TUBE BRIDGES.

Hawke's Bay Herald, Volume 15, Issue 2014, 16 December 1870, Page 3

TUBE BRIDGES.

The following description of the light bridge over the Rakaia, contained in a letter from a gentleman in Christchurch, has been kindly placed at our disposal by his Honor the Superintendent. It will, without doubt, be read with interest: — Christchurch, October 10, 1870. My Dear — — — , Perhaps it would be convenient that I should put into writing the substance of what was said in conversation a few days ago about the light bridge over the Rakaia. Mr. White, the contractor for the permanent bridge over the river, put up this light_sti-uoture to servo as a scaffold merely " to carry the pile engine and other implements required in the driving of the piles of the permanent work. It was found after it was finished to have a greater degree of strength and stability than it might have been expected to possess, considering the lightness of the materials put into it. I believe it was erected something in this way : — A 2j inch gas pipe was driven down through the loose shingle of the river bed into the hard ground, below it. The driving was done in the same way that artesian wells are put down, and with such a monkey as is generally used for that purpose. The pipe had at the lower end of it an iron plug, and it was driven until it got well into the hard ground below the loose shingle. Then a frame of timber, ' consisting of three pairs of studs 4x3 and braces rather stronger, was put over the ' pipe that had been driven, and the other two pipes to complete the pier driven down ' between the other two pairs of studs. The : lower horizontal brace being put as far ' down into the shingle as it could be con- ! veniently got, and the longitudinal bearers : laid on the capsills in the ordinary way. ' There have been several heavy freshes in the river since the temporary bridge was finished, and I am told by Mr. White that the piers have not suffered the slightest i disturbance. The experiment, for so it i may as a piece of bridge building be considered, has teen so far successful that the i Canterbury Government purpose to have ■ one of the same kind put up across the Ashburton. But, as the Ashburton bridge < will be of the nature of a permanent structure, so far aa aucli a atructuve as this can i

be made 'permanent, -it is interidedt tSfiiei^ somewhat heavier materials. Sajr3:'m6b?fe pipes instead of 2j incb, and 5 x 4 studs - instead of 4 x 3. The capsills should be ■„•- --10 inch wide and 4 inch thick, and the '; lower walings Bx3 and braces about 6; x 3. If New Zealand timber is used it should be : . hard wood, and, as the scantling of it is ,_- --light, it should be clean and straight Jin jthe ; grain. ." ... [-„ : You may ask how long is such a struc- ■ ture likely to last, what weight will ifc 7 carry, and what will be the cost per foot lineal? ; ~ ;... ; I don't know how long it will last, nor . does anybody else. It will depend on/the < endurance of the pipes. In salt water the pipes would corrode, in a very short time, if they were plain iron, and rather sooner, perhaps, if they were galvanized. I believe they would last many years if they were entirely under water, but bow long they . will last where they are alternately wet and dry, it is impossible to say. I do not know why they should not last as long as the ordinary artesian pipes do. The conditions to which they are submitted are very similar— one in water, and the other generally in mud, and both sometimes wefc and sometimes dry. It is some 10 years ■ ago since artesian pipes were first put down in Christchurch, and I have heard of no instance in which renewal has become neces- * sary through the old pipe being worn out. I think it is not unreasonable to expect that a stout 3 inch pipe would last .10 years in fresh water. It might assist it somewhat to coat the inside with coa! tar before driving it, — pour it in hot TaE'one' end, and let what would run "'but : at the other. Whether galvanising "would help it, I don't know. There is no fear of the current displacing it — I mean such a current aa there is in the Eakaia when in flood, that is, 10 miles or 12 miles an hour, — if it is driven well down into the hard ground. It would not of course do for a river in which there was drift timber, nor would it do for a heavy stream of great depth,— lo feet of water going 10 miles an hour would not hurt it ; but I would not use such a structure at all where the t)p of the pier required to be more than 14 feet or 16 feet above the bed of the river, — that is, there should not be more than 14 feet or 16 feet of pipe out of the ground. As to weight, it will, so far as the pipes are concerned, carry almost any weight that is likely to come on to a cart bridge, and the superstructure may be anything you please. If it has to carry such a weight as a loaded bullock dray, and six or eight bullocks, the piers ought to be no more than 15 feet or 16 feet apart. The longitudinal bearers carrying the flow, of which there should be four in a bridge 12 feet wide, should be at least 12 x d or 12 x 16, and the floor itself 3 inches thick. If a pipe is well driven — say until 20 blows of a l^r cwt. monkey, with a fall of 6 feet, will sink it less than half an inch,— there is no fear that any weight that could come on the bridge in a bullock dray would press it further down. Tbe cost of such a bridge here would be about £2 or £2 sa. per foot lineal ; and it would be considerably less if timber could be got near at hand, so that carriage may be avoided. There is in most of the rivers here a stratum, more or less thick, of hard ground under the loose shingle, and wherever such ground exists, at a depth of a few feet, say 5 feet or 6 feet below the surface of the shingle, such a bridge as this may be put up. The shingle itself is not holding ground, and may as well be away altogether, so far as the piers of the bridge are concerned. I need hardly say that it would not do for a bottom of sand or mud. Though such a bridge cannot be deemed other than temporary, it might be worth while to put it up in some place where it is intended to put a permanent work at some future time. It will serve the purpose of a bridge for some years, and when it is getting shaky it will still serve as a scaffold for putting in the piles of the permanent structure.