Tímarit Verkfræðingafélags Íslands


Tímarit Verkfræðingafélags Íslands - 01.12.1964, Qupperneq 35

Tímarit Verkfræðingafélags Íslands - 01.12.1964, Qupperneq 35
TlMARIT VFl 1964 91 7. Niðurstaða. Niðurstaða þessarar tilraunar bendir ótví- rætt til þess, að eiginleiki aurmettaðs vatns til þess að þétta undirlag sitt sé atriði, sem taka má tillit til við hönnun vatnsvirkj- ana á hraunum. Yfirleitt eru aðstæður þannig við stíflugerð á hraunum, að varla er hætta á, að undirstaðan grafist af lekavatni. Er áhætta því mjög lítil að gefa ánum sjálfum tækifæri til þess að þétta jafnvel mjög lek hraun. Reynslan frá millilaginu í holu DI-3 bendir til þess að sandsía þéttist betur og fljótar en hraunin sjálf. Þetta atriði getur verið vert að hafa í huga í sambandi við hönnun á stórum uppistöðulónum, þar sem sandurinn í aurburðinum mun ekki ná upp á grynningar fjarri innrennsli. Þesskonar sandsíur eru raunverulega til á yfirborði margra gamalla hrauna, sem sandorpin eru orðin eða gróin. Aurburður er réttilega talinn mikill bölvaldur, því hann fyllir uppistöðulón og eyðileggur með því vatnsmiðlanir. Sennilega mun aurburður í Þjórsá og Jökulsá á Fjöllum teljast í milljónum m3 árlega, sem að töluverðu leyti mun setjast í væntanleg rafstöðvalón. En á móti leysir aur- burðurinn annað vandamál jarðtæknilegs eðlis, sem allajafna er mjög kostnaðarsamt að leysa á annan hátt. Þetta vandamál er leki úr uppi- stöðulónum á hraunum runnum eftir ísöld, en einmitt á hraunasvæðunum eru margir að öðru leyti mjög álitlegir virkjunarstaðir. Heimildir: Bureau of Reclamation 1960: Earth Manual. Denver. Einarsson, í>. og Tómasson, H. 1962: Búrfell, General Geology (skýrsla til raforkumálastjóra). Harza Engineering Company International 1963: Búr- fell Project, volume II, appendix B, Geology. (Skýrsla til raforkumálastjóra). Harr M. E. 1962: Groundwater and Seepage, New York. Kjartansson, G. 1953: tír sögu Helliskvíslar. Nátt- úrufræðingurinn 23: 1—13. Kjartansson, G. 1961: Tungnaá, skýrsla um jarðfræði- atnuganir á hugsanlegum virkjunarstöðum. (Skýrsla til raforkumálastjóra). Thoroddsen, S. og Tómasson, H. 1964: Dam construct- ion in a geology of bedded basalt lava flows, as eluci- dated by conditions in Iceland. Eight International Con- gress on Large Dams, Edinburgh. S U M M A R Y Seepage anil sediment lo:ul. Considerable part of the water power potentials in Iceland is located in areas where rivers are flowing on very young and highly permeable strata, which are mainly postglacial lava flows. Many of these rivers have considerable sediment load of which the more fine grained part largely comes from the glacier fed com- ponents of the rivers. It seems that the sediment load has tightened the river channels to such extent, that at present the rivers have no connection with the ground water table which may be tens of meters below the riverbed. The tightening effect of the sediment load might be of great value in connection with dam projects in these areas, but the rapidity of the tightening process is un- known. In 1963 an experiment was performed in order to find out whether the tightening by the river Thjórsá is rapid enough to be taken into account when dams are projected on postglacial lavas. Firstly the permeability of the lava fields is described, and it seems that their permeability is of the same order as for open work gravel. This is elucidated both from tests in bore holes and by the ground water flow through the restricted area between Búrfell and Sauda- fell, which is fairly well known. Secondly it is known from geological evidence that the permeability of the lava fields is different, in vertical and horizontal direction and much higher in the horizontal one. The vertical permeability can be decreased in two manners, either by accumulation of sand and silt, (which is much less permeable than the lava) on top of the lavabed or by seepage of sediment ladden water through the lava, where the sediment load is left behind in joints and openings in the lavabed and gradually seals it. There is also geological evidence of big lakes having been dammed up by lava flows and probably in due time then became tight. Krókslón is the biggest of these former lakes and had an area of about 17.5 km= and existed for an interval of about 1500 to 2000 years. The test was performed by pumping water from the river Thjórsá into a borehole situated in the lavafields at Búrfell. The hole is considered as a seepage path through the lavabeds and the tightening of the hole as an example of how natural leakage paths are sealed by seeping water. The test was performed in the following manner: 1) Permeability test I 2) Tightening test I Pumping of water from Thjórsá river into the hole for 17 days. (Samples were taken from the water and its sediment load analysed. The results are shown in table IV and fig. 4). 3) Permeability test II 4) Tightening test II Pumping of water as previously for three days. The results of tightening test I show that the water take of the hole decreased to 1/100 part of what it was when at maximum. Then 3500 ms of water had been pumped into the hole, containing approximately 950 kg of sediment load. In permeability test II the sediment fillings were washed out again, but still the permea- bility was much lower than previously. In tightening test II, the water take was similar to that of the llth, 12th and 13th day of tightening I. The interbed was completely sealed by tightening test I and allowed a pressure of 6—7 kg in permeability test II without

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