; diameter of screw flange, three feet three inches; pitch, six inches. The form of screw is conical, the thread commencing at nothing, near the pointed end of the pile, and attaining its full width of fourteen and a half inches in one and a half turns, it then made one turn more at the full diameter. The conical shape was found of great service in penetrating the hard crust existing a little way below the surface. Imbedded timber was the only contingency to be feared, and its actual existence at the bottom was proved by one of the borings made by Captain Burgess. Several tests were made before screwing down, to guard against this contingency, and the nature of the ground may in a measure be judged from the fact that little more than the weight of the rods was necessary to send them from the surface to the hard. This
required the best efforts of four men about ten minutes to penetrate, which being effected, a further depth of twenty feet was easily done in about two minutes more. The crust was, in these trials, estimated at a thickness of four inches.
A temporary platform having been built on the site, the screwing down was effected by fitting a strong capstan embracing the body of the pile, which was moved upwards a few feet at a time as the pile descended. This capstan was nineteen feet in diameter over the ends of the bars, which were fitted so that a rope could encircle them, as round a drum. An ordinary single and double power winch was placed in a convenient position, and the power transmitted to the capstan by an endless rope coiled three times round the winch barrel, and twice round the capstan bars; the slack was usually taken up by hand. The pile could be, and usually was, screwed down about five feet, before fleeting the capstan up another lift. The winch was usually placed in single gear, and with this four men could work easily, making twenty-eight revolutions of the handles per minute. The ratios of the winch handles and radius of the barrel, and of the single gearing compounded, show that a force on the handles is increased a little over twenty-five times at the circumference of the barrel. Hence four men at the handles were equal to 100 exerting the same force at the ends of the capstan bars. The circumference of the capstan was sixty feet, and it revolved at the rate of ten feet per minute, or one turn in six minutes. As the average descent of the screw was three inches per revolution, its rate was thus one half inch per minute, requiring for actually screwing the full depth of twenty feet, a period of eight hours. A man's power, working at a winch eight hours per day, is usually taken at 2,600 foot pounds per minute, but as the men in this case never worked more than from one to two hours without stopping, and adjusting the capstan, or other parts, and rests often occurred, we may take the power exerted at 3,000 foot pounds. The circumference of the path of the handles being 8.6 feet, twenty-eight revolutions per minute give 240.8 feet, by which dividing 3,000, we have nearly thirteen pounds as continuous pressure exerted by the hands. This again is equal to (with four men) 1,300lbs. at the capstan rope, moving ten feet per minute, and as each foot required twenty-four minutes to screw down, it follows that it was also attained after an expenditure of 312,000 minute foot pounds, or a force equal to raising 139 tons one foot high had to be exerted to screw the piles down one foot. The slip is stated at an average of three inches per revolution, being 50 per cent. of the pitch. The actual descent was about two and a half inches at the beginning, one and three-quarter inches in passing through the crust, and from three inches to five in the mud, for each revolution.
The total weight of the structure is about sixty tons; this is mainly sup-
ported on the six outer piles, the centre one being relieved of nearly all direct weight, and serving to take the thrust of the other through the bracing, when the building is subjected to high winds. The area of each screw is 8.38 feet, and 1.2 tons thus fall to be borne by each square foot of surface, an amount shown by the screwing force to be far within the limits of safety. The interiors of the piles are filled with good cement mortar, to preserve the iron from rust, and the heads and nuts of the joint bolts are imbedded in the same. Tubular cast iron braces, and the lower timber frame, three feet above high water, complete the foundation which has answered all expectations in the recent exceptionally stormy season.