Ársrit Verkfræðingafjelags Íslands - 01.01.1914, Blaðsíða 29

Ársrit Verkfræðingafjelags Íslands - 01.01.1914, Blaðsíða 29
29 IV. Reports. — Referate. 1. The Bridge across Ytri-Rangá. A lecture given in the Association of Civil-Engineers in Iceland on Okt. 15th 1912 by Jon Thorlaksson civil engineer, 1. The site of the bridge. Ytri-Rangá has its source in the western edge of the lava round Hekla and fiows in a southwesterly direction through Rangárvalla-district, between Holt and Land on the west side and Rangárvellir on the east side. It joins Þverá a little distance above the latter's mouths. Already in 1899 a road passable for carriages had been carried forward to the western bank of the river at Ægissíða in Holt. The highway led across the river there, which was passed by a ford. It was very difficult to cross the ford with carriages, but the country to the east of the river is dry and flat, all easily passable for carriages, so that no roads are needed there. The need for a bridge across Ytri-Rangá was therefore seriously felt, and after some delay caused by difíerence of opinion abont the most suitable site for the bridge, the Rudget for 1912 provided 45 thousand "krónur" *) for the building of it. The site of the bridge is immediately above the ford, mentioned above. A cross-seclion of the site is shown on the map, Plate 1 (pag 8). At the western bank there is a rock level with the water, but everywhere else a layer of sand on a cliff of sandstone. The current progresses at a rate of 0,9 metre per second at the deepest parts when the river is lowest, and the ave- rage rate is probably about 0,7 m. per sec. So the least quantity of water is very Iikely about 50 cbn,/Sec. The sandlayer above the cliff of sandslone is very lose and consequently usatisfactory as found- ation, and so it was thought necessary to dig it away and rest the piers immediatly on the cliff. The length of the bridge is 92 metres, and equal to the breadth of the channel at the site. Two piers are built in the river, and the spans measure 31,5 -29—31,5 metres. The rivers is much narrower bolh a little above and some way below the site, at Ár- bæjarfoss and Ægissíðufoss; but at either of these places it would have been necessary to build a bridge with one span only (very likely a suspensionbridge), and such a bridge, although shorter, would not have been correspondingly cheaper. Resides, both these places are at some distance from the higd-road. Stone was not to be had above ground at a shorter dislance from the site than about 3 kilometres to the east of the river, and it is very unsatisfaclory, ') ca. 2500 pounds. small, cracked, very hard and easily broken. Still some of it was transported to the site the preceding winter and employed in the eastern pier and abut- ment. At the western end of the bridge there is a rock in the ground; a part of it was blasted and the stone employed in the western pier and abutment. It is dolerite, very solid and easily dressed, resembling in quality the best dolerite to be had near Reykjavík. 2. The abutments and piers. The western abutment is built on a rock, and made of dolerite on the outside with a backing of stone concrete. The eastern abutment is founded on a firm layer of sand, which was reached by digging to the depth of 1 metre. It was built of the same ma- terials and in the same way as the western abutment. The western pier stands near the rapidest cur- rent and greatest depth of the river. It was built in the following manner. A casing of piles, 4 by 8 metres wide, was rammed down round the site of the pier with a 300 kg. monkey. The planks were 3 inches thick, and 6 by 6 in at the corners. Then the layer of sand was remowed with a bagdredger. At a depth of 2,20 metres below the level of the waler, solid bottom was reached, impenetrable with the bag. Then the boltom was levelled, by dragging across it a channel iron ([ iron) hanging in a wooden frame which was driven to and fro on the top of the pile- planking. In this manner the bottom was dressed completely level. Then a caisson of concrete was made in the following way. A platform was placed below tfae surface of the water, hanging in 8 screw-bolts of ll/8 in. iron. The studs under the upper beams were placed, some on the pile planking and some driven down into the bottom inside it. The caisson is ex- hibited on Plate 2 (page 8); its height was 2,30 metres. It was all reinforced. Eight days after it was finished, it was screwed down till it floated on the water within the pile-planking, drawing then about 1 metre. Then the platform was removed from under it, and with a first layer of concrete filled in. A liltle before the caisson had reached the bottom, it was sunk by filling it with water, in order to trj' if it settled on the bottom wilhout leaning. It stood per- feclly plumb; and then the water was pumped out of it and it was filled with stone concrete. Then the pier was built on this support. The total height of the pier from bottom is 5,2 metres.

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